ICEAF VII - Book of Abstracts

e-book of Abstracts Conference co-Chairmen Prof. Emeritus Spiros Pantelakis, University of Patras Prof. Michael Vormwald, Technische Universität Darmstadt

1 Contents Performance and characterization of materials and structures (Abstracts 1-83)................... 2 Fracture of materials and structures, and failure analysis (Abstracts 84-162)................... 106 Manufacturing, Joining Technologies, and Surface Engineering (Abstracts 163-209).... 213 Environmental Degradation, NDT, and SHM (Abstract 210-246)....................................... 273

2 Performance and characterization of materials and structures (Abstracts 1-83)

3 1 A study of methods for fatigue parameters and behavior estimation of quenched and tempered steels Basan R1 , Marohnić T1 , Marković E1 , Iljkić D1 1University Of Rijeka, Faculty Of Engineering Quenched and tempered (Q&T) steels group comprises both unalloyed and alloyed steels possessing a chemical composition suitable for quenching and tempering with achievable hardness depending primarily on their carbon content. Quenching and tempering processes can be controlled to produce a wide range of properties within a single material, which makes Q&T steels suitable and regularly used in high-performance components as found in the automotive, aerospace, and industrial equipment industries. Since such components need to be lightweight and withstand demanding conditions, specific properties at different locations within a component are needed in order to achieve a good combination of strength, toughness, fatigue and wear resistance. Hence, they are often surface hardened and as such, have functionally graded materials (FGM) featuring a gradient in microstructure and material properties across their volume. For modeling and FEA simulations of FGM materials and components number of models can be used, such as homogenization models, multi-constituent models that represent FGMs as a series of different materials with distinct properties, graded element models as well as microstructural models. Main challenges when modeling functionally graded materials arise due to their varying properties and the lack of detailed information on their material behavior. For multi-constituent models and graded element models in particular, methods for estimation of materials stress-strain and fatigue parameters and behavior may provide a way to overcome these difficulties during initial analyses early in the product design process. This study addresses estimation of fatigue behavior and parameters specifically of Q&T steels group regarding their FGM applications. Selected estimation methods are re-evaluated considering the fact that most of them were developed on and for groups including wider range of metallic materials. Even methods that differentiate between unalloyed and low-alloy steels were developed so that unalloyed steels datasets included also structural steels which are not intended for heat treatment and carbon steels such as casehardening steels, while low-alloy steels group in some cases included other, non-tempering steel grades. Results of analysis including determined suitability of individual estimation methods for Q&T steels are presented and outlook for development of methods tailored for this important group of steels is given with the aim to improve estimation accuracy and improve simulation of stress-strain response and fatigue behavior of Q&T steel components both with homogeneous and functionally graded materials. Acknowledgments: This work has been fully supported by Croatian Science Foundation under the project IP-2020-02-5764 and by the University of Rijeka under the project number uniri tehnic-18-116. The work of doctoral student Ela Marković has been fully supported by the „Young researchers’ career development project – training of doctoral students” of the Croatian Science Foundation.

4 2 Fatigue life estimation of corroded welded steel joint using probabilistic approach Pastorcic D1 , Vukelic G2 , Bozic Z3 1University of Zadar, Maritime Department, 2 Faculty of Maritime Studies University of Rijeka, 3 Faculty of Mechanical Engineering and Naval Architecture University of Zagreb This study presents a probabilistic model to estimate fatigue life of the free corroded welded joint using the concept of the limit state function. In addition to limit state function for the S-N analysis, the linear elastic fracture mechanics approach is also considered in order to give information about the crack propagation, which is important for NDT in-service inspections and repairs of the offshore structures. Nondestructive testing methods detect the cracks with certain probability which is also taken into account. Probabilities are assessed using Monte Carlo method in specially devised computer routines, where the long term stress distribution and the accumulated fatigue damage are stochastic variables. The calibration of parameters of the fracture mechanics theory was performed for the case when the initial size of the crack is unknown and considered as a stochastic variable and for the case when is set as a corrosion pit. This can be assumed because of the similarities in the shape and dimensions between pits and cracks that emanate from them and small fatigue cracks. In the first case fatigue life is equivalent to crack propagation life, while in the second case fatigue life comprises crack initiation and crack growth periods. The size of the corrosion pit is obtained from the real long term experiment whereby welded shipbuilding steel specimens were exposed to the corrosive environment (sea and sea splash) in the north Adriatic for 6,12, 24 and 36 months. The threshold stress intensity factor range and threshold stress range for the pit to crack transition were estimated with El Haddad's model for small cracks. It is shown that calibrated LEFM material parameter C, hence the crack growth rate is very sensitive to the choice of the initial crack size. Consequently, there are differences in probability assessment when updating failure probability after nondestructive test. Therefore, this can lead to inadequate and over conservative estimations of the NDT inspection intervals. For given threshold probability, this difference can be noticed after the fourth and fifth year of service life for the example in this research.

5 3 Fatigue strength analysis of thin steel plates Chmelko V1 , Semeš M 1 Slovak University Of Technology Many load-bearing components in transport and construction technology are made of sheet metal. The assessment of the fatigue life of such components has its own specificities. The initiation point of fatigue cracks is often the edges that have been formed by splitting from the original sheet metal. The cutting technology significantly affects the condition and properties of the material, as it introduces residual stresses and strain hardening into the material. In this paper, three different low carbon steel sheet materials (S355, S500, S700) will be analyzed. Comparison of the results of cyclic tests of the sheet metal specimens and different cutting technologies (punching, laser cutting) as well as analysis of fatigue crack initiation sites will be presented. Edge of sheet metal after cutting Fatigue crack initiation

6 4 Improving the Fatigue Design of Mechanical Systems such as Refrigerator Woo S1 1Ethiopian Technical Uniersity To enhance the lifetime of mechanical system such as automobile, new reliability methodology – parametric Accelerated Life Testing (ALT) – suggests to produce the reliability quantitative (RQ) specifications—mission cycle—for identifying the design defects and modifying them. It incorporates: (1) a parametric ALT plan formed on system BX lifetime that will be X percent of the cumulated failure, (2) a load examination for ALT, (3) a customized parametric ALTs with the design alternatives, and (4) an assessment if the system design(s) fulfil the objective BX lifetime. So we suggest a BX life concept, life-stress (LS) model with a new effort idea, accelerated factor, and sample size equation. This new parametric ALT should help an engineer to discover the missing design parameters of the mechanical system influencing reliability in the design process. As the improper designs are experimentally identified, the mechanical system can recognize the reliability as computed by the growth in lifetime, LB, and the decrease in failure rate. Consequently, companies can escape recalls due to the product failures from the marketplace. As an experiment instance, two cases were investigated: 1) problematic reciprocating compressors in the French-door refrigerators returned from the marketplace and 2) the redesign of hinge kit system (HKS) in a domestic refrigerator. After a customized parametric ALT, the mechanical systems such as compressor and HKS with design alternatives were anticipated to fulfil the lifetime – B1 life 10 year.

7 5 Influence of Asymmetric Fillet Geometry on Spur Gear Fatigue Life Trumbić N1 , Vučković K2 , Čular I2 , Galić I2 1Rimac Technology d.o.o., 2University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture Introduction Gear root load-carrying capacity is primarily influenced by fillet geometry [1]. Many different fillet geometries and optimization approaches have been tried to increase gears' load-carrying capacity. One such approach was proposed by Akpolat [2] and Yilmaz et al. [3]. Authors proposed using an asymmetric root profile to minimize root stresses in spur gears. Such a method is reported to improve the load-carrying capacity by up to 12% [3]. Root stress was determined in [2,3] using static FE analysis by applying load to the highest point of single tooth contact. In this research, the effect of asymmetric fillet geometry on root fatigue life was investigated. Moreover, the effect of the adjacent tooth was also explored. Methods Referent and modified gear geometry are specified in Table 1. Linear-elastic root stresses and strains are obtained from the finite element analysis performed in ABAQUS. Then, the stresses and strains are fed into the bending fatigue model proposed by Vučković et al. [4]. Results After analyzing the results obtained by the numerical model, it was observed that tooth root stresses in modified gears are lower than those in referent gears. The reduction in tooth root stresses is comparable with previously published works. On the other hand, the difference in fatigue life is not as big as expected. It is assumed that the main reason for this is the effect of the adjacent tooth. The slight bi-directional loading experienced by the tooth root is considered (Fig. 1) with this effect [5]. Conclusion The results have shown that asymmetric fillet geometry can benefit the tooth root load-carrying capacity of spur gears. Tensile stresses are reduced due to the larger radius of curvature in the root of the gear's working flank. On the other hand, the actual differences in fatigue life might not be as high as expected. It is also worth noting that the benefits of such geometry are only present in gears that transmit torque unidirectionally because the load-carrying capacity of one flank is increased at the expense of the capacity of the other flank. References [1] – Sanders, A., Houser, D., Kahraman, A., Harianto, J., Shon, S. (2011). An Experimental Investigation of the Effect of Tooth Asymmetry and Tooth Root Shape on Root Stresses and Single Tooth Bending Fatigue Life of Gear Teeth. Proceedings of the ASME Design Engineering Technical Conference. 8. DOI: 10.1115/DETC2011-48303. [2] – Akpolat, A., Yildirim, N., Sahin, B., Yildirim, O., Karatas, B., Erdogan, F., (2018) Reduction of tooth root bending stresses in gears generated by symmetric cutter with asymmetric tip radii. Gear Solutions. [3] – Yılmaz, T., Doğan, O., Karpat, F. (2017). Stress Analysis of Thin Rimmed Spur Gear with Asymmetric Trochoid. DOI: 10.11159/icmie17.132. [4] – Vučković, K., Čular, I., Mašović, R., Galić, I., Zezelj, D. (2021). Numerical model for bending fatigue life estimation of carburized spur gears with consideration of the adjacent tooth effect. International Journal of Fatigue. 153. 13. DOI: 10.1016/j.ijfatigue.2021.106515.

8 [5] – Linke, H., Börner, J. (1994) The Influence of Neighbouring Teeth on the Tooth Root Capacity. Table 1 - Gear geometry Figure 1 - Adjacent tooth effect – illustration

9 6 Influence of residual stresses caused be shrink fit on strength of hollow shafts Le L1 , Suchy L1 , Hasse A1 1Universtiy of Technology Chemnitz Shafts are essential for driving components in all machinery and equipment. Friction connec-tions, such as shrink fits, are a proven and widely used option for transmitting loads in drive trains. With the increase in trend factors in design, hollow shafts are becoming more important in areas such as lightweight design, downsizing and reduced material consumption. Two possi-ble design rules based on nominal strength for determining the fatigue strength of shrink fit hollow shafts are DIN 743 and the FKM guideline. This paper shows the limitations in the appli-cation of DIN 743 and the FKM guideline for hollow shafts with high interference. The fatigue strength of the shrink fit was determined by experimental tests and simulation. The experimental long-term strength was determined using the staircase method at a load cycle limit of 10e+6 cycles. The materials used were C45 and 42CrMo4. The picture shows examples of fractured shafts under rotating bending and torsion. The shaft fails at the undercut in front of the shaft shoulder radius. In addition to the experimental databases, long-term strength was also determined using DIN 743 and the FKM guideline. For both materials, the fatigue strength according to the guideline is 20-40% higher than the experimental fatigue strength. In order to obtain more information on these differences, simulations are also carried out for both materials and geometries. The elastic-plastic material properties in the simulations were obtained from constant amplitude strain control experiments and incremental step tests. The diagram shows the stresses of the shrink fit at the undercut. The normal mean stress in the y and z directions caused by the interference is at the radius of the undercut at its peaks. The simulation result shows the location of the failure at the undercut. Due to the high pressure caused by the interference fit, a mean stress is already present in the unloaded state. This mean stress is superimposed on the cyclic stress and therefore reduces the fatigue strength. In DIN 743 and FKM, the mean stress caused by the interference is not included in the calculation. Therefore, there is a difference between the guidelines and the experiment. In further research, a material model with kinematic and isotropic hardening is included in the simulation. Another important factor is the ratcheting of the mean contact stress over many cyclic loads. Key words: shaft-hub connection, hollow shaft, kinematic and isotropic hardening, long-term strength

10 Failure at hollow shaft under bending (left, middle); stress distribution of amplitude and mean stresses (right)

11 7 Minimizing Porosity in 17-4 PH Stainless Steel Through Cold Pressing and Sintering for Improved Resistance to Static and Dynamic Loads Dr. Mikó T1 , Markatos D2 , Koulouris K2 , Troiani E3 , Gacsi Z1 1University of Miskolc, Institute of Physical Metallurgy, Metalforming and Nanotechnology, Hungary, 2 Laboratory of Technology & Strength of Materials (LTSM) Department of Mechanical Engineering and Aeronautics, 3Department of Industrial Engineering DIN Discontinuities in metals and alloys are commonly recognized as the primary cause of failures, as cracks, porosities, and inhomogeneities inside the structures negatively affect their physical, chemical, and mechanical properties. These defects typically result from the solidification process, but traditional Ingot Metallurgy (IM) methods can often eliminate them via plastic deformation and extended heat treatments. However, Powder Metallurgy (PM) and Additive Manufacturing (AM) techniques offer fewer opportunities for this purpose. Despite this, these processing methods are increasingly important in various industries, including automotive, aeronautical, medical, and other industrial sectors mainly due to the design flexibility they offer and the reduced costs and waste. Yet, insufficient sintering or welding leading to porosity is widely recognized as the most significant challenge in producing bulk metallic materials using powder-based methods such as PM or AM. Therefore, one of the primary objectives of researchers is to minimize porosity to obtain properties similar to or better than wrought products. In this context, the present study introduces a novel approach to enhance the properties of samples produced with gas atomized 17-4 PH stainless steel powder, using the cold pressing and sintering technique. To this end, the spherical morphology of the powder was modified to irregular by short time milling before the cold pressing, to achieve better pressability properties and minimize the amount of porosity in the green sample. Cold pressed Charpy, rotating bending, and tensile samples were produced using the modified powder, and were subsequently sintered using induction heating at different temperatures. The porosity of the produced specimens was measured to evaluate the effectiveness of the sintering process in reducing the amount of voids or gaps in the material. The fracture behavior of the specimens was also analyzed to assess their resistance to failure under static and dynamic loads. Preliminary results have shown that higher sintering temperatures and longer sintering times significantly reduced porosity while enhancing the material's strength and toughness. In addition, tafel tests will be carried out in order to access the corrosion resistance of the samples.

8 An interesting fatigue phenomenon in 316L stainless steel processed by surface mechanical rolling treatment Jiang Y1 , Henderson S1 , Liu S2 , Wang X2 1University Of Nevada, Reno, 2 Zhejiang University of Technology Tests were conducted to study the fatigue behavior of 316L stainless steel with a gradient nanostructured surface layer formed by surface mechanical rolling treatment (SMRT). SMRT is a surface process by repeated and deep rolling at a high strain rate, which results in a nano-grained thin layer near the surface of the processed workpiece. The plate specimen has a cross section of 7.0mmx5.5mm, and only two opposing surfaces were processed by SMRT. The round specimen has a diameter of 10mm in the gage section. The grain sizes of the nanostructured surface layer range from 30 nm to 300 nm while the coarse-grained base material has an average grain size of 70 μm. The stress-life fatigue curves show a significant enhancement of fatigue strength due to the surface process, especially on the round shaped testing specimens. The strainlife fatigue curve of the plate SMRT specimens is practically identical to that of the base material. However, the strain-life curve of the round SMRT specimens is significantly higher than that of the base material particularly in the high cycle fatigue regime. When a strain amplitude is lower than 0.4%, fatigue cracks are found to initiate in the base material. Cracks are initiated in the SMRT layer when the strain amplitude is higher than 0.4%. For the round specimens, the strain-life curve displays a distinguishable kink point at the demarcation strain amplitude of 0.4%, while the strain-life curve of the plate specimens does not clearly show such a kink point. Mechanisms associated with the enhanced fatigue property by SMRT are discussed in light of the mechanics of the gradient material and the microstructures. Residual stresses and the stress states in the nano-grained layer and the coarse-grained base material are attributed to the observed phenomenon.

13 9 Microstructure evaluation of cryogenically hardened and tempered 5%Cr hot-work tool steel Papageorgiou D1 , Tsarouxa A2 , Mouzakis D3 , Manolakos D1 1 Laboratory of Manufacturing Technology, School of Mechanical Engineering, National Technical University of Athens, 2 Shipbuilding Technology Laboratory, School of Naval Architecture and Marine Engineering, National Technical University of Athens, 3Mechanics of Materials Laboratory, Sector of Mathematics and Engineering Applications, Department of Military Sciences, Hellenic Army Academy The tools manufactured of hot work steel must process a combination of strength, wear resistance and toughness [1]. During cryogenic hardening sequence, the crystallographic and microstructural changes on tempering result in precipitation of a finer distribution of carbides in tempered martensite, with subsequent increases in both wear resistance and toughness [2]. The greatest improvement in hardness, thus wear resistance, is obtained by carrying out a deep cryogenic treatment between quenching and tempering whilst the increase is not as high after tempering [3]. In this context, a PhD dissertation on improving the performance of H13 hot work tool steel used in aluminum extrusion dies by optimizing its wear resistance and toughness though hardening via cryogenic treatment sequence is currently carried out. In the present work, the microstructure of the specific tool steel grade after shallow and deep cryogenic treatment (SCT and DCT) is studied. While the transformation of retained austenite into martensite is primarily pursued in the shallow cryogenic treatment, the precipitation behaviour of carbides is modified due to DCT in addition to the transformation of retained austenite [4]. Initially, groups of cryogenically hardened specimens were reheated covering the tempering range of 1800C to 6300C for the SCT and DCT respectively in order the tempering diagrams of the material to be determined. Core samples after tempering to 1800C and 2500C, as well as to the range of temperatures of the secondary hardening peak to 500, 525 and 5500C were investigated by light and scanning electron microscopy. Carbides are shown to be enriched by Chromium and nanocarbides were found in both SCT and DCT samples. The majority of precipitated carbides are formed in-between martenisitic laths. Martensite of fine needles can be seen in both cryogenic procedures. Nanocarbides are secondary precipitates, which are distinguished due to their very small size. The retained austenite is present in all groups of samples. The spherical vanadium carbides are desirable as they increase the wear resistance of the steel. In the context of this investigation, DCT leads to a more homogeneous distribution of secondary carbides in comparison to SCT. In terms of cryogenic temperature and soaking time, the microstructure of the material was investigated and will support the design of the cryogenic hardening sequence (austenitizing temperature, cryotreatment, tempering stages) for the specimens for mechanical testing (wear and toughness). 1. https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=234 (accessed on 3/12/2022). 2. T. Holm, Cryotreatment – State of the art 1997 – an Update, AGA Internal Report, 1997. 3. S. Li, Y. Xie, X. Wu, Hardness and toughness investigations of deep cryogenic treated cold work die steel, Cryogenics,50, 2010. 4. S. Acar, G. Gerstein, F. Nürnberger, C. Cui, A. Schulz, M. Steinbacher, M.Wunde, J. Kurzynski, Deep Cryogenic Treatment of X153CrMoV12 Cold-work Tool Steel, Proceedings of 11th Tooling Conference, RWTH, Aachen, Germany, 2019.

14 10 On the high-cycle fatigue properties of a near-net shape manufactured high-nitrogen tool steel Ms. Faezeh Javadzadeh Kalahroudi1 , Dr. Mohamed Sadek1 , Giulio Maistro2 , Krishnan Hariramabadran Anantha2 , Thomas Mikael Grehk1 1Karlstad University, 2Uddeholms AB. Near-net shape manufacturing is one of the promising techniques for producing components to precise tolerances with a cost-effective production with only a few machining steps and small amounts of wasted material. In this study, a high-nitrogen tool steel was produced by near-net shape manufacturing using powder metallurgy (PM) and hot isostatic pressing (HIP) process. This high-strength martensitic steel has the capabilities of high corrosion resistance, good machinability, and high wear resistance. It is composed of about 12% hard phase particles, including nitrides and carbonitrides, which allows a higher amount of chromium in the solid solution; hence, improves the corrosion resistance of the steel. In the HIP process, high temperature and high gas pressure are simultaneously applied to the materials powder, leading to a pore-free component with fully isotropic properties. To evaluate the fatigue properties of the produced HIPed materials, high-cycle fatigue testing was performed using a four-point bending test under a stress ratio of 0.1 and a frequency of 10 Hz. The effect of surface quality on the fatigue strength of the HIPed samples was evaluated. To evaluate the quality of the HIPed sample, the fatigue life was compared to the conventionally produced materials (HIPed and forged). Fatigue strength data (using the stair-case method) and S-N diagrams were determined to the ultimate number of load cycles of 106. In addition, fractography was used to determine the failure mechanisms for the different material conditions. The results showed that the dominant failure mechanism in both HIPed and HIPed + forged samples was inclusions. The location (surface or subsurface) and geometry of the inclusions had a great impact on the fatigue performance of the samples. Investigation of the samples with different surface conditions revealed that the fatigue properties were very sensitive to the surface quality, and the major failure mechanism turned out to be surface defects.

15 11 Nano-scale Characterisation, Deformation and Failure Mechanisms in Enhanced-Performance Modern Steels Kaldellis A1 , Makris N1 , Fourlaris G1 , Tsakiridis P1 1 Lab. of Physical Metallurgy & Center for Electron Microscopy, School of Mining & Metallurgical Engineering, National Technical University Of Athens This research project aims to experimentally study the nature and fundamental characteristics of microstructure, deformation and failure mechanisms, focused on various and complex nanoscale phases, such as precipitations and paraequilibrium phases, not yet completely explained in modern nanophase steels by state-of-the-art literature. Moreover, this work focuses on the investigation of the correlation of the microstructure with its impact on the mechanical properties in laboratory-developed novel nanophase steels, which mainly consist of a ductile ferrite matrix, and are strongly affected by appropriate alloy design and thermomechanical treatment differentiations. The microstructural investigation will be accomplished by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), in conjunction with Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) analysis, as well as mechanical properties determination via tensile and hardness tests. The experimental approach has been designed based on the -nano to macro-examination of the materials under investigation to overall understand and explain their nature and properties. Finally, the outcome endeavors to interpret the nanophases’ influence, as well as the contribution of alloying elements and thermomechanical routes on the microstructure-properties relationship of these novel nanophase steel

16 12 Characterisation of TCP Precipitation Sequences of Superaustenitic Stainless Steel and Correlation with Electrochemical and Mechanical Properties Kountouris N1 , Makris N1 , Ioannidou D1 , Alexandratou A1 , Deligiannis S1 , Kaldellis A1 , Tsakiridis P1 , Fourlaris G1 1 Lab. of Physical Metallurgy & Center of Electron Microscopy, School of Mining & Metallurgical Engineering, National Technical University Of Athens In this study, microstructural characterization along with electrochemical corrosion testing was carried out to investigate the influence of precipitation sequences formed at critical high-temperature range for various ageing time of super austenitic stainless steel 654SMO against the evolution of its corrosive resistance performance. The specific type of stainless-steel alloy is one of the most corrosion resistant along with exceptional mechanical properties, tailor-made for pressurized and erosive systems handling chlorinated sea water, plate heat exchangers, and fuel gas cleaning applications and an excellent case study of in-depth phase transformations-electrochemical properties characterization phenomena. The microstructure evolution was characterized by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), Vicker’s hardness, and potentiodynamic polarization curves. The corresponding precipitation characteristics and sequences of secondary phases were discussed and correlated with electrochemical properties. The results demonstrate σ phase with various morphologies formed during isothermal aging and the formation of three more secondary topologically close pack (TCP) phases, such as chi phase (χ), Laves phase (A2B), and Cr2N against corrosive resistance and mechanical performance, which are of primary importance for understanding electrochemical phenomena of different phases and useful for optimizing such alloys for various applications.

17 13 Performance Analysis of High-speed and High-pressure Non-contact Mechanical Seals under Typical Failure Conditions Zhao X1 , Liu Y1 , Liao H1 , Li H1 , Huang A1 , Liang Z1 1Tsinghua University Non-contact mechanical seals rely on the micron fluid film between the rotating ring and stationary ring, which plays a key role in sealing leakage and safety, to achieve non-contact operation under high-speed and high-pressure conditions, as shown in Fig.1. The typical seal failures are mainly due to the defect damages of the seal face and the loss of following characteristic of the compensation ring. The impact mechanism analysis of the surface defect damages and the dynamic compensation ability for sealing characteristics are not very clear at present. In this paper, a thermo-elasto-hydrodynamic model is established, to calculate and analyze the influence on sealing performance with the surface defects like scratches, pits, and roughness, which are damaged by impurities in the medium during operation. The performance parameters of liquid film thickness, pressure distribution and seal leakage under various defects are compared in order to obtain the influence rule, dangerous position and critical threshold of every defect. For the dynamic compensation failure of the seal, Iwan model is introduced to calculate the damping at the secondary seal which determines the dynamic characteristics of the seal, as shown in Fig.2. The kinetic equations, coupled flow field analysis, are established to quantitatively calculate the effect of the variation of the secondary seal parameters on the axial compensation performance of the main seal. The performance of mechanical seal under typical failure conditions is revealed theoretically, which has potential application value in sealing ring repair and reuse. It provides a guidance method on structural design of the secondary seal to ensure excellent dynamic following performance of main seal based on the Iwan model. Diagram of non-contact mechanical seal Diagram of Iwan model

18 14 Phase field modelling of low cycle fatigue in the framework of nonconventional thermodynamics Tsakmakis A1 , Vormwald M1 1Technische Universität Darmstadt In fracture mechanics, phase field theories have been introduced for the first time for brittle materials, in order to regularise the sharp crack topology. Specifically, the crack surface part of the total energy functional is regularised by using a phase field variable D. Then, the governing equations are obtained by minimising the total energy functional. There are other approaches for phase field fracture, e.g., by employing concepts of microforces or the Ginzburg-Landau equation or non-conventional thermodynamics. In the present work, a phase field approach for ductile fracture in the framework of non-conventional thermodynamics is proposed. In contrast to brittle fracture, the physical mechanisms for fracture are supposed to be initiation, growth and coalescence of voids, driven by plastic deformation. Thus, the aim of the paper is to demonstrate how well-established ingredients of continuum damage mechanics and fatigue phase field theories can be fit in the proposed framework. For instance, incorporation of Lode-angle and stress triaxiality into the constitutive model allows to capture a wide range of crack patterns, see Fig. 1. Generally speaking, the choice of material parameters has a very big influence on the predicted crack paths. Finally, several numerical examples and experimental comparison are used to demonstrate the capabilities of the proposed theoretical framework in predicting crack propagation phenomena. Predicted crack paths for different constitutive functions of Lode-angle and stress triaxiality.

19 15 Statistical analysis of parameters and behavior of quenched and tempered steels Marohnic T1 , Basan R1 , Marković E1 , Smokvina Hanza S1 1University Of Rijeka, Faculty Of Engineering Previous research on parameters and behavior of steels showed that statistically significant differences exist among steels divided according to their alloying element content, i.e. to unalloyed, low-alloy and high-alloy steels. Feature selection of properties that could potentially be used to estimate those parameters and behavior showed that different properties are relevant for estimation of parameters and behavior of each steel subgroup. Thus, the existing methods for estimation should be selected carefully, and new estimation methods should be proposed separately for each steel subgroup. Steels of different alloying element content are suitable for different heat treatment processes, which result in specific microstructure that determines their mechanical properties such as strength, ductility, and toughness. By controlling the heat treatment of steels, it is possible to tailor their structure and properties to meet specific design requirements. For example, the microstructure of steel after quenching and tempering (Q&T) depends on the chemical composition of the steel and the specific het treatment process used. Q&T steels are commonly used in applications where high performance is required and are thus of great interest to engineers and researchers. Q&T steels can be found within both, unalloyed and low-alloy steels, but regardless of the chemical composition, Q&T mainly results in a microstructure that is a combination of martensite and tempered martensite. Since data on microstructure are often unavailable or incomplete, a good alternative when estimating parameters and behavior of steels is to take into account the combination of alloying element content and heat treatment. A thorough statistical analyses were performed within the framework of this research in order to establish whether the parameters and behavior of Q&T steels show statistically significant differences from unalloyed and low-alloy steels. Feature selection techniques were used to determine which properties are relevant for estimation of parameters and behavior of Q&T steels and compared to those that proved to be relevant for estimation of parameters and behavior of unalloyed and low-alloy steels. Considering such design-relevant subgroup of steels separately can be used to improve the estimations. Acknowledgments: This research has been fully supported by Croatian Science Foundation under the project IP-2020-02-5764 and by the University of Rijeka under the project number uniri-tehnic-18-116. The work of doctoral student Ela Marković has been fully supported by the „Young researchers’ career development project – training of doctoral students” of the Croatian Science Foundation.

20 16 Fatigue behaviour of advanced high strength steels Sierra-Soraluce A1 , Gomez A1 , Banis A2 , Petrov R2 , Molina-Aldareguia J3,1, Dutta A4 , Sabirov I1 1 IMDEA Materials Institute, 2Department of Electromechanical, Systems and Metal Engineering, Ghent University, 3Department of Mechanical Engineering, Universidad Politécnica de Madrid, 4ArcelorMittal Global Research and Development Advanced high-strength steels (AHSS) are widely used in automotive, aerospace, and construction industries due to their high strength-to-weight ratio and good formability. However, they are susceptible to high-cycle fatigue failure. The high-cycle fatigue behaviour of AHSS is influenced by various factors such as microstructure, loading conditions, and environmental conditions. The microstructure of AHSS, including grain size, phase distribution, and inclusion content, affects the fatigue strength and crack initiation and propagation behaviour. High cycle fatigue life is generally controlled by fatigue crack initiation, and delaying it one can improve fatigue performance of materials. Several approaches have been proposed to enhance fatigue resistance of AHSSs, including microstructural design, surface treatments, and optimization of loading conditions. Microstructural modification involves optimizing the microstructure of AHSS through heat treatment or alloying to improve fatigue performance. Surface treatments such as shot peening or surface coatings can also improve the fatigue strength of AHSS. Optimization of loading conditions such as reducing stress amplitude, controlling mean stress, and adjusting loading frequency can also improve the fatigue life of AHSS. Our work aims to understand the mechanisms of the fatigue crack initiation and propagation in the newly developed AHSSs to improve their fatigue life via further microstructural design. We studied the high cycle fatigue behaviour of multiphase quenched and partitioned (Q&P) martensitic stainless steels and austenitic Fe-Mn-Al-C steels. We have shown that the volume fraction of retained austenite and fresh martensite determine the fatigue performance of the multiphase Q&P treated steels. Their content can be tuned via alloy and Q&P treatment design. Meanwhile, kappa-carbides play the key role in fatigue resistance of the austenitic Fe-Mn-Al-C steels. Their morphology and content can be effectively controlled by aging treatments.

21 17 Texture and anisotropy investigation on austenitic lightweight steel Villa G1 , Mombelli D1 , Barella S1 , Gruttadauria A1 , Mapelli C1 1Politecnico Di Milano A wide and ongoing research is focalised on High Mn and high Al steel alloys because of their lightweightness (density 16% lower than a typical stainless austenitic steel like AISI 306L) and their very high mechanical properties (up to 800MPa of Yield Strength and up to 55% of elongation at fracture). Such high mechanical properties are reached due to precipitation hardening of carbides typically observed in FeMnAlC systems. To exploit such feature the alloys usually undergo thermo-mechanical treatments. As a consequence of their properties, such steels could be very interesting for the automotive sector, but to be employed in any industrial sectors specific properties must be checked and studied e.g. the formability. In this study, the evolution of the anisotropy and the texture in an austenitic lightweight steel alloy (X100MnAl30-9) have been studied considering different thermo-mechanical conditions. And it was observed that, while the solution treatment significantly affects the texture, the ageing treatments haven’t affected the microstructure preferential orientations.

22 18 GTAW effect on austenitic lightweight steel’s microstructure and mechanical properties Villa G1 , Mombelli D1 , Barella S1 , Gruttadauria A1 , Mapelli C1 1Politecnico Di Milano A wide and ongoing research is focalised on High Mn and high Al steel alloys because of their lightweightness (density 16% lower than a typical stainless austenitic steel like AISI 306L) and their very high mechanical properties (up to 800Mpa of Yield Strength and up to 55% of elongation at fracture). Such high mechanical properties are reached due to precipitation hardening of carbides typically observed in FeMnAlC systems, but such precipitation can either have positive or detrimental effects on the mechanical properties. Because the most likely application would be in automotive sector, also further properties must be checked and studied e.g., the weldability. The welding process could be very problematic as the FeMnAlC steels are usually high alloyed, prone to grain growth and it could lead to uncontrolled precipitation or microstructural transformations. In this study an austenitic lightweight steel alloy (X100MnAl30-9) has been tested with GTA welding technique and different material starting conditions. Microstructure and mechanical properties of the joints have been investigated and despite the pessimistic premises, robust welding was observed and the expected detrimental microstructural transformations haven’t occurred.

23 19 Study of edge cracking during hot rolling of lightweight Fe-Mn-Al-C steels using high-speed camera Dutta A1 , Duprez L1 , Waterschoot T1 1Arcelormittal Global R&d Gent Previously, some of the problems of cracking as well as edge cracking during two phase hot rolling (e.g. duplex stainless, high Mn steels) have been attributed to the effect of hot ductility, difference of flow behaviour between the different phases. There is limited work and knowledge regarding the exact cracking mechanism as a function of rolling pass number, temperature, mean flow stress or reduction percentages. Using a high-speed camera, it is possible to address this and extract valuable information for optimizing the rolling process for such steels. In a lab environment, a high-speed camera setup was used while rolling high manganese and aluminium steel concepts (Fe-Mn-Al-C alloys). Data such as crack initiation, crack propagation with each rolling pass were obtained for various rolling schemes, considering different reduction ratios, starting rolling temperatures etc. Information from such experimental schemes was analyzed based on alloying composition and thermodynamic equilibrium diagrams in the rolling temperature regions. Process maps were created incorporating data from the high-speed camera and phase equilibrium information. Insights from such studies were used to avoid edge cracking in some alloy compositions by adjusting the processing parameters during rolling. Such studies can be used to further optimize the processing of such alloys or variation in alloying elements to avoid such dual phase regimes which are prone to hot ductility. High-speed camera usage to understand the cracking phenomenon during rolling

24 20 The effect of aging treatment on the microstructure and mechanical properties of Fe-Mn-Al-C lightweight steels on macro- and microscale Gomez-fernandez A1 , Sabirov I1 , Monclús M1 , Avella M1 , Dutta A2 , Molina-Aldareguia J3,1 1 IMDEA Materials Institute, 2AccelorMittal Global Research and Development, 3Department of Mechanical Engineering, Polytechnical University of Madrid The automotive industry is interested in Fe-Mn-Al-C lightweight steels. Low density Fe-Mn-Al-C steels are one of the emerging class of structural materials for applications in the automotive industry. These steels exhibit outstanding tensile mechanical performance at room and cryogenic temperatures while offering weight reduction of up to 18% owing to the high Al content. Moreover, they exhibit attractive properties such as high energy absorption behavior and toughness at room temperatures. Their application would allow significant lightweighting of automotive structures. The austenitic lightweight steels are the most promising in terms of their processability and properties. Artificial aging increases their mechanical strength due to the formation of kappa-carbides via spinoidal decomposition. This work focuses on the effect of artificial aging on the microstructure and mechanical properties of a Fe30Mn-9Al-1C alloy. The microstructure is analyzed using SEM, EBSD, and HRTEM techniques. Mechanical properties are studied on macroscale via tensile testing and microscale via nanoindentation. The relationship between microstructure and mechanical properties on macro- and microscale is analyzed. HRTEM image (dark-field) showing kappa-carbides in the Fe-30Mn-9Al-1C alloy after artificial aging at 550º C for 8 h Typical engineering stress-strain curves from tensile testing

25 21 Direct Quenching and Partitioning for Novel Tough Ultrahigh Strength Steels Somani M1 , Ghosh S1 , Kantanen P1 , Porter D1 , Kömi J1 1University Of Oulu In recent years, the potential use of quenching and partitioning (Q&P) as a way of improving the work hardening capacity and uniform elongation of hot-rolled structural steel has been extensively explored. In the Q&P processing, the steel is austenitized, quenched to a temperature between the martensite start (Ms) and finish (Mf) temperatures and held at a suitable temperature for a requisite time to allow the partitioning of carbon from martensite to austenite, which can thereby be partially or fully stabilized down to room temperature. A martensitic matrix has the potential to provide the required strength, while a small fraction of retained austenite finely divided between the martensitic laths is expected to provide improved work hardening and uniform elongation without a loss of impact toughness. At the University of Oulu, a novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) has recently been established that has shown good potential for the development of tough ductile ultra-high strength structural as well as hard abrasion-resistant steels. TMRDQP processing route has come a long way, ever since the first experiments were successfully conducted using 0.2C (high) Si and/or Al steels. Right from designing appropriate compositions to establishing the DQP parameters with the aid of physical simulation, the emphasis was essentially pointed at ensuring the applicability of the process for industrial hot strip production. Detailed interpretation of dilatation curves, supported by comprehensive metallography and calculations, showed that besides carbon partitioning, isothermal martensite and bainite form at the partitioning temperatures. Small volume contractions seen at the start of partitioning in some unstrained high-aluminium steels have been tentatively attributed to the interface migration from austenite to martensite. Controlled ausforming in the no-recrystallization regime resulted in extensive refining and randomization of transformed martensite packets/laths, besides fine division of austenite, thus resulting in an all-round improvement of mechanical properties. The potential of TMR-DQP process has since been extended to process medium carbon steels with special emphasis on developing innovative approaches to realize excellent combination of properties, though low Ms temperatures often require changes in partitioning approaches. The key research strategy encompasses studying structural refinement approaches, phase transformation characteristics, and accompanying microstructural mechanisms using advanced metallography as well as analytical techniques including atom probe tomography. The accomplishment of reasonable ductility and high toughness with these complex microstructures has been characterized in the light of associated deformation mechanisms. A brief account of the status and merits of direct quenching and partitioning processing will be presented in this paper.

26 22 Understanding Mn and C segregation at the phase boundary in medium Mn steel Syed F1 , Sun B1 , Ponge D1 , Raabe D1 1Max-planck-institut Für Eisenforschung Gmbh Understanding the solute decoration state of Mn and C at the ferrite-austenite interface is crucial for enhancing the performance of medium Mn steel. Despite the significant strides in the thermodynamics of grain boundary segregation, phase boundary thermodynamics has not received similar attention. Therefore, this study aims to identify and quantify the dominant mechanism responsible for phase boundary segregation in a medium Mn alloy. A detailed investigation using APT analysis was performed on a Fe-10Mn3Al-0.2C wt‰ alloy with a two-phase microstructure of ferrite-austenite, which underwent a low temperature annealing at 450⁰C for varying durations (2, 50 and 200 hours). In-depth analysis concerning the fundamental mechanism promoting the observed segregation will be discussed. Furthermore, an analysis on how the orientation relationship between adjacent ferrite-austenite grain impacts the C and Mn segregation behaviour will be presented.

27 23 The effect of κ-carbides on the deformation of Fe-Mn-Al-C steel after aging Banis A1 , Gomez A2 , Sabirov I2 , Petrov R1,3 1Ghent University, 2 IMDEA Materials Institute, 3Delft University of Technology The current work focuses on the interaction between κ-carbides and dislocations during the deformation of low-density steels. Such steels contain high amounts of manganese and aluminum, alloying elements that tend to decrease the overall density of the material. In this work, the studied steel alloy contains, 28% manganese, 8.7% aluminum, and 0.9% carbon. Upon aging, fluctuations in the chemical composition and short-range ordering lead to the precipitation of κ-carbides. These carbides have a perovskite-type lattice and a lattice misfit with the austenite matrix of less than 3%. Due to their nano-scale size, coherency with the matrix, and small interparticle spacing, the gliding dislocations cannot bypass the carbides, and the pinning effect occurs. Then, with higher strains, the shearing of the carbides takes place, which leads to a decrease in the total elongation and work-hardening rate at a larger particle size and higher volume fraction. On the contrary, at the initial stages of aging, where the κ-carbides are finer and at a lower volume fraction, the dislocations can move more freely in the matrix, leading to increased ductility. This work aims to determine the threshold at which the size and fraction of κ-carbides favor the balance between stress and strain to avoid undesired loss of elongation. To do so, High-Resolution Transmission Electron Microscopy (HR-TEM) and Transmission Kikuchi Diffraction (TKD) are employed to study the effect of the κcarbides in deformed samples under different applied tensile strains. Due to the high aluminum and manganese content, the Stacking Fault Energy is high in the studied material, and the deformation mechanism observed is the Micro-Band Induced Plasticity (MBIP). According to this, due to the reduced dislocation mobility caused by the κ-carbides, the dislocations rearrange themselves to accumulate the strain energy and create dislocation walls and micro-bands. These bands will rotate at even higher strains, creating misorientations inside the grains. Such misorientations have been observed in TKD, as shown in Figure 1, while a sheared κ-carbide is shown in Figure 2. TKD Inverse Pole Figure map showing the misorientations formed in the grain from the rotation of the micro-bands

28 Inverse Fast Fourier Transformation TEM image showing the atoms that belong to a single κ-carbide, which has been sheared along the direction indicated by the arrows

29 24 Microstructure, texture, and properties correlation of an Ultra-Fast Heattreated commercial grade steel Papaefthymiou S2 , Banis A1 , Sabirov I3 , Petrov R2,4 1Ghent University, 2National Technical University of Athens, 3 IMDEA Materials Institute, 4Delft University of Technology Ultra-Fast Heating (UFH) has become a subject of interest in the automotive industry due to its potential to produce complex steel microstructures that improve both strength and ductility. Parts with such microstructures can be used in the body-in-white of a vehicle, thereby reducing its overall weight and carbon emissions without compromising the safety of the passengers. This study aims to investigate whether UFH can replace the conventional methods currently used in the automotive industry. To achieve this, a commercial steel grade typically used in vehicle bodies was heat-treated with high heating rates of 800 °C/s without or with very short austenitization times. The microstructure and mechanical properties of the UFH-treated samples were compared to the conventionally heat-treated steel of the same commercial grade. The microstructure of the UFH-treated samples after intercritical annealing with ultrafast heating showed partial spheroidization and dissolution of the cementite found in the pearlitic colonies of the initial microstructure. This was due to the significantly shorter diffusion distances resulting from the high heating rates and the short austenitization times, which favored the refinement of the ferrite, parent austenite grains, and martensite of the final microstructure. In contrast, the microstructure of the conventional sample was typical of a Dual Phase steel, with martensite islands on a ferritic matrix. The UFH-treated sample, on the other hand, consisted of non-dissolved cementite, ferrite, martensite, and some traces of retained austenite. The recovery and recrystallization processes during ultrafast heating are initiated at higher temperatures in comparison to the conventional heating rate (10 °C/s). These processes overlap with the austenitization and remain incomplete. Consequently, the UFH-treated samples showed crystallographic textures similar to those of initial cold-rolled steel sheets, indicating that the recrystallization process was impeded to a great extent. The anisotropy of the heat-treated steel sheets was also studied through texture and tensile tests. The UFH-treated samples showed increased strength, up to 270 MPa, while the elongation remained in the range of 13%-24%, which is the same as in the conventionally heat-treated steel. However, the average normal anisotropy (Lankford value) of the UFH steel sheets was higher than the one measured in the conventionally heat-treated sheets. The fractographic analysis showed that the fracture type is mixed ductile and brittle when the tensile axis is parallel to the rolling sheet direction but brittle when the tensile axis is perpendicular to the rolling direction (i.e., parallel to the sample transverse direction). In conclusion, this study shows that UFH has the potential to replace conventional methods currently used in the automotive industry. The UFH-treated sample showed improvements in strength, without compromising its ductility, due to the formation of very fine, multiphase structures.

30 The microstructure of the conventional heated (CH) and ultra-fast heated (UFH) samples consists of ferrite (F) plus martensite (M), and ferrite, martensite, and undissolved spheroidized cementite (SC), respectively.

31 25 Fracture characterization of structural steel S690Q by using mini-CT specimens Sánchez M1 , Cicero S1 , Arroyo B1 1 LADICIM (Laboratory of Materials Science and Engineering), University of Cantabria, E.T.S. de Ingenieros de Caminos, Canales y Puertos, Av/ Los Castros 44, Santander, 39005 Cantabria, Spain, Santander, Spain Mini-CT specimens (see Figure 1) are an interesting alternative when characterising the fracture behaviour of structural materials and there are issues with regard to, for example, the amount of available material, the irradiation level (in nuclear materials) or material inhomogeneities. Furthermore, in ferritic-pearlitic steels, the characterisation of the fracture behaviour within the ductile-to-brittle transition zone (DBTZ) is of particular interest, given that the material may behave very different, in terms of fracture toughness, when operating at different but relatively close temperatures within this zone. Methods In many occasions, the definition of the DBTZ behaviour is performed through the Master Curve (MC) methodology and, thus, by testing standardised fracture specimens (e.g., CT, SENB) and determining the material Reference Temperature (T0). The use of mini-CT specimens to define T0 has been validated in a wide range of steels used in nuclear industry, but its application to structural steels has been scarce. Thus, this work gathers the fracture characterisation results (T0) obtained in structural steel S690Q, comparing them to those obtained by using conventional standardized SENB specimens. It is shown that, for this particular structural steel, the use of miniaturized specimens provides T0 estimations which are comparable to those values obtained from conventional larger specimens. Thus, mini-CT specimens may be used to characterise the DBTZ of steel S690Q. Original half conventional CT specimen and extracted mini-CT and Small Punch specimens.

32 26 Formation of the Residual Stresses at Welding Seams at and under the surface at S235 and S355 Müller E1 , Hermann T1 1Bochum University Of Applied Sciences Welding seams are often the origin of cracks under dynamic load that may give a failure of the component. Because of high temperature near the welding seam during the welding procedure structural changes appear and the residual stress stage changes significant. The cooling process depending on the thickness of the material, the kind of welding and further parameter influence the formation of the residual stress stages in the welding zone. If tensile residual stresses are obtained the propability the crack starting rises. [1] In this case two different materials (S235; S355) were investigated. The samples were MAG-welded with and without fixation. The sample size was 180 mm * 100 mm * 10 mm. The residual stress was determined by X-ray diffraction parallel and perpendicular to the welding seem up to 35 mm distance in a step wide of 1.5 mm for all 4 parameter combinations and at most of the measuring spots a residual stress profile up to 200 µm in the depth was determined. Figure 1 shows one whole distribution of residual stresses with the measuring direction perpendicular to the seam. The heat-effected zone (with the different colors) is corresponding with the residual stresses. The investigation gives a contribution to the simulation of residual stresses by welding that is very sensitive to the parameters during welding and the material. References [1] V. Schuler, J. Twrdek, Praxiswissen Schweißtechnik. Springer Fachmedien, Wiesbaden, 2019. Keywords: welding; seam; residual stresses; S235; S355 Remark: The talk was already given at the ICRS in Nancy but no proceedings are planned. Residual stresses perpendicular to the seam with the corresponding sample

33 27 Cold-forming of quenched and partitioned martensitic stainless steels: from Nakajima to simulation Sierra-soraluce A1 , de Pablos J1,2, Smith A3 , Muratori M4 , Sabirov I1 1 Institute Imdea Materials, 2Universidad Politécnica de Madrid, 3RINA Consulting - Centro Sviluppo Materiali, 4ACERINOX S.A.U. An approach to cutting down transport emissions is to lighten the vehicles themselves. Reducing crosssections of the frame elements needs to be compensated with materials presenting improved strength. With the novel heat treatment of "quenching and partitioning" (Q&P), it is possible to improve the mechanical properties of steel balancing strength and ductility. This is achieved by introducing retained austenite in the microstructure, which enables the transformation-induced plasticity (TRIP) effect. Q&P-treated martensitic stainless steel (MSS) is a promising option for manufacturers, showcasing improved ductility while maintaining strength and corrosion resistance. This work explores the formability of Q&P-treated MSS, as it has not been studied previously. Three martensitic stainless steel alloys (410, 420 and 420ma) have been treated via Q&P. The resulting materials have been microstructurally characterized via SEM, EBSD and TEM, and their basic mechanical properties via uniaxial tensile testing to assess the result of the heat treatment. Nakajima testing is used to obtain the forming limit diagram of the Q&P-treated MSSs. Then, the microstructure of deformed samples is characterized to observe its evolution during formability testing. FEM models are fed with the experimental tensile data, using the Johnson-Cook model to simulate the Nakajima experiments. Finally, two automotive parts are simulated with the fitted model to asses their formability during cold-forming. From Nakajima experiments, the FLD showcases that the best and worst formability belongs to alloys 410 and 420ma, respectively. Alloy 410 presents 9.6 % of retained austenite, and alloy 420ma 18.7% of retained austenite. The microstructures of selected Nakajima specimens of these two alloys are characterized. A progressive consumption of the retained austenite phase is observed, increasing stress triaxiality in both alloys. In alloy 420ma deformed in biaxial mode, the remaining volume fraction of retained austenite is more significant than in similar samples of alloy 410, showing that the benefits of the TRIP effect are not utilized to their full potential. The tensile properties do present an improvement, reaching uniform elongations up to 17.9 % with UTS of 1642 MPa (alloy 420). The developed computational models have been validated to a satisfactory degree against Nakajima testing. The validated model is applied to the simulation of cold-forming two automotive parts: a B-pillar and a tunnel. The tunnel simulation is successful, while the B-pillar fails due to the latter's complexity. From the results of the previous work, the following conclusions are drawn: • Local plastic strain tends to increase with increasing stress triaxiality value in all studied alloys. • The TRIP effect plays an essential role as the volume fraction of RA is inversely proportional to the local plastic strain. Increasing stress triaxiality promotes RA => M phase transformation, as the former is less stable under a complex stress state.

34 • The simulations fit reasonably well with the observed low ductility of these alloys during the experiments. • Complex geometries are not achievable with the cold forming of the studied steels. Graphical Abstract

35 28 The deceit of steel strength ductility diagrams: A case study on high manganese lightweight steel. Elkot M1 , Sun B2 , Ponge D1 , Raabe D1 1Max-planck-institut Für Eisenforschung, 2Key Laboratory of Pressure Systems and Safety, School of Mechanical and Power Engineering Strength-ductility diagrams are the most common way of demonstrating the potential of advanced high strength steels (AHSS). High strength-ductility combination is attractive as it offers weight reduction in structural applications. However, the impressive mechanical properties of AHSS are usually associated with microstructural changes that could negatively influence other technical properties. In this work, we studied the case of austenitic high manganese lightweight steels. The strength-ductility product of these steels can be significantly increased via the precipitation of κ-carbides. It is believed that selecting the optimum age hardening temperature would favour the homogeneous precipitation of the beneficial grain interior κcarbides and hinder the heterogeneous formation of the detrimental grain boundary (GB) κ-carbides. In this work, we offer a new understanding of the mechanism of the formation and growth of GB of κ-carbides based on atom probe tomography (APT) of samples containing GBs of both solution-treated and age hardened conditions. We explain why it would be challenging to completely avoid the precipitation of these precipitates. We also demonstrate the mechanism connecting GB carbides and the decrease in impact toughness, low temperature toughness and hydrogen embrittlement resistivity.

36 29 Improving resistance against hydrogen embrittlement of high strength medium Manganese TRIP steels by heterogeneous Mn distribution Part I: Hydrogen damage mechanisms in medium Mn TRIP steel Ponge D1 , Sun B1,2, Raabe D1 1Max-Planck-Institut fuer Eisenforschung GmbH, 2 School of Mechanical and Power Engineering, East China University of Science and Technology Medium Manganese TRIP steels combining high strength with high ductility are attractive for weight reduction of cars. However, the strain induced martensitic transformation (TRIP effect) decreases the resistance against hydrogen embrittlement (HE). We present the damage mechanisms due to hydrogen (Part I) and a new approach to increase the HE resistance by tailoring the microstructure (Part II). The HE resistance can be significantly increased by adding mechanically stable austenite into an ultrafine microstructure. This ductile phase serves as a dead end for microcracks by blunting them. However, in order to increase ductility by a TRIP effect, additional metastable austenite is required. Such a tailored microstructure with stable AND metastable austenite and ferrite can be produced by multistep annealing: Atom probe results reveal that during intercritical annealing reverted austenite is formed with the equilibrium partitioning of Mn. This is due to the local equilibrium at the moving ferrite/austenite interface during austenite reversion. We exploited this for a medium Mn steel (0.2C–10Mn–3Al–1Si in wt.%) to form stable austenite (high Mn partitioning) at 700°C for crack blunting and metastable austenite (lower Mn partitioning) at 750°C to enhance the ductility by a TRIP effect. This approach is based on achieving a heterogeneous Mn distribution in the austenite and results in a significantly increased HE resistance and still a similar strength and ductility for the tailored microstructure (ferrite+stable austenite+metastable austenite) in comparison to the conventional microstructure (ferrite+metastable austenite).

37 30 Improving resistance against hydrogen embrittlement of high strength medium Manganese TRIP steels by heterogeneous Mn distribution Part II: New approach to increase the hydrogen embrittlement resistance of medium Mn TRIP steel Ponge D1 , Sun B1,2, Raabe D1 1Max-Planck-Institut fuer Eisenforschung GmbH, 2 School of Mechanical and Power Engineering, East China University of Science and Technology, China Medium Manganese TRIP steels combining high strength with high ductility are attractive for weight reduction of cars. However, the strain induced martensitic transformation (TRIP effect) decreases the resistance against hydrogen embrittlement (HE). We present the damage mechanisms due to hydrogen (Part I) and a new approach to increase the HE resistance by tailoring the microstructure (Part II). The HE resistance can be significantly increased by adding mechanically stable austenite into an ultrafine microstructure. This ductile phase serves as a dead end for microcracks by blunting them. However, in order to increase ductility by a TRIP effect, additional metastable austenite is required. Such a tailored microstructure with stable AND metastable austenite and ferrite can be produced by multistep annealing: Atom probe results reveal that during intercritical annealing reverted austenite is formed with the equilibrium partitioning of Mn. This is due to the local equilibrium at the moving ferrite/austenite interface during austenite reversion. We exploited this for a medium Mn steel (0.2C–10Mn–3Al–1Si in wt.%) to form stable austenite (high Mn partitioning) at 700°C for crack blunting and metastable austenite (lower Mn partitioning) at 750°C to enhance the ductility by a TRIP effect. This approach is based on achieving a heterogeneous Mn distribution in the austenite and results in a significantly increased HE resistance and still a similar strength and ductility for the tailored microstructure (ferrite+stable austenite+metastable austenite) in comparison to the conventional microstructure (ferrite+metastable austenite).

38 31 Parametric study of guided wave propagation in honeycomb sandwich panel for model-assisted damage assessment method Fiborek P1 , Kudela P1 1 Institute of Fluid Flow Machinery, Polish Academy of Sciences Objective The research subject was a parametric analysis of a model-assisted damage identification function (MADIF) in a honeycomb sandwich panel. The MADIF, obtained using computer simulations, determines how the size of the damage affects the propagation of the guided wave in the inspected panel. The analysis included various parameters of the core, the skin and the piezoelectric transducers affecting wave propagation. Methods The numerical simulations determined the MADIF based on the spectral element method for various widths of the rectangular disbonds of the core and the skin. The sandwich consisted of the aluminium honeycomb core, the carbon fibre-reinforced polymer skin, the adhesive layer between the core and the skin and piezoelectric transducers (PZT) attached to the skin by cyanoacrylate glue. The simulations assumed the following models of each panel component: full core geometry model with shell elements, shell elements for the adhesive layer and cyanoacrylate glue, and solid elements for the skin and the PZT. Moreover, the interface elements were used to join all components together. The disbonds were modelled by removing the interface elements within the damaged area between the core and the adhesive layer. To determine MADIF, the damage index based on the root mean square deviation (RMSD) was used. Once the function was validated experimentally, the numerical simulations were performed in various parameters of the core geometry and material properties of the components. Results The MADIF was obtained for a rectangular 500×500×1.6 mm3 panel with a damage width in the range of 0- 120 mm. Two PZTs were attached to the panel top surface at a distance of 200 mm from each other. Damage was symmetrically located between the PZTs. The comparison of the MADIF and the corresponding experimentally obtained function (EDIF) is presented in Figure 1. It can be seen that it is in very good agreement, achieving an absolute error of less than 4 mm. A parametric study was then conducted to indicate how the various simulation parameters affect wave propagation. Since the values of some of these parameters can change due to environmental conditions, they must be considered when assessing the damage. An example of the signals obtained for different dielectric permittivity of the PZT is presented in Figure 2. Conclusion The research developed the function for damage severity assessment in the honeycomb sandwich panel. Then a parametric study was performed for various factors of the specimen components. The analysis

39 showed that the phenomenon of elastic wave propagation in the panel is very complex, and signal response strongly depends on many parameters. The model-assisted damage identification function (MADIF) and the experimental damage identification function (EDIF) based on the root mean square deviation (RMSD) The signal envelopes for the dielectric permittivity in the range of 80-120% of the reference value

40 32 Experimental and Numerical Investigation of the In-Plane Shear Behavior of A-5052 Honeycomb Core under monotonic tension loading Pikilidis J1 , Tsirigotis A2 , Sevastianos N2 , Kermanidis A1 , Labeas G2 1 Laboratory of Mechanics and Strength of Materials Department of Mechanical Engineering University of Thessaly, 2 Laboratory of Technology and Strength of Materials Department of Mechanical Engineering and Aeronautics University of Patras Abstract Honeycomb sandwich structures are increasingly used in lightweight transport applications [1],[2], due to their excellent stiffness to weight ratio, good impact behaviour and attractive mechanical properties. For primary structural applications the honeycomb core, which contributes to the significant density reduction in the lightweight design, needs to be optimized under different types of loading conditions. In the present work the in-plane shear properties of aluminium 5052 honeycomb core structure have been investigated under macroscopic tension loading according to the ASTM-C273 standard (Figure 1). The stiffness, shear modulus, elastic limit and shear strength of honeycomb core have been evaluated in longitudinal and transverse honeycomb directions to examine the anisotropy in shear material behaviour. The experimental results indicate that the ribbon direction (double-wall direction) of the honeycomb core plays significant role in the behaviour of the honeycomb structure, increasing the shear modulus, elastic limit and maximum strength with regard to the transverse honeycomb direction. A numerical analysis was performed to simulate the experimentally investigated in-plane shear behavior of the honeycomb material. For this purpose, firstly a suitable unit cell finite element model has been assembled in order to perform a buckling analysis and a nonlinear analysis, and then a finite element model of the entire honeycomb core panel was developed. Comparison of the numerical with the experimental results showed that the numerical simulation can satisfactorily describe the elastic deformation modes (Figure 2), onset of elastic buckling and predict reasonably well the shear modulus and elastic limit in the honeycomb panel. Key words: Honeycomb core, Shear behaviour, Fracture behavior, Elastic loading unloading, Elastic properties References [1] B. Castanie, C. Bouvet, M. Ginot, Review of composite sandwich structure in aeronautic applications, Composites Part C: Open Access, 1, 2020, 100004, ISSN 2666-6820 [2] D. Verstraete, P. Hendrick, P. Pilidis, K. Ramsden, Hydrogen fuel tanks for subsonic transport aircraft, International Journal of Hydrogen Energy, 35, 20, 2010, 11085-11098, ISSN 0360-3199.

41 Figure 1 In plane shear of honeycomb core. Experimental setup in tension Figure 2 Deformation modes under in plane shear i) experimental observation ii) numerical analysis

42 33 Mechanical behavior of perforated and unperforated aluminum honeycomb core under shear loading Gastens M1 , Dafnis A1 , Schröder K1 1 Institute For Structural Mechanics And Lightweight Design - Rwth Aachen University The increasing demand for lightweight structures in air and space transportation applications has led to the development and use of sandwich structures often using metallic honeycomb cells as core. These structures combine low weight with exceptionally high stiffness and strength behavior. Perforated cells of the sandwich core are often used in aerospace systems to allow pressure compensation during flight, but it is unclear how this affects the mechanical properties of the sandwich structure. This study aims to investigate experimentally the behavior of aluminum honeycomb core under shear loading with a focus on perforated and unperforated cells. For this purpose, an experimental setup based on the standards DIN 53294 and ASTM C273-00 are used introducing shear loading into the specimens under compression direction. The objective of this investigation is to identify important parameters of both the experimental setup and the specimen itself which influence the mechanical properties of the specimen such as shear modulus and shear strength. In relation to the specimen, a specific objective was also to identify and describe mechanisms and boundary conditions which occur during the production process and influence its mechanical properties. To achieve these objectives, quasi-static shear tests was carried out in both W- and L-direction using aluminum core of the same geometry as well as with and without cell perforation. The evaluation of the test campaigns showed that loading in the W-direction no significant influence on the shear modulus due to the perforations was observed. Different in L direction where an increase of the shear modulus due to the perforations was identified. Furthermore, the maximum shear stress was only influenced by the load direction and not by the presence of perforations. The experimental reperformances also demonstrated that only consistent boundary conditions and nominal alignment of the core shapes during the test ensure reliable comparison results. The test results showed a good agreement with numerical investigations where local boundary conditions of the specimen are implemented in the simulations. Noticeable differences in the global quasi-static behavior between perforated and unperforated cells were recognized, demonstrating the effect of the presence of perforations on the mechanical properties of honeycomb aluminum cores under shear loading.

43 34 Multi-physics finite element model of a general aviation liquid hydrogen fuel tank Tzoumakis G1 , Lampeas G1 1University Of Patras As the effects of climate change are becoming evident, guidelines regarding significant emission reductions emerge from entities all around the world. As transportation accounts for a significant part of the overall emissions, several alternative fuels are investigated, with hydrogen currently considered as one of the most promising. When produced carbon-free, it allows the complete elimination of CO₂ and CO emissions. Its usage in fuel cells emits only water vapor and provides extremely high efficiency. When combusted in a thermal engine, low particle and NOx emissions can be expected which can be further reduced with an optimized combustion process. Hydrogen is exceptionally attractive as an aviation fuel due to its heating value of 120 MJ/kg, while fuels like Jet A-1 or Avgas have a heating value around 42 MJ/kg, meaning hydrogen has about 2.8 times more energy per mass unit. However, hydrogen has an extremely low energy density of just 10 MJ/m³ under ambient conditions (25 ⁰C, 1 atm) while liquid hydrocarbons have energy densities in the range of 31-36 GJ/m³. Compression can significantly increase the energy density up to 4.5 GJ/m³ at 700 bar, while liquid hydrogen (LH₂), at a temperature of -253 °C, has an energy density of 8.5 GJ/m³, making it the most viable solution volume wise of storing hydrogen inside aircraft. LH₂ tanks should meet conflicting requirements, as they have to demonstrate low heat losses, high strength and a low weight, while having a low cost. The application of LH₂ as an aviation fuel has been investigated in only a limited number of projects and still has a low Technology Readiness Level. While prototypes like the Tupolev Tu-155 have demonstrated the feasibility of LH₂ as a fuel, advanced parametric modelling is required to support the design and optimization of LH₂ related components like the large cryogenic tanks. In this direction, the present work refers to the development of a parametric multi-physics finite element model of a cryogenic fuel tank designed to store liquid hydrogen with an energy content equivalent to the average amount of avgas carried by 4-6 seat general aviation aircraft. The developed model comprises of a thermal part for calculation of temperature distributions and heat flux, and a structural part that uses the results of the thermal as inputs and combines them with other mechanical loads in order to perform stress/strain analysis. The simulation results are used to assess both structural and thermal performance of the tank and its mass efficiency as a function of its design parameters, providing valuable inputs for the optimization process.

44 35 Development of a numerical methodology for the analysis of the postbuckling and failure behaviour of composite stiffened panels considering the effect of initial debonding Psihoyos H1 , Fotopoulos K1 , Lampeas G1 , Waleson J2 , Brethouwer M2 1ATHENA-RC, 2 Fokker/GKN Aerospace Composite stiffened panels are engineering structures usually used in aerospace applications. The presence of defects formed during the manufacturing or scheduled/unscheduled maintenance of these structures and service conditions, such as barely visible impact damage (BVID) [1], have detrimental effect on the performance of the structure during its service conditions [2]. For this reason, it is crucial to investigate the combined effect of these defects with the failure mechanism of the composite materials on the residual strength and behaviour of the structures in order to assess their damage tolerance, providing information for their design. The failure behaviour of composite stiffened panels has been studied in literature for various combinations of materials and configurations. The observed damage modes have been quite complex and interacted, including intra-laminar damage (matrix cracking, fibre breakage and fibre-matrix shear failure), inter-laminar damage (debonding) between two adjacent layers and buckling failure [3]. Moreover, the debonding between the skin and the stiffeners have been observed which dramatically decreases the load capacity of the structure [4]. For the modelling of these phenomena various modelling approaches have been utilized. Cohesive Zone Modelling (CZM), Virtual Crack Closure Technique (VCCT) [1] for the modeling of debonding between the structural components and Progressive Damage Modelling (PDM) for the modelling of intralayer damage of the composite material. In the current work, a numerical modelling framework for the analysis of a panel fabricated by a AS4D/PEKKFC composite material is presented. An initial damage is embedded between the stiffener and the skin to investigate the effect of defects on the panel’s compressive behavior and the panel has been experimentally tested. PDM and a bilinear CZM procedures have been employed for the modelling of intra- and interlaminar damage in the composite material, respectively. The PDM/CZM framework aims at the prediction of initiation and propagation of the damage developed during the compression. Meanwhile, a large deformation analysis has been combined with the previous methods for the capture of buckling and postbuckling response of the structure. The predicted force-displacement results and buckling modes correlate quite well with experimental ones validating the ability of the method to provide an insight into the failure mechanism of the composite panel. Acknowledgements: This research work was supported by the EU Clean Sky 2 programme TAILTEST [no. 865123]. References: [1] Van Dooren, K.S., Tijs, B.H.A.H., Waleson, J.E.A., Bisagni, C. Skin-stringer separation in post-buckling of butt-joint stiffened thermoplastic composite panels. Compos. Struct. 304, 2023, 116294.

45 [2] Riccio A, Raimondo A, Fragale S, Camerlingo F, Gambino B, Toscano C, et al. Delamination buckling and growth phenomena in stiffened composite panels under compression. Part I: an experimental study. J Compos Mater 48 (23), 2014, 2843–55. [3] Zhao L, Wang K, Ding F, Qin T, Xu J, Liu F, et al. A post-buckling compressive failure analysis framework for composite stiffened panels considering intra-, inter- laminar damage and stiffener debonding. Results Phys 13, 2019, 102205. [4] Yetman JE, Sobey AJ, Blake JIR, Shenoi RA. Investigation into skin stiffener debonding of top-hat stiffened composite structures. Compos Struct 132, 2015, 1168–81.

46 36 Stiffened panel crack propagation simulation by representative fuselage fatigue spectrum Kordas P1 , Lampeas G1 1Athena Research Center A typical flight-mission profile of a passenger airplane consists of several phases during which different structural components experience complex sequences of alternating loads. When focusing on a specific section of a fuselage, it is usually the case that such sequences will be comparable between them in shape and form, especially during the cruise portion of the flight. More specifically, it is expected that the stiffened thin shell of the fuselage will develop membrane stresses due to cabin pressurization, as it is simultaneously affected by bending, caused by the change in lift due to wind gusts and maneuvers. For a substantial part of their operational usage, most short and medium range aircrafts are cruising at relative high altitudes, making this phase of the flight the predominant period during which flaws can either develop or grow inside the fuselage structure. A large amount of fatigue data has been collected and organized by various studies in exceedance diagrams. The loading parameters are first normalized by the average 1-g flight reference and then arranged so that all cumulative occurrences of positive-negative stress excursions are catalogued for a set number of flights executed or hours flown. By selecting one of these diagrams which is characteristic to fuselage loading and adapting it to a pre-specified loading combination, it is possible to define a fatigue spectrum for any individual part of the fuselage. The resultant spectrum consists of pressurization-depressurization cycles, on which semi-stochastically defined bending cycles will be superimposed. In the present work a curved stiffened panel, manufactured using a 3rd generation AL-Li alloy by means of Friction-Stir-Welding-Method (FSW), along with a novel for panel-testing experimental arrangement, are simulated in a Finite Element (FE) environment and then subjected to a combination of pressure and axial loads to simulate realistic fuselage fatigue loading. The FSW welding methodology is a cutting-edge solidstate joining process and its effect on fatigue propagation of cracks located in these areas is currently an important research topic. After a presentation of the test-rig, detailed FE static analyses are carried out to set the de-normalization stress values for the spectrum. Subsequently, preliminary analytical predictions of crack growth are carried out, with regard to through-cracks located at critical locations inside the panel. Acknowledgement: This research has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007881 (under the call H2020-CS2-CFP11-2020-01): DEMONSTRATE ‘Demonstration of Novel Fuselage Structural Integrity‘

47 37 Utilization of nanostructured coatings for tribocorrosive applications E.P. Georgiou1 , A. Koutsomichalis1 , D. Drees2 , J.-P. Celis3 Fountas N, Papantoniou I, Kechagias J, Manolakos D, Vaxevanidis N 1 Department of Aeronautical Sciences, Hellenic Air Force Academy, Dekelia Air Base, 13671, Attika, Greece 2 Falex Tribology NV, Wingepark 23B, 3110 Rotselaar, Belgium 3 Department of Materials Engineering (MTM), K.U. Leuven, Kasteelpark Arenberg 44, 3001 Leuven, Belgium Corrosion is a naturally occurring phenomenon that results in the degradation of materials due to a chemical and/or electrochemical reaction of their surface with the surrounding environment. Corrosion phenomena can be significantly accelerated by the simultaneous occurrence of a mechanical load onto the surface of a material, as the formation of cracks and surface defects, along with surface strain and stress fields lead to faster diffusion of corrosive ions or the destruction of protective layers (depassivation). Thus, the synergism between corrosion and wear phenomena can lead to accelerated failure of components. In recent decades, nanostructured materials are finding their way into various industrial and technological applications, due to their superior mechanical and corrosion properties over conventional materials. This combination makes them excellent candidate materials for tribo-corrosive applications where both phenomena coexist. In this work, we present some indicative examples of how nanostructured coatings can be used in applications to provide improved protection compared to existing benchmark materials. The tribo-corrosive ranking of the materials is done under relevant to the application conditions, whereas the dominant failure mechanisms are analysed in order to better understand the behaviour of the materials.

48 38 Investigation and calculation of longitudinal compressive strength of unidirectional glass fiber reinforced plastic considering fiber orientation distribution Blümel T1 , Sahr R1 , Krimmer A2 1Technische Universität Berlin, 2TPI Composites Germany GmbH The mechanical behavior of a fiber-reinforced plastic (FRP) composite can be flexibly and precisely adapted to structural requirements by adjusting the fiber orientation and combining differently oriented layers. However, due to textile fiber materials and manufacturing-related imperfections, the fibers are not perfectly aligned but have stochastic orientations, which can be described using the fiber orientation distribution (FOD). This stochastic fiber arrangement affects all mechanical properties of the FRP - especially the longitudinal compressive strength is strongly affected. The objective of the presented work is to develop a mechanical material model which adequately represents the influence of the FOD and correctly predicts the longitudinal compressive strength of unidirectional FRP. For this purpose, an analytical stochastic material model with a multiscale structure was developed, in which the FOD is represented by a bi-normal distribution function. With this model, the three-dimensional elasticity and strength properties can be calculated as a function of the FOD. The results of this calculation show a strong dependence of the fiber-parallel compressive strength on the extent of the undulations. This correlation was investigated experimentally on standardized compression specimens made of unidirectional glass fiber-epoxy composite to validate the model. For measuring the FOD, the microstructure of each compression specimen was imaged using micro-X-ray computed tomography (µCT) and then analyzed using the open-source image analysis software ImageJ. Subsequently, the compression specimens were mechanically tested. This procedure allows direct comparison of the experimental results with the individual predictions of the model, considering the internal structure of each sample. To investigate the failure mechanisms and to validate the model assumptions, finite element (FE) analyses were performed on representative elements with stochastic FOD under longitudinal compressive loading. The results of the FE analyses indicate matrix fracture-initiated failure and thus confirm the assumptions of the analytical model. In the considered series of specimens, the FE analyses and the analytical stochastic model provide similar results. Compared to the experimental data, the analytically calculated compressive strength shows an average error of 6%, which outperforms existing analytical approaches. Overall, the results of this work show that the analytical stochastic material model realistically determines the fiber-parallel compressive strength of unidirectional GFRP. In this presentation, the analytical stochastic material model and the FE analyses are presented. Furthermore, the experimental procedure including the determination of the FOD by µCT as well as the assessment of the compressive strengths by mechanical tests will be described. Finally, the results for the longitudinal compressive strength from the experiments as well as from the calculations will be compared and discussed.

49 3D data from micro-X-ray computed tomography on an 8 mm³ GFRP cube Representation of longitudinal compressive strength as a function of the fiber orientation distribution: comparison between analytical model (with fiber volume fraction of 56%) and experimental data