150 Overview of the range of application of the FKM-Guideline, basing on applicant's questions to one of the authors between 2010 and 2022, assigned to fields of application Structure of analytical strength assessment according to FKM-Guideline
151 119 Fatigue strength assessment of steel welded joints and components: A comparison of different mechanical engineering guidelines and standards Fiedler M1 , Rennert R2 , Kästner M1 1Chair of numerical and experimental solid mechanics, TU Dresden, 2 IMA Materialforschung und Anwendungstechnik GmbH To estimate the fatigue life or the degree of utilization of welded joints and components, various guidelines and standards can be applied according to the state of the art in Germany. The codes can differ in the fatigue assessment including the applied fatigue concepts, the definition of the degree of utilization, the hypothesis for multiaxial stress states, the considered influencing factors and of course the applied safety factors and concepts. This presentation compares the modules of - FKM guideline linear - IIW recommendations - DVS 0905 - DIN EN 1993-1-9 (Eurocode 3) - EN 17149 (rail vehicle construction) - DVS 1612 (rail vehicle construction) - DIN EN 13001-3-1 (crane construction) in order to give an overview for industrial users.
152 120 Suggestions for Correcting the Stress Parallel to the Weld Seam Feickert W1 , Kirchhoff T1 , Schlitzer T1 1 ihf Ingenieurgesellschaft mbH Welds under cyclic loads are often the weak points of a structure. The verification is carried out according to rules, such as: IIW Recommendations, Eurocode, FKM guideline or DVS. Methods based on structural stresses or notch stresses have become state of the art. With these methods, the stresses at the weld toe or in the weld root can be verified directly. In the notch stress method, a fictitious notch radius at the weld toe and in the root is introduced into the simulation model. When verifying the local stresses using the notch stress method, three stress components must be determined with correct direction. For this purpose, the directions normal and parallel to the weld must be determined at all verification points. Within the IIW-Recommendations the notch stress method is not applicable if there is a significant stress component parallel to the weld. In the FKM the nominal stresses is used for the stress component parallel to the weld. In DVS 0905 the stress value taken at a distance of 2 mm next to the weld toe is applied. The local stresses along the seam do not correspond to the nominal stress due to the obstruction of transverse contraction caused by the stress perpendicular to the seam, depending on the notch radius and the seam opening angle. The local stresses are usually determined with the help of a Finite Element Analysis. The nominal stresses then have to be determined in another way and cannot be clearly defined. Three different concepts are proposed to meet this challenge. It is assumed that it is possible to calculate an undisturbed stress component parallel to the weld seam in the notch radius as an approximation for the nominal stress. This approximation should be on the save side but less conservative than the notch stress itself. First approach: It is proposed to determine the stress in the direction parallel to the seam at two points inside the structure and to extrapolate it linearly towards the surface, thereby nonlinear effects due to the transverse contraction will be subtracted out (Fig.1) Second approach: Investigations on a notched rod show that the part of the stress parallel to the seam caused by the obstruction of the transverse contraction depends on the notch opening angle and the notch radius. If the seam opening angle and the radius are known, a correction factor for the stress parallel to the seam can be specified (Fig.2).
153 Third approach: The seam opening angle is not always constant over the seam length. With an automated evaluation of the weld, the local seam opening angle would have to be determined for each verification point, which is relatively complex. It is shown that a correction is also possible with the help of the stress gradient in the notch if the radius is known (Fig.2). The three methods are presented and their accuracy and and suitability is discussed. Some of the concepts were developed as part of a project funded by the German federal state Hessen (DISTR@L). Figure 1: Extrapolation of stress component parallel to weld seam Figure 2: Stress ratio parallel to perpendicular stress over seam opening angle and stress gradient
154 121 Fatigue strength assessment of preloaded cross-toothed flange connections based on the FKM guidelines Härtel H1 , Ulrich C1 , Fiedler M2 , Kästner M2 , Schlecht B1 1Chair of Machine Elements, TU Dresden, 2Chair of Computational and Experimental Solid Mechanics, TU Dresden Cross-toothed flange connections are used in drivetrains for form-fit coupling of shafts. They belong to the group of non-shiftable couplings. Compared to frictionally connections, tooth flange connections can transmit significantly higher torques with the same design space. In contrast to other tooth couplings, the crossed tooth flange coupling investigated here is characterized by a particularly complex load distribution. The preload force, which is necessary to transmit operational loads, is generated by tensioning bolts, which can be located centrally or decentralized. Thus, the stress condition in the notch of the coupling is always determined by a non-proportional synchronous superposition of stresses from preload force and operating loads. A calculation concept for the load-bearing capacity of this connection is currently not available. Therefore, extensive experimental and simulative investigations were carried out as a part of a research project. In addition to the development of a pragmatic, specially adapted calculation concept for cross-toothed couplings for practice, the experimental tests were accompanied by calculations with the FKM guideline "Analytical Strength Assessment" and the FKM guideline "non-linear". The determination of local stresses was carried out with the help of elasticity theoretical and elastic-plastic FE full-contact calculations based on experimentally determined material data. This article deals with the possibilities of calculation based on the FE simulation results using the above mentioned FKM guidelines. The experimentally determined load-carrying capacity is critically compared to the calculation results. The focus is on the implementation of the fatigue strength analysis in the context of the corresponding boundary conditions. Topics such as the assumed knee point of the Wöhler curve to the fatigue strength range are discussed as well as the evaluation options for elastic-plastic FE results and nonproportional stress states for the guideline „non-linear“.
155 122 Development of a software for fatigue strength assessment of welded joints Beier H1 , Fällgren C1 , Vormwald M1 , Baumgartner J2 , Faß M2 , Melz T2 , Kirchhoff T3 , Schlitzer T3 , Bös F3 , Feickert W3 1Technische Universität Darmstadt - IFSW, 2Technische Universität Darmstadt - SAM, 3 ihf Ingenieurgesellschaft mbH Topic of the project "Entwicklung einer Software für den Festigkeitsnachweis von Schweißnähten" (development of a software for fatigue strength assessment of welded joints) was to develope a software for the fatigue strength assessment of welded joints / structures based on local concepts as part of finite elements (FE) calculations. The project can be devided into 4 working packages: (1) Fatigue tests on a welded specimen type (load-bearing plugged-in plate) which is not listed in recommendations for fatigue assessment like IIW [1], FKM [2] or EC3 [3]. (2) Construcion of a database with results from (1) and data from literature, including a large number of different welded joints, for validation of local concept and verification of software to be developed. (3) Adaption of the local stress concept including size effects for the fatigue assessment of the specimens from the database. Design of FE-models of the database specimens using local radii of 0,05 mm and 0,3 mm, calculation of fatigue strength based on the local concept and validate concept. (4) Development of software for fatigue assessment of welded joints based on the local stress concept. Programming of a user interface for standard CAE software Ansys, ABAQUS and HyperView. Verification of the software using results from the database. The project was funded by the federal State of Hessen within the framework of the DISTR@L funding line as a cooperative project with the partners ihf Ingenieurgesellschaft mbH and the departments IFSW and SAM from Technische Universität Darmstadt. [1] Guideline IIW „Recommendations for fatigue design of welded joints and components“, Hobbacher, Springer Nature, 2016 [2] Rennert, R.; Kullig, E.; Vormwald, M.; Esderts, A.; Luke, M.: FKM-Richtlinie Rechnerischer Festigkeitsnachweis für Maschinenbauteile, VDMA Verlag, 7. revised edition, 2020 [3] DIN EN 1999-1-3: Eurocode 3: Bemessung und Konstruktion von Stahlbauten – Teil 1-9: Ermüdung; Deutsche Fassung EN 1993-1-9:2005 + AC:2009; Beuth Verlag GmbH, Berlin; Issue December 2010 [4] Ansys: Ansys Mechanical. Structural FEA Analysis Software. ANSYS Inc. https://www.ansys.com/ [5] ABAQUS: Finite Element Analysis for Mechanical Engineering. Dassault Systèmes. https://www.3ds.com/products-services/simulia/products/abaqus/ [6] Altair HyperView: Comprehensive CAE Post-processing and Engineering Data Visualization. Altair Engineering Inc. https://www.altair.com/hyperview
156 123 The low-cycle fatigue behavior of high-Manganese twinning-induced plasticity steels with various loading orientations Song D1 , Beier H2 , Vormwald M2 1 School of Mechanical and Electrical Engineering, University of Electronic Science and Technology of China, 2Materials Mechanics Group, IFSW, Department of Civil and Environmental Engineering Sciences,Technical University of Darmstadt The low-cycle fatigue behavior of high-manganese twinning-induced plasticity (TWIP) steels with five orientations (i.e., the specimen axial direction of 0°, 22.5°, 45°, 67.5°, and 90° to the rolling direction) are investigated, and the strain amplitudes are from 0.6% to 4.4%. Experimental results show that the cyclic hardening and softening behavior and fatigue lives for various specimen orientations present significant differences. The dependence of the fatigue life on the specimen orientation is very obvious for lower strain amplitudes, and it gradually reduces with the increase of the applied strain level. The stress response for the 90° orientation is the highest and that of the 0° orientation is the lowest for each strain level. These two orientations also show much shorter fatigue lives, even one third to one half compared to the life for the 45° orientation.
157 124 The FKM guideline nonlinear - strength assessment considering elasticplastic material behaviour Fiedler M1 , Wächter M2 , Varfolomeev I3 , Esderts A2 , Vormwald M4 1TU Dresden, Chair of numerical and experimental solid mechanics, 2TU Clausthal, Institute for Plant Engineering and Fatigue Analysis, 3 Fraunhofer IWM Freiburg, 4TU Darmstadt, Material Mechanics Group The FKM guideline nonlinear was first published in 2019 and enables users to do static and fatigue strength assessments considering elastic-plastic material behaviour for metallic components. The static strength assessment requires an elastic-plastic FE-calculation using the true stress-strain-curve from a tension test as the basis of calculation. Different loading steps have to be applied until one calculation point reaches a critical value. The critical values are described by a failure limiting curve which can be approximated from the material properties of a uniaxial tension test and considers the local multiaxial factor h= σ_h/σ_V for each calculation point. The fatigue strength assessment requires only the ultimate tensile strength and a linear-elastic FEcalculation in order to calculate the fatigue life of a component under constant or variable amplitude loading. Although the FE-calculation is linear-elastic, the fatigue assessment is based on elastic-plastic material behaviour. The plastic effects can be considered in the post-processing of the FE-calculation using 1.) cyclic elastic-plastic material behaviour approximated from the ultimate tensile strength in combination with 2.) a method for notched specimens transmitting stresses and strains from elastic to elastic plastic material behaviour and 3.) a hysteresis counting method. In order to consider the damage amount of each applied loading cycle, damage parameters P are used to formulate damage based on elastic-plastic calculated stresses and strains. The P-N-curves can be approximated by the ultimate tensile strength in combination with component effects like size effects or the surface roughness. Further, the linear damage accumulation by Miner is used for the fatigue life calculation. This presentation gives an overview about the current state and although about new developments as a preview for the second edition of the guideline.
158 125 PRACTICAL FATIGUE STRENGTH DIAGRAMS FOR COMPRESSION SPRINGS BASED ON THE FKM-GUIDELINE “ANALYTIC STRENGTH ASSESSMENT FOR SPRINGS“ Petrich M1 , Kletzin U1 1 Ilmenau University Of Technology Introduction The mathematical relationships and Goodman diagrams contained in the standard DIN EN 13906-1 form the essential basis for the design and calculation of cylindrical helical compression springs. They are not only used nationally, but throughout Europe and internationally in the spring industry and by spring users. However, the diagrams are over 50 years old and no longer reflect the current state of modern spring materials and spring manufacturing processes. Since then, on the one hand, the wire materials have improved significantly (e.g. metallurgy, purity, drawing process, heat treatments, tensile strength, tolerances and uniformity) and, on the other hand, it is now known that the spring geometry (spring index, number of coils, etc.) process sequences and process parameters in the spring production (heat treatment, pre-setting, shot peening, etc.) as well as the operating conditions have a significant influence on the fatigue strength of the springs. According to the EN standard, these influencing factors are currently not taken into account in the spring design. For the users of the standard, this results in a certain uncertainty, as the 50 years old graphs are partly very conservative (oil tempered wires), but partly also critical (stainless wires), which currently has to be compensated by cost-intensive fatigue tests. Small and medium-sized enterprises (SMEs) are absolutely dependent on meaningful and appropriate fatigue strength values and guidelines when designing their springs. Methods In order to overcome those inadequacies, the deduction of new fatigue strength diagrams has been the subject of an extensive research program in cooperation with the German Spring Manufacturers Association (VDFI). Endurable stresses were calculated with the FKM-guideline "Analytic strength assessment for springs and spring elements", which was published in 2020. The calculations were validated by numerous existing and newly obtained results from fatigue tests on compression springs. The application of a newly developed safety concept, which is part of the FKM-guideline, results in new fatigue strength diagrams (permissible stresses) that are conservative with regard to real test data.
159 Results These new fatigue strength diagrams developed as part of the research project can be used directly for standard springs in standard applications as before but with much better accuracy. If required, the quality of the results for springs with properties (geometry, manufacture) that differ from the reference springs can be further improved using basic analytical calculations. Conclusion The aim of the VDFI and the research group is to update the EN standard with the new fatigue strength diagrams for compression springs. This contribution shows approaches to design competitive springs in a shorter time and with less testing effort according to the current state of research.
160 126 Investigations on permissible plastic strains using the example of feather key connections Muhammedi B1 , Hasse A1 , Suchý L1 1Chemnitz University Of Technology International competition is forcing companies to use resources sparingly and economically in production, which is leading to the use of increasingly high-performance materials for shaft-hub connections (SHC). Due to its easy assembly and disassembly, the feather key connection (FKC) is widely used, especially in gearbox construction. For the simple dimensioning of FKCs by means of method C of DIN 6892, the maximum transmittable torsional moment of a feather key connection must be determined, whereby a constant surface pressure at the shaft notch is assumed, whereas method B, based on the work of MILITZER, takes into account a nonlinear course of the surface pressure. A new limiting load criterion based on a permissible expansion of the shaft notch is proposed, which is examined by means of numerical investigations accompanied by torsion tests. Here, the shaft as a critical component is in special focus, which is why an advanced elastic-plastic material law with hardening properties according to VOCE and CHABOCHE is implemented to describe the shaft material. The material behaviour of the hub is estimated by means of a uniform material law according to BÄUMEL and SEEGER and the feather key is described as an ideal plastic component. For the quasi-static transmission of the torque, 2-D and 3-D models are constructed and a torsional moment or a torsional angle is specified as an input variable. The torsion of the shaft in the hub causes the featherkey to tilt, which leads to the system settling and triggers a stiffness jump in the system. After the specified load maximum is reached, the system is unloaded as in the test bench. The notch expansion is measured throughout the simulation. The plastic deformation is considered permissible if no visible cracks appear in the area of the shaft notch or if a defined plastic strain is not exceeded. This limiting load criterion is to be tested on the basis of the component stress-strain curves. KRESINSKY shows strength potentials between 20 % for a ratio of the load-bearing key length (l_tr ) to the joint diameter (d) of l_tr/d=0.95 up to 50 % for l_tr/d=0.5 (Figure 1). It is assumed that below 10,000 cycles a saturation of the hardening and settling behaviour of the FKCs occurs. The new proposed limiting load criterion based on permissible plastic notch expansion promises potential for the design of FKCs (Figure 2). Further investigations are necessary to describe influencing variables such as hub wall thickness, key shape and stress ratio.
161 keywords: feather key connection, fatigue criterion, notch expansion, strain-based criterion Maximum transmittable torque moment as a function of the key length Comparison of the component and the material stress-strain-curves at different axial position for the material C45+N
162 127 Robust calculation method for fretting fatigue strength assessment of steel, cast iron and aluminum contacts Knabner D1 , Suchý1 L1 , Hasse A1 1Technische Universität Chemnitz The risk in the use of component connections is the possible occurrence of the phenomenon of fretting fatigue. In that case, a characteristic tribological stress arises due to reversing micro slip movements of the components, which results in an enormous reduction of the tolerable stress. For engineers and designers, the challenge in the development process is how to dimension such parts efficiently and with economical effort. Complex strength evaluations, for example, according to critical plane methods, are no option for these users as long as they are not implemented in commercial finite elemente softwares. Although there are already proven standards for the evaluation of fatigue strength, these are either only applicable to plain fatigue problems or only take into account the fretting related strength reduction in the form of notch numbers for specific component geometries (e.g. shrink-fit connections). The aim of present research is therefore to develop a robust calculation method for components with fretting fatigue. However, the shape of fretting fatigue endangered component connections is manifold, which makes the use of numerical methods inevitable. The internationally known standard FKM guideline offers, among other issues, the possibility to calculate the fatigue strength calculation with local stresses. Authors extended this guideline in recent years to include the aspect of fretting fatigue for the materials steel, cast iron and aluminum. From the literature it is known that the fretting fatigue strength depends significantly on the parameters slip amplitude s and contact pressure p. Using a double-actuated fretting pad test bench, it was possible to carry out a separate analysis of the influencing parameters. Figure 1 shows the test principle of the fretting fatigue laboratory tests with flat-flat contact constellation. For the material groups steel, cast iron and aluminum, the worst-case parameter combination of s and p was determined experimentally. Based on this worst case, the fretting fatigue limits for different materials within the material groups were determined. The influence of the mean stress sensitivity and the influence of the counter-body material on the fretting fatigue strength was determined on a sample basis for individual materials. Subsequently fretting factors for the individual materials were generated from the findings and implemented in a suitable place in the FKM guideline. Finally the newly developed calculation method was then validated in experimental tests with connecting rods. The worst case parameter combination for the different material groups were determined as follows: for steel and cast iron the critical parameters are s=10 µm; p=40 MPa. For aluminum the worst case was found at the parameter combination s=5 µm; p=10 MPa. The determined fretting factors representing the strength reduction are shown in Figure 2. The maximum error in calculating the degree of utilization on the connecting rods was 18,7%.
163 Test principle of the fretting fatigue test on the fretting pad test bench and exemplary observed failures Determined fretting factors of the different materials investigated
164 128 Fatigue Life Assessment of Automotive Leafsprings GIANNAKIS E1 , Savaidis G1 , Fragoudakis R2 , Savaidis A3 1Aristotle University of Thessaloniki, School of Mechanical Engineering, 2Merrimack College, Department of Mechanical Engineering, 3 School of Pedagogical and Technological Education, Department of Mechanical Engineering Educators Introduction The creation of a high performance leaf spring is a very chalenging procedure that consists of many stages over a long timeframe from concept to production. High performance leaf springs are components with high complexity in refer to the multiple manufacturing processes that the raw material undergoes to reach its final geometrical and microstructural form and mechanical properties. Currently, the fatigue life assessment of such load-carrying components is performed purely experimentally on multiaxial test rigs. The recently published German guideline for springs and spring elements covers wire springs, e.g. coil springs, not leaf springs. The “conventional” FKM guideline for strength assessments of mechanical components is restricted to metallic materials with ultimate tensile strength not greater than 1200 MPa. Therewith, the intenational literature lacks on appropriate standards or guidelines for theoretical fatigue performance assessments. Methods The paper shows a simple fatigue life calculation procedure applicable to leaf springs, following the strategy of the FKM guideline, with adjusted parameters for the various manufacturing processes of leaf springs. The fatigue life calculation procedure uses the FKM guideline as a basis. Results The procedure’s calcualtion accuracy and, therewith, its usefulness for industrial applications, is demonstrated against a comprehensive fatigue life database of leaf springs (more than 300 results), covering different raw materials and manufacturers, different geometrical and loading parameters. Conclusion The new calculating procedure can be used to calculate the fatigue life of leaf springs during any stage of its development. It is a really effective tool for better understanding of the manufacturing processes, accelerating the development of these high performance leaf springs and reducing the testing and prototyping costs. Acknowledgement The work has been realized within the framework of the research project T7DKI-00054 (bilateral scientific and technological cooperation between Greece and P.R. China) funded by the European Union and Greek national funds (Competitiveness, Entrepreneurship, and Innovation).
165 129 Finite Element Modelling of Stress Shot Peening: Application on Leaf Springs Gakias C1 , Fragoudakis R2 , Savaidis G1 1Aristotle University Of Thessaloniki, 2Merrimack College Introduction Shot peening (SP) is a widely used surface treatment process that involves the repetitive impacts of small spheres on the surface of a component. This process results in a permanent deformation that creates a compressive stress field up to a depth of 300-400 μm, which can significantly improve the fatigue life of the component. Stress shot peening (SSP) is an enhanced version of SP that involves applying a pretension to the treated part. SSP has been proven to be beneficial for the fatigue life and durability of metallic components. In the case of automotive leaf springs, this process is crucial for adequate endurance and further weight reduction. Methods This study presents a structured modeling 2D approach for stress shot peening by means of finite elements. The approach is capable of implementing important process parameters, including the stochastic nature of the used shots, the elastoplastic material properties of both the shots and the treated part as well as degraded surface properties, e.g. due to surface decarburization. This approach significantly reduces calculation time and allows for the examination of multiple scenarios, as compared to more complicated 3D FEA models. The model is validated on both shot and stress-shot peening processes, and a comparison with measured residual stress values is presented. Results The present study provides an overview of stress shot peening and compares it to the conventional shot peening process in terms of compressive residual stresses and surface roughness. The results demonstrate that SSP produces a deeper and more homogeneous compressive stress field than SP, and the surface roughness is comparable to that of SP. Calculated residual stress profiles are in good agreement with corresponding experimental ones determined from metallic leaf spring specimens. The model accurately predicts the residual stress values for both shot and stress peening processes, which indicates that -though simplified- it is a useful tool for optimizing SSP processes. Moreover, the outcomes of this model (residual stress and surface roughness profiles) can be directly implemented into fatigue life calculation models to assess the effects resulting from SP and SSP processes on fatigue life. Conclusion This study contains a novel and efficient approach for modeling stress shot peening that considers important process parameters and the stochastic nature of the used shots. The results demonstrate that SSP produces a more uniform compressive stress field than SP and is therefore beneficial for improving the fatigue life and durability of components. The validated model can be used as a tool for optimizing SSP and further reducing the weight of load-carrying engineering components, e.g. leaf springs, which may have significant implications for the automotive industry.
166 130 Proof of structural durability for surface-hardened components based on the local strain approach Yadegari P1 , Beier H1 , Vormwald M1 1Technical University of Darmstadt, Materials Mechanics Group The "Guideline Non-linear" of the German Research Association for Mechanical Engineering provides, among other things, a proof of structural durability for machine components based on the local strain approach. Currently, only homogeneous components can be evaluated, the influence of the surface on the component fatigue life is only estimated via the surface roughness and it is not possible to take residual stresses into account. Since highly stressed components in mechanical engineering often require the use of surface hardening measures in order to guarantee their resistance to wear and fatigue (e.g. by shotpeening, case-hardening, induction-hardening), the consideration of the resulting inhomogeneous material properties and introduced residual stress states is of particular interest. Furthermore, due to these inhomogeneous material, the failure of components with surface layers may initiate from the notch root as well as from the transition area between the low-strength core material and the high-strength surface layer. Therefore, an adaptation of the fatigue strength assessment based on the local strain approach was developed for the detection of this second failure-relevant point, which is the subject of the present publication. The algorithms were extended to include a two-point assessment as well as the consideration of residual stresses. An adapted approximation method was also developed to estimate elastic-plastic strains at both assessment points based on the stresses determined by elasticity theory. These investigations serve as preliminary work to extend the “Guideline Non-linear” and to allow the proof of structural durability of surface-hardened components the consideration of residual stresses and inhomogeneous material properties.
167 131 FATIGUE LIFE ESTIMATION OF NOTCHED COMPONENTS UNDER MULTIAXIAL NON-PROPORTIONAL LOADING Kraft J1 , Vormwald M1 1Technische Universität Darmstadt Within the framework of the FKM project "Multiaxial Local", calculation concepts for the determination of component crack lifetimes under non-proportional loading were investigated. The fatigue life of a notched component up to the technical crack initiation can be described with the framework of the local strain approach. For the fatigue life calculation in the case of local non-proportional stresses and strains, the concept of critical planes has proven to be effective. Here, the fatigue life in different cutting planes through the material element in the notch root is investigated. The plane that exhibits the lowest fatigue life is considered the critical plane. The presented algorithm consists of two parts, namely, a notch simulation to determine the local multiaxial elastic-plastic stresses and a subsequent damage evaluation using the critical plane concept.In the present work, the Fatemi/Socie parameter, the damage parameter P_RAM from the FKM Guideline nonlinear and a damage parameter based on short-crack fracture mechanics were investigated for accuracy. The accuracy evaluation is based on a broad database. This contains test results of notched specimens under proportional and non-proportional loading under constant and variable amplitudes. It is shown that by considering non-proportional strain hardening in the estimation of local stresses and strains, the accuracy of the results under non-proportional loading can be improved regardless of the applied multiaxiality hypothesis. The accuracy of the SWT approach is highly dependent on the loading situation. For tests under nonproportional and pure torsional loading, excessively long fatigue lives are predicted. The Fatemi/Socie and short-crack parameter predict fatigue lives with high accuracy regardless of the applied load.
168 132 Notch approximation methods for components under thermomechanical stresses Hamacher J1 , Vormwald M1 1Technische Universität Darmstadt Welded joints represent one of the most important joining techniques in mechanical engineering. Nevertheless, no validated, mechanism-oriented or fracture-mechanical concepts exist for the real-time service life assessment of welded joints for variable, thermomechanical stresses. An extension of the knowledge has an immediate relevance in the field of manufacturing, non-destructive testing, evaluation and monitoring of welded joints, as well as in accompanying material and sensor-based component testing under operational stresses. The aim of the present work ist to develope real-time capable lifetime models for welded joints made of the austenitic steel material 1.4550 for thermomechanical stresses with variable amplitudes and plastic deformations. For this purpose, approximate solutions for welded joints are developed based on Neubertype corrections to the cyclic stress-strain path. The resulting estimation serves as a basis for a subsequent damage assessment. The methodology to be developed for real-time fatigue assessment of welded joints has a generic character and can therefore be transferred to other thermomechanical high-stress components such as power plant components, gas, aircraft and steam turbines, combustion engines, high-temperature fuel cells and hot forming tools. This applies both to the cyclic plasticity models to be developed and to the fracture mechanics-based real-time capable lifetime models. Therefore it will be possible to transfer the methodology to other materials apart from the austenitic steel material 1.4550 to be dealt with here.
169 133 Black-box system identification approach for damage detection in a smallscale wind turbine blade Liu M1 , Hultmann Ayala H1 1Mechanical Engineering Department, Pontifical Catholic University of Rio de Janeiro System identification techniques play an important role in structural health monitoring of engineering components. In this study, we used a set of measurements of vibration-based experiments of a small-scale wind turbine (WT) blade to assess its structural condition under healthy and damaged scenarios. Firstly, a black-box system identification with ordinary least squares was implemented, considering a linear autoregressive exogenous (ARX) model to represent the dynamics of the WT blade. This method required a high-order ARX model. Nonlinear ARX (NARX) modeling was also performed, however, it did not demonstrate advantages over the linear one in terms of reducing model complexity or prediction errors. In addition, a recursive least squares algorithm was implemented to identify changes in the structural dynamics induced by the presence of crack-like defects. The procedure was carried out under various defect severity scenarios and showed satisfactory performance in monitoring the health condition changes. The applied system identification strategy performs well and is a reliable monitoring tool for diagnosing damage in the WT blade.
170 134 Comparison of the strength assessments of the FKM guidelines Fiedler M1 , Rennert R2 1Tu Dresden, Chair of numerical and experimental solid mechanics, 2 IMA Materialforschung und Anwendungstechnik GmbH There are currently four guidelines published by the Forschungskuratorium Maschinenbau (FKM) in Germany for static and fatigue strength assessment in mechanical engineering. One guideline especially for springs and three guidelines for general mechanical components subdivided first according to the applied failure criteria for fatigue failure: crack initiation (guideline nonlinear), crack growth (guideline fracture mechanics) and fracture (guideline linear). In addition to the failure criteria, the guidelines nonlinear and linear also differ in the material behaviour considered, the definition of damage and the consideration of sequence effects, which leads to differences in the calculation procedures. This presentation gives an overview of the FKM guidelines linear and nonlinear and compares the calculation procedures and assumptions.
171 135 A novel reliability evaluation method combing improved subset simulation and adaptive Kriging model for rare failure events Meng D1,2, Yang S1,2, Nie P1 , Guo Y1 1 School of Mechanical and Electrical Engineering, University Of Electronic Science And Technology Of China, 2 Institute of Electronic and Information Engineering of UESTC in Guangdong Reliability evaluation is of great significance for the safety of products. With the continuous development of the industrial level, the accurate reliability assessment for rare failure events becomes more and more important. Subset Simulation (SS) was known for its efficiency and accuracy in estimating reliability for small failure probability events. However, SS is extremely dependent on the geometry of the limit state function, which may cause conditional samples that are used to calculate intermediate failure probability to iterate in an unreasonable direction. Moreover, Markov Chain Monte Carlo which is adapted to generate conditional samples in SS will induce the correlations among the samples in each subset, which may impact the computational efficiency of SS. In view of the above situation, this article proposes a novel reliability evaluation method combing improved subset simulation and an adaptive Kriging model to address these limitations. Specifically, the technology of magnifying the variability of the input variable is used to ensure that the algorithm iterates in a reasonable direction. The importance sampling method is adopted to generate conditional samples. Furthermore, an adaptive Kriging model is utilized to improve efficiency. Four cases are used to illustrate the performance of the proposed method. The result is shown that the proposed method is more efficient for rare failure events compared with selected existing methods.
172 136 A hybrid adaptive strategy for support vector machine-based structural reliability analysis Yang S1,2, Meng D1,2, Nie P1 , Wang H1 1University Of Electronic Science And Technology Of China, 2 Institute of Electronic and Information Engineering of UESTC in Guangdong The classical reliability analysis methods, due to the ever-increasing complexity of engineering structure, may lead to higher and higher calculation errors and costs. The support vector machine-based reliability analysis has a satisfactory performance, which has been widely used in reliability evaluation. In this study, a novel hybrid adaptive updating strategy for support vector machine-based structural reliability analysis is proposed to adaptively estimate the failure probability. In the proposed method, the K-means clustering technique is used to identify efficient sampling regions. Then, the novel ensemble of learning functions is introduced to achieve adaptive learning for the support vector machine model, which can comprehensively consider the global and local uncertainty of the model. Furthermore, an effective stopping criterion based on the coefficient of variation of failure probability is adopted to improve the robustness of the proposed method. Three examples, including one mathematical case and two practical engineering examples, are applied to illustrate the performance of the proposed method.
173 137 Plastic stress concentration effects in the tolerance to short fatigue cracks Castro J1 , Liu M1 , Bandeira C1 , Miranda A2 , Vieira R1 , Meggiolaro M1 1PUC-Rio, 2UnB Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which de-pends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature. Moreover, high sensitivity thermo-cameras are used to detect the crack formation at the notch tip and its initial growth, until it eventually stops and becomes non-propagating due to the stress gradients ahead of the notch tip.
174 138 Modeling ductile failure -- A non-local plasticity model for porous metals with deformation-induced anisotropy Prof. Nikolaos Aravas1,2, Dr. Ioanna Papadioti1 1University Of Thessaly, 2 International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University A non-local (gradient) plasticity model for porous metals that accounts for deformation-induced anisotropy is presented [1]. The model is based on the work of Ponte Castañeda and co-workers on porous materials containing randomly distributed ellipsoidal voids. It takes into account the evolution of porosity and the evolution/development of anisotropy due to changes in the shape and the orientation of the voids during plastic deformation. A material length is introduced and a non-local porosity is defined from the solution of a modified Helmholtz equation with appropriate boundary conditions, as proposed by [2, 3]. At a material point located at x, the non-local porosity, can be identified with the average value of the “local” porosity over a sphere of radius R centered at x. The same approach is used to formulate a non-local version of the Gurson isotropic model. The mathematical character of the resulting incremental elastoplastic partial differential equations of the non-local model is analyzed. It is shown that the hardening modulus of the nonlocal model is always larger than the corresponding hardening modulus of the local model; therefore, the non-local incremental problem retains its elliptic character, and the possibility of discontinuous solutions is eliminated. A rate-dependent version of the non-local model is also developed. An algorithm for the numerical integration of the non-local constitutive equations is developed, and the numerical implementation of the boundary value problem in a finite element environment is discussed. An analytical method for the required calculation of the eigenvectors of symmetric second-order tensors is presented. The non-local model is implemented in ABAQUS via a material “user subroutine” (UMAT or VUMAT) and the coupled thermo-mechanical solution procedure, in which temperature is identified with the non-local porosity. Several example problems are solved numerically and the effects of the non-local formulation on the solution are discussed. In particular, the problems of plastic flow localization in plane strain tension, the plane strain mode-I blunt crack tip under small-scale-yielding conditions, the cup-andcone fracture of a round bar, and the Charpy V-notch test specimen are analyzed. References [1] Aravas, N. and Papadioti, I., “A non-local plasticity model for porous metals with deformation-induced anisotropy: Mathematical and computational issues”, J. Mech. Phys. Solids 146, 104190 (2021). [2] Geers, M.G.D., Engelen, R.A.B., and Ubachs, R.J.M., “On the numerical modelling of ductile damage with an implicit gradient-enhanced formulation”, Rev. Européenne Élém. Finis 10, pp. 173–191 (2001). [3] Peerlings, R.H.J., Geers, M.G.D., de Borst, R., and Brekelmans, W.A.M., “A critical comparison of nonlocal and gradient-enhanced softening continua”, Int. J. Solids Struct. 38, pp. 7723–7746 (2001).
175 139 EXPERIMENTAL AND NUMERICAL ASSESSMENT OF FAILURE IN AIRCRAFT COMPONENTS MANUFACTURED BY LIQUID RESIN INFUSION (LRI) E. Karachalios1 , T.S. Plagianakos1 , K. Muñoz2 , M. Jiménez2 , V. Prentzias1 1Hellenic Aerospace Industry, P.O. Box 23, 32009 Schimatari, Greece 2Element Materials Technology, Wilbur y Orville Wright 1, Aeropolis, Seville, San Jose de la Rinconada 41300, Spain Liquid Resin Infusion (LRI) is an Out of Autoclave process for composites manufacturing, involving impregnation of dry fabrics with liquid resin under the application of vacuum and heating. Hence, it combines several advantages compared to traditional manufacturing processes involving prepreg and autoclave limitations. The present work focuses on the assessment of failure of LRI-manufactured aircraft components. Based on mechanical characterization at composite ply level, which includes measurements of interlaminar fracture toughness, the static response of an aft-bulkhead joint and a floor beam subjected to bending loads is simulated and measured.
176 140 Probabilistic buckling assessment and reliability of FML, composite and aluminum cylindrical panels under compression with load and fabrication uncertainties Kalfountzos C1 , Bikakis G1 , Theotokoglou E1 1Department of Mechanics, Laboratory of Testing and Materias, School of Applied Mathematical and Physical Sciences, National Technical University of Athens Fiber Metal Laminates (FMLs) are hybrid composite structures built up from alternating plies of thin metal alloy sheets and fiber reinforced layers. FMLs have critical benefits such as low density, high fatigue resistance, excellent impact and corrosion resistance, which make them ideal for various thin-walled structures, especially in the aerospace industry. The elastic buckling of slender structures is a problem of major practical importance, since it is a primary cause of failure and, as a result, an accurate prediction of their buckling behavior is required for their design. It is known that the variability in the manufacturing process of a structural component, due to unavoidable practical imperfections, leads to deviations of the actual from its nominal dimensions [1]. For example, the curing of composite materials has technical difficulties which cause deviations from the desired structural characteristics. The manufacturing uncertainties affect the buckling strength of slender structural components and, as a result, they must be taken into account in order to predict the buckling behavior and reliability of FML panels. The purpose of this article is to study the elastic buckling behavior and reliability of FML, composite and aluminum cylindrical panels under uniaxial compression taking into account fabrication and loading uncertainties. A 3D finite element modeling with ANSYS software [2] is implemented for this purpose. The panels are discretized using shell elements and the eigenvalue buckling analysis is conducted for the prediction of their elastic buckling and of the critical deterministic buckling load (Ndet), corresponding to constant axial load distribution. The probabilistic buckling behavior of the considered panels is investigated with the Probabilistic Design System (PDS) of ANSYS software [2]. With sensitivity analysis it is found that the thickness of aluminum layers is the most significant uncertain variable for the critical buckling load factor (NC) of FML panels, whereas the fiber orientation angle of their composite layers is insignificant. The previously unknown role of the MVF (Metal Volume Fraction) of the panels is revealed. As the MVF decreases, the sensitivity of the elastic buckling load factor to variations of the axial load distribution is reduced whereas its sensitivity to variations of the thickness of composite layers is increased. Consequently, the MVF is an important design parameter for the uncertain buckling behavior of the panels. A significant new result is the calculation of the reliability of the panels with the considered uncertainties (Figure 1).
177 Graphical definition of reliability R 1. Blischke WR and Murthy DNP. Reliability Modeling, Prediction and Optimization, John Wiley & Sons, Inc. 2000, New Jersey. 2. ANSYS Documentation, Structural Analysis Guide, Software Package, ANSYS Inc. 2014.
178 141 Investigation of the size effect on the critical distance and fatigue life using the highly stressed volume approach He j1 , Zhu S1 , Niu X1,2, Liao D1 1University Of Electronic Science And Technology Of China, 2Department of Chemical Engineering, Materials and Environment, Sapienza University of Rome ABSTRACT: Structures with different sizes/geometries normally present different fatigue performances, namely, the size effect. Under external loads, different geometric details generally show different local stress states, and thus pose non-negligible influence on fatigue strengths of engineering components. As an efficient method for notch effect modelling, the theory of critical distance has been commonly utilized for notch fatigue analysis and the next level of this method includes its mature applications in engineering practice. In this study, a probabilistic model for deducing fatigue life distribution of notched components is established by combining the theory of critical distance with the Weibull distribution. In addition, the influence of size effect on critical distance and predicting performance is characterized by the highly stressed volume approach. Then, fatigue tests on notched specimens of GH4169 alloy are conducted for model comparison and evaluation as well as experimental data of two other types of metallic materials with different notch geometries. Results indicate that the predicted P-S-N curves by the proposed model agree well with the tested ones. Keywords: Critical distance; Size effect; Fatigue; Probabilistic; Notch; Highly stressed volume.
179 142 Probabilistic lifetime assessment of a lost foam cast Al-Si alloy based on microcomputed tomographic measurements Wagner M1 , Barth G2 , Mösenbacher A3 , Hoyer M4 , Riva M4 , Christ H5 1MTU Aero Engines AG, 2 justairtech mbH, 3 IABG mbH, 4BMW AG, 5Universität Siegen In Al-Si cylinder heads, start-stop cycles lead to thermomechanical fatigue (TMF), which is accompanied by the initiation and growth of cracks in the critical web regions. A major issue is the high scatter of the resulting lifetime. The cylinder heads investigated in the framework of this study were manufactured by the lost foam casting process. Due to the low cooling rates caused by the process, the material contains a high number of large and sharp-edged pores. Under cyclic loading, these defects have a significant influence on crack initiation and growth and thus on the lifetime. During the work, uniaxial TMF tests were performed on specimens extracted from the thermomechanically critically loaded region of the cylinder heads. For the determination of the statistical distribution of the initial crack length a method based on the detection of pores using microcomputed tomography (μCT) was developed. In combination with scanning electron microscopy of the fracture surfaces, the pores in the primary crack origin and the relevant criteria for crack initiation were identified. A program developed in this work enables the computational localization of the primary crack origin from the μCT data. The program contains the identified crack initiation criteria (Figure 1, left). The statistical distribution of the elastic-plastic crack propagation rate was determined by uniaxial TMF crack propagation tests (Figure 1, right). On the basis of the experimentally obtained results, a new method was developed for the assessment of the lifetime distribution of thermomechanically loaded cylinder heads based on the distribution of defects within the material. For this purpose, a lifetime model based on fracture mechanics was used (Figure 1, center). This model estimates the lifetime distribution from the distribution of the initial crack length and the crack propagation rate. The deviation between the calculation and test results is quite small for both the lifetime and lifetime scatter (Figure 2). The combination of the μCT for the detection of pores within the material and the lifetime assessment method developed enables a non-destructive quality assurance. In the series process, negative effects of the material quality on the lifetime can be identified at an early stage.
180 Fracture mechanics based model for the probabilistic lifetime assessment under uniaxial thermomechanical loading; input variables for the model from experimental and methodical results. Damage parameter vs. lifetime; the experimental data represent the performed TMF tests; the straight lines correspond to the average lifetime and scatter band of the TMF tests and lifetime calculation.
181 143 The Generalized Local Model: Review of applications during the last 5 years Muñiz Calvente M1 , Fernandez Canteli A1 1University of Oviedo The Generalized local model was developed to provide an objective definition of the material characterization and guarantee the transferability of the material strength from experimental results to the component design, irrespective of the test chosen and the shape and size of the specimen. This methodology represents a general solution without restrictions of former particular local models and provides the statistical information related to the distribution of the reference parameter considered, i.e. the primary failure cumulative distribution (PFCDF), which is identified as a characteristic crack driving force of the material under study. During the last 5 years, the GLM has been applied to a large variety of materials, test conditions and failure criteria. In this work, the authors resume the main applications of the methodology, covering the characterization and prediction of failure under static (fracture) or dynamic (fatigue) conditions of different materials, such as glass, steel or composites. Particularly, the concepts and advances related to the GLM that will be reviewed in this work are the following: a) Application of the GLM on the characterization of brittle materials (glass and epoxy). b) Join test evaluation based on the GLM (mix different types of tests/geometries to obtain and check the material properties). c) Application of the GLM to the characterization of fatigue curves (S-N curves). c) Application of the GLM to the characterization of HCF and VHCF fatigue curves. d) Proposal of application of the GLM on multiaxial fatigue problems. e) Proposal of application of the GLM on fretting fatigue problems. e) Proposal of application of the GLM for notched components. Finally, a brief resume of the main contributions made in the field and the future works proposed by the authors will be presented.
182 144 Probabilistic assessment of rolling contact fatigue including the scale effect Escalero M1 , Zabala H1 , Olave M1 1 Ikerlan Rolling contact fatigue displays a marked scale effect and is characterized by a considerable scatter typical in fatigue phenomena. However, the most used procedures according to NREL or ISO guidelines fail to consider such effect rigorously. In this study, the rolling contact fatigue is evaluated by an in-house software called iKonPro, based on the determination of the stress time series at the different points of the raceways and the quantification of failure probability by the Generalized Local Model (GLM). The software is applied to a 2 ball-row bearing tested under controlled conditions. The ability to consider the scale effect is demonstrated and results are compared to experimental values.
183 145 Fatigue crack growth prediction: from lab to real component Calvín García G1,2, Escalero M1 , Zabala H1 , Muñiz-Calvente M2 1 Ikerlan Technology Research Center, 2Department of Construction and Manufacturing Engineering, University of Oviedo Accurate fatigue crack growth prediction (FCGP) results from an optimum material characterisation and practical use of finite element models (FEM) or analytical equations available in the literature. In practice, two-dimensional hypotheses are assumed, and local phenomena, such as plasticity-induced crack closure (PICC) that significantly influence crack growth due to the contact between the crack surfaces, are not adequately considered. For this reason, this work aims to make predictions of crack growth considering plastic effects around the crack tip. For this end, S275 structural steel, commonly used in industrial applications, was chosen because of i) its beneficial mechanical qualities, such as weldability and malleability; ii) has a low creep that promotes the appearance of plastic effects, such as PICC, which are the main feature to be analysed in this work. The evaluation of crack growth was carried out using three procedures based on the calculation of fracture parameters: - ∆K, which does not take plasticity into account - ∆K_(eff, Newman) using the Newman model, which includes plastic effects - ∆K_(eff, new method ) based on the analytical equations developed by the authors in previous works that consider the 3D PICC The main conclusion of this work is that a proper consideration of the PICC is vital in predicting the fatigue behaviour of real components.
184 146 An extension of the Weibull regression model to cover the LCF, HCF and VHCF regimes for fatigue results performed at different stress ratios R. Fernández-Canteli A1 , Castillo E2 , Blasón S3 , de Jesus A4 , Correia J4 1University of Oviedo, 2Royal Spanish Academy of Engineering, 3BAM, 4University of Porto Introduction Among the S-N models, those capable to cover the three typical fatigue regimes, namely, LCF, HCF and VHCF, are referred to as Class III, according to Strzelecki [1]. The sigmoidal regression proposals, which are intended to represent the entire S-N field, do not fulfil the compatibility condition between the distributions F(Δσ;N) and F(N;Δσ), and, as a result, do not provide a satisfactory probabilistic definition of the Wöhler field. Methods As an alternative to the sigmoidal models, which obviate the non-linear relation between stress and strain of the material, a novel generalized reference variable, GRV=E·σmax·dε/dσ is applied to the extended Weibull regression model proposed by Castillo and Canteli [2,3]. The strain gradients, dε/dσ, may be calculated from the monotonic or cyclic stress-strain curve or even from the hysteresis cyclic curves of the material. In this way, the sigmoidal trend in the experimental S-N field is avoided and replaced by the asymptotic definition of the S-N field while maintaining the compatibility condition along the S-N field. Results The results of an external program on concrete [4] and of an experimental campaign performed on P355NL1 steel [5] for different stress ratios, R, are assessed according to the proposed probabilistic model with the ProFatigue program [4] taking into account the new reference variable. The study confirms the applicability of the new proposed model. Conclusions - A compatible S-N model is proposed, which ensures the probabilistic definition for the LCF, HCF and VHCF domains. - The model replaces the conventional Δσ by a new generalized reference, GRV=E·σmax·dε/dσ. - The inconsistency of the sigmoidal regression models in the probabilistic definition of the S-N field is discussed. - The New model is satisfactorily applied to two fatigue campaigns using the ProFatigue software program. Bibliography [1] Strzelecki P., Accuracy of determined S-N curve for constructional steel by selected models. Fatigue Fract. Eng. Mater. Struct., Vol. 43, Issue 3, 550-557, 2020. [2] Castillo E., Fernández-Canteli A., A unified statistical methodology for modeling fatigue damage. Springer, 2009. [3] Fernández-Canteli A., Castillo E., Blasón S., Correia J.A.F.O., de Jesus A.M.P. Generalization of the Weibull probabilistic compatible model to assess fatigue data into three domains: LCF, HCF and VHCF. Int. Journal of Fatigue, 159, 106771, 2022. [4] Holmen, J. O., Fatigue of concrete by constant and variable amplitude loading, The Norwegian Institute of Technology. Ph.D. Thesis, University of Trondheim, 1979. [5] Correia J.A.F.O., Calvente M., Blasón S., Lesiuk G., Brás I.M.C., De Jesus A.M.P., Fatigue life response of P355NL1 steel under uniaxial loading using Kohout-Vêchet model. XVIII ICMFM, 2016.
185 [6] Fernández-Canteli A., Przybilla C., Nogal M., López-Aenlle M., Castillo E., ProFatigue: a software program for probabilistic Representation of the probabilistic S-N field performed on P355NL1 steel for R= -1 ; -0.5 and 0.
186 147 A probabilistic fatigue model based on nonlinear Kohout-Věchet function: Application to 42CrMo4+QT steel Marques J1 , Papuga J1 , Růžička M1 , Benasciutti D2 1 FME, Czech Technical University in Prague, 2Department of Engineering, University of Ferrara Introduction The Kohout-Věchet (K-V) function consists of a few parameters (fatigue limit, Δσ_∞, number of cycles where the curve bends, N_1 and N_2, and exponent, b) giving a straightforward formula [1]: Δσ=Δσ_∞ ((N+N_1)/(N+N_2 ))^b (1) When describing the fatigue (or S-N) curves, the K-V function offers several advantages over classical functions, e.g., the Basquin function. It can capture the nonlinearity of the S-N curve in different regions of cycles, and it provides a better fit for experimental results in the transition to the fatigue limit region above all, but also in the quasi-static domain. Moreover, it is more suitable for interpolating fatigue curves when there is no fatigue data in a specific region of cycles. Despite its advantages, the K-V function only represents a median curve. The whole family of percentile curves, so typical for the commonly used power-law S-N functions, was not derived for the K-V function till now. To overcome this limitation, this study proposes a probabilistic fatigue model to obtain the percentile S-N curves for the K-V function. Proposed probabilistic fatigue model The proposed model assumes, as in Weibull's model [2], that the deviations, x=Δσ-Δσ ̃, of the observed stress range values Δσ from the values Δσ ̃ belonging to the median curve follow a Weibull distribution. By using Equation (1) to determine the values of Δσ ̃, the proposed model in terms of the probability of failure is: p_f=1-exp[-((Δσ-Δσ_∞ ((N+N_1)/(N+N_2 ))^b-λ)/δ)^β ] (2) where p_f=F(Δσ;N) is also the cumulative distribution function of Δσ for given N. The δ, λ and β are the three Weibull parameters. Example of application This example demonstrates the proposed model applied to fatigue data from four series (S1 to S4) of cylindrical specimens made of 42CrMo4+QT steel, see [3]. The K-V model parameters were determined by minimizing the root mean square error. The deviation values were calculated using the formula x_i=Δσ_i-Δσ_∞ [((N_i+N_1 ))⁄((N_i+N_2 ) )]^b, where i=1,2,…,n and n is the number of tested specimens. The Weibull parameters were estimated by maximizing the log-likelihood. Substituting the parameter values into Equation (2), S-N curves were obtained.
187 Figure 1 shows the S-N curves, estimated without run-outs and for 2.3%, 50% and 97.7% probability of failure. The median S-N curve fits the data well, with almost all points falling within the 2.3% to 97.7% probability of failure range. Although there are no fatigue data to further evaluate the model's agreement in the low- and high-cycle regions, the percentile S-N curves show a promising trend in such regions. Conclusion The model was applied satisfactorily to the fatigue data, demonstrating its robustness in obtaining the S-N curves for different percentiles and cycles to failure. References [1] J. Kohout, S. Vechet, A new function for fatigue curves characterization and its multiple merits. International Journal of Fatigue, 2001. [2] W. Weibull, Fatigue testing and analysis of results. Pergamon Press, 1961. [3] J.M.E. Marques, et al. Evaluating size effects on fatigue life of 42CrMo4+QT steel using a statistical SN model with highly-stressed volume and surface. Procedia Structural Integrity, 2022. Proposed probabilistic model applied to constant amplitude fatigue data from [3]
188 148 Design of a TRL 3 Concept for Supersonic Variable Pitot Inlets with a Safe Design Approach for Academic Environments Kazula S1,2, Höschler K2 1 Institute of Electrified Aero Engines, German Aerospace Center (DLR), 2Chair of Aero Engine Design, Brandenburg University of Technology Abstract Achieving high efficiency, reliability and safety are major goals for future aviation. This affects projected applications for supersonic transport (SST) in particular. Variable pitot inlets that adjust the inlet geometries for different flight conditions can be a way to attain high efficiency by reducing the aircraft drag [1]. Furthermore, they can support the prevention of engine surge during operation at high angles of incidence or crosswind. Hence, variable inlets have been investigated in a number of subsonic studies [2], [3], [4]. As subsonic variable inlets are currently not in use, potential reasons for this absence of application are that their limitations concerning complexity, reliability and costs outweigh their provided aerodynamic benefits. Variable pitot inlets for SST applications up to Mach 1.6 offer higher aerodynamic benefits, which could compensate these limitations [5]. Furthermore, variable pitot inlets have several advantages over other supersonic inlet types in terms of flow uniformity, length and weight. The increased complexity of a variable inlet system can entail reliability and safety issues. These issues can be addressed by the application of a safe design approach. This paper presents the development of a variable inlet concept for SST by means of a safe design approach. First, an overview of aero engine inlet design is given. The design process in aviation according to Aerospace Recommended Practices ARP 4754A [6] is described. Subsequently, the applied safe design process for academic environments is introduced. Based on this process, the development of a feasible variable inlet concept is explained highlighting the results of safety and reliability methods. The feasibility of the concept is proved by analyses and functional demonstrators, whereby TRL 3 is achieved. These results highlight the high potential of variable pitot inlets for future SST. References 1. Baier, H. Morphelle - Project Final Report [online]. Morphing Enabling Technologies for Propulsion System Nacelles, 2015 [http://cordis.europa.eu/docs/results/341/341509/final1-morphelle_final_report.pdf 2. Ozdemir, N.G., F. Scarpa, M. Craciun, C. Remillat, C. Lira, Y. Jagessur und L. Da Rocha-Schmidt. Morphing nacelle inlet lip with pneumatic actuators and a flexible nano composite sandwich panel [online]. Smart Materials and Structures, 2015, 24(12), 125018. ISSN 0964-1726. doi:10.1088/0964- 1726/24/12/125018.
189 3. Kling, U., A. Seitz, J. Bijewitz, A. Hermanutz, L. da Rocha-Schmidt, F. Scarpa, F. Majić, G. Efraimsson und C.J. O'Reilly. Shape adaptive technology for aircraft engine nacelle inlets. Proc. of the Royal Aeronautical Society's 5th Aircraft Structural Design Conference, 2016. 4. Kondor, S., B. Englar, W. Lee und M. Moore. Experimental Investigation of Circulation Control on a Shrouded Fan. In: 21st AIAA Applied Aerodynamics Conference. Reston, Virigina: American Institute of Aeronautics and Astronautics, 2003. ISBN 978-1-62410-092-5. doi:10.2514/6.2003-3409. 5. Kazula, S., M. Wöllner, D. Grasselt und K. Höschler. Parametric design and aerodynamic analysis of circular variable aero engine inlets for transonic and supersonic civil aviation [online]. Proc. of the 24th International Symposium on Air Breathing Engines, 2019, (ISABE-2019-24018) 6. SAE Aerospace. ARP4754A. Guidelines for Development of Civil Aircraft and Systems. Warrendale, PA, United States: SAE International, 2010. doi:10.4271/ARP4754A. Safe design approach
190 149 Common Cause Analysis of the Air Supply System of Fuel Cell-Powered Propulsion Systems in Electrified Aviation Kazula S1 1 Institute of Electrified Aero Engines, German Aerospace Center (DLR) Abstract The aviation industry has to contribute to achieving the political goals of limiting the effects of climate change from the Paris Agreement. Hence, the European Commission published Flightpath 2050 to reduce carbon dioxide emissions of aircraft [1]. As a consequence, sustainable and regenerative energy sources, such as green hydrogen, are being investigated for utilisation in aviation. Thus, the aircraft powertrain topology needs to evolve. A variety of electrified powertrain topologies have been identified for different passenger capacity and flight range requirements [2], some of them including hydrogen fuel cell systems (FCSs). These FCSs are intended to provide electric power to electrically-driven propulsors. Numerous challenges concerning air, fuel, water and thermal management still need to be solved to comply with the strict reliability, safety and weight requirements in aviation. Hence, FCSs have not been applied in commercial aviation yet. This paper analyses polymer electrolyte membrane fuel cell systems (PEMFCSs) as main energy provider for electrified aircraft propulsion, identifies potential weaknesses as well as safety challenges and presents potential solutions. The general design, operating principles and main characteristics of hydrogen-fuelled low temperature PEMFCSs are described. The safety assessment process in aviation according to Aerospace Recommended Practices ARP4754A [3], selected methods according to ARP4761 [4] are introduced and analyses, which have previously been conducted, are summarised. The application of the methods Functional Hazard Assessment (FHA) and Fault Tree Analysis (FTA) as well as subsequent design adaptations were performed by Kazula et al. [5]. The Common Cause Analysis CCA is an additional iterative safety assessment method that is performed from early stages of the development process onward. A CCA comprises of: the ZSA to determine hazards by adjacent systems failure modes, maintenance and installation errors, the PRA to identify external events, and the CMA to verify independence of functions. The process, advantages and limitations of these methods are presented. Additionally, these methods are applied to the air system of fuel cell-powered propulsion systems. The results of the mentioned analyses reveal design challenges associated with the application of fuel cell systems in electrified aircraft propulsion; for instance, concerning functional independence as well as solutions for foreign object damage mitigation, heat transfer and lightweight design.
191 References 1. European Commission. Flightpath 2050. Europe's vision for aviation. Luxembourg: Publ. Off. of the Europ. Union, 2011. ISBN 9789279197246. doi:10.2777/50266. 2. Jansen, R., C. Bowman, A. Jankovsky, R. Dyson und J. Felder. Overview of NASA Electrified Aircraft Propulsion (EAP) Research for Large Subsonic Transports. In: 53rd AIAA/SAE/ASEE Joint Propulsion Conference. Reston, Virginia: AIAA, 2017. ISBN 978-1-62410-511-1. 3. SAE Aerospace. ARP4754A. Guidelines for Development of Civil Aircraft and Systems. Warrendale, PA, United States: SAE International, 2010. 4. SAE Aerospace. ARP4761. Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment. Warrendale, PA, United States: SAE International, 1996. doi:10.4271/ARP4761. 5. Kazula, S., S. de Graaf und L. Enghardt. Preliminary Safety Assessment of PEM Fuel Cell Systems for Electrified Propulsion Systems in Commercial Aviation. Proceedings of the 32nd European Safety and Reliability Conference (ESREL 2022), 2022. doi:10.3850/978-981-18-5183-4_S16-02-019-cd.
192 150 Review of Potential Safety Challenges Associated with Electromagnetic Interference for Future Electrified Aero Engines de Graaf S 1German Aerospace Center e.V., Institute of Electrified Aero Engines With the introduction of all-electric and hybrid-electric propulsion systems into the aviation sector, to meet the Flightpath 2050 and ATAG Waypoint 2050 goals, a number of challenges arise. An immediate topic becomes the development of enabling technologies and the modification of existing technologies to the stringent requirements of aviation. It is necessary to maximise power density, to increase reliability and to tailor technologies towards the environmental conditions associated with the cruise altitude as well as hottest day and coldest day requirements. However, by introducing electrification to aviation the design space for aircraft is being opened up, giving way to novel aircraft and propulsion system integration concepts. An iterative design process will allow for a holistic approach to the development of aircraft, their electrified propulsion architectures as well as the components therein. Safety and reliability are key design and decision-making metrics, alongside high efficiency and power density. One aspect of ever-rising importance is the safeguard of electromagnetic compatibility given the high level of bus voltage being considered and the amount of electric power necessary. In addition, the number of electrical components on board increases and thereby the number of potential sources and sinks of interference. This work provides an overview of the various challenges associated with high-voltage and electrification in terms of electromagnetic interference in an aircraft. These potentials for interference were first reviewed and analysed in terms of their potential coupling paths: capacitive, inductive, conductive and radiated coupling. Furthermore, a diagram is presented to map the interactions of various emissions and the potential sinks to show their relevance for the overall system design. To identify the design level and design stage at which the interference potentials and their coupling pathways need to be considered, they are categorised by the system level at which their influence is most prominent. Means of mitigation and prevention of electromagnetic interference as well as suggestions regarding design considerations to be applied in an early system design stage are being presented in this paper. In the case of many power electronics components a trade-off will be the reliability and electromagnetic compatibility versus achieving high power density. Enabling zero-emission aviation will require finding the optimal overall system design. This paper delivers guidelines to be used as a starting point for preliminary system design and development of integration concepts for electrified aero engines.
193 151 Analysis of Critical Loss of Thrust for Hybrid-Electric Aircraft Configurations and Implications on Preliminary Aircraft Design Mangold J1 , Strohmayer A1 1University of Stuttgart Hybrid-electric aircraft offer several advantages, such as reduced fuel consumption and lower emissions. The Horizon 2020 Project FUTPRINT50 investigates a regional aircraft with an entry-into-service by 2035/2040, whose goals are to be more environmentally friendly, as requested in Flightpath 2050. The study considers a 50 seat aircraft with a design range of 400 km. However, the development of such systems presents several challenges, including the need for different propulsion architectures and the definition of the critical loss of thrust (CLOT) for the one engine inoperative (OEI) failure case. In addition, the safety implications on preliminary aircraft design as well as the transfer to the sizing chart and the selection of power and wing loading need to be considered. The challenge is to fulfill all top-level aircraft requirements (TLAR) and certification requirements with a CLOT. This work presents a study on the critical loss of thrust for hybrid-electric propulsion (HEP) aircraft configurations and its implications on preliminary aircraft design. The propulsion architecture is a parallel hybrid-electric with electric wing tip propellers, battery, and combustion engines. One critical aspect of hybrid-electric aircraft design is the definition of the critical one engine inoperative case or the CLOT, which is not applicable with HEP due to the presence of multiple power and energy sources. Hence, through the lack of handbook methods, the definition in preliminary aircraft design is more complex than for conventional aircraft. To address these challenges, new equations for power requirements based on the hybridization factor of installed power H_P and energy management strategies are proposed. Furthermore, a reasonable hybridization level is determined to ensure that the system is optimized for performance, cost, and environmental impact. The results of the study show that the hybrid-electric propulsion system can meet the TLARs and certification requirements, as well as the critical loss of thrust for the OEI scenario with transfer to the sizing chart. The proposed equations for power loading and energy management strategies provide a useful tool for preliminary aircraft design. The study also highlights the importance of considering the safety implications of HEP systems. In conclusion, hybrid-electric aircraft configurations have the potential to reduce emissions. However, their development presents several challenges that need to be carefully considered. The proposed study provides insights into the CLOT for hybrid-electric aircraft configurations and its implications on preliminary aircraft design, which can be useful for researchers and engineers in this field.
194 152 Zonal safety analysis for the powertrain and fuel supply system of a hydrogen-powered aircraft Moebs N1 , Mangold J1 , Strohmayer A1 1University of Stuttgart The rapid pace of change in aviation and ambitious climate protection targets call for radical technical changes on the next generation of aircraft. In order to meet the ambitious requirements towards CO2-free or even zero-emission aviation, new approaches have to be explored and alternative concepts need to be examined. In this context, the potential of hydrogen propulsion is being investigated more closely in current aviation research. The new form of hydrogen-based propulsion offers unconventional systems architectures that include placing primary parts of the powertrain within the fuselage, wing, or nacelles. In this presentation, a thorough zonal safety analysis (ZSA) will be performed for the powertrain of a hydrogen-powered aircraft to grasp the compatibility of mutually influencing components during operation. This needs to be done as the use of hydrogen as fuel introduces various hazards and risks that might not apply to conventional aircraft concepts. First, the state of the art for hydrogen-propelled aircraft and the method of ZSA will be examined in more detail. In order to be able to use a representative reference in the work, a class of aircraft to be considered in more detail is introduced. For this aircraft class, a more in-depth investigation of all necessary aspects of the newly developed powertrain is then performed. Initially, this includes an estimation of the technical characteristics of essential subsystems in terms of geometry, mass and performance. This allows a thorough segmentation of all relevant zones in the aircraft which feature interconnections and dependencies. This supports the investigation of the general requirements of the powertrain components with regard to integration criteria, operational challenges and safety aspects. Furthermore, a systematic analysis of applicable explosion protection guidelines is included in the study. Thus, a safety assessment of the zones can be performed to set requirements for the system design, like spillage or tightness criteria. From this work, a thorough catalogue of resulting recommendations for powertrain integration will be presented, serving as a universal reference for future hydrogen-based aircraft design efforts.
195 153 Textile Reinforced Mortar strengthening as effective prevention against premature failure of masonry walls made of AAC blocks subjected to diagonal compression Kałuża M1 1 Silesian University Of Technology Introduction The technology of erecting walls made of Autoclaved Aerated Concrete (AAC) blocks ensures fast and effortless execution. In addition, very good physical parameters (thermal properties) and low weight make it a commonly used building material for erecting load-bearing walls in low-rise residential and public buildings. Unfortunately, the great advantage of this technology (the use of thin and unfilled joints) is at the same time its disadvantage. Such AAC walls, subjected to shearing, undergo extensive damage, which leads to their premature failure. Therefore, effective methods to improve the shear parameters are being explored, which is regarded as a preventive activity. Experimental program The research includes basic material tests of masonry blocks and strengthening components and model tests performed on small masonry wallets with dimensions of 800 x 900 x 240 mm subjected to diagonal compression. Unstrengthened elements (one series) and five series of elements strengthened on both sides with the use of systemic strengthening dedicated to masonry structures (three different composite meshes) and nonstructural solutions used in thermal insulation systems (plaster meshes laid on a thin-layer adhesive mortar) were made and tested. The measurement of structure deformation during the test was carried out using the traditional method (inductive gauges) and non-contact optical system. Experimental results / discussion On the basis of the obtained results, the cracking and strength characteristics, the stress-strain behavior of the elements, as well as the failure process were analyzed. The application of full-surface strengthening clearly changed the behavior of AAC walls from brittle one (in the unreinforced series) to "pseudo-ductile" observed after the appearance of the first cracks (Fig. 1). The type of mesh used affected the stiffness of the wall in the initial phase (before cracking) and the behavior of the elements after reaching their full load-bearing capacity. Some of the strengthening made it possible to safely absorb significant deformations of the wall, without any visible damage to its surface.
196 In all strengthened models, a significant increase in the load-bearing capacity and a “post-peak” phase was observed. An interesting finding was a very high efficiency of non-structural materials, which was comparable to the effects of using systemic solutions. The existence of unfilled and thin joints in the strengthened elements was insignificant, unlike in the case of unstrengthened elements, since the failure pattern was of a diffuse nature. Figure 2 presents the area of strong deformation of the strengthened wall after reaching its bearing capacity. The use of optical measurement made it possible to analyze the deformation growth and to recognize the failure process, together with the identification of the moment of mesh delamination. In addition, based on the results of previous studies, the influence of the loading speed, which is not specified in the standard, on the obtained shear resistance values was analyzed. Conclusions In conclusions, the evaluation of the suitability and effectiveness of the strengthening solutions was made. Non-structural materials were found to be the best solution both in terms of the mechanical benefits and the financial input. Fig. 1 Stress-strain relationship of the strengthened wall Fig. 2. The deformation of strengthened wall before failure
197 154 Simulation of the degraded (steel - concrete) bond strength due to corrosion via modeling pull out tests Koulouris K1 , Charalampopoulos A1 , Apostolopoulos C1 1 Laboratory of Technology and Strength of Materials, University of Patras Steel corrosion is recognised as major degradation factor of structural capacity and durability of reinforced concrete (RC) structures, especially in coastal areas. Corroded structures face different wear problems that affect their performance, such as loss of the cross-sectional area of rein-forcement, cracks in concrete and degradation of bond mechanism between steel and the sur-rounding concrete. In the present work, a threedimensional (3D) model was developed, using finite element analysis (FE) in ABAQUS to simulate the degradation of bond strength of corroded structural components, taking into account the recommendations of fib Model Code 2010. As a basis for the model validation, an existing experimental study on the effect of corrosion and stirrups spacing on bond behavior was considered. In the abovementioned study corroded speci-mens were used with transverse reinforcement Φ8/120mm (confined) and without stirrups (unconfined) and subjected to pull-out tests of the main eccentricallyplaced corroded bar. A detailed study was made for the effective way of modeling the complex corroded interface, by selecting the cohesive behavior between the two surfaces to meet those criteria, with modifications appro-priate to the parameters, for each level of corrosion of the reinforcement. The modeling of the damage in the concrete was done as found in the literature by reducing its strength (tension and compression) to an area of corrosion influence that is estimated by a separate code found in the relevant literature. Accordingly, the same was applied for the damage of the reinforcement using empirical relations for the degradation of the mechanical properties of the steel. After extracting the bond – slip curves of the simulations, a comparison was made with the experimental ones, resulting in good predictions from the developed model. In terms of maximum strength, the larg-est deviation is at 9.38% while at the same time positive conclusions are drawn about the failure shapes of the specimens with good predictions of crack formation but not for its representative-ness in order of magnitude. An important observation is that the specific model can take into account the density of the transverse reinforcement with a future for its extension to real con-structions and that the instructions of the Model Code 2010 find good agreement for moderately confined specimens. However further investigation needs to be done into slip values that cannot be predicted of increasing corrosion level and to validate the proposed model with other experi-ments.
198 155 Effect of shot blasting treatment on mechanical behavior of steel reinforcement Basdeki M1 , Apostolopoulos C1 1University Of Patras As it is widely known, chloride induced corrosion of steel reinforcement constitutes a real threat for reinforced concrete structures located in coastal areas. The corrosion phenomena, in synergy with repeated loads as in the case of earthquake prone areas, negatively influence their useful service life. The current experimental study demonstrates the anticorrosive resistance and the mechanical behavior under dynamic loads of steel reinforcing steel bars following the application of the shot blasting treatment, parameters of which were adopted pursuant to several experimental tests. For this purpose, accelerated corrosion tests were conducted to bare and shot blasted steel bars at various times of exposure and low cycle fatigue tests at amplitudes of ± 0.5%, ± 0.75% and ± 1.25% were executed. The outcomes of the present manuscript exhibited the beneficial effect of the shot blasting treatment on B500c steel, hereby providing corrosion resistance, but mainly enhancing the mechanical response under cyclic loading. Besides, an effort has been made to evaluate bare and shot blasted rebars, via a dynamic material index proposed by the authors, in terms of material quality and durability. The findings demonstrated the improved mechanical performance of shot blasted specimens vs. bare specimens, for medium range-imposed deformation in long terms.
199 156 Structural Health Monitoring implementation in standards and application to historic masonry structures Diamantidis D1 , Sýkora M2 1OTH Regensburg, 2CTU Prague Structural Health Monitoring (SHM) is the process of measuring at spot times, periodically or continuously parameters affecting the structural performance of the structural system. Thereby the investigated parameter can be a single variable such as material strength, a function of variables such as displacement, the global condition of the structure such as scanning of signs of deterioration or even a network of structures subjected to traffic monitoring. The present contribution addresses important issues to be implemented in civil engineering standards: a) The decision process of SHM such as the definition of the time and duration of SHM implementation, the selection of the SHM system, the data evaluation etc. b) The choice of the SHM procedure depending on the failure consequences of the structure, on the environmental conditions, on the complexity of the structural system, on measurement uncertainty and related cost of SHM. c) The Value of Information (VoI) gained through SHM and its quantification for decision making including pre-posterior decision assessment (see Figure 1), probabilistic reliability analysis and selection of a monitoring strategy [1]. The presented procedure aims at a wide field of application by providing generally valid recommendations and is illustrated by the case of historic masonry structure based on data from representative walls of historic stone and brick masonry structures in Central Europe. The general decision tree presented in figure 1 is employed in order to show how to make use of all information available i.e. uncertainties related to performed tests and information on the basic variables affecting the reliability of the masonry wall and how to rank the preferences amongst different options i.e. strengthen the wall or keep as it is. It is thereby investigated how the number of destructive tests (DTs) used to calibrate non-destructive tests (NDTs) can be optimised to provide the basis for the reliability assessment of a particular structure: the preliminary assessment is based on non-calibrated NDTs. The number of DTs for calibration is then optimised by means of a probabilistic cost optimisation, considering also possible subsequent actions – ‘do nothing’ or ‘strengthen the structure’. It is shown that the estimates of masonry unit strength based on NDTs are associated with a large dispersion and they may be significantly biased. Consequently, it is often beneficial to conduct at least one DT. Obviously, the larger failure consequences, the higher number of DTs should be taken. General conclusions regarding the implementation of the outcomes in future standards related to assessment of existing structures are also drawn. [1] Diamantidis, D., Sykora, M., & Sousa, H. (2019). Quantifying the Value of Structural Health Information (SHI) for Decision Support - Guide for practicing engineers (COST Action TU1402). Retrieved from https://www.cost-tu1402.eu/action/deliverables/guidelines