usyd_abaqus_2006

ABAQUS: Selected Topics L8.63

Some Important Contact Issues

• Contact with Friction
– Friction is a highly nonlinear effect.
• Solutions are more difficult to obtain.
• Do not use unless physically important.
– Friction is nonconservative ⇒ unsymmetric equation system.
• ABAQUS/Standard will automatically use the unsymmetric solver

when µ > 0.2 or when contact pressure dependency is detected.

• The unsymmetric solver will be used for the entire analysis.

Copyright 2005 ABAQUS, Inc.

ABAQUS: Selected Topics L8.64

Some Important Contact Issues F2 F1

• Rigid body motion F1

– Many mechanical assemblies use
contact between bodies to prevent rigid
body motions.

– This is not effective until the bodies are
actually in contact.

– If loads are applied to the body, typically
a singular system is produced, with
unbounded rigid body motion resulting.

– Can add stabilization to help in such
cases:

• Contact pair damping

• Automatic viscous damping (whole
model)

Copyright 2005 ABAQUS, Inc.

ABAQUS: Selected Topics L8.65

Some Important Contact Issues master surface 1
slave node
• Overconstraining the model master surface 2
– A slave node involved in contact is
subject to contact constraints. Slave node is overconstrained
– The user should be careful not to
overconstrain a slave surface:
• A particular slave surface should
only be paired with one master
surface (it cannot be slave to two
masters).
• Do not apply boundary conditions
or other constraints to nodes on
the slave surface
– Overconstraining the model can lead to
non-convergence or unreliable results

Copyright 2005 ABAQUS, Inc.

ABAQUS: Selected Topics

Workshop 8a: Hinge Model
Workshop 8b: Clip and Plate Model

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ABAQUS: Selected Topics L8.67

Workshop 8a: Hinge Model

Copyright 2005 ABAQUS, Inc. L8.68

ABAQUS: Selected Topics

Workshop 8b: Clip and Plate Model

Copyright 2005 ABAQUS, Inc.

ABAQUS: Selected Topics

Lecture 9

Bolted Connection Modeling

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.2

Overview

• Modeling Bolted Connections
• Mesh-Independent Point Fasteners
• Beam + Coupling Technique
• Modeling Bolts with Solid Elements
• Bolt Pre-tension Loads

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.3

Modeling Bolted Connections

• Modeling bolted connections usually involves:
– Modeling the bolts and their interaction with the bolted components; and
– Modeling contact between the bolted components

• Contact between the components is achieved through surface-to-surface
contact, as usual

• Several modeling techniques are available for considering the bolts and
their interactions with the other components.
– The appropriate choice of modeling technique depends on the desired
outcomes of the analysis. For example:
– Is it sufficient to have correct load transfer across the joint?
– Or are accurate local stress solutions require in and around the bolts?

• In this lecture, we aim to give a brief overview of some common modeling
techniques

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.4

Modeling Bolted Connections Coarse system
models
– Rigid or deformable point fasteners Not interested in local
(ignores holes in parent components) detail, just overall
load transfer
– Deformable beam fasteners with
couplings Detailed component
models
– Deformable solid fasteners with tie Interested in accurate
constraints local results in vicinity
of bolts
– Deformable solid fasteners with threaded
bolt interaction capability (new in v6.6)

– Deformable solid fasteners with detailed
thread modeling

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics

Mesh-Independent Point Fasteners

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.6

Mesh-Independent Point Fasteners multiple
surfaces
• Mesh-independent point fasteners allow
you to conveniently define point-to- radius of
point connections between surfaces. influence

– They can be located anywhere
between surfaces.

• They need not be defined at attachment
nodal locations. points

– They can connect multiple layers;

• i.e., the number of connected
surfaces is not restricted.

– The fastener acts over a specified
radius of influence.

– The meshes on the surface do not
need to match

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.7

Mesh-Independent Point Fasteners coupling
constraint
– The fastener capability combines either:
• connector elements or connector
• beam multi-point constraints element
or MPC
with distributing coupling constraints to
define a connection.

– Translation and rotation of the
attachment points are related to the
translation of nodes on the shell surface.

– Fasteners can model either rigid, elastic, or inelastic connections with
failure by using the generality of connector behavior definitions.

– Fasteners are not currently supported by ABAQUS/CAE

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.8

Mesh-Independent Point Fasteners Fasteners

• Example: Rail crush with fasteners

Video Clip

Nodes involved in couplings for
mesh-independent fasteners

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.9

Mesh-Independent Point Fasteners

• A major advantage of mesh-independent fasteners is that the necessary
elements and couplings are generated automatically by ABAQUS
– The user need only define:
• The coordinates of a single point to locate each fastener
• The properties to be used for the fasteners (eg rigid, deformable,
failure, etc.)

• This is only a very brief description of mesh-independent fasteners
• For more detailed information, see

– Section 20.3.4 in the ABAQUS Analysis User’s Manual, “Mesh-
independent fasteners”

– Section 1.2.3 in the ABAQUS Example Problems Manual, “Buckling of a
column with spot welds”

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics

Beam + Coupling Technique

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.11

Beam + Coupling Technique manually defined
coupling constraint
• This is quite a common technique for
approximating a bolted connection manually created
beam element
• Similar to the mesh-independent point
fastener technique, but requires more
manual work
– Manually create beam elements to
approximate the bolts
– Connect the ends of the beams to the
circumference of the hole using
coupling constraints (manually
defined)
– Commonly used in joining
components modeled with shell
elements, where the holes are
included

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics

Modeling Bolts with Solid Elements

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.13

Modeling Bolts with Solid Elements

• If accurate local stress solution is
required in and around the bolt,
use a detailed component model

Contact pressure
distribution due to
interference
resolution

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.14

Modeling Bolts with Solid Elements

• Detailed component models are good for local stress analysis
– Can give good accuracy of local stress and contact pressure on threads

• However, they are computationally expensive
– All components are modeled as deformable bodies
– 3D finite-sliding contact with friction
– Threads must be meshed quite finely ⇒ large model

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.15

Modeling Bolts with Solid Elements

• Coarse system model approach
– Only remote stresses in the bolted components are of interest (not
detailed local stresses around the bolt)
– The bolt and other components are still modeled with solid elements
– The bolt thread interaction is approximated by tie constraints
– A much coarser mesh can be used for the bolt

• The advantages with this technique are
– Good for fast system response
– Low computational effort
– Ease of modeling

These pairs of surfaces are Tied to
approximate the meshed threads

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.16

Modeling Bolts with Solid Elements

• Tie Constraints
– A tie constraint provides a simple way to bond surfaces together
permanently.
– Surface-based constraint using a master-slave formulation.
– The constraint prevents slave nodes from separating or sliding
relative to the master surface.
– Boundary conditions should not be applied to the nodes on the
slave surface of a tie constraint pair; doing so will overconstrain
the model at those nodes.

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.17

Modeling Bolts with Solid Elements

*TIE,NAME=TubePlateTie, POSITION TOLERANCE=0.01, ADJUST=YES

Surface-based constraint.
(Can select either predefined
surfaces or regions directly in
the viewport.)

Only slave nodes The surface-to-surface
within this distance method is used by default
from the master
surface are tied to Warnings will be issued in the
the master surface. data (.dat) file for these nodes.

Both translational and Slave nodes can be moved
rotational degrees of onto the master surface in the
freedom can be initial configuration without any
constrained. strain.

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.18

Modeling Bolts with Solid Elements Bolt
Tension
• Starting in V6.6, ABAQUS provides a new
threaded bolt interaction capability Axial force

• Provides accurate detailed response within a Radial
system model framework force
– Compromise between meshing and ignoring
threads Bolt Bolt hole
– Mesh-independent surface interaction using
bolt-thread specification
– Capture 3-D thread interactions
• Frictional contact at thread angle (axial
and radial)
• Model radial spread and load due to axial
bolt load
• Asymmetric torsion response due to
thread helix angle

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.19
L9.20
Modeling Bolts with Solid Elements

• Supported through ABAQUS/CAE as
an extension of surface-to-surface
contact

• User enters common bolt parameters
to define the interaction, such as
– Clearance region on slave surface
– Bolt direction vector
– Half-thread angle
– Pitch
– Bolt diameter

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics

Modeling Bolts with Solid Elements

• Example comparison (Mises stress)

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.21

Modeling Bolts with Solid Elements

• Total axial load – differ by 2.3%

Axial bolt load versus engaged threads

45
40 38.7

34.8
35
Axial load in bolt (% of total)
30

25

20 Meshed threads
New bolt feature
15.6
15 14.0

10.1 9.9
10 7.7 8.1

6.1 6.2 5.1 4.8

5 4.3 4.2

0 23456 7
1 Number of engaged threads

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics

Bolt Pre-tension Loads

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ABAQUS: Selected Topics pre-tension L9.23
section bolt
Bolt Pre-tension Loads
gasket A
– The pre-tensioning capability
can be used to simulate the
tightening of fasteners that are
used to assemble a structure.

– A fastener is identified by
means of a pre-tension
section, across which a
desired load is applied to
tighten the fastener.

Example of a fastener

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.24

Bolt Pre-tension Loads

– The pre-tension section is controlled with a “pre-tension node” that has
only one degree of freedom and is used to:

• apply a load across the pre-tension section; or to

• apply a tightening adjustment (displacement) of the pre-tension
section, which also results in a preload of the fastener; and to

• maintain the tightening adjustment so that the load across the fastener
can increase or decrease upon loading of the entire structure.

– The load or tightening adjustment acts along the normal to the pre-tension
section.

– Use concentrated loads to prescribe a pre-tension load at the pre-tension
node.

– Use boundary conditions to prescribe a pre-tension tightening or to
maintain the pre-tension adjustment during further analysis.

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.25

Bolt Pre-tension Loads

– The total force transmitted over the pre-tension section is the sum of the
reaction force (identified as RF1) at the pre-tension node, plus any
concentrated load (identified as CF1) at that node.
• The stress distribution across the pre-tension section can be obtained
from the underlying elements.

– The tightening of the pre-tension section appears as the U1 displacement
of the pre-tension node.

– Pre-tension sections can be defined in fasteners modeled with:
• Continuum elements
• Beam or truss elements

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ABAQUS: Selected Topics L9.26

Bolt Pre-tension Loads beam or truss n
element 2
• Fastener modeled with beam or
truss elements 1 pre-tension
– If a beam or truss element is pre-tension section
used to model the fastener, the node
pre-tension section is chosen to ∗
be inside an element.
• The pre-tension direction
points from the first node of
the element to the last
node (following the element
connectivity).

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.27

Bolt Pre-tension Loads

– The normal n to the pre-tension section:

• Is by default a unit vector oriented from the first node to the last node
of the element

• Can be given directly by the user
• Remains fixed, even for large displacement analysis

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.28

Bolt Pre-tension Loads n

• Fastener modeled with continuum pre-tension node
elements pre-tension section

– If continuum elements are used to
model the fastener, the pre-tension
section is defined with a surface across
the fastener.

elements chosen by
user to describe the
pre-tension section

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.29

Bolt Pre-tension Loads

– The pre-tension section:
• Does not have to be continuous.
• May be connected to elements other than continuum elements as long
as only continuum elements are used to define the pre-tension
section.

– The normal n to the pre-tension section:

• By default is oriented in the direction of the positive surface normal.
• May be given directly by the user.

– Hence, the pre-tension section does not have to be orthogonal to
the pre-tension direction.

• Remains fixed, even for large-displacement applications.

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.30

Bolt Pre-tension Loads

• Usage in ABAQUS/CAE

Create a new Select an internal Choose how to specify
bolt load surface for the pre- the pre-tension

Copyright 2004 ABAQUS, Inc. tension section

ABAQUS: Selected Topics L9.31

Bolt Pre-tension Loads

• In subsequent steps, you can modify the bolt
load
– Option to fix the bolt at its current length
– This maintains the initial adjustment of the
pre-tension section by fixing the degrees of
freedom at their current values
– This technique enables the load across the
pre-tension section to change according to
the externally applied loads.
– If the initial adjustment of a section is not
maintained, the force in the fastener will
remain constant.

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.32

Bolt Pre-tension Loads

• Example: Bolted pipe joint
– A pre-tension load of 15 kN
is applied across the bolt to
place the gasket under
compression.

flange

gasket

bolt

Assembly load between bolt, flange, and gasket

Copyright 2004 ABAQUS, Inc.

ABAQUS: Selected Topics L9.33

Bolt Pre-tension Loads

– Contour of
stress S22

Copyright 2004 ABAQUS, Inc.