2011 ANSYS, Inc. May 28, 2014 1

An overview of methods for modelling bolts in ANSYS V15

Dragana Jandric ANSYS Inc Technical Support Engineer

Presented at the 2014 ANSYS Regional Conference Chicago May 23, 2014

2011 ANSYS, Inc. May 28, 2014 2

Introduction

Bolt Modeling - Bolt Modeling Method 1 3D bolt representation

- Bolt Modeling Method 2 3D bolt representation

- Bolt Modeling Method 3 bolt thread contact

- Bolt Modeling Method 4 bolt thread contact

- Bolt Modeling Method 5 screw joint

- Bolt Modeling Method 6 line body representation

- Bolt Modeling Method 7 line body representation

- Bolt Modeling Method 8 beam connection

Result Comparison

Summary

Outline

2011 ANSYS, Inc. May 28, 2014 3

Bolted connections are common to many industries

Basic requirements of a bolted connections are:

Bolt should transfer the load realistically across the connecting elements

Bolt must have adequate strength

Joint must remain intact

Connection must have adequate fatigue and fraction life

Bolted analysis is no different than any other FEA calculation

This presentation shows different approaches of modeling bolted connection using full 3D models to beam models

It also explores examples and best practices and explains enhancements in ANSYS v15

Introduction

2011 ANSYS, Inc. May 28, 2014 6

To demonstrate different ways to model bolts, a simple eight bolt flange is taken. In the following slides following aspects will be considered:

Geometry Meshing Contact Pre-tension loading Post processing

Bolt modeling

2011 ANSYS, Inc. May 28, 2014 7

Bolt modeling

Bolt can be modeled as:

- Solid body - Line body - Beam connection

Solid Body + Most accurate + All contact details available + Easy post-processing - Geometry preparation - Mesh refinement - High computational time

Line Body + Easy to set up + Low computational time + Some post-processing tools - Creating line bodies - No contact detail - No stress detail in flange

Beam Connection + Easy to set up + Low computational time + No geometry required - No stress detail in flange - No contact detail - APDL post-processing

2011 ANSYS, Inc. May 28, 2014 8

Approach to modelling the bolts usually involve making engineering decisions about the following:

Prepare geometry

Bolt and flange

Mesh

Minimum DOF for best representation

Consider contact areas for load transfer/stress

Hex / tet

Three step analysis:

Step 1: preload by load or adjustment

Step 2: fix the pretension, release any temporary restraining boundary conditions

Step 3: Apply in-service loads

Model and analysis considerations

2011 ANSYS, Inc. May 28, 2014 9

Eight sectors, each has a different method of modelling the bolt

Upper / lower flanges are multi-body, sweep-able parts

All contacts are asymmetric & bonded

Analysis settings:

Upper / lower flanges fixed at pipe OD

2 step (load/lock ), linear analysis

500N pre-load to all bolts

Overview of model

2011 ANSYS, Inc. May 28, 2014 10

Key features of this approach:

No/very little geometry preparation

Full thread on bolt and nut

Good geometric representation of stiffness of bolt/nut will be captured if mesh is dense enough

Contact areas give accurate representation of bolt head and nut contact area to flange

Most cases will produce a tetrahedral mesh, check element quality

Bolt model 1:

2011 ANSYS, Inc. May 28, 2014 11

Key features of this approach:

Some geometry preparation, threads removed on bolt and nut

Care should be taken not to alter bolt shank stiffness as this will affect bolt deflection and load transfer in the system during pre-tension and in-service loading

Contact areas give accurate representation of bolt head and nut contact area to flange

Load between bolt and nut is transferred via bonded contact.

Most cases will produce a tetrahedral mesh, check element quality

Bolt model 2:

2011 ANSYS, Inc. May 28, 2014 12

Key features of this approach:

Geometry same as bolt model 2

New V15 bolt thread contact applied (recommended 4 elements span 1 thread width)

Contact sizing option to increase number of elements in thread area

Contact results show helical load transfer at threads

Bolt model 3:

2011 ANSYS, Inc. May 28, 2014 13

Key features of this approach:

Significant amount of geometry preparation on bolt and nut

De-feature, respecting size of contact area under bolt head/nut and bolt shank diameter

Decompose to sweep-able bodies

Multi-body back together

Prepare 1 fastener and use pattern to replace others

Can take quite a few mesh controls to get a good quality mesh

Bodies are modified to mesh them with hex mesh.

New V15 bolt thread contact applied (recommended 4 elements span 1 thread width)

Bolt model 4:

2011 ANSYS, Inc. May 28, 2014 14

Key features of this approach:

Geometry and mesh same as bolt 4

Bolt thread contact replaced with a cylindrical joint

APDL commands to redefine joint as a screw joint

Bolt model 5:

keyo,_jid,1,17 sectype,_jid,joint,screw,_wbjoint pi=acos(-1) secjoin,,12 pas=1 secjoin,pitch,(pas/2/pi)

2011 ANSYS, Inc. May 28, 2014 15

Key features of this approach:

Geometry preparation

Bolt/nut geometry replaced with a line body

Line body meshed as beam elements, model size significantly reduced

Contact, end of bolt to cylindrical edge of bolt hole, MPC couple U-Rot inside pinball, note: for edge contacts WB automatically extends spider out 1 element for load transfer

Bolt model 6: