Modeling plastics in ANSYS

© 2011 ANSYS, Inc. August 27, 20141 Release 14.0

14. 0 Release

ANSYS Mechanical

Experimental Elastomers

Modeling Plastics in ANSYS

© 2011 ANSYS, Inc. August 27, 20142 Release 14.0

Introduction

The large strain nonlinear stress-strain behavior of thermoplastic exhibits the following:

• Strong hysteresis

• Rate dependence

• Softening after yielding

• Brittle failure at low temperatures and ductile behavior at higher temperature

Thermoplastics usually show different material behavior under different loading and environmental conditions. Thus usually one single material model could not be used to predict plastic nature. Following slides will discuss the thermoplastic behavior under different conditions and ANSYS material models which could be used to model such behavior.

© 2011 ANSYS, Inc. August 27, 20143

Uniaxial behavior of thermoplastics

Under monotonically increasing uniaxial testing conditions, generally thermoplastics exhibits following behavior:

• Stress increases monotonically without any softening after yield (sample 1).

• Stress softening after yield and then resumes hardening (sample 2).

This is typical behavior of

thermoplastics under uniaxial tensile

loading.


Behavior under Loading-unloading condition

Under loading-unloading uniaxial testing conditions, generally thermoplastics exhibits strong hysteresis effect which is followed by permanent (plastic) deformations. While considering plastic components, it is very important to determine if there is any possibility of that component experiencing loading-unloading conditions.

Generally the reverse loading slope is not same as the loading curve slope.


Creep and relaxation

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material.

It is more severe in materials that are subjected to heat for long periods, and near melting point and always increases with temperature.

Thermoplastic also shows creep and relaxation behavior. If the material is known to show creep behavior at operating conditions, it is always advisable to consider creep during numerical modeling of that component.


Available ANSYS Material Models

ANSYS offers range of material models which could be used to model different thermoplastic behavior under different conditions. Choice of material model dependents on:

1) Experimental data available

2) Operating conditions

Different models which could be used are:• Small deformation plasticity (Elasto-plastic)• Pressure dependent plasticity models (Drucker Prager/ Extendend Drucker

Prager)• Large deformation elasticity models (Hyperelastic)• Bergstrom Boyce Model• Rate dependent plasticity models (Viscoplastic)• Viscoelastic • Creep


Choosing right model


Choosing right model..

ANSYS offers various models which could help to describes the material behavior of

thermoplastics.

However issue remains on how to choose the ‘Correct’ model!

Key is to first narrow down all the material behavior user wish to include for the

material.

List all the material behavior

Brittle/Ductile/high strain elastic

Different loading –unloading behavior

Permanent deformation

Stress relaxation/Creep




thermoplastics.


Second step is to list all the loading conditions for which the structural component needs

to be designed.

Cyclic/non-cyclic loading

Loading rate

Temperature loading

List all the loading condition

© 2011 ANSYS, Inc. August 27, 201410



thermoplastics.


Final step is to list down the simulation objective. This is very important steps since it

also determine the material model.

Stress Analysis

Life prediction

Design OptimizationList all the simulation objective

© 2011 ANSYS, Inc. August 27, 201411


As mentioned earlier there is no single ‘Material Model’ which could model all the behavior described in the last slides. Thus it is advisable to gather all experimental data and list all the operating conditions and choose material model. Let consider few scenarios:

.

© 2011 ANSYS, Inc. August 27, 201412

Case 1

Structure is subjected to monotonically increasing load where Stress increases monotonically without any softening after yield. There is no reverse loading and temperature variation is not much ie. Stress-strain behavior is similar in the vicinity of the operating temperature.

Model Recommendations (in increasing order of complexity):

1) Small strain metal plasticity (elasto-plastic):

Pros: Easiest to use.

Cons: Not advisable for large strains and may not be easy to define yield point.

2) Large Strain hyperelastic:

Pros: Include large strain effect in the equation

Cons: Need more experimental data to properly define the model.

3) Bergstrom-Boyce:

Pros: Include large strain effects

Cons: Need more experimental data to properly define the model and currently no curve fitting in ANSYS

© 2011 ANSYS, Inc. August 27, 201413

Case 2

Structure is subjected to monotonically increasing load where Stress softening is occuring after yield and then resumes hardening . There is no reverse loading and temperature variation is not much ie. Stress-strain behavior is similar in the vicinity of the operating temperature.

Model Recommendations (in increasing order of complexity):

1) Large Strain hyperelastic:

Pros: Include large strain effect in the equation

Cons: Need more experimental data to properly define the model.

2) Bergstrom-Boyce:


Cons: Need more experimental data to properly define the model and currently no curve fitting in ANSYS.

Metal Plasticity cannot be used since “Softening” requires decrease in stress-strain slope which is not allowed in this model.

© 2011 ANSYS, Inc. August 27, 201414

Case 3

Structure is subjected to loading –unloading load, unloading slope is not same as the loading slope and permanent deformation is present. It is assumed that temperature variation is not much ie. Stress-strain behavior is similar in the vicinity of the operating temperature.

Model Recommendation:

1) Bergstrom-Boyce:


Cons: Need more experimental data to properly define the model and currently no curve fitting in ANSYS.

Metal Plasticity cannot be used since “Softening” requires decrease in stress-strain slope which is not allowed in this model and also unloading slope is same as the loading slope.

Hyperelastic models do not show any hysteresis and thus cannot be used here.

Documents

Modeling plastics in ANSYS