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Creep Rupture of Thermoplastics

Introduction

Failure behavior of polymers can be divided into three different categories: (1) If the load is monotonically increasing, then the failure is caused by overload; (2) If the load is held constant for a long time then the failure is caused by creep rupture; and finally, (3) if the load is cyclic then the failure is caused by fatigue. In this article I will discuss how thermoplastics fail by creep rupture, and how you can predict creep rupture in a Finite Element (FE) simulation.

Polymer failure types.

Experimental Creep Rupture Data: Polystyrene (PS)

The following figures show typically uniaxial tension creep data and creep rupture data for a PS material. The data is from “The Effect of Creep” handbook. Since the creep curves have different shape at different applied stresses, the material is NOT linear viscoelastic.

Experimental Creep Rupture Data: PEEK

The following figures show the stress-strain response and creep rupture response for a PEEK material (VICTREX 450G). Also in this case the creep rupture stress becomes almost linear when plotted as a function of logarithmic time.

Predicting the Creep Rupture Response in FEA

To demonstrate how to predict creep rupture of thermoplastics, I will analyze experimental stress-strain data for a polycarbonate (PC) material. The stress-strain data in the figure below is available in MCalibration. I selected a suitable PolyUMod TNV material model and calibrated it to the experimental data. The results are shown in the figure below.

In then created 4 virtual creep tests in MCalibration to explore how the calibrated TNV model predicts creep behavior. The results are shown in the figure below. The creep strain rate is initially almost constant (corresponding to steady state creep), and then at a stress-dependent time the creep strain starts to increase rapidly. This corresponds to tertiary creep which leads to the ultimate failure. Note that it is possible to estimate the failure time (for each stress level) without having a well-defined critical strain value. The strain-time curve becomes almost vertical (on a logarithmic time), which leads to the creep rupture.

From the creep data I can then create a creep rupture curve (see image below). This figure shows that the creep rupture response becomes almost linear when plotted against a logarithmic time scale (just like the experimental data). In other words, a suitable viscoplastic material model (like the TNV model) can predict creep rupture! The calibrated material model can then be used in FE-based creep rupture simulations of any product you are interested in.

Summary

  • Failure of a material under constant load is called creep rupture.
  • It is possible to extrapolate experimental creep rupture data to longer times.
  • The creep rupture response of a FE model can be predicted using a viscoplastic material model (like the PolyUMod TNV model).
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