Fatigue and Crack Growth Testing of Plastics

Introduction

Fatigue failure in plastic components can be difficult to understand. Plastic parts in service can fail at strains that are only a fraction of the tensile failure. This may occur because of cracking, fatigue, self heating, creep or chemical degradation. This page is dedicated to the mechanisms of crack growth and fatigue.

Tear and Crack Growth Experiments:

  • Critical Crack Growth Measurement
  • Fatigue under Cyclic Loading
  • Fatigue Crack Growth Testing


  • Critical Crack Growth Measurement

    For rigid plastics, a monotonic crack experiment can be performed to get a measure of the critical stress and energy required to propagate a crack in a compact tension specimen. A conservative approach to predicting failure is to use maximum stress and tearing energy information. In this experiment, a cut is introduced into a compact tension test specimen and stretched until the crack grows. This experiment is often performed at multiple temperatures.

    CT data

    Force Displacement Data from a Monotonic Crack Test

     

    Plastic CT cracked
    Plastic Compact Tension Specimen Cracked in Tension using a Pre-cut Crack During Loading

    Fatigue under Cyclic Loading

    Classic s-n curves may be developed for plastics. In these experiments tensile specimens are cycled until failure at various stress levels and mean stresses. To avoid heating, these experiments must be run slowly. As such, plastic fatigue tests take a long time to run and can be prohibitively expensive.

    s-n Curve
    s-n Plastic Data. 

     

    Fatigue Frames
    Basic Fatigue Frames.

     

    Fatigue Crack Growth Testing

    Based on the work of Wyzgoski and Novak, Axel Products is developing automated test systems and methods for the measurement of crack growth in structural plastics under static and fatigue loadings. The overall objective of this effort is to use fatigue crack growth experimental data to effectively expand traditional single condition s-n fatigue data to a data set containing multiple temperatures and rates. A compact tension test specimen is pre-cracked and loaded with a constant force controlled sine wave. The compliance of the test specimen is captured with a clip-on extensometer. The crack size is measured using a camera system and customized software which extracts the crack dimentions from images.

    Plot of crack growth rate with increasing tearing energy.
    Plot of Crack Growth vs. Tearing Energy

     Actual Crack in Plastic
    The Advancing Crack Front During a Fatigue Crack Growth Experiment

     Compact Tension Plastic
    Compact Tension Test Specimen with a Pre-cut Crack During Dynamic Loading