Durability and Crack Growth Testing

Introduction

Failure in rubber components is difficult to understand. The strain at failure in a tensile test can be at several hundred percent yet the parts in service can fail at strains that are only a fraction of the tensile failure. This may be because cracks that already exist or cracks to initiate and grow and cause a part failure. A couple of experiments that help understand these issues follow.

Tear and Crack Growth Experiments:

  • Static Tearing Energy
  • Dynamic Fatigue Crack Growth


  • Static Tearing Energy

    Failure in elastomeric parts is hard to predict and understand. Tensile failure strain data can be very misleading when materials are exposed to cuts or defects. A conservative approach to predicting failure is to use maximum strain, maximum stress and tearing energy information developed in a static tearing energy experiment. In this experiment, a cut is introduced into a planar tension test specimen and stretched until the crack grows. This experiment is often performed at multiple temperatures.

    Graph of stress and strain in a rubber tearing test.
    Stress Strain Loading to Crack Initiation

     

    Image of a static tearing test in planar tension (pure shear).
    Planar Tension Test Specimen with a Pre-cut Crack During Loading

    Dynamic Fatigue Crack Growth

    The following snapshots show a few of the testing services available to those researching durability and the growth of cracks in elastomeric materials. CCD camera based vision systems have been integrated with digital servohydraulic instruments to provide crack measurements during testing. Crack images are captured at precise strain levels and at preset cyclic intervals and then processed to provide crack width and crack contour measurements along with stress and strain measurements as cyclic loading progresses.

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

     Image of a rubber specimen and camera in a dynamic crack growth experiment.
    Planar Tension Test Specimen with a Pre-cut Crack During Dynamic Loading