Durability and Crack Growth Testing of Elastomers

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 that initiate, grow and cause a part to fail.

Simple static tearing tests provide valuable design maximum strain and maximum stress. Fatigue of elastomers is more complex and dependent on many factors. Through a combination of testing and material related fatigue analysis, one can generate a “map” of these dependencies. Testing isn’t a matter of cycling tensile specimens under all conditions. Generating elastomeric strain-life (s-n) fatigue data sets is extremely time consuming. The use of shorter duration crack growth experiments and advanced fatigue analysis software from Endurica provides a faster and more versatile method to get important fatigue data.

Axel Products will work with Endurica to provide a general elastomer fatigue map. Axel Products will also provide individual experiments as desired.

Tear and Crack Growth Experiments:

  • Elastomer Fatigue Properties Map
  • Static Tearing Energy
  • Creep Crack Growth
  • Cutting Resistance Measurement
  • Dynamic Fatigue Crack Growth


  • Elastomer Fatigue Properties Map

    Fatigue of elastomers is complex and dependent on many factors. Through a combination of testing and material related fatigue analysis, one can generate a “map” of these dependencies. Testing isn’t a matter of cycling tensile specimens under all conditions. Generating elastomeric strain-life (s-n) fatigue data sets is extremely time consuming. The use of shorter duration crack growth experiments and advanced fatigue analysis software from Endurica provides a faster and more versatile method to get important fatigue data.

    Graph of stress and strain in a rubber tearing test.
    Comparison of observed fatigue life with Endurica-computed strain-life curves for 3 flaw sizes. 

     

    Image of a static tearing test in planar tension (pure shear).
    Typical images of crack tip evolution during testing.

    Image of a static tearing test in planar tension (pure shear).
    The effect of temperature on the strength of 2 rubber compounds.

     

    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

    Creep Crack Growth

    The growth of a pre-cut crack in an elastomer specimen is observed under a constant or very slowly increasing strain.  The creep crack growth may occur at smaller stresses and strains than expected.  Valuable information may be obtained in experiments lasting less than a day.

    creep crack growth.
    Crack Length Resulting from Very Slow Straining

     

    Graph of slow creep ramp.
    Very Slow Loading of a Crack Test Specimen

    Cutting Resistance Measurement

    Cutting resistance is measured by applying a sharp blade against pre-strained elastomer specimens while observing the force required to cut. The cutting resistance forces can be used directly to evaluate materials. The resulting data may also be used as part of an intrinsic energy calculation. The experiment is a modern version of the experiment performed by G.J.Lake and O.H. Yeoh referenced in the International Journal of Fracture 14, 509 (1978). The experiment was re-discovered by Will Mars of Endurica in recent years.

    Graph of cutting resistance force vs time.
    Stress Strain Loading to Crack Initiation

     

    Image of cutting resistance experiment.
    A Sharp Blade Cutting a Pre-strained Elastomer Test Specimen.

    Dynamic Fatigue Crack Growth

    The purpose of the fatigue crack growth experiment is to measure the rate at which a pre-cut crack grows during cyclic loading. The main idea here is that if we know the starting crack size and we know how fast cracks grow , we can predict part failure. There are many ways to design a crack growth experiment and at Axel Products we perform them under many conditions. For simplicity, a common approach is described herein. A pure shear (planar tension) test specimen is selected because the strain state at the crack tip stays the same as the crack grows and the calculations of crack tip energy is simple. A cut is placed in the specimen far from the edge to avoid edge effects. The specimen is stretched in a loading-unloading pattern such that the minimum strain is zero and the maximum strain slowly increases over 20 hours. Throughout the entire duration of the experiment, the width of the crack is measured.

    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