Short Term Stress Relaxation

If an elastomer is stretched to a particular strain and held, the stress in the elastomer will decrease over time. This decrease in stress over time is referred to as stress relaxation. This reduction in stress can be a significant fraction of the initial stress. For many elastomers, the normalized shape of the stress-time function is relatively insensitive to the absolute strain level and to the strain state. This behavior, viscoelastic behavior, is typically modeled separately from the hyperelastic behavior.

As a result of this behavior, analytical predictions based solely on frequency or rate effects will fall short if the effects of mean strain and dynamic amplitude are ignored.

At frequencies above approximately 500 HZ, it may not be reasonable to measure dynamic material properties assuming a simple specimen model as shown above. The short wavelength and mass effects of higher frequency requires a different approach.

One technique is to use an infinite length specimen technique whereby longitudinal waves are transmitted along a long (greater than 300 mm) specimen. The wave speed and wave attenuation are then determined at points along the specimen to determine the dynamic properties using basic wave equations.

This technique has been used at Axel Products for measurements between 500 Hz and 10,000 Hz.