Vibration and Viscoelastic Experiments

Introduction

Elastomer and plastic response to vibration and short time events depends significantly on the mean strain condition and amplitude of the perturbation; perhaps more than the rate or frequency of the event. As such, it is critical to first characterize the static condition upon which dynamic events are imposed. Also, vibrational and viscoelastic behaviors are sensitive to temperature and must be measured at the temperature of the application.

This section is divided into sections as follows:


Viscoelastic Decay (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.

img

Dynamic Stress Response to a Stepped Strain Sequence

img

Short Time Stress Decay at Constant Strain


Dynamic Vibrations

Elastomeric components often experience dynamic sinusoidal loading superimposed on a larger mean strain as shown herein. This is common for mounts, bushings and body seals. The response to the dynamic loading is such that higher frequencies result in higher stiffness values. However, for most engineered elastomers, the effects of mean strain amplitude and dynamic sinusoidal amplitude may be greater. 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.

img

Dynamic Tensile Test on a High Stiffness Instrument

img

The Small Amplitude on Large Mean Strain Typical of Many Elastomeric Applications


Acoustic Vibrations

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.

img

Wave Propagation Measurement System with Laser Vibrometer