Creep and Stress Relaxation

Creep

When subjected to load, all elastomers exhibit an increasing deformation with time, know as creep. This occurs at any stress level and takes place in compression, tension and shear loadings and varies for each type of loading. In service, creep can be minimized by using low working stresses and avoiding high temperatures. No rapid method has been developed for its measurement because there is no know way of accelerating time effects without introducing inaccuracies in predicting rate of creep.

Creep is usually expressed in percent of deformation after the part is loaded rather than the unloaded dimension. Determination of creep takes place after some arbitrary short time interval such as one minute, five minutes or even on day after applying the load. Creep, expressed as a percent, equals total deformation minus initial deformation divided by initial deformation, time 100. In the initial stage, creep occurs at a relatively high rate and then continues at a very slow rate. Failure can occur after an extended period of high stress. Figure 1 illustrates characteristic creep curves. AB in the high stress creep curve indicates the failure phase where actual fracture can occur.

FIG 1

Below the failure zone, when stress is removed, the part will attempt to return to its original dimension; however, it will never fully recover. The unrecoverable portion is called permanent set. Loads which allow intermittent recovery will exhibit less creep than if continuously loaded. However, continuous vibratory loading will increase creep since internal heat is generated.

Strain relaxation is important in applications such as engine mountings since it influences the alignment of various parts of the equipment. Yet, it is difficult to predict these properties for a given application without resorting to simulated service tests because several factors have an important effect on them. Chief among these are amount of strain, operating temperature and changes in these two resulting from vibration.

The relative effects of variables have not yet been correlated so that results of tests under one set of conditions will permit accurate prediction of creep under another set of conditions. It has been established that the higher the initial strain, the higher the creep; also, the higher the temperature, the higher the creep. In general, the degree of creep is dependent on the type of strain. Creep is greater under tension strain then under equal compression strain. Creep is also increased more under dynamic loading then under static loading.

The creep characteristics of two urethane polymers, over a ten-month period, are shown on Figure 2. After approximately 3000 hours (18 weeks) creep reaches a plateau and becomes almost constant. The amount of creep is a function of stress level. This involves a stress of 400 psi. Creep will continue at a very low rate after this point, which is the classic behavior of elastomers.

FIG 2

The actual creep of the 95durometer A compound was 0.033 inches after ten months compared with and initial deflection of 0.200 for sample 0.500 inches thick. After the initial loading, creep is only 6.6%.

The creep rate of rubber materials of all kinds increases at elevated temperatures. Where dimensions are important, operating temperatures must be kept below 150F (66C).

Stress Relaxation

Stress relaxation is the loss in stress when it is held at a constant strain over a period of time. It is usually expressed in terms of percent stress remaining after an arbitrary length of time at a given temperature.

There is no standard method for determining stress relaxation. However, many laboratories have developed relaxation cells. These cells utilize the compression set specimen and the test procedure parallels ASTM D-395 Method B. Stress relaxation for 50A urethane is shown in Figure 3.

FIG 3