Polymers fatigue testing

Polymers, like other materials, are sensitive to fatigue. Figure 1.7 plots some examples of fatigue test results according to the logarithm of the number of cycles leading to failure. 

Altstadt V, Loth W, Schlarb A (1996) Comparison of fatigue test methods for research and development of polymers and polymer composites. In Cardon AH, Fuduka H, Reifsnider KL (eds) Progress in Durability Analysis of Composite Systems. Balkema, Rotterdam, pp 75-80... 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

Brena, S. E, S. L. Wood and M. L. Kreger (2002). Fatigue tests of reinforced polymer composites. Second International Conference on Durability of Fiber Reinforced Polymer (FRP) Composites for Construction, Sherbrooke, Qudbec, Canada, pp. 575-586. 

Fatigue testing of polymers cannot be accelerated by simply increasing the loading frequency. The reason is the relatively high level of mechanical damping (internal friction) in common polymers which would produce an excessive heating of the specimen. 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

E. H.Andrews, Fatigue in Polymers, in Testing of Polymers, Vol.I V John Wiley Sons, N.Y., 1969. 

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