Cyclic Deformation and Failure of Polymers

The phenomenological fatigue behavior of many metals can be described by equations in which the material constants are determined from cyclic deformation and monotonic fracture properties. This approach makes it possible to describe and predict fatigue behavior with a minimum of testing and provides a rational method for selecting metals to resist fatigue. There have been few attempts to determine what cyclic and monotonic properties are necessary to describe and predict the fatigue failure of polymers, however, and this knowledge may be useful when and if these materials are modified to be used for structural purposes. To this end the cyclic deformation and failure behavior of some common plastics is examined.

At least three failure phenomena occur in polycarbonate and polymethylmethacrylate tested in completely reversed strain control at room temperature and at low frequencies: catastrophic fracture, cyclic fracture, and delayed cyclic softening. At high cyclic strains the definition of failure is often arbitrary, and the onset of failure can depend upon the test frequency. In this region it does not appear feasible to relate the fatigue behavior to any monotonic material properties. At low cyclic strains the number of cycles to fracture appears to be independent of the frequency of testing, and the behavior is often described by an equation similar to the Coffin-Manson equation. Moreover, when log strain is plotted against log life, the slope appears to be relatively constant for a variety of polymers. In addition, the plot of stress amplitude versus log life is linear for nylon and polycarbonate in the low strain region and the extrapolation to one reversal is very well approximated by the fracture load divided by the cross sectional area after fracture.