2. Deformation behavior of Polymers:
Unlike metals and alloys, polymers consist of long organic chains that undergo viscoelastic deformation depending on their composition, molecular structure, and external variables such as frequency of cycling and test temperature [18]. Hence yielding in polymers is controlled by their chain mobility. Polymer deformation can still be differentiated broadly into three types depending on the molecular structure. First is the shear-band-like deformation at stress concentrations due to low mobility of the chain segments. The bands can be 38oto 45o to the tensile axis. The second is the crazing, which is somewhat inhomogeneous deformation depending on the relative mobility of the molecular chains. It can lead to cavitation in the deformed region. Crazes typically form in planes perpendicular to the maximum applied stress and vary depending on the polymer’s molecular structure. At low stains, polymers can undergo viscoelastic deformations that are time and temperature-dependent, similar to power-law creep in metals. Hence test frequency and temperature become important. At high strains, some viscoelastic deformation can still occur in competition with the crazing, depending on the structure of polymers. Some polymers are also somewhat brittle at room or low temperatures. Hence fatigue behavior of polymers can differ from material to material and vary with test conditions. Nevertheless, we show here that the two-parametric analysis for FCG is applicable to all cases considered.
Under cyclic load, we can still expect monotonic and cyclic plastic zones as in metals. The extent of each depends on the strength of the polymers and the applied stresses. Hence the characteristic cyclic, Δσ (or ΔK) and monotonic σmax (or Kmax) stresses determine their fatigue damage. Therefore, the load ratio effects on fatigue damage can be significant since it changes with the relative ratio of ΔK and Kmax. There is little work on the R-ratio effects in S-N fatigue of polymers. Hence, this study will be concentrated on the FCG behavior using the available R-ratio data in the literature.