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.