Low-cycle fatigue of unidirectional composites:: Bi-linear S–N curves

Low-cycle fatigue (LCF) behavior of polymer matrix composites (PMCs) is investigated in an experimental study of unidirectional glass/epoxy composites subjected to axial tensile loading along longitudinal 0° orientation of fibers. Under high LCF loads, fatigue life of PMCs is found to be less than 104 loading cycles due to the high property degradation rates that are noticeably higher than those seen during high-cycle fatigue (HCF). In PMC response, unique LCF features have been identified and linked with damage accumulation patterns in unidirectional composites. At high loads near the ultimate strength of specimens, large strains and finite strain rates are found to be significant under semi-rectangular loading so the LCF behavior is affected. Lower and upper limits for the LCF life impose some restrictions on the S–N curves that are obtained for the LCF life assessment. A bi-linear S–N curve is used to approximate the data in the LCF and HCF regions. The bi-linear S–N relationship and the associated fatigue model are described by a proposed analytical formula. The concept of pre-LCF damage state is introduced.