Cyclic fatigue delamination of carbon fiber-reinforced polymer-matrix composites: Data analysis and design considerations

Abstract

Activities toward standardization of fracture mechanics tests on carbon fiber-reinforced polymer-matrix (CFRP) composites have recently focused on cyclic fatigue under mode I (tensile opening), mode II (in-plane shear) and mixed-mode I/II loading. Data from recent round robins performed by Technical Committee 4 (TC4) of the European Structural Integrity Society (ESIS) and from preliminary testing of additional CFRP epoxy laminates at the authors’ laboratories are analyzed with different approaches in attempts to reduce scatter and to identify parameters for CFRP structural design. Selected test data comparing load and displacement control for the cyclic fatigue tests are also discussed. Specifically, threshold values from Paris-law data fitting are compared with values from fitting with a modified Hartman–Schijve approach. Independent of the approach used for the analysis, mode I threshold values of selected CFRP seem to be in the range between about 30 and 100J/m2, i.e., roughly around the range of critical mode I energy release rate values (denoted by GIC) obtained from fracture testing of neat commercial epoxy resins, but clearly below quasi-static initiation GIC-values for unidirectional CFRP composites. Implications for CFRP structural design based on mode I fatigue fracture mechanics test data are briefly discussed.