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Cohesive-Shear-Lag Model for Cycling Stress-Strain Behavior of Unidirectional Ceramic Matrix Composites

During fatigue tests of fiber-reinforced ceramic matrix composites (CMCs), the stress-strain hysteresis loops undergo typically through various changes in shape and size before they stabilize after many cycles. These hysteresis loops are a good representation of intrinsic deformation and damage modes occurring in the materials. The present study proposes a cohesive-shear-lag model to analyze the cyclic stress-strain behavior of unidirectional fiber-reinforced CMCs. The model, as a modification to classical shear-lag model, takes into account the effects of spatially distributed fracture of a partial number of fibers as well as matrix cracking and partial interfacial debonding. The effect of the distributed fibers fracture is modeled in an average sense by using a cohesive damage law. The proposed model has been utilized to investigate the stress-strain hysteresis loops of a unidirectional fiber-reinforced ceramic-glass matrix composite, SiC/1723. The results demonstrate its capability not only in characterizing the cycling stress-strain behavior but also in tracking the progressive damage process that occurred during the tension-tension fatigue of the composite. The dominant progressive damage mechanisms in this case were found to be accumulation of fibers breakage, growth of fiber-matrix interfacial debond, and smoothening of frictional debonded interface, following matrix cracking, which presumably occurred in the first few cycles.