An energy-based fatigue damage parameter for off-axis unidirectional FRP composites

An energy-based fatigue damage parameter has been developed to assess the fatigue damage of unidirectional glass-reinforced plastic (GRP) and carbon-fiber reinforced plastic (CFRP) composites. The proposed parameter is based on the physics and the mechanism of fatigue cracking within three damage regions of matrix (I), fiber–matrix interface (II), and fiber (III) in these materials as the number of cycles progresses. The parameter involves the shear and normal energies calculated from stress and strain components acting on these region. In region I the damage was initiated in the form of microcracks within the matrix. For region II, the damage progress took place along the matrix–fiber interface leading to fiber fracture in region III. The proposed fatigue damage model successfully correlated fatigue lives of unidirectional GRP and CFRP composites at various off-axis angles θ and stress ratios R. The results of fatigue damage assessment revealed that the proposed damage parameter correlated fatigue data with a higher degree of success as compared with fatigue damage parameters developed earlier.