Determination of damage evolution in CFRP subjected to cyclic loading using DIC

Damage evolution in carbon fibre reinforced plastic subjected to fatigue loading (R = 0.5, −1, 2) has been studied using digital image correlation to obtain full-field surface strains. Damage initiation being a local phenomenon, its effect on global parameters is not significant. The local transverse strain is a better indicator of delamination which affects transverse strain more than the longitudinal strain. Variation of normalized local transverse strain (ratio of local transverse strain to applied stress) near the initiated delamination indicates that the damage evolution occurs over 2 to 3 stages. Each stage has a stable damage growth with a drastic increase between the stages. The stages correspond to different damage mechanisms (matrix cracking, fibre-matrix debonding, delamination, and fibre breakage) dominating at different periods during the fatigue life. Scatter in normalized local transverse strain plots due to large relative displacements was eliminated using different reference images for DIC. Waviness due to shift in the time at which the images are captured during the loading cycles was avoided using a sine curve fit to obtain maximum transverse strain in a cycle. Normalized local transverse strain plots were found to qualitatively reflect the physical extent of damage, thereby providing confidence in the methodology. Fatigue life curves were generated and run-out lives were determined.