FATIGUE AND DAMAGE-TOLERANCE ANALYSIS OF COMPOSITE LAMINATES - STIFFNESS LOSS, DAMAGE-MODELING, AND LIFE PREDICTION

The prediction of fatigue life and evaluation of progressive damage for general composite laminates are studied analytically. From theories of damage tolerance, residual-modulus degradation, and residual-strength degradation, a simple and approximate approach is proposed for the prediction of progressive stiffness loss, matrix-crack density, and delamination area in terms of tension-tension fatigue load and number of cycles for general laminates containing 0-degrees plies. The proposed approach provides four choices for predicting tension-tension allowable fatigue life and for assessing fail-safety for structures made of composite laminates, namely, a residual-modulus criterion, a matrix-cracking criterion, a delamination-size criterion, and a residual-strength criterion. The ap-proximate analytical relationship between S-N curves for general laminates containing 0-degrees plies and for a unidirectional 0-degrees-ply laminate is proposed. Three glass/epoxy laminates of [0/90]s, [+/-45/0/90]s, and [0/+/-45]s lay-ups, and one [0/90/+/-45]s T300/5208 graphite/epoxy laminate under tension-tension fatigue are used to illustrate and verify the proposed approach. The analytical results are in good agreement with experimental data.