16. Damage Evolution and Local Strain Redistribution in Composite Laminate with Various Fiber Arrangements

The initiation and gradual development of damage in composites is associated with the degradation of the composite laminate properties. Understanding the characteristics of damage evolution in composite laminates has been one of the major interest in composite studies. There is a lot of progress in this regard, however still there is a lack of clear understanding on how damages are initiated, grown and transformed from one form to another. In this study experiments are conducted to capture the strain localization and cracks formation on the free-edge of composite laminates. Laminates that have a stacking arrangement of (0/−Ɵ/+Ɵ/90)_s with plies that have different fiber angles (Ɵ = 15^°, 30^° and 45^°) are manufactured. Coupon samples are made from these laminates and subjected to a uniaxial tension loading until final fracture. Using digital image correlation technique at high magnification, the local deformation field is determined. From the test, it is observed that the strain/stress response of the composite is influenced by the arrangement of the fiber angle of the off-axis plies. From the strain contours obtained on the free-edge, the gradual initiation and growth of matrix cracks is observed to be localized in the 90^° plies. In addition, these matrix cracks grow and lead to cause delamination between the 90^° plies and neighboring plies. The local strains in each individual ply are seen to fluctuate along with the emergence of cracks at the vicinity of the damage as a result of stress redistribution.