ANALYSIS OF MATRIX CRACKING AND LOCAL DELAMINATION IN (O THETA/-THETA)S GRAPHITE-EPOXY LAMINATES UNDER TENSILE LOAD/

Several three-dimensional element analyses of (0/theta/ - theta)s grap hite epoxy laminates, where theta = 15, 20, 25, 30, and 45-degrees, su bjected to axial tensile load, were performed. The interlaminar stress es in the theta/ - theta interface were calculated with and without a matrix crack in the central - theta plies. The interlaminar normal str ess changes from a small compressive stress when no matrix crack is pr esent to a high tensile stress at the intersection of the matrix crack and the free edge. The analysis of local delamination from the - thet a matrix crack indicates a high strain energy release rate and a local ized Mode I component near the free edge, within one-ply distance from the matrix crack. To examine the stress state causing the matrix crac king, the maximum principal normal stress in a plane perpendicular to the fiber direction in the - theta ply was calculated in an uncracked laminate. The corresponding shear stress parallel to the fiber was als o calculated. The principal normal stress at the laminate edge increas ed through the ply thickness and reached a very high tensile value at the theta/ - theta interface indicating that the crack in the - theta ply may initiate at the theta/ - theta interface. Predicted crack prof iles on the laminate edge in the - theta ply were constructed from the principal stress directions. The cracks were found to be more curved for layups with smaller theta angles, which is consistent with experim ental observations in the literature.