On the effect of matrix cracks on laminate strength

Matrix cracks and their effect on ultimate laminate strength in (0/90), ($\pm$45/90), and ($\pm$45/0) type graphite/epoxy laminates under uniaxial tension were investigated. Initial off-axis ply transverse strength was found to increase with decreasing off-axis ply thickness. The subsequent change of transverse strength for multiple cracking was considered a result of volume change. Therefore, matrix crack growth can be predicted from comparisons of the finite element results and the subsequent corresponding transverse strength. The initiation of matrix crack-induced delamination was identified experimentally and compared with the initiation load predicted using a crack closure method in finite element analysis. Good agreement was found between the predicted and experimental data. Unstable delamination growth was observed when delamination initiates; afterwards delamination growth became stable and competed with matrix crack growth. Saturation crack density was obtained when matrix crack growth could not compete with delamination growth. Otherwise, saturation crack density was not found; only the rate of matrix crack growth slowed, and ultimate crack density was used instead of saturation crack density. A simple linear law was proposed to monitor delamination growth when delamination initiates. Good agreement was found when the analysis was compared to the delamination area observed with optical microscope. Without considering the unstable delamination growth, the stable delamination growth can be predicted using this simple model. A theoretical model of predicting ultimate laminate strength, which considers matrix crack density, matrix crack size, and matrix crack-induced stress concentration has been proposed. The mechanisms leading to final failure have been discussed extensively. All derivations were based on the observed previously phenomena. Predictions were compared with experimental data. Good agreements were found. To estimate the effect of off-axis matrix cracks on the load-carrying capacity of adjacent plies, a soft but tough film was placed along the interfaces in some laminates to isolate the effect of the matrix cracks. The use of adhesive layers in (0/90), ($\pm$45/90), and ($\pm$45/0) laminates tested in this study raised laminate strength by about 30%, 55%, and 16%, respectively