Effects of large damage on residual strength of carbon fiber reinforced composite laminates

Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace EngineeringTension or compression fracture behavior studies are normally initiated with open hole tension or compression tests performed at the laminate level. While these test data serve as an excellent starting point in residual strength studies in small notch sizes, the need to evaluate and verify the residual strength for larger structures and at larger notch sizes region is still there. An experimental study was performed with the focus on large notch sizes of circular hole and narrow slit flaw configurations on laminates fabricated with a carbon/epoxy oven-cure capable prepreg material system. The current part of the study focused on uniaxial tension loading with a constant width and height to flaw dimension ratio. Experimental test data were then combined with lamina and laminate level data for residual strength curves generation and the curves were validated against Whitney-Nuismer and Mar-Lin fracture mechanics models. The effects of flaw sizes and the flaw type towards the residual strength capability of a laminate turn out to be substantial as the notch size gets larger than one inch. Test results showed that a narrow slit or saw cut damage is far more critical than a circular hole cut-out. The notch sensitivity order of a saw cut flaw is also roughly three times more than that of a circular hole. Analytical studies were also performed to evaluate several finite element method variable effects on residual strength prediction and to discover the best practice in stress analyses of notched composite laminates. Point Stress failure criterion was used in the analyses and predicted failure loads were compared to the experimental data. Good agreements and correlations were found between the analytical predictions and the experimental data. It can be concluded that the residual strength of a notched laminated composite can be reasonably predicted from finite element analyses