Crack growth from notched unidirectional graphite/epoxy tubes subjected to axial, torsional, and combined loading conditions was studied experimentally and theoretically. Tubes with fiber orientations of 2.5, 15, 45, and 87.5° were notched with slots 0.5 cm long, parallel to the tube axis.
The stress state at the notch of the thin tubes was determined using an elasticity analysis of an infinite plate with an elliptical hole. The normal stress ratio theory was used to predict the far-field stresses and the direction of crack growth at crack initiation. The slots in the tubes were modeled as an ellipse having the same notch tip radius as the slot, an ellipse having the same length and width, or as a circular hole depending on the loading regime. The normal stress ratio theory correctly predicted the direction of crack growth parallel to the fibers, but did not provide good correlation with the experimental far-field stress for crack initiation in all cases. It is believed that this lack of correlation for the critical stresses is due to the different notch geometries in the theoretical model and actual tubes.
The fracture surfaces of the failed tubes were examined using a scanning electron microscope. It was found that failure occurred primarily with the cracks growing from the notch either fully within the matrix or along the fiber/matrix interface depending upon the far-field loading.