Failure of carbon/epoxy composite tubes under combined axial and torsional loading 2. Fracture morphology and failure mechanism

Abstract

Failure mechanisms of cross-ply composite tubes made by the lapped moulding technique were investigated following biaxial testing, as reported in an earlier study (Part 1). Before mechanical testing the undamaged specimens were inspected to characterize their microstructure, and the source of first material damage was also inspected. From phenomenological failure analysis three types of failure mode were exhibited, depending on the biaxial ratio, and the corresponding failure mechanisms are suggested. By means of fractographic observations of the fracture surface, microscopic failure was investigated as a function of biaxial ratio, and it is suggested how the performance of fiber reinforced composite materials tube for engineering applications might be improved. The main factors involved at low biaxial ratio are matrix strength, the bond strength of the seam, and uniform distribution of fiber and matrix, while at a high biaxial ratio the fiber strength is the main factor.