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

Failure mechanisms of cross-ply composite tubes made by the lapped moulding technique were investigated following biaxial testing, as reported in an e arlier study (Part I). Before mechanical testing the undamaged specimens we re 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 fract ographic observations of the fracture surface, microscopic failure was inve stigated as a. function of biaxial ratio, and it is suggested how the perfo rmance of fiber reinforced composite materials tube for engineering applica tions might be improved. The main factors involved at low biaxial ratio are matrix strength, the bond strength of the seam, and uniform distribution o f fiber and matrix, while at a high biaxial ratio the fiber strength is the main factor. (C) 1999 Elsevier Science Ltd. All rights reserved.