Micromechanics of reinforcement and damage initiation in carbon fibre/epoxy composites under fatigue loading

A model single carbon fibre/epoxy composite geometry was subjected to cycli c loading at a maximum strain below the critical fatigue limit of the matri x material. The carbon fibres were pre-strained prior to incorporation in t he resin to ensure that they were free of thermally induced compression str esses in the axial direction. A strain controlled cyclic experiment from 0 to 0.5% applied strain was performed up to a maximum life of 10(6) cycles. At discrete fatigue levels of 10(0). 10(3), 10(4) 10(5), 5 x 10(5) and 10(6 ) cycles the fibre normal stress distributions of a specified window of obs ervation were obtained by means of remote laser Raman microscopy. The fibre normal stress distributions at each fatigue level were converted to interf acial shear stress (ISS) distributions from which important parameters, suc h as the maximum ISS the system could accommodate, the transfer length for efficient stress built up and the length required for the attainment of max imum ISS were obtained. The results showed that up to the level of full fib re fragmentation the main fatigue damage parameter that affected the stress transfer efficiency at the interface was the fibre fracture process itself . (C) 2001 Elsevier Science Ltd. All rights reserved.