A fractographic analysis of delamination within multidirectional carbon/epoxy laminates

Multidirectional laminates of the continuously reinforced carbon/epoxy composite T300H/914C have been tested under static and fatigue conditions by the use of fracture mechanics coupons. Loadings of pure mode I, pure mode II and different ratios of mixed mode I/II, i.e. opening tension and sliding shear, have been applied to double cantilever beam (DCB), end-loaded split (ELS), fixed-ratio mixed-mode (FRMM), and mixed-mode bending (MMB) specimens. Optical and scanning electron microscopy techniques were used to identify distinguishing fractographic features and to establish the differences between static and fatigue fracture, as well as the differences between the various modes of fracture. The cusp angle and the amount of fibre pull-out on the fracture surface can be used to characterise the different loading modes. A large amount of fibre pull-out is the dominant feature of a mode I fracture whereas cusps are characteristic features of a mode II fracture. Fatigue fracture surfaces exhibited slightly more debris than static fracture surfaces: this is thought to be due to the fretting nature of fatigue. For the same reason, shear cusps (hackles) were more rounded and less distinct in fatigue than on static fracture surfaces. In certain fatigue cases, it was observed that polyethersulphone (PES), a toughening agent used in the formulation of the epoxy resin, came out of the matrix; the reason for this is not fully understood.