Transverse (interlaminar) cracking under tensile loading in pultruded CFRP

Abstract

The examination of microstructure of tensile specimens of pultruded 60% V _f carbon fibre-reinforced epoxide of up to 6 mm unreduced diameter shows that transverse cracking precedes the tensile failure of groups of fibres. In material whose strength is ∼2 GN m⁻², the process can commence in waisted specimens at stresses as low as 1 GN m⁻²; in those of unreduced section it was not detected below 1.5 GN m⁻². This failure initiation stage can be associated with the decrease in the slope of the load-extension curve. With increasing load the inter-tow cracks were observed to grow and some surface fibre bundles detached. It is suggested that misaligned fibres in these surface bundles were straightened out and contributed to the load-carrying capacity of the rod. Only following detachment of numerous bundles (for the specimens with unreduced section) or growth of interlaminar cracks into the specimen shoulders (for those with a reduced gauge diameter) did tensile failure of fibre bundles lead to catastrophic fracture. It is to this last propagation stage that statistical models of failure of bundles at different cross-sections should refer.