RESIDUAL STRENGTH AND TOUGHNESS OF DAMAGED COMPOSITES

A study has been made of the effect of prior damage on the tensile strength and toughness of a number of carbon and glass fibre reinforced plastics materials. The methods of introducing this prior damage were by tension fatigue cycling, by transverse compression, by repeated impact, and by stress corrosion. The effects of the damage were assessed by measuring the un-notched and notched tensile strengths and, in some instances, the work of fracture of notched samples. Acoustic emission monitoring and microstructural studies were carried out in support of the mechanical tests. It appears that under various circumstances, the effects of microstructural damage may result in independent changes in the notched and un-notched strengths of a composite, although the pattern of changes is not simple. The toughness to strength ratio, K(Q)/sigma-f, may rise or fall, depending on the material and the damaging conditions. However, the results for all of the tests presented here still fall within the 90% confidence limits for the K(Q)/-sigma-f relationship previously identified.