An Investigation of the Failure Mechanisms of a Carbon Fibre Reinforced Composite Material in Bending

Abstract

The mode of failure of a carbon fibre reinforced concrete beam has been studied. Early investigations show that there were two distinct types of failure, tensile and compressive, depending on which of these properties was the weaker. Composite specimens were prepared under controlled conditions and their tensile and compressive strengths investigated using optimum test procedures. A large scatter exists. A large scatter exists on the tensile strengths of the composite such that the 'law of mixtures' gives as reliable a prediction of strength as the more sophisticated analysis. However this simple analysis does not predict the gauge length effect or the loss in efficiency in transferring the load from fibre to fibre at the high fibre volume fractions shown by the test results. As indicated by the macro and micro-stability analysis the compressive strength is directly dependent upon the shear modulus of the matrix. As the shear modulus is increased there is a similar improvement in the compressive strength. The test results also indicate a dependence of the compressive strength upon voidage. Both the macro and micro-analysis predict a compressive strength approximately three times the measured strength. A tentative sequence of failure is suggested to explain this disparity which combines the macro-analysis and the effect of voidage, where the compressive buckle initiates at a stress concentrating flaw. The test results show that the mode of failure when a beam fails in tension is gradual as when it fails in compression it exhibits brittle behaviour. Possible ways of improving the compressive strength over the tensile strength of the composite by increasing the shear modulus of the matrix have been suggested, thus giving the more controlled tensile mode of bending.