Size effects in the testing of fibre-composite materials

Abstract

The effect of test specimen size on the unnotched strength of continuous fibre reinforced composites is considered. The different fundamental failure mechanisms of fibre direction tension, fibre direction compression, and matrix dominated transverse tension and shear are discussed in turn. The available experimental data is reviewed, concentrating on tests on unidirectional carbon and glass fibre/epoxy materials. The results show a tendency for the strength to decrease with increasing specimen volume for all failure modes. Size effects in tensile failure have been found in both tensile and flexural tests, and from higher strengths in bending than in tension. The magnitude of the effect is consistent with Weibull moduli in the range 13–29 for the materials considered. There are some indications that the size effect in tension diminishes with increasing scale. Lower compressive strengths have been found in large components than in small test coupons, and attributed to defects. Size effects observed in compressive failure in bending are believed to be mainly due to the stress gradient through the thickness. There are large size effects for matrix dominated failures, and Weibull strength theory fits the data reasonably well. It is important to consider the effect of specimen size on strength when carrying out tests on composites, and to take account of it when using strength data from small coupons in the design of large structures.