The effect of specimen size on the bending strength of unidirectional carbon fibre-epoxy

Four-point bending and pinned-end buckling tests were carried out on scaled specimens of 25, 50 and 100 plies. Both types of test gave similar results and showed a significant decrease in strength with increasing specimen size. The smaller specimens tended to fail progressively in tension. The tensile strain to failure decreased by about 8% for each doubling of specimen size. This corresponds to a Weibull modulus of about 25. However the amount of scatter in the results was much smaller than that expected based on Weibull strength theory. The larger specimens tended to fail catastrophically in compression. This suggests a reduction in compressive strength with specimen size which may be even larger than the reduction in tensile strength.