Scaling effects in the flexural response and failure of composite beams

An experimental program was conducted in which scale model graphite-epoxy composite beams were loaded in bending until failure to investigate the effects of specimen size on flexural response and strength. Tests were performed on unidirectiona l, angle-ply, cross-ply, and quasi-isotropic beams of eight different scaled sizes ranging from 1/6 scale to full scale. The beams were subjected to an eccentric axial compressive load designed to promote large bending deformations and rotations. Results indicated that, although normalized flexural response was independent of specimen size, a significant scale effect in strength was observed for all laminate types. Typically, failure stresses and strains decreased as the size of the beam specimen increased from subscale to the full-scale prototype. Standard failure criteria for composite materials, such as the maximum stress, maximum strain, and tensor polynomial criteria, cannot predict the strength degradation with increasing specimen size. Consequently, a Weibull statistical model and a fracture mechanics model were used to analyze the strength scale effect.