Yield Point and Stress Relaxation Behavior of Ply-Ply Interaction of Thermoset Prepregs

The forming of continuous-fiber composites is an automated manufacturing process capable of high production rates. Its greatest weakness lies in the tendency to develop wrinkle defects. The formation of wrinkle defects is the effect of a combination of multiple material deformation mechanisms. This paper investigates possible improvements for current manufacturing techniques by studying the interply properties of continuous-fiber thermoset prepreg. In this research, the inter-ply slip deformation behavior is tested via double lap shear tests. The results show that pressure, deformation rate, and moisture have a strong effect on the ply-ply interaction response. The multi-stage nature of the loaddisplacement response suggests the complex nature of the deformation. To better understand each stage of deformation, the total amount of slip deformation of the double lap shear test is measured. The total slip versus displacement profile indicates the presence of a yield point in the ply-ply interface. The yield point indicates both the softening of the ply-ply interface and the initiation of plastic deformation. The plastic deformation past the yield point can be further increased if the preform is held in a deformed state for some time before removing the external load. A new manufacturing process is proposed based on these results. One can take advantage of the yield point of the ply-ply interface and conform a preform onto an intermediate tool before conforming it onto the final tool. As deformation can be preserved once the material deforms past the yield point, this sequential forming approach can reduce the risk of wrinkling by reducing the slip required to complete the forming operation.