Experimental and Numerical Investigations of Stochastic Thickness Effects in Discontinuous Fiber Composites

Abstract

Discontinuous fiber composites (DFCs) made of chopped prepreg tapes have recently drawn the attention of the aerospace and automobile industries thanks to their flexible manufacturing capability. Because of the discontinuity in a form of prepreg tapes, the fibers easily follow the mold contours while maintaining comparable stiffness and strength to continuous quasi-isotropic laminates. Furthermore, the high production rates and part complexities enabled by DFCs make them a competitive alternative to the use of metals in several applications. However, one of the current roadblocks for the use of DFCs is the lack of reliable analysis methods to predict their mechanical behavior, which depends on different parameters such as platelet sizes, aspect ratios, and spatial distribution. In this paper, we first experimentally investigated tensile elastic modulus and strength of unnotched coupons made of two different platelet aspect ratios (square and narrow) for varying coupon thicknesses. From the experiments, the square platelets showed significant thickness effects on both elastic modulus and strength. The narrow platelets also had significant thickness effects on strength but relatively constant modulus with varying thicknesses. In both modulus and strength, the narrow platelets had higher average values with larger deviations. Next, we computationally examined the relationship between the platelet distributions and the corresponding thickness effects. To get a thorough understanding of the effects of the platelet distribution on the mechanical behavior, the analysis was performed in two steps. In the first step, computational models were generated utilizing a uniform platelet distribution. In the second step, the models were generated leveraging platelet orientation tensors obtained from X-ray micro-computed tomography characterization. It was found that the assumption of a uniform orientation distribution condition was sufficient to capture the average modulus and strength with varying thicknesses for both platelet sizes. However, the associated Coefficient of Variation (CoV) of the results were significantly underpredicted, especially in the case of narrow platelets. On the other hand, numerical results using the orientation tensor obtained via micro-CT provided significantly improved predictions of the CoVs with varying thicknesses. These numerical investigations suggest that, for parts manufactured in conditions of limited platelet flow, the average mechanical performance can be accurately predicted by stochastic Finite Element models featuring a uniform platelet orientation distribution. On the other hand, the prediction of the CoV of moduli and strengths urges the use of an accurate representation of the real platelet morphology.