Short predicting the strength and toughness of fiber composites

Abstract

Predictions of strength and toughness for short-fiber-reinforced plastic systems are complex but industrially crucial problems. In this contribution, we utilize a new approach which accounts for the large stress concentration penalties in a perfectly aligned short-fiber composite. Although empirical, the approach permits calculation of a strength reduction factor which can then be utilized with an appropriate failure criterion to calculate the strength of a wide range of short-fiber composite systems. A similar approach is used for the toughness problem, with the fracture toughness of an isotropic, random-in-aplane, short-fiber composite being expressed in terms of the fracture toughness of single unidirectionally oriented plies with cracks oriented along the two orthotropic axes. For both strength and toughness, a laminate analogy approach is used. The short-fiber system is thought of as being composed of several plies or layers, each containing uniaxially aligned short fibers. The plies are oriented in the laminate to replicate the actual system fiber orientation distribution, and the linear stress-strain or fracture toughness properties are calculated by analyzing the individual ply responses to the overall applied stress. Comparison with experimental data for random-in-a-plane fiber orientation and for biased in-plane orientations at practical fiber volume loadings shows good agreement. In the case o f fracture toughness, there was qualitative agreement between theory and experiment, but the data scatter precluded a rigorous comparison. INTRODUCTION TO THE STRENGTH PROBLEMPrediction of strength for short-fiber-reinforced plastic systems is a complex but industrially crucial problem. Even in the case of unidirectionally aligned fibers with tensile stress applied in the fiber direction, failure may occur in the fibers, in the matrix phase, or at the interface. Furthermore, failure may take place in a tensile or shear mode and may be brittle or ductile in nature. The problem of strength prediction is best illustrated in Figure 1, wherein we see that unlike for stiffness, continuous-fiber composite strengths cannot be attained in discontinuousfiber systems, even at very high aspect ratios. A second massive drop in strength occurs due solely to fiber orientation. Thus in going from a continuous aligned fiber system to a

Up-cycling end-of-use materials: Highly filled thermoplastic composites obtained by loading waste carbon fiber composite into fluidified recycled polystyrene

“…The fiber fracture energy and the interaction term are negligible for short or weak fibers. In this case, the overall impact strength of the composite is worsened independently from the interfacial adhesion . This assumption justifies the resilience data obtained in the case of CF‐25 based materials, in which the filler is mainly composed of small particles.…”