Effects of Shape and Surface Area of Fiber in Short Fiber Composites

Abstract

The efficiency of load transfer in short fibers in composite materials was analyzed. The variables of reinforcement like shape (wavy/straight fibers) and surface area were investigated theoretically and verified experimentally. Tension tests were performed on the short fiber composite specimens. A two dimensional finite element model containing a single fiber with the application of appropriate boundary conditions was constructed for analysis. Using this model the stress distributions at the fiber/matrix interface was studied. The wavy fiber composite were found to be much stronger than the straight fiber composite. It was also found that, for straight fiber composites, thinner fibers make a stronger composite than thicker fibers. I. Introduction n short fiber composites, the load transfer efficiency among fibers is crucial in effecting superior composite properties. It is conceivable that this load transfer efficiency depends on the shape, aspect ratio, and surface area of the fiber. Recently, some researchers have used short fibers with two enlarged ends to reinforce polymer composites. 1-2 and demonstrated the potential to improve significantly both strength and fracture toughness of the composite. The primary toughening mechanism provided by the bone-shaped short fiber is the large scale deformation in the surrounding matrix as the enlarged fiber ends pull out. In an earlier study, Zeng and Sun 3 found that a wavy lap joint configuration could yield much greater joint strengths than the conventional flat lap joints. This improvement in strength of the wavy joint was found to be the result of the interfacial stresses that were altered by the wavy geometry. The aforementioned success in wavy lap joints has motivated the present study in the shape of the reinforcement. The effect of surface area of the reinforcing element is of particular importance because of the increasing use of nano particles to form nanocomposites. It is well known that for the same volume, a material at nano scale possesses much greater surface areas than at larger scales. It is evident that more surface areas mean more load transfer paths and, thus, lower interfacial stresses between the reinforcement and the matrix. The lowering of interfacial stresses is expected to lead to higher composite strengths. The objective of this research is to study the efficiency of load transfer in short fibers in composite materials. The variables of the reinforcement such as its shape and surface area/weight ratio are investigated both theoretically and verified experimentally. It is anticipated that the result of this research will benefit the design of the conventional short fiber composites as well as the emerging nanocomposites in which nano particles have extremely high surface/volume ratios. Platelet type reinforcements are considered. Attention is focused on the effect of shape and surface area of the platelet on the mechanical properties of the composite. To simplify the experiment and observation, model composites are manufactured with platelets whose lateral dimensions are on the order of centimeters and thickness in the range of 0.01 - 1.0 mm. Micromechanics models are developed to help us understand the load transfer mechanisms and to achieve the optimal platelet shape and thickness.