Journal of Composite Materials

Static crushing tests were conducted on graphite composite tubes to examine the in fluence of fiber and matrix maximum strain at failure on the energy absorption capability of graphite reinforced composite material. Fiber and matrix maximum strain at failure were determined to significantly effect energy absorption. The higher strain at failure composite material system, AS-4/5245, exhibited superior energy absorption capability compared to AS-4/934, T300/5245 or T300/934 composite material. Results of this in vestigation suggest that to achieve maximum energy from a composite material a matrix material that has a higher strain at failure than the fiber reinforcement should be used.

Surface waviness in glass fiber reinforced composites is predominately due to dif ferences in the thermal expansion properties of the resin and the reinforcement. A model has been developed which simulates the surface deformation of glass fiber reinforced composites as a function of factors such as molding temperature, thickness of glass fibers, and the thermal expansion properties of the resin and the glass. Application of a top layer helps to conceal fiber readout which is a type of surface waviness. Based on the predic tions of the model, increasing the top layer thickness is more effective than increasing the top layer stiffness.

A compact finite element formulation based on singularity transformation is presented and is used to analyze the stress singularity at the boundary-layer of an interface between adjacent layers in a laminated composite. The composite is subjected to uniform axial ex tension in the plane of the layers. For illustration, results from few elements are shown to demonstrate the accuracy and efficiency of the present approach. Excellent agreement between the present results and the analytical solutions is obtained.

The split Hopkinson bar was used to determine the stress, strain and strain rate response of woven graphite-epoxy laminates during high shear strain rate deformation. The specimens were oriented and tested in both interlaminar shear and in transverse shear at strain rates ranging from 6000 to 18000 sec^-1. The results show that the strain rate effect in interlaminar shear is small. The fracture surface is characterized primarily by interfacial failure between the fibers and the matrix. In transverse shear, however, the strain rate effect is significant due to the greater role played by the resin matrix in the deformation process. In both cases, at the high strain rates imposed in this investigation, sparks were produced by the specimens during deformation. These are believed to be caused by the combustion of fragments resulting from the high rate deformation.

The purpose of this paper is to develop an efficient boundary element method (BEM) for use in the analysis of composite structures, principally loaded holes in mechanically fastened composites. The idea is to modify the BEM by an analytical scheme to evaluate boundary stresses and deformations accurately. The validity of the method is verified by computing the stress concentration factor in a double lap joint for different hole sizes and various boundary conditions.

Static crushing tests were conducted on graphite and Kevlar¹ reinforced epoxy tubes to examine the influence of specimen geometry on the energy absorption capability of com posite materials. Tube inside diameter to wall thickness (D/t) ratio was determined to sig nificantly affect the energy absorption capability of composite materials. As D/t ratio decreases, the energy absorption capability increases nonlinearly. The energy absorption capability of K/E tubes was found to be geometrically scalable but energy absorption of Gr/E tubes was not geometrically scalable.

An analysis was performed to evaluate the effect of the assumed pin load distribution on the calculated strength and predicted failure mode of pin loaded holes in laminated composites. The calculations utilized a finite element method of stress analysis combined with the Yamada-Sun failure criterion applied along the Chang-Scott-Springer char acteristic curve. Strengths and failure modes were calculated for both a cosine load distri bution and a load distribution determined by an iterative procedure. The strengths and failure modes calculated by the two types of load distributions agreed closely.