Journal of Reinforced Plastics and Composites

A modified variational analysis based on the principle of minimum com plementary energy, is used to describe the stress state around a through-the-thickness transverse crack in [0 _m/90_n]_s cross-ply laminates. The analysis takes into account nonlinear stress distribution in the thickness direction of the 0°-plies and contains a thermoelastic consideration. A failure criterion based on linear elastic fracture mechanics is used to pre dict the onset of transverse matrix cracking in cross-ply laminates. The predictions are compared with experimental data and with previous analyses developed in the framework of the shear lag theory or the variational approach suggested by Hashin.

Composite repair patches are often cured under vacuum pressure only, and are therefore more porous than their autoclaved counterparts. Thus it is common practice to add extra plies to a composite patch, over and above the number in the original struc ture. However, there is little published work on how the number of extra plies and their orientation should be chosen. By comparing the strength and stiffness of autoclaved and vacuum cured material, using classical laminate analysis, some guidelines for the selec tion of extra plies have been deduced.

This paper is to investigate the failure process and pin-bearing strength of laminated composites under different temperatures. An incremental finite element code, based on the updated Lagrangian approach, was developed for determining the strains dis tribution of laminates under the consideration of the temperature dependent material prop erties. The quadratic failure criterion is adopted for failure predictions. Numerical results include the failure process, failure mode and ultimate bearing strength for two differ ent laminates, S-500 glass/polyester and graphite/epoxy, with three different lay-ups, [0°/45°/90°/-45°]s, [0° ₂/ ±45°]_s and [90°₂/ ±45°]_s, as well as with different end distances (e = d-4d) and widths (W = 1.5 d-6d). In order to verify the accuracy of the numerical calculations, the bearing strength model test is also made for understanding the effect of temperature on the failure behavior of the laminated composites with pin-loaded hole.

Resin Transfer Molding (RTM) is a commonly used fabrication method for large parts of fiber reinforced composites. The reinforcement consists of fiber mats or woven rovings first laid inside a mold cavity, then catalyzed resin is injected through prop erly positioned injection ports. A numerical model based on Darcy's law has been devel oped. It permits to simulate, by a boundary-fitted finite difference method, the filling of two-dimensional molds of arbitrary shapes. The resin pressure distribution and the resin front positions can be obtained at each time step. Calculated results on selected mold geometries are compared with the experimental observations and discussed with those ob tained by other investigators. Finally the experimental and computational limitations of the proposed simulation are pointed out and illustrated for particular examples.

Compression and compression-after-impact (CAI) tests were conducted on seven different AS4-3501-6 [0/90] 0.64-cm thick composite laminates. Four of the seven laminates had through-the-thickness (TTT) reinforcement fibers. Two TTT reinforcement methods, stitching and integral weaving, and two reinforcement fibers, Kevlar and carbon, were used. The remaining three laminates were made without TTT reinforcements and were tested to establish a baseline for comparison with the laminates having TTT rein forcement. Six of the seven laminates consisted of nine thick layers whereas the seventh material was composed of 46 thin plies. The use of thick-layer material has the potential for reducing structural part cost because of the reduced part count (layers of material).

The use of a thin rubber coating on the fiber in a fibrous composite has been shown to increase the composite low-speed impact resistance [1,2]. The coating used was very thin compared to the fiber diameter, in order to preserve the other composite properties. From elastic consideration, the thin rubber coating was understood to have a direct effect upon the reduction of the stress concentration in the region near the fiber/matrix interface, especially at the chopped fiber end or at the matrix crack front reaching the fiber [3,4]. If the possible existence of localized high stress zones is reduced, the material can be subjected to higher loading before global failure occurs. With the rub ber coating the high stress zones inside the material can spread out to a larger volume. There will be less chance for local damage to initiate under the same loading level. The material will therefore exhibit higher resistance to failure, i.e., it will absorb more energy before failure. Three injection-molded glass/nylon composites were tested for tensile fail ing energy. One is reinforced with bare chopped fibers and the other two are with rubber- coated fibers. Tensile experiments showed significant increase of energy absorption in one sample with a rubber coating. Micrograph study was extensively carried out on the failed samples to identify the parameters of this desirable consequence. The rupture of the rub ber interphase and the limited plastic flow of the nylon matrix on the fractured surface of failed specimens were clearly evidenced. Meanwhile, interfacial bonding measured through microdebonding tests [4] revealed reasonable relation of the characteristic inter facial bonding and the energy absorption.