Journal of Thermoplastic Composite Materials

In this paper, a method is presented that facilitates the determination of stress intensity factors in composite bonded joints with interfacial or interlaminar cracks. The method is based on a global/local concept, where the displacements and stresses away from the crack tip are found by a conventional finite element analysis. In the neighborhood of the crack tip singularity, where the finite element results alone no longer converge, an elasticity solution is constructed using an eigenfunction expansion of the stresses and displacements. It was shown that the method allows accurate determination of stresses and stress intensity factors at the crack tip (within 5% of reference solutions) with a relatively small computational effort by using a coarse FE mesh. In addition, the method does not require special finitelements and can be used with any existing finite element code. An equivalent intensity factor [ILLEGIBLE] that captures the combined effect of K_I and K_II on failure was defined. Numerical results indicated that, out of various interfacial and interlaminar cracks studied, the largest value of the equivalent intensity factor occurs at the interlaminar crack in the first ply interface of the adherend near the adhesive, but not at the adherend-adhesive interface. These results are consistent with the experimental observation that failure of composite bonded joints typically occurs by delamination of the adherend near, but not at, the adhesive.

This series of papers reports a new, general, and unified micromechanical model for estimating the three-dimensional mechanical properties of a composite made from two constituent materials, i.e., continuous fibers and matrix. Various mechanical properties, i.e., elastic, elasto-plastic, strength, and rubber-elastic properties, of unidirectional laminae made using different constituent materials have been simulated in Parts I-IV. The present paper addresses the problem of failure analysis and strength prediction of laminated composites consisting of different unidirectional laminae. By using the classical laminate theory, the stress shared by each lamina ply in the laminate can be determined, and the lamina analysis theory reported previously can be applied. The progressive failure process in the laminate is treated through stiffness reduction. As soon as one ply fails, it has no further contribution to the overall instantaneous stiffness matrix of the laminate, and the total strength of the laminate corresponds to a load level at which all the plies have failed. A generalized maximum normal stress criterion, which accounts for equibiaxial and equitriaxial tensions, is used to detect the lamina tensile failure, whereas the lamina compressive failure is controlled using a criterion similar to the original maximum normal stress criterion. The present modeling procedure has been applied to four different angle-ply laminates subjected to various biaxial load conditions. Favorable correlation between predictions and available test data has been found.

This work characterizes moisture absorption and its influence on elastic moduli for various composites. Three thermoset-matrix composites and one thermoplasticmatrix composite were examined. The first was a traditional carbon/epoxy composite; the second was similar except that the epoxy was a polyurethane-modified type. The third was an interleaved composite, adding a Nylon 6 non-woven fabric in between the carbon/epoxy plies. The last was a carbon/Nylon 6 composite made from commingled yarns. Composite panels with various fiber orientations were fabricated for material characterization. Based on these materials, prototypes of tennis rackets were fabricated for studying the loss of stiffness due to moisture absorption. Fabrication of the panels and rackets has been discussed. Two hygrothermal environments have been applied to the rackets; one is a normal condition and the other is a wet-and-hot condition. Histories of moisture absorption and the induced softening effect have been studied. Two standard tests of the racket flexibility have been adopted for measuring the loss of stiffness. The coefficients of hygroscopic diffusion were calculated for each material using the Fickian type of moisture diffusion. Compared with the thermoset composites, the carbon/Nylon 6 absorbed much more moisture and resulted in a much greater reduction in stiffness. Because of the non-woven layers, the interleaved composite was more prone to moisture absorption than the two epoxy-based composites. The relationship between moisture content and stiffness loss differs greatly for the material systems under different conditions.

In this study, cross-winding equipment has been designed to produce ABS + 20 wt% PC/Al flake metallized plastic pellets with a high aspect ratio. There are two aspect ratios, 150 and 200, for the pellets, and each of them has three different weight percentages of Al flakes, 20 wt%, 27 wt%, and 33 wt%. Through injection molding, specimens for tensile and impact strength tests, and shielding effectiveness tests are produced from the metallized plastic pellets. The effects of Al flake content and aspect ratio on properties are also discussed. According to experiments, a high aspect ratio causes better impact strength, shielding effectiveness, heat distortion temperature, and volume resistivity but reduced tensile and impact strength. High Al flake content worsens tensile and impact strength but achieves better shielding effectiveness, heat deflection temperature, and volume resistivity.

Three sizes of A-glass beads, 6000 (4 [im), 3000 (35 prm), and 1922 (219 μm), were chosen in this work to identify the effects of filler content and size on the drop-weight dart impact fracture behavior of filled polypropylene (PP) composites at room temperature. At higher filler concentration, the values of the maximum impact load and the crack initiation energy for the PP filled with smaller glass beads were higher than those of the unfilled PP and the systems filled with larger glass beads. However, the crack propagation energy of the former was lower than that of the latter. The influence of the filler content and size on the impact fracture energy of these composites was insignificant. Comparatively, the drop-weight dart impact resistance of the systems filled with smaller glass beads was somewhat improved. Furthermore, the impact fracture morphology of the specimens was observed and analyzed. The results indicated that the smaller particles were more beneficial to initiate and increase microcracks in the PP matrix interlayer around them and thus to improve the impact resistance of the composites at high filler concentrations.