Uniaxial compressive failure of unidirectional composites with small imperfections
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INTRODUCTION
THE design of structures and components to withstand the initiation of failure is of prime concern to engineers. This article discusses the initiation of compressive failure from small defects (simulated by relatively tiny holes of different sizes) in unidirectional, carbon-fiber epoxy composites. This is an important problem in engineering design because unidirectional composites form the basic building blocks of multidirectional composites, and little work has been undertaken to establish the role of small defects in compression. The present investigation is experimental in nature. Plane rectangular specimens containing a relatively small central hole have been tested in uniaxial compression (along the fiber direction) to gain insight into how these imperfections reduce the compressive strength of composites. The primary aim of our investigation is to examine the effect of hole size on failure mode and unidirectional compressive strength. Even small imperfections tend to initiate failure at lower loads, thus reducing the compressive strength of composites. Such imperfections are simulated in the present experiments by relatively small holes. Each small hole can be viewed as the conglomerated effect of a multitude of tiny defects concentrated within the composite. In situ scanning electron microscope (SEM) examination has been used to examine damage development from these holes. II. EXPERIMENTAL TECHNIQUES The material used in this study was a modern commercial composite referred to as T800/924C. The matrix was a (924C) Ciba Geigy epoxy matrix, and the fibers are Toray (T800) intermediate-strength carbon fibers. Small, uniform holes of diameters ranging from 0.3 to 2.0 mm were
S. SIVASHANKER, Assistant Professor, is with the School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798. S.O. OSIYEMI, Research Fellow, is with GINTEC Institute of Manufacturing Technology, Nanyang Technological University, Singapore 639798. Manuscript submitted December 2, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
drilled at the center of Celanese-type specimens of 3 mm nominal thickness (Figure 1(a)). The specimens are 10 mm wide and have a gage length of 10 mm, in order to compromise between the need to overcome Euler macrobuckling and end-tab effects. If the gage length is too short, the end tabs are too close to each other to enable a uniform distribution of stresses in the central region, and, if the gage length is too long, Euler macrobuckling becomes significant. The unidirectional panels were constructed from prepreg material cut to size and laid up to make a 24-ply laminate. The holes were drilled using high-speed tungsten carbide drills, with the specimen sandwiched between perspex plates to minimize machining damage induced by the drilling process. Drilling was performed without lubricant, but in small steps, to keep the bits sufficiently cool. Note that the smallest practical hole diameter that could be made using this technique is 0.3 mm. Each specimen was loade
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