Papers by Author: Dong Joo Lee

Abstract: During the fabric preform and/or mold closure processes of the resin transfer molded composites (RTMCs), the discontinuous fabric patterns such as wrinkling and overlapping can be occurred and influenced the failure strength. It is found that the composites with discontinuous fabric preform had two failure mechanisms as functions of fabric ply number and discontinuous fabric length under the three-point bending. First, the failure modes can be related to the bend strengths that were controlled by the interfacial bonding strength depending on the discontinuous fabric length. Second, the failures were controlled by the potential strength of fabric when the discontinuous length reached the critical value. Moreover, the experimentally measured results of the normal and discontinuous preform models under bending were compared to examine the safety conditions as functions of fiber content and other factors.

Abstract: Five methods are used to measure values of circumferential Young’s modulus and three methods are used to obtain shear modulus using the closed and open ring specimens. Based on the assumption that the technique using a strain gage must yield the correct value of Young’s modulus, the technique using a closed ring specimen gives more consistent results than the technique using an open ring specimen. For the simplicity of measurement, the technique using a closed ring specimen under compression can be used as long as the thickness of the used specimen remains thin enough to neglect the effects of shear deformation. To measure the shear modulus, G12, both techniques using the open and closed ring seems to have the same trend as a function of thickness. As expected from the loading scheme, the technique using an open ring gives more consistent results for the shear modulus.

Abstract: The tensile, fatigue and creep properties of carbon fiber filled poly(ether-ether-ketone) or PEEK were compared by employing injection and compression molding processes. The specimens with different initial fiber lengths were fabricated using compression and injection molding methods. The effects of fiber length on mechanical properties are evident in cases of specimens made by compression molding. But, the injection molded samples show no differences in tensile and fatigue strengths contingent on the initial fiber length. Similarly, the effects of fiber volume fraction on tensile and fatigue strengths are more evident for the compression molded specimens than the injection molded specimen. For the creep properties, even the injection molded specimens show some improvement for long reinforcing fibers especially as temperature is increased. When the influence of time and temperature is considered, the effects of fiber length on mechanical properties are very significant. Also, these results represent the significant fiber damage during injection molding.

Abstract: The hygrothermal degradation of glass fiber/nylon composite was investigated after aged at 25°C, 50°C, 75°C and 100°C up to 1 month of total exposure in aqueous solution. The effects of moisture absorption and thermal aging on mechanical properties are compared as functions of temperature, fiber volume and concentration of sodium chloride. The amount of water absorption increases when the aging temperature is increased and the concentration of NaCl is lowered. In general, the mechanical properties decrease with amount of water absorption. The degradation rate of various mechanical properties is different depending on the temperature, fiber volume and the concentration of NaCl. The diffusion mechanisms of water in short-fiber reinforced nylon are discussed as functions of fiber volume, molding conditions and concentration of sodium chloride.