Browsing by Author "Li, Min-Chung"

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    Autohesion model for thermoplastic composites
    Li, Min-Chung (Virginia Polytechnic Institute and State University, 1989)
    A non-isothermal autohesion model was developed by combining a transient finite element heat transfer model with the isothermal autohesion model. Heat transfer analyses and the interfacial strength development analyses were conducted using the non-isothermal autohesion model on a polysulfone (Udel P1700) compact tension specimen, a 64-ply graphic (Thomel T300)/P1700 unidirectional composite, and a 192-ply graphite (Hercules AS4)/P1700 unidirectional composite. A 64-ply T300/P1700 unidirectional composite was processed in a matched metal mold. Temperature data were taken and compared with the calculated values. Good agreement was observed between the calculated and the measured temperature values. A healing test which aimed at studying the interplay bond development in AS4/P1700 unidirectional composites was performed. The double cantilevered beam (DCD) Mode l fracture toughness test was selected. The DCB specimens were fractured and healed in a special fixture with different combinations of temperature pressure, and time. The healed DCB specimens were refractured and the critical strain energy release rates (GIC) were measured. The pressure was found to be a key factor in the healing process. Temperature and time dependencies of the interply bond development were also observed. The non-isothermal autohesion model predicted a higher strength achieved in a shorter time. This was due to the extra time which was needed for the fracture interface to achieve intimate contact, and the assumption of the initial intimate contact achievement of the non-isothermal autohesion model.
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    Thermoplastic composite consolidation
    Li, Min-Chung (Virginia Tech, 1993-10-05)
    Fabrication of high-quality composites from thennoplastic prepregs requires careful selection of the processing cycles so that intimate contact at the ply interfaces is achieved resulting in the formation of strong interply bonds and the process-induced residual stress is minimized to ensure superior mechanical performance. The void formation and the consolidation mechanism were studied experimentally. A refined model was developed to relate the processing parameters of pressure, temperature and time to the interply intimate contact of thermoplastic composites. The model was developed by integrating a prepreg surface topology characterization with a resin flow analysis. Both unidirectional and cross-ply lay-ups were modeled. Two-ply unidirectional laminae fabricated from graphite-polysulfone and graphite-PEEK prepregs and [0/90/0]T laminates were consolidated using different processing cycles. Optical microscopy and scanning acoustic microscopy were used to obtain the degree of intimate contact data. Agreement between the measured and calculated degree of intimate contact was good. A finite element model was developed to analyze residual stresses in thermoplastic composites by combining a plane-strain elasticity analysis and a temperature-dependent matrix properties. The residual stress model takes into account the mismatch of the thermal expansion coefficients and the crystallization shrinkage of the matrix. [O₁₀/90₆]T graphite-PEEK laminates were manufactured at different cooling rates to verify the model. The induced residual thermal defonnations were measured by a shadow moire system. The model accurately estimated the out-of-plane displacement of the non-symmetrical laminates.