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Date of Award
Doctor of Philosophy (PhD)
Composite materials are becoming prevalent in aerospace industries as the uniqueness of the composite structure allows the composite to be tailored specifically for individual applications. Many fabrication techniques produce defects in composite parts such as wrinkles, fiber waviness, fiber misalignment, and porosity. The driving mechanisms behind these defects occurring during forming processes are not fully understood and, thus, characterization formation of these defects in a uncured state is beneficial to optimize composite processing. This work primarily investigated the influence of how uncured pre-impregnated carbon ply properties affect the wrinkling behavior of a composite laminates. Several factors affecting composite ply forming were investigated include intra-laminar shearing, material tack, and inter-laminar shearing. A series of hot drape forming trials were conducted with uni-directional tapes (UD), plain weaves (PW), and eight harness satin (8HS) prepregs which showed that intra-laminar shearing was a factor in the wrinkling. The 8HS laminate formed without wrinkles while the PW laminate had smaller wrinkles than the tape laminate which followed the trend of drapability of individual plies. Focusing only on PW prepregs, variations of Cycom 970’s PW prepreg was analyzed for tack and inter-laminar shearing. The tack test showed the transition between inter-facial and a cohesive failure which corresponds to the temperature at which the resin starts to flow. The inter-laminar shearing characterization provided strong correlations between viscosity, strain rate, initial pressure, and fiber twist on the shear modulus and shear stress. The characterization of the prepreg properties was also used to develop an uncured laminate model derived from the first-order shear displacement theory. Verification of this model was conducted via experimental hot drape forming of laminates at various Hersey numbers which accurately predicted the experimental wrinkle formation.
Richard Axelbaum, Kathy Flores, Kenneth Jerina, Shankar Sastry,
Available for download on Thursday, December 12, 2019