Experimental observations and finite element analysis of the initiation of fiber microbuckling in notched composite laminates

The objectives of this dissertation research are to develop a better understanding of the factors that affect the semi-circular edge-notched compressive strength and to identify the associated failure mode(s) of thermoplastic composite laminates with multidirectional stacking sequences.; The primary variables in this investigation are the resin nonlinear shear constitutive behavior, stacking sequence (orientation of plies adjacent to the 0{dollar}spcirc{dollar} plies), resin-rich regions between the 0{dollar}spcirc{dollar} plies and the off-axis supporting plies, fiber/matrix interfacial bond strength, and initial fiber waviness. Two thermoplastic composite material systems are used in this investigation. The materials are the commercial APC-2 (AS4/PEEK) and a poor interface experimental material, AU4U/PEEK, designed for this investigation. Notched compression specimens are studied at 21{dollar}spcirc{dollar}C, 77{dollar}spcirc{dollar}C, and 132{dollar}spcirc{dollar}C.; Geometric and material nonlinear two-dimensional finite element analysis (implementing ABAQUS) is used to model the initiation of fiber microbuckling of both the ideal straight fiber and the more realistic initially wavy fiber. The effects of free surface, fiber constitutive properties, matrix constitutive behavior, initial fiber curvature, and fiber/matrix interfacial bond strength on fiber microbuckling initiation strain levels are considered.; Observations indicate that the notch radius controls fiber microbuckling initiation, and thus compression strength, by dictating the unsupported fiber length at the notch. Experimental and finite element results show that increasing the test temperature, locating 0{dollar}spcirc{dollar} plies at the free surface of the laminate, and degrading the fiber/matrix interfacial bond strength reduce the resistance to fiber microbuckling initiation in these notched laminates. Experimental results show that resin-rich regions reduce the resistance to fiber microbuckling initiation. The fiber microbuckling initiation strain is shown to be constant, regardless of stacking sequence, for these notched laminates.; Finite element results show that both larger amplitudes of initial fiber wavinesses and debonds cause increases in the localized matrix shear strains. These increases lead to premature fiber microbuckling initiation. The analyses also show that an energy analysis of fiber microbuckling should include the axial and shear strain energy components in the matrix and the axial strain energy component in the fiber.