The objectives of this research focus on the effects of nonlinear matrix constitutive behavior, initial fiber waviness, and fiber/matrix interfacial bond strength on fiber microbuckling initiation in thermoplastic composites. Nonlinear geometric and non linear material two-dimensional finite element analysis is used to model the initiation of fiber microbuckling of an initially wavy fiber. Results show that reductions in the resin shear tangent modulus, larger amplitudes of initial fiber wavinesses, and debonds each cause increases in the localized matrix shear strains; these increases lead to premature fiber microbuckling initiation. These numerical results are compared with experimental data obtained during this investigation. These experimental results and comparisons are presented in a companion paper [1] .
A combination of macroscopic delamination fracture toughness, scanning electron microscope (SEM) real-time fracture observations, and postfracture morphology were used to study the micromechanical processes of delamination failure in several graphite/epoxy systems.
Strain energy release rate GIc, GIIc, and mixed mode GI&IIc were obtained from unidirectional double cantilever beam specimens. Comparisons of these energy release rates with the resulting fracture surface mophology are used to clarify the relative importance of the formation of hackles and the fiber matrix interface adhesion to delamination toughness under Mode I, Mode II, and mixed Mode I & II loading conditions.
Real-time fracture observations of composite delamination in the SEM revealed the microprocesses of microcrack formation and coalescence. These observations coupled with GIc for the neat material, determined from compact tension specimens, provide insight on how resin toughness can be translated into composite delamination toughness.
The implications of hackle formation and their importance in the failure analysis of graphite/epoxy systems are discussed.
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