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] .

No abstract

Compression failures in laminates with holes are often preceded by the development of a damage zone which grows with increasing compressive load. This damage zone is due to local fiber buckling and/or shear crippling. The local softening in the damage zone has been modeled using a Dugdale-type approach. This paper compares this analytical model to the actual development of the damage zone size as a function of load. Comparison of the model predictions to the experimental observations indicates that the Dugdale model does not accurately predict the load-damage zone size relationship in open hole composite specimens loaded in compression. Hole size and resin ductility were the two primary variables used in this investigation. Three hole diameters were used in specimens 2.54 cm (1 in.) wide containing 5.08 cm (2 in.) gauge length. Matrix ductility and stiffness were also varied by using matrices of polyetheretherketone (PEEK) and Hexcel F155. Very brittle resins were not studied because the damage zone development that precedes unstable damage extension is too small to record. Several coupon tests were interrupted prior to catastrophic damage growth to allow for a more careful examination of the fiber shear crippling and/or buckling. These observations were accomplished using nondestructive examination (C-scan and X-ray) and scanning electron microscopy. A preliminary sectioning study was conducted through the thickness of the damage zone of an AS4/PEEK specimen; each section was observed in the scanning electron microscope.

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing com pressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling into the edges of the hole. The length or size of the damage zone increases with increasing load, propagating stably until it reaches a critical size. Then, unstable growth begins, and the zone completely traverses the specimen. Subsequently, a more cata strophic failure occurs giving brooming and/or delamination. Several tests of 2.54 cm wide AS4/PEEK containing a 0.16 cm diameter hole were inter rupted prior to catastrophic damage growth to allow for a careful examination of the fiber shear crippling and/or microbuckling. These observations were accomplished using scan ning electron microscopy. Direct observation of the specimen revealed the surface size of the shear crippling zone. To determine the extent of this damage through the thickness of the laminate, systematic sectioning techniques were employed. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the damage zone was developed.

The objectives of this research are to investigate the effects of stacking sequence (orientation of plies adjacent to the 0° plies), free surfaces, fiber/matrix interfacial bond strength, initial fiber waviness, resin-rich regions, and nonlinear resin shear constitutive behavior on fiber microbuckling initiation. Three thermoplastic composite material systems are used in this investigation. The materials are the commercial APC-2 (AS4/ PEEK), QUADRAX Interlaced Tape, and a poor interface experimental material, AU4U/ PEEK. Notched compression specimens are studied at 21°C, 77 ° C, and 132°C. Observations indicate that the notch radius controlled fiber microbuckling initiation, and thus compression strength, by dictating the unsupported fiber length at the notch. The numerical results from a companion paper [1] are compared qualitatively with these experimental results. The results show that increasing the test temperature, locating 0° 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. The fiber microbuckling initiation strain is shown to be a constant, regardless of stacking sequence, for these notched laminates. Experimental results show that resin-rich regions also reduced the resistance to fiber microbuckling initiation. NE PROBLEM IMPEDING the widespread application of composites, particularly thermoplastic composites, is their inherent weakness