A Comparison of Experimental Observations and Numerical Predictions for the Initiation of Fiber Microbuckling in Notched Composite Laminates

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Abstract
Previous experimental observations indicate that the compressive failure of open-hole multidirectional thermoplastic composites begins with in-plane fiber microbuckling at the hole. Growth of this damage requires little additional load, suggesting that compressive strength is controlled by initiation, rather than propagation, of in-plane microbuckling. The significant role of local constraint on the initiation of fiber microbuckling is indicated in this work.Stacking sequence and resin ductility (varied as a function of the test temperature) are the two primary variables in this investigation, both of which provide systematic variation of support to the load-bearing 0° plies. The material system selected for this investigation is a thermoplastic composite, APC-2: AS4/PEEK.The compression specimens used are 2.54 cm wide with a 2.54 cm long gage length. Each specimen contains a 0.3175-cm-diameter semicircular notch at each free edge, centered along the gage length. This polished notch facilitates the observation of the initiation of damage in the form of in-plane fiber microbuckling into the notch. High-temperature tests are conducted using a controlled forced-air heat gun rather than an environmental chamber to allow high-magnification observation of the failure processes at the higher temperatures.Geometric and material nonlinear two-dimensional finite element analysis is used to model the effects of the initial fiber waviness in combination with matrix nonlinearity on the fiber microbuckling initiation strain levels and the resulting shear strain developed in the matrix.