Comparative study of thermosetting CF/epoxy and thermoplastic CF/PAEK composite laminates: Effects of stacking sequences on open-hole strength

Abstract

This study experimentally and numerically investigated the effect of stacking sequences on the open-hole compressive (OHC) and tensile (OHT) strength and failure mechanisms in carbon-fiber-reinforced thermoplastic (CFRTP) laminates. Specimens with stacking sequences containing a higher proportion of 0° layers and those with more ±45° layers were considered, denoted as “hard” and “soft” specimens, respectively. Strength prediction simulations for CFRTP were conducted using the three-dimensional extended finite element method, previously validated for carbon-fiber-reinforced thermosets (CFRTS). In the OHC test, kink-bands formed before matrix-dominated damage occurred for both hard and soft CFRTP specimens, while the opposite was found for CFRTS specimens due to the higher matrix toughness of CFRTP. Consequently, in soft CFRTP specimens, stress redistribution to the ±45° layers after kink-band formation led to gradual damage progression, achieving a strength comparable to CFRTS despite the higher 0°-layer compressive strength in CFRTS. The OHC simulation effectively predicted CFRTP failure using the maximum stress criterion. In the OHT test, CFRTP exhibited a higher strength than CFRTS in both hard and soft specimens due to the higher 0°-layer tensile strength in CFRTP. The OHT simulation accurately predicted abrupt failure in hard CFRTP specimens using the Weibull failure criterion but underestimated the strength of soft CFRTP specimens. This discrepancy arose because CFRTP exhibited less matrix-dominated damage, allowing stress redistribution to the ±45° layers after fiber breakage, leading to gradual damage progression. X-ray micro-computed-tomography observations confirmed that dispersed fiber breakages progressively connected through splitting cracks and propagated perpendicular to the fibers.

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