Circular Cutouts Effect the on Crashworthiness of CFRP Rectangular Tubes with Different Stacking Sequences

The presence of holes in the crash boxes of vehicle parts is sometimes unavoidable to accommodate connections, cables, and maintenance access. The effect of circular cutouts on the crashworthiness of a Carbon-Fiber Reinforced Polymer (CFRP) rectangular laminate used as a crash box, consisting of identical plies with different stacking sequences, was investigated. Five tubes with different stacking sequences were prepared: an outer ply of 0°/90° twill-woven fiber and inner plies of 0°/0°, ±45°, and 0°/90° unidirectional fibers. A second set of tubes was prepared with circular cutouts of 6 mm diameter in the tube walls. A quasi-static axial crushing test was done on the tubes. Crashworthiness indicators, including initial peak load (Pip), mean crushing load (Pm), energy absorption (EA), specific energy absorption (SEA), and crushing-force efficiency (CFE), were evaluated and statistically analyzed using ANOVA. The results show that the neat tubes generally exhibit higher energy absorption and more stable progressive crushing than the tubes with circular cutouts. The presence of cutouts reduces the initial Pip, Pm, EA, and SEA, while also changing the failure mode from progressive splaying-dominated crushing to localized micro-buckling, segmentation, sliding, and slicing. Among the tested configurations, samples with 0°/0° plies in the mid-laminate region showed the best crashworthiness performance in both neat and with cutout tubes. These findings demonstrate that both stacking sequence and geometric discontinuity strongly influence the crash response of CFRP tubes and should be considered together in crashworthy design.