Experimental and Numerical Investigation of End-Tab Geometry Effects on the Compressive Response of Ultra-Thick CFRP Laminates

Compression testing of ultra-thick carbon-fibre-reinforced polymer (CFRP) laminates requires high loads to be transferred over a limited gripping length, while the stiffness discontinuity at the end-tab termination can initiate unintended local failure. This study examines the effects of taper geometry and tab thickness through combined-loading compression tests on four end-tab configurations, surface-strain measurements, fractography, and three-dimensional finite-element analysis. Multiple intralaminar failure indices were evaluated, and a Ye-type index was used to assess interlaminar initiation risk at the free edge. For the 20 mm thick tabs, introducing a 25 mm taper increased the mean compressive strength by 9.76%. At comparable nominal stresses near 330 MPa, the representative abrupt-ended specimen exhibited approximately twice the strain ranges in the shear and through-thickness directions of the representative tapered specimen. The numerical results showed that the tab-end stiffness gradient governed interlaminar load transfer: increasing taper length reduced the axial-stress gradient and the local interlaminar shear and through-thickness stress concentrations, with the calculated peak S13 decreasing from 48.01 to 18.36 MPa, while shifting the governing Hashin initiation mode from matrix compression to fibre compression. The measured strain localization and the observed fibre compression–shear fracture, matrix shear cracking, and delamination were qualitatively consistent with the numerically identified three-dimensional stress concentrations. These results demonstrate that end-tab transition geometry governs local failure initiation and the measured load-carrying capacity of ultra-thick CFRP compression specimens.