Compression Testing of High-Performance Carbon Fiber Composites Using Cross-Ply Laminates: A Multi-Scale Investigation of the In Situ Effect

Compression testing of high-performance carbon fiber composites remains challenging due to premature failure modes in unidirectional laminates, which can underestimate true material strength. This study investigates the compressive behavior of T800-grade carbon fiber-reinforced polymer (CFRP) cross-ply ([90/0]2s) and unidirectional ([0]8) laminates using finite element simulation and experimental testing following the SACMA SRM-1R-94 standard, combined with macroscopic and microscopic failure analysis. The results show that cross-ply laminates consistently exhibit valid mid-gauge failure with lower data dispersion (coefficient of variation: 3.44%), whereas unidirectional laminates are prone to invalid root failures (crushing or shear). The compressive strength derived from cross-ply laminates using the back-out factor (2040 MPa) is 13% higher than that from direct unidirectional testing (1802 MPa), attributed to the in situ effect where adjacent 90-degree plies suppress fiber microbuckling. The cross-ply approach provides a more reliable and practical method for characterizing the true in situ compressive strength of high-performance CFRP composites.