Compressive and shear behavior of cutout frames in integrated composite panels: A comparison of three local reinforcement configurations

Stress concentration easily occurs in thickness transition zones of cutout frames in integrated composite panels, and reinforcement configurations dominate the compressive and shear behavior of components. Three reinforcement schemes including ply drop-off (PD), direct lay-up (DL) and machined (M) were designed. In-plane compression and diagonal shear tests were conducted to compare the ultimate bearing capacity, stiffness degradation and macroscopic failure characteristics of each configuration. An SC8R continuum shell progressive damage model based on the Hashin criterion was adopted to rapidly locate damage zones. Combined with SEM observations to identify mesoscopic damage, the failure modes of cutout frames were revealed. The results show that fiber truncation of the M configuration induces edge stress concentration, with an ultimate shear load of only 53.75 kN. Without ply transitions, the DL configuration possesses the lowest ultimate compressive load of 34.54 kN. Benefiting from continuous fibers and stepped ply transitions for uniform load transfer, the PD configuration reaches ultimate compressive and shear loads of 40.87 kN and 60.31 kN, with maximum increments of 18.33% and 12.20% compared with the other two schemes. Simulated concentrated damage areas match the actual failure positions of specimens. Microscopic observations verify that structural failure is dominated by massive intralaminar matrix cracking instead of fiber strength failure. This study demonstrates that fiber continuity and stepped plies can effectively restrain buckling and improve damage tolerance. The numerical model can quickly predict critical structural regions, which provides a reference for the optimal design of lightweight composite cutout frames.