Principal stress lines-driven design of variable stiffness composite structures with cutouts

Abstract

To reduce the stress concentration of composite structures with cutouts, this paper presents a principal stress lines-driven (PSLs-driven) design method for variable stiffness (VS) composite structures with cutouts. To address local directional discontinuities in the principal stress direction field (PSDF), particularly in high-stress gradient regions under complex non-uniform loading, an adaptive local smoothing criterion considering principal stress magnitudes is developed to improve directional continuity while preserving load transfer characteristics. Following PSL generation obtained from the Euler method, a geometric interpolation-based fiber placement strategy is introduced to translate continuous PSLs into element-wise fiber orientations, achieving close synergy between structural stiffness and the principal stress field (PSF). Numerical results show that the proposed VS laminates reduce stress concentration at the cutout boundary by at least 10.2% compared with constant stiffness (CS) laminates, and also outperform streamline‑driven fiber laminates. For a composite cylindrical shell with a single cutout under non‑uniform loading, the method reduces circumferential deformation by 13.7% and cutout boundary stress concentration by 34.3%. The research results indicate that the PSLs-driven design method for VS composite structures with cutouts proves to be highly applicable and effective, offering a viable approach for developing high-performance composite structures with cutouts.