Hygrothermal reconfiguration of radial chemo-mechanical gradients in arc-shaped natural fiber-reinforced bamboo composite for flattening and panel manufacturing

It remains unclear whether hygrothermal softening controls the deformation of bamboo as a natural fiber-reinforced composite by reshaping its radial physicochemical structures and mechanical-gradient heterogeneity. Here, hygrothermally softened arc-shaped bamboo strips were investigated using a multilayer radial slicing strategy to link cell-wall chemistry, component-level mechanics, and macroscopic deformation across scales, and to clarify the vascular bundle (fiber)-matrix-interface synergy governing radial mechanical-gradient evolution. Results show that hygrothermal softening triggers layer-dependent chemical-crystalline-mechanical reconfiguration, involving amorphous matrix degradation, crystalline-structure adjustment, and interfacial synergy rearrangement. Hygrothermal softening index (HSI) analysis revealed that transverse compressive strength decreased by up to 47.34%, indicating the high sensitivity of matrix-rich layers. Gradient index (GI) analysis showed that the gradients of longitudinal shear strength and longitudinal tensile modulus increased by 27.14% and 18.21%, respectively, demonstrating selective enhancement of radial mechanical heterogeneity. Meanwhile, the strength synergy index (SSI) increased from 1.284 to 1.545, suggesting improved structural efficiency through fiber load bearing, matrix compliance, and interfacial coordination. Thus, a classic composite design principle is built. A layer-parameterized finite element model further visualized stress/strain redistribution and potential failure evolution under hygrothermal conditions. This study proposes a hygrothermally induced radial-gradient reconfiguration mechanism that provides a mechanistic basis for understanding deformation and stress redistribution during subsequent bamboo flattening and panel-oriented processing, while offering insights into the processing of natural fiber-reinforced composites with graded architectures.