Quantifying the Roles of Yarn–Yarn Interfacial Stiffness and Friction in Load Transfer of Plain-Woven Kevlar 49 Fabrics

Micro-Raman spectroscopy offers an optical, non-contact route to stress metrology in fibrous materials: the Raman peak of aramid fiber shifts linearly with axial strain, so the spectral signal acquired at a laser-probed point directly reflects the local stress state. Here, Raman spectra of Kevlar 49 plain-woven fabrics were acquired with a 785 nm laser at 10 mW through a 10× objective over 1540–1650 cm−1, and the G′ band was tracked point by point along a drawn warp yarn to build spatially resolved stress profiles under drawing loads of 3–9 N and transverse preloads of 0 and 30 N. The optically measured profiles were inverted, through a two-zone friction–adhesion model, into interfacial shear parameters and a stress-transfer coefficient, and then analyzed via sensitivity analysis. The spectra show that transverse preload raises the effective sliding-friction shear stress by 57–78% and the maximum static interfacial shear stress by 60–75%. Sensitivity analysis shows that yarn–yarn interfacial stiffness governs the spatial decay of stress far more strongly than yarn elastic modulus, and defines a characteristic load transfer length. This spectral-shift-to-mechanics framework links optical measurements to interfacial design of high-performance fabrics.