The local mechanical effects of UV degradation on tubular braided composites intended for low earth orbit applications

Tubular braided composites (TBCs) offer low mass, damage tolerance, and tailorable structural performance for spacecraft applications; however, their local mechanical response to low Earth orbit ultraviolet (UV) radiation remains poorly understood. This study investigates the effects of simulated Air Mass Zero solar exposure on localized tensile behaviour of carbon/epoxy and Kevlar®/epoxy 2D diamond TBCs. Specimens with 35°, 45°, and 55° braid angles, together with neat epoxy dog-bone specimens, were exposed continuously for 336 h to radiation between 350 and 1200 nm and compared with unexposed controls. Tensile tests combined stereo digital image correlation and region-by-region strain analysis to quantify modulus, yield strength, and yield strain in resin-rich and fibre-reinforced regions. Neat epoxy showed no statistically significant bulk-property changes after exposure, although modulus increased by 3.87%, UTS decreased by 6.84%, and strain-at-failure decreased by 16.40%. Carbon/epoxy TBCs exhibited matrix-dominated degradation: resin-rich-region modulus increased significantly at 35° (+90.32%) and 55° (+39.89%), with yield-strain reductions of 38.34 and 30.31%, respectively. Fibre-reinforced regions stiffened, consistent with proposed matrix embrittlement and altered local load transfer rather than direct carbon-fibre degradation. Kevlar®/epoxy TBCs showed smaller, architecture-dependent changes. Matrix-rich-region modulus changes were not statistically significant, whereas fibre-reinforced regions showed significant exposure- and braid-angle-dependent responses. At 35°, fibre-region modulus and yield strength increased by 17.17 and 19.06%; at 55°, yield strength and yield strain decreased by 14.72 and 15.23%. These findings demonstrate that fibre type, braid angle, and local braid region strongly influence the UV-degradation behaviour and should be considered when qualifying TBCs for long-duration space structures.