Strength and failure modes of surface treated CFRP secondary bonded single-lap joints in static and fatigue tensile loading regimes

•Sanding, grit blasting and peel-ply plus grit blasting surface characterization.•Failure mode from cohesive (static) to thin-layer cohesive (high-cycle fatigue).•Sanding treatment had better fatigue endurance and uniform failure surface. Adherend surface treatment prior to bonding is pivotal for efficient assembly or repair of carbon fiber reinforced polymer (CFRP) composites. The effect of CFRP surface treatment has gathered much attention in the literature but reported test results on fatigue performance are less prevalent. The effect of surface treatment was extended in this work to cover the static and also fatigue strength of single-lap bonded joints. Surface treatments of manual sanding, grit blasting, and peel-ply plus grit blasting were produced and their surface roughness and energy were measured. The sanding treatment produced the lowest static strength but the best static strength retention at one million cycles with 55.4%, compared to 50.3% for peel-ply plus grit blasting, and 47.5% for grit blasting. The failure surfaces of the fatigued sanding specimens were the most uniform, mostly of cohesive failure mode for static and low-cycle fatigue, progressing to thin-layer cohesive for high-cycle fatigue.