Influence of surface treatment and design of 3D-reinforcements on delamination resistance & mechanical properties of CFRP/CFRP joints under static & fatigue loading

Structural joint design strongly determines the weight saving potential of carbon fiber reinforced polymers (CFRP) in primary aircraft structures. The introduction of through-thickness reinforcements into the joining area addresses the issue of an efficient load transfer between laminates and adherents to increase delamination resistance and thus improve damage tolerance. In the present work, an innovative process is presented for a cost and time-efficient manufacture, with multidimensional metallic structures enhancing the joint's interlaminar fracture toughness. Spikes bent in the out-of-plane direction made of stainless steel and titanium were exposed to wet-chemical (HF/HNO₃, Turco 5578®) and physical (pulsed laser irradiation; Nd:YVO4, 1064 nm) surface treatments to create micro- and nano-scaled oxide layer morphologies respectively. A single lap shear (SLS) geometry was chosen to determine an optimized spike density and layout under static and fatigue load for the aimed application of the stringer run-out area in the lower aircraft fuselage. mode II strain energy release rate (SERR) was then determined and proved to increase considerably through the introduction of pretreated reinforcements into an end notched flexure (ENF) joint geometry. Laser irradiation on titanium reinforcements shows a superior performance when compared to alkaline etched or baseline sandblasted spikes. Surface analytics through scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS) was employed to further correlate fracture mechanical results to the oxide surface chemistry, morphology and contamination, resulting from the applied co-bonding process.