Shear strength of epoxy adhesive joint between steel substrates with indented patterns

Indented patterns, about 40 m wide and 30 m deep, were created by pressing down a hardened steel file onto the surface of steel substrates using Instron test machine. We examined the penetration of epoxy into those micro-indented patterns and their interactions with Kevlar fibres (diameter of around 12 m), and their influences on the shear strength of epoxy adhesive joints. To ensure complete wetting of the patterned substrate surface, resin pre-coating was applied first. Three different acetone/resin (without hardener) solutions were prepared for this purpose: they contained 1 wt%, 3 wt% and 5 wt% of resin in acetone, respectively. For the purpose of comparison, plain steel substrates polished by #120 sandpaper were also prepared. It had been found that the patterned substrate coated by the 3 wt% resin/acetone solution yielded the highest shear strength, showing about 20% improvement in comparison with that of polished steel substrates. Only limited Kevlar fibres were found within the indented patterns due to their relatively large spacing and shallowness, and, furthermore, due to the random fibre distribution of Kevlar fibres of around 14 mm in length. The limited Kevlar fibre bridging activities were reflected by only around 4% improvement in the shear strength.