Effect of surface treatment for steel adherends on static and fatigue crack growth of epoxy adhesives

The fracture toughness of unmodified, glass-bead-reinforced and CNBR-modifi ed epoxy adhesives under mode I loading is not improved by acid surface tre atment of steel adherends since cohesive failure always occurs for all adhe sives with or without acid surface treatment. On the other hand, the fatigu e crack growth resistance greatly increases due to acid surface treatment o f steel adherends. Especially, the threshold dramatically increases. The cr ack grows cohesively at all stages of crack velocity for DCB specimens trea ted with acids while it grows at the interface between the adherend and the adhesive layer for the specimens whose polished surface of adherends is on ly decreased with solvent. An optical microscope observation revealed that adherend surfaces treated with acids were rougher than ones without acid tr eatment, although XPS examination for the surfaces did not show significant difference in their chemical elements among the specimens with and without acid treatment.