A study on the effects of surface roughness on the strength of single lap joints

In order to gain an insight into the aspects of surface roughness, surface profile, adhesive viscosity, and surface morphology, experiments were conducted using two different steels (1018 cold-rolled and hot-rolled weld steels). Four different surface modifications were performed on these samples by sand-blasting at 552 kPa from a distance of 25.4 mm, and etching for 2 and 10 min with a chemical recipe containing chromic acid. Two different adhesives were used, namely Epon 815 and Epon 830 with viscosities of 5-7 and 170-225 Poise, respectively, to assess the effect of viscosity in bonding to adherends with different surface topographies. Contact angles were measured on all surfaces using the two adhesives. To gain an insight into the effect of surface topography, scanning electron microscopy (SEM) was utilized to observe the modified surfaces at different magnifications. Both average and maximum peak-to-valley height values were measured using a profilometer and surface profiles were also obtained for all the samples. The single lap joint strength and displacement values were measured at two different crosshead speeds of 1 and 100 mm/min, to assess the interrelationship between the failure mechanisms and joint displacements, surface topography and adhesive viscosity. The results of our work showed a general decrease in the contact angle values with average surface roughness for the liquid, Epon/DETA, thermoset/hardener combination on both cold- and hot-rolled steel specimens. When tested at a 100 mm/min loading rate, and compared with the 1 mm/min loading rate condition the cold-rolled specimens, which had smaller surface roughness averages in comparison with the hot-rolled condition, exhibited a higher reduction in failure load values. It is believed that this indicates a higher proportion of interfacial failure in cold-rolled specimens at higher loading rates.