Correlation of single lap joint strength, and deformation with joint resistance, surface, and cure conditions

For achieving a reliable adhesive bond, the adhesive must properly wet the surface asperities on the adherend before any other physical or chemical phenomena can take place. The physical phenomenon of mechanical interlocking is related to the surface morphology of the adherend. Generally, a rough surface increases adhesive joint strength, by enhancing the surface area and the extent of mechanical interlocking up to a limit. Particle loading and particle type can also influence surface wetting and the overall joint performance. In order to gain insight into the aspects of surface roughness, surface profile, filler particle shape, size, surface morphology, and joint resistance, experiments were conducted using 1018 cold rolled steel, which is convenient to etch and test. Surface modification was done by chemical etching with a recipe of chromic acid for 10 minutes. As a result, the average surface roughness changed from 0.45 microns to 1.65 microns, and the maximum roughness changed from 8.9 microns to 21 microns. Epon 830 adhesive with a viscosity of 170-225 Poise was used, filled with Nickel particles at 35% and 60% by volume. To elucidate the effect of particles, three different nickel particles were used, namely irregularly ellipsoidal Ni 104 particles of size 3 to 35 micron, spherical Ni 110 particles of size 3-8 micron in etched and unetched form. The metal specimens were bonded using 4 different adhesive thicknesses and cured. To gain insight into the effect of surface topography, SEM, and a profilometer were utilized to observe the surfaces. Strength and displacement values were measured at two different crosshead speeds of 1 and 100 mm/min, to assess the extent of interfacial failure.