On Relationship Between the Topographic Characteristics of Aluminum Adherends and Their Joint Properties
A proprietary mechano-optical system was used to quantitatively characterize the topographies of aluminum adherends with the aim of identifying topographic parameters that relate to joint properties. The data obtained by the system are the coordinates of the topography function, i.e., z equals z(x,y), where z are the heights on the surface as a function of their x,y coordinates in the midplane bisecting the surface undulations of the test region. The spatial interval between coordinates is an experimental variable. The topographies and surface oxidation states of the aluminum substrates were altered by irradiation from a KrF excimer laser, where the radiant intensity, incident to the surface was 1.67 X 1013 W/m2/p. In this procedure mm2 regions were irradiated for either 5 or 20 pulses (p) per region with no overlap between regions. Irradiation with 20 p/reg. resulted in a 24% increase in ultimate joint strength and approximately 1.7 times the strain at fracture over those of the control--where the adhesive used was the same. Two experimentally determined topographic parameters are proposed to explain these results. These are the fractions of solid and void volume within the topographic surface region [i.e., between (zmax - zmin)] and the angular distributions of surface inclinations. These, we propose affect both the probabilities of crack initiation within the topographic surface region and the extents to which crack propagation to failure are inhibited. A preliminary elastic model of joint deformation is given where these parameters are shown to affect both crack initiation rates and crack propagation paths.