The resolution dependence of measured fractal characteristics for a real un-dismantled electrical contact interface

An electrical contact interface is visualized by rendering “contact maps” from X-ray Computer Tomography (CT) images without the need for dismantling the specimens. The contact maps consist of approximately 1500 pixels × 1500 pixels with each pixel relating to an 8.0 μm × 8.0 μm × 8.0 μm volume at the interface. The specimens consist of bolting a cable lug to a printed circuit board. The resolution dependence of measured fractal characteristics is studied for a contact interface with a normal force of 1.6 kN. The total mechanical area of contact, Am, is found to be invariant with resolution whereas the largest contact spot size, AL, is found to decrease with higher resolution. The number of spots on the apparent area of contact is found to increase with resolution but a spot increasingly has areas of non-contact within itself at higher resolution. The fractal dimension, D, of the spot area is found to converge to 1 at lower resolution consistent with self-affinity behaviour. At higher resolution D converges to a value <2.