Dynamic resistance test of spot welds

The quality of a spot weld is determined by applying a periodic pressure while a test current is flowing through it. Poor quality is indicated by a variation in the weld resistance which results in a periodic potential across the weld. This potential is amplified for ease of detection. When a sinusoidal alternating current is used the presence of harmonic potentials indicates a poor weld. Removal of the fundamental by a filter is desirable before amplification.