Patent ID: 7210329

Claim:
A method for determining threshold values of gates for permitting to evaluate a good sound coupling during ultrasound testing of a series of spot welding joints of a selected type, said spot welding joints being configured between at least one first metal sheet and at least one second metal sheet, the first sheet having a first sheet thickness and the second metal sheet having a second sheet thickness, the method comprising: a) establishing a geometrical minimum thickness dMin for the spot welding joint, said minimum thickness being smaller than an overall sheet thickness dT of the metal sheets joined together with the spot welding joint, b) coupling an ultrasonic probe to a spot welding joint to be tested, emitting at least one ultrasound pulse onto a front surface of the spot welding joint and receiving echo signals, the echo signals received comprising at least one entrance echo on the front surface received at a time tT, one echo of a first reflection on a rear wall of the spot welding joint (first rear wall echo) and at least one further (n th ) rear wall echo, c) defining a propagation time tMin of the ultrasound pulse for a path from the front surface to the rear wall and back to the front surface at the minimum thickness dMin as well as the propagation time tT for the overall sheet thickness dT, d) setting a first gate B 1 for the signal of the first rear wall echo that starts at a time t 1 S=tE+tMin−tSA and ends at a time t 1 T=tE+tT+tSE, with tSA and tSE being small safety allowances, e) fixing an n th gate for the n th rear wall echo that begins at a time tnS=tE+n*tMin−tSA and ends at a time tnT=tE+n*tT+tSE, and f) coupling the probe in different ways, whereby at least an emission angle and/or emission site of the probe is modified thereby achieving a sufficiently large amplitude of either the signal of the first rear wall echo within the first gate or the signal of the n th rear wall echo within the n th gate and the achieved maximum is used, and stored, for evaluation.