Non-destructive quality-testing of a joint between sheets made by electric spot welding

A method is described for the non-destructive testing of a joint obtained by the electrical spot-welding of two sheets of substantially the same material joined together in a local zone with the formation of a weld core which has a thermal diffusivity substantially different from the thermal diffusivity of the material of the two sheets before welding, such a core being obtained only when the welding is of a good quality. The testing is effected by a device the main characteristic of which is an electrode which delivers heat and a detector electrode located at two conveniently chosen points on one surface or opposite surfaces of the two welded sheets. The detector electrode is connected through an amplifier and an analog-digital converter to a computer which processes the signal to provide an indication of the weld quality on the basis of the measured value of the thermal diffusivity.