Attenuation of ultrasonic signals
During the resistance spot weld process, the metals rapidly melt and cool. The resulting weld nugget has a crystalline structure that is relatively coarser than the unmelted sheet metal. This coarse crystalline structure in the weld nugget has a much higher attenuation rate than the unmelted sheet metal. The attenuation rate is the rate at which the ultrasound signal deteriorates due to absorption and scattering of the signals at the interface between materials or grain structures in the materials.
You can see the differences in attenuation rate by looking at an A-scan. The rate that the echoing signals decrease in amplitude as the signal echoes off the bottom and top surface of the spot weld indicate the attenuation rate. If the signal has a good-sized nugget, the signal will decrease more rapidly than if there is little or no signal as with the case of a stuck weld or cold weld.
Ultrasonic NDT (Not-Destructive Test) systems
The automotive industry has been using NDT inspections of spot welds have been used for over 20 years and have proven it to be an effective way to evaluate the integrity of a spot weld. However, older technology in ultrasonic NDT systems require a well-trained operator to determine if a spot weld is good or not. Advancements in technology have provided software algorithms and automation that have resulted in more advanced ultrasonic NDT systems that automate the evaluation of a spot weld to determine if the spot weld is good.
There are still some areas where training and experience can help in the evaluation of a spot weld. For example, the system may determine that the diameter and the thickness of the spot weld meet the predetermined criteria for a good weld, but the operator may notice that the amplitude of the ultrasonic signals on the A-scan remain strong and are perpetuating longer than normal. This suggests that this may be a stuck weld or cold weld and it is worth further inspection using a manual strength test.
The right ultrasonic NDT system that has effective automation and algorithms will catch most flawed spot welds; coupling the right system with proper operator training results in inspections that rarely miss a spot weld that does not meet the predetermined quality criteria.