An important correlation between the attenuation of an Ultrasonic A-scan reading and the strength of a spot weld can help you identify “stuck welds” or “cold welds”. Many studies have been done over the years to better understand how Matrix Phased Array (MPA) ultrasonic NDT systems can pick up on different characteristics of a spot weld. You want to know the diameter and thickness of a weld nugget and ultimately, it’s strength. Many different approaches have been taken to find the best solution for you to do NDT testing of spot welds.
Attenuation as a way for you to tell if you have a stuck weld
You want to be able to tell if the spot weld is big enough and if it has defects that can cause it to be weaker than desired. One important defect you want to be able to find is a stuck weld, where the surface of the sheets are stuck, but there is not a sufficient nugget to make the weld strong.
Research has shown a strong correlation between your nugget thickness and the attenuation of an ultrasonic A-scan reading. When you form a spot weld, the metal sheets melt and reform into a nugget, the crystalline structure of the resulting nugget causes more attenuation of an ultrasonic signal due to the scattering of the ultrasonic waves. When you look at the A-scan of a well-formed nugget, the amplitude of the subsequent waves will drop off more quickly than it will with a stuck weld because the ultrasonic waves have to travel through more of the nugget which can scatter the signals.
If you have an advanced ultrasonic NDT system, it will automatically calculate the nugget diameter and the overall thickness for you. If you train your operators on what to look for, they can also visually pick up on the difference between a stuck weld and a good weld based on how quickly the amplitude of the A-scan drops off. The quicker it drops off, the more of a weld nugget it must travel through, indicating that your weld is stronger than if the A-scan amplitude remains high for a long time. Because the attenuation factor of metals and weld nuggets can vary, it is necessary for your operators to have the right training and experience to be able to identify a stuck weld.
If you have a good ultrasonic NDT system, it will determine the size of a weld nugget, but not whether it is a stuck weld or not. Your operators will need the right training and experience with different welds to be able to visually pick up on whether a weld is a good weld or a stuck weld. This training is not difficult and your good operators should not have a problem identifying a stuck weld with a moderate amount of experience.