There are advantages and limitations for you to consider with ultrasonic technology to do NDT (Non-Destructive Testing) of spot welds. For the most part, you can count on a well-made Ultrasonic system to be a good solution to provide accurate results for spot welds Non-Destructive Testing. Fortunately, you don’t have to be an expert to understand the science of ultrasonic NDT systems.
The transmission of ultrasonic signals through materials is just like the transmission of audible sound waves; it is straightforward and easy for you or anyone to understand. Ultrasound is simply a high frequency sound wave and the rate of travel through different materials is very well understood and documented; and like sound waves, ultrasonic waves travel through different materials at different rates. If you are in the automotive industry, you are probably using spot welds with some form of steel or aluminum. The speed of travel of ultrasonic waves is very similar through most steel and aluminum, so you don’t have to make adjustments due to the material you are working with.
You want the right Ultrasonic NDT System
Since the method and speed of travel of ultrasonic waves through material is well understood, you can easily calculate the thickness of the material the signal goes through by dividing the speed of travel by the time it takes to travel that distance. Since you likely care about the diameter of the weld nugget, you will want a system that can differentiate between the acceptable and unacceptable parts of a weld nugget. You will likely want an advanced ultrasonic system that has an array of ultrasonic elements that send and receive an array of signals and use a C-scan to determine the diameter of a weld nugget.
A good matrix phased array system can tell you which parts of an area have a thickness that meets your specified thickness requirement and which parts do not. The parts that meet the specified thickness requirements are considered part of a good weld nugget and contribute to the size, or diameter of your weld nugget.
For example, if you are welding three sheets together with thicknesses of 0.8 mm, 1.0 mm and 1.2 mm then the total stackup is 2.0 mm. If you consider a good spot weld to create a little concave on each side of the upper and lower surfaces to produce a weld nugget of between 1.5 to 1.8 mm, then you could set a minimum requirement of 1.5 mm and any thickness reading that is not greater than 1.5 mm would not qualify to be part of a good nugget. If you choose the right advanced system, it will look at many areas using the array of elements and determine how large your weld nugget is based on the parameters that you provide.
Because the science of ultrasonic travel is so basic and so well understood, you can have confidence in the readings of a good system that is designed for NDT inspection of spot welds. You will want to find a system that uses a matrix phased array of elements and have advanced algorithms that accurately calculate the size of the weld nugget and compare it to your minimum requirements to determine if the spot weld is good or not.