Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating spot welds without destructing the weld, so the part can still be used after the inspection or test is completed. Destructive testing can be done on a limited number of samples to confirm the accuracy of the NDT testing method.
Modern nondestructive tests of spot welds are used in manufacturing to ensure product integrity and reliability of the welds and to provide valuable feedback to control the welding processes, lower production costs and to maintain a uniform quality level. Testing the integrity of a spot weld is necessary to ensure the integrity, longevity, and safety of the automobile.
While many different methods of NDT have been used over the last several years, see NDT article, one that is most effective methods, which is gaining much usage in the automotive industry is Ultrasonic Matrix Phased Array systems. Ultrasonic systems use ultra-high frequency sound that is projected into the spot weld being inspected and when the sound hits a material with a different acoustic impedance (density and acoustic velocity), some of the sound will reflect back to the sending unit and can be presented on a visual display. The sound frequencies used in UT are too high to be heard and do not travel through air. The ultrasonic sound waves signals travel through the spot weld and the reflection of these sound waves can provide the user a variety of valuable information, including, the diameter of the weld nugget, as well as the thickness and whether there are any significant flaws in the spot weld.
The sound is introduced into the spot weld using an ultrasonic transducer (“probe”) that converts electrical impulses from the UT machine into sound waves, then converts returning sound back into electric impulses that can be displayed as a visual representation on a monitor. If the machine is properly calibrated, the operator can determine the distance from the transducer to the reflector, and in many cases, can determine the type of discontinuity (like slag, porosity or cracks in a weld) that caused the reflector. Because ultrasound will not travel through air (the atoms in air molecules are too far apart to transmit ultrasound), a liquid or gel called “couplant” is used between the face of the transducer and the surface of the part to allow the sound to be transmitted into the part.
Matrix Phased Array (MPA) ultrasonic systems use a probe with multiple elements that can be individually activated. By varying the time when each element is activated, the resulting sound beam can be “steered”, and the resulting data can be processed by a variety of computer algorithms to provide more accurate and more detailed information about the integrity of the weld.
Ultrasonic inspection systems can be operated by a person or a robot. To see examples of how these systems work, see these SpotSight system videos.