Analogy to Light waves:
You can think of the ultrasonic waves (or signals) as being similar to light waves. Light waves emitted from the sun or another source travel through the air until they hit something and then some or all of the light waves are reflected off of the surface of the object, and if someone is observing the object, they see the light that is reflected and that person’s brain processes it as an image. When light hits the surface of water, some of the light is reflected off the surface of the water and some of it penetrates the surface and reflects off of objects in the water or off the bottom of the container and the reflected light is received by the eye of the person observing it. Ultrasonic signals travel in a similar manner; some of the signals are reflected at the surface of a spot weld and some travel through the spot weld and are reflected off of either a flaw in the spot weld or off the bottom of the spot weld.
Three-Dimensional Matrix Phased Array Probe
The spherical geometry of a curved three-dimensional probe elements includes an 8 x 8 array of piezoelectric elements arranged in a spherical geometry and has a flexible membrane to conform to the contoured surface of a spot weld while enabling the sound energy to be transferred directly into the spot weld being tested.
An excitation is sent to a subset group of transducer elements, and are combined to send an ultrasonic beam toward a spot weld. Each transducer element in a subset group may be pulsed at different time intervals (phase delay) and their individual waves summed to produce a focusing effect of the beam as well as a steering effect if needed. Other three-dimensional arrangements are possible for optimizing the performance for specific applications. The total number of elements, overall dimension, and operating frequency determine the overall three-dimensional surface contour shape and its operating characteristics and parameters.
When inspecting a spot weld, the end of the probe is dipped in water to make good connection to the spot weld (this water is in place of the gel couplant that most systems require). The flexible membrane allows the tip of the probe to conform to the contour of the welded area. The fluid filled chamber acts as a pathway for focusing and steering ultrasonic beams. A curved array of ultrasonic elements transmits ultrasonic beams into the welded area to capture the associated reflections of those ultrasonic beams. The Matrix Phased Array (MPA) electronics unit communicates with the ultrasonic elements through signal pathways and the computer processes the incoming ultrasonic data and generates a readout of the spot weld thickness and average diameter as well as a visual representation of the fused area.
Ultrasonic Signal Processing Methodology
NDT systems use a combination of A-Scans and C-Scans to generate the signals and resulting reflections that provide the necessary information to determine thickness and diameter of the spot weld. A-Scans use a single signal and reflection that is valuable in determining thickness. C-Scans use an array of signals and reflections to give a more complete picture of the area of the spot weld. Some systems use both A-Scans and C-Scans along with complex algorithms to process all of the data to provide a complete representation of the spot weld.
Ultrasonic Image Processing
The system directs ultrasonic energy into a spot weld where some of the Ultrasonic signals are reflected from the surface of the Top Sheet of the welded part and some travel through the weld and are reflected off of the bottom of the Lower Sheet. The system receives the reflected signals and process the data to produce an A-Scan image, a C-Scan image, and the resulting average Diameter and Thickness of the spot weld. The system then compares this information to the minimum acceptable Diameter and Thickness settings that are provide to it by the user in order to determine if the spot weld is OK or No Good (NG).
Some systems use “Gates” to determine which ultrasonic signals the system will look at when determining if a spot weld is good or not. This differs from other systems, which utilize an attenuation coefficient compensation method. In recent use at an automobile manufacturer, a round of inspection was performed with a system that uses Gates on a part from spot weld #1 through #22 without stopping while the total inspection time was timed with a stopwatch. The average inspection time was estimated to be 13 seconds per spot weld.
References:
Some of the ideas of this article reference the article Design and Development of High Frequency Matrix Phased-Array Ultrasonic Probes. For a detailed version of this with graphics, see article Ultrasonic system with 3-D Matrix Phased Array Probe for testing spot welds.