Air-Coupled Ultrasonic Testing of Welding Seams in Automotive Applications

A radiationless and contact-free non-destructive testing method is wanted for detection of interruption-type defects of automatically welded seams. In the present work welded flat steel sheets with a thickness between 0.8 and 1.6 mm were investigated. These are typical dimensions in the automotive industry. The interruptions of the welding seam have a length from 1 mm up to several centimeters and the gap between the sheets is usually smaller than 1 mm. For testing an air-coupled ultrasonic technique was used. The ultrasonic probes for generation and detection are situated on the same side of the steel sheet. This kind of single-sided method is one of the main demands of the production people. The probes are oriented perpendicular to welding seam on both sides of the seam, respectively. The acoustical axis of the transmitter is align with a definite angle to the surface to propagate a guided lamb wave in the sheet. The angle depends only on the velocity of the ultrasonic wave within the plate and determines the signal quality. After transmitting the welding seam the guided wave is detected by an air-coupled receiver probe. A main challenge in this setup is to separate the relatively small signal from the reemitted guided wave from the strong direct signal through the air. In such cases an additional shielding is normally used to suppress this direct sound fraction. This shielding is in direct contact with the sample. In this special case for edge-to-edge welded plates the distinct geometry allows a special setup without an additional shading which enables a complete contactless method. Any discontinuity within the sound path leads to changes of the ultrasonic signal. Due to the fact that the plates itself should have zero defects the changes of the signal indicate directly a change of the welding seam. This enables a relatively large distance between the two probes. This leads to a separation of both signals within the A-scan. An appropriate signal analysis is used to draw conclusions about the quality of the welding seam and to characterize the defect types and to estimate their dimensions.

References

1. Liu, Z., “Lamb Wave Analysis of Acousto-Ultrasonic Signals in Plate”, Proceedings of the 15th WCNDT, Roma, 2000.

2. Gaal, M., J. Doering, G. Brekow, and M. Kreutzbruck, “Detection of flaws with lam waves and air-coupled ultrasound”, NDT in progress, Prague, 2009. 

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