NDTnet 1998 June, Vol.3 No.6

Automotive Industry Turns to Ultrasonics for Spot Weld Examination

Nondestructive examination can now be applied to spot welds that typically require destructive testing

Based on information provided by Panametrics, Inc., NDT Division, Waltham, Mass.


Spot welding is now widely used in the fabrication of sheet metals, mainly due to cost and time considerations. Spot welds are found in nearly all products where sheet metal is joined. Examples range from a simple metal tool box to the more than 10,000 spot welds found in a typical passenger car.

Obviously, the quality of the spot weld has a direct impact on the quality of the product. If the weld nuggets are improperly or incompletely formed, or the area surrounding the nugget is smaller than required, the structural integrity of the entire part may be uncertain. Furthermore, these inconsistencies are usually internal and are seldom visible to optical inspection.

In the past, spot weld inspections consisted of destructive tests, induding tension shear, twist peel and hammer chisel tests. Recent advances in ultrasonic instrumentation and transducer design now allow spot weld testing to be performed quickly, accurately, and more importantly, in a nondestructive manner. Using ultrasonic testing can significantly reduce process time and material costs in many cases.

The primary goal behind ultrasonic spot weld inspections is to utilize the pulse-echo technique to classify welds into unacceptable (Fig. 1), undersized (Fig. 2) and acceptable (Fig. 3) categories. A key factor in the inspection process is to select a transducer with an element diameter that closely matches the theoretical weld nugget diameter. This approach allows the user to identify acceptable and unacceptable weld conditions and to be more sensitive to undersized welds.

Fig. 1 - An unacceptable weld is indicated by reflected signals returned from only the first plate thickness. Note that the echoes come from the back wall of the top layer only. This characteristic waveform would indicate an unacceptable weld.
Fig. 2 - An undersized weld would be indicated by reflections from the entire thickness of troth plates and from the first plate thickness at the outer edges of the undersized weld nugget. This characteristic waveform pattern would indicate an undersized weld.
Fig. 3 - An acceptable weld is indicated by echoes returned from the entire thickness of the two fused plates without intermediate echoes occurring Note the dramatic change in signal amplitude from the first to second back wall echoes and lack of intermediate echoes. The rapid decay of reverberations is caused by the comparatively large amount of course-grained structure present in a good weld. This characteristic waveform would be attributed to a good weld.

There are transducers specifically developed for spot weld inspections. They come in a wide range of active element diameters and are compatible with either fixed delay lines or captive water column delay lines to meet the varying surface conditions encountered in spot weld inspections.

The fixed delay configuration consists of a hard plastic wave guide (usually having the same tip diameter as the active element). which is firmly held in place with a threaded retaining ring. If weld surface con ditions allow, the fixed delay option is often favored due to its ease of alignment and quick setup time. Otherwise, when weld conditions are extremely rough and nonuniform, the captive water column provides optimal coupling. The uniquely designed captive water column assembly consists of a small amount of water that is kept in place by a thin urethane membrane. When assembled, the transducer housing creates water pressure on the membrane that expands and conforms to the front surface of the weld.

At the Saab plant in Trollhättan, Sweden, an ultrasonic pulse-echo technique is used to classify spot welds as having no nugget, undersized nugget or an acceptable nugget on the basis of echo patterns. The ultrasonic unit used is a Panametrics Epoch III with 20-MHz transducers. One of the reasons for choosing this unit was that its portability allowed the spot welds to be inspected directly on the production floor. The unit has been in use for more than a year and in that time inspectors have achieved results that correlated up to 95% with the destructive teardown testing.

Based on information provided by Panametrics, Inc., NDT Division, Waltham, Mass.
Panametrics offers an abundant line of transducers specifically developed for applications such as spotweld inspections.

Acknowledgement:
The Paper was first published
in the Welding Journal Nondestructive Examination
published by the The American Welding Society,
550 NW LeJeune Road, Miami, FL 33126. |Top|


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Copyright © Rolf Diederichs, rd@ndt.net 1. June 1998
/DB:Article /AU:et_al /IN:Panametrics /CN:US /CT:UT /CT:spot_weld /CT:weld /CT:automotive /ED:1998-06