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Can resistance spot welds be non-destructively tested?

   

Frequently asked questions

Periodic destructive tests provide the normal means of confirming weld quality. Such tests are detailed in BS 1140 and ISO 10447. They include peel and chisel tests for routine quality checks and the sampling frequency depends on the component type, the quality required and the production volume. However, there is a small percentage of scrap associated with such tests and a non-destructive test alternative is attractive.

Thin sheet steel assemblies may be tested using the so-called 'non-destructive' chisel test. This involves driving a chisel between spots or around a spot weld until the material deforms, but without tearing, so that examination of the crevice reveals the apparent size of the weld. A stuck weld (non-fused) should break under this test. The deformed metal can then be hammered flat and the component used as normal. The test is less reliable on coated steel welds but gives a reasonable indication of strength. High strength steels and thicker materials risk damage to weld strength by such a test.

Before using such a test to replace some of the full destructive testing on a new component or material, the indications of this chisel test should be checked against the normal destructive test results.

Ultrasonic testing has proved in some automotive applications to be a good indicator of weld size as a post weld NDT method. Such tests are now common in the larger European automotive manufacturers and suppliers.

Ultrasonic testing of spot welds requires a special purpose high frequency transducer, with a water column retained by a plastic membrane, see Fig.1. The bubble so formed provides a soft nose to the probe, which is placed in the spot weld indentation. Multiple reflections of the ultrasonic signal are then displayed on the flaw detector or PC screen, as illustrated in Fig. 2. Great skill is required to interpret these signals in terms of weld quality. However, substantial reduction of destructive testing has been achieved by some of the major automotive manufacturers by using this method.

Fig.1 Typical ultrasonic probe for testing spot welds
Fig.1 Typical ultrasonic probe for testing spot welds
Fig.2 Ultrasonic testing of resistance spot welds.
Fig.2 Ultrasonic testing of resistance spot welds.

Substantial training and practice is required to achieve high reliability of weld quality indication and smaller companies may have problems maintaining a team of trained, full time ultrasonic testing operators.

The ultrasonic indication is affected by the quality of the surface indentation, the material type and thickness. However, access is only required from one side and the probes are easily manipulated, being about 15mm diameter and 50mm long.

The PC based units now available simplify setting up procedures and provide additional features. Some automatic evaluation of the ultrasonic signal is displayed to assist the operator. However, there is still a strong reliance on the skill of the operator to achieve a suitable signal and provide the final interpretation. Modern systems also provide a comprehensive data logging capability and the weld test data can be linked directly to the part drawings.

  1. BS1140: - British Standard specification for Resistance spot welding of uncoated and coated low carbon steel.
  2. ISO 10447: - Welding - Peel and chisel testing of resistance spot, projection and seam welds.