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Assessment Of Resistance Spot Welds In Low Carbon And High Strength Steel Sheet - Part 1 Static Properties

 

TWI Industrial Member Report Summary 193/1982

By R M Rivett

Background

The work covered in this report details the development of rational static 'type tests' and shop floor assessment techniques for determining the quality of resistance spot welds in low carbon and high strength steels.

It has been established that the maximum specimen dimensions recommended by the International Institute of Welding for the testing of resistance spot welds under shear and crosstension loading are suitable for use with 1-5mm low carbon steels and 2 and 4mm high strength steels with yield strengths up to 600N/mm2.

Chisel, peel and metallurgical techniques were assessed as possible shop floor tests for determining weld size. Although an optimum chisel design was determined, the required angle of the chisel tip varied for each of the steels examined. Furthermore, the test would be sensitive to operator technique such as centralisation of the chisel between the welds, impact force, etc. Therefore, the chisel test would not be practicable for assessing the quality of welds in high strength steels made under mass production conditions.

The peel test, conventional transverse metallurgical sections and plan sections prepared using a high speed metallurgical technique permitted the direct measurement of weld diameter even when interface failures were achieved. However, when peel testing, a certain amount of skill was required to differentiate between fused and non-fused areas for such welds. In contrast, both the conventional and high speed metallurgical preparation techniques permitted the size of the fused zone to be easily determined. However, the preparation time for the conventional metallurgical sections was ~15-20min/sample and therefore might not be suitable for use in mass production industries. The preparation time for the high speed metallurgical technique was ~3min/sample and therefore might be suitable as an alternative shop floor testing technique.

The static properties of welds in a range of 2mm high strength steels of nominally 500N/mm2 yield strength were assessed using the optimised specimen dimensions. For welds tested in shear it was established that the failure force was independent of steel composition and the mode of failure. However, the total failure energy of 5 root t diameter welds which gave plug failure was ~250% of that obtained for welds which failed across the interface. For welds tested in crosstension it was found that the results differed for each of the steels examined. There was no relationship between either the failure force or total failure energy and the mode of weld failure.

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