Mechanical properties of stainless steel clinched joints

Abstract: Clinching is a joining method for sheet material where the joint is created without need of heat or additional material. The material is joined by local plastic deformation creating a geometrical locking. Since the materi al is not heated during joining, problems caused by HAZ or phase transformation during heating or cooling can be avoided.Clinching has mainly been used for joints where other properties than the load carrying ability are the main point, for example in the white goods industry where rectangular clinches can make necessary flanges smaller since they can be placed very close to other geometries and the use of round clinches when joining coated sheets keeping the coating intact. In recent years the focus have moved more and more towards using clinching in load carrying details.Clinching can be divided in two main groups, round and rectangular. The main difference is, as the names suggest, the geometry of the joints. To make a rectangular clinch the sheets have to be cut which is not needed when making a round joint.In this work it has been investigated if clinching is a suitable joining method for austenitic stainless steels or not. Overlap joints in three different steels; AISI 301, AISI 304 and AISI 316 in thicknesses ranging from 0,8 mm to 1,5 mm have been used. The high load required to produce a clinch (95 kN for round clinch in 1.0 mm AISI 304) might be a problem since only a few tools are designed for such high loads. It is very important to keep good position accuracy when clinching, otherwise cracks in the neck area might appear.During clinching the material is highly plastically deformed, the hardness in a round clinch in AISI 304 reaches values of up to 500 HV as compared to the unclinched material with a hardness of around 200 HV. If this hardening effect is removed, in this case by heat treatment, the tensile strength of the joint is lowered somewhat, but the main part of the strength remains and can thus be assigned to the geometrical locking.Strength of rectangular clinches is direction dependent, shear tests show that when loaded along the cuts made during production a rectangular clinch loses 20-30 % of its strength. When round and rectangular clinches are compared the strength of the round clinches reach higher values but they also show a higher variation in strength.In fatigue the difference in life between round and rectangular clinches is high. Staircase testing was used to find the fatigue limit, for 2 million cycles it is more than twice as high for round joints compared to rectangular ones. The explanation for the lower fatigue limit in rectangular clinches is in that the sheets are cut during forming of the clinch and thus already contain macro cracks when loading begins.In rectangular clinches fatigue cracks grow from the ends of the cuts already present in the material. In round clinches fatigue cracks initiate in fretting damages on the surface between the sheets. Measurements of residual stresses in the area where cracks initiate suggest that residual stresses do not play a major role in fatigue failure.