What it is all about. We want to obtain a strong interlocking of the sheets and profiles as shown by the S-shaped interface between the layers as shown on the photo below.
A key quality control parameter for clinch joints, be it round or rectangular is the residual bottom thickness ST, see figures below.
We can measure the ST-value, using a thickness gauge. back to top Λ
If we take a closer look at a cut through the clinch joint, we can see that there are two parameters that will determine the strength of the joint. The thickness of the neck portion of the upper, i.e. punch side layer, is called s1 and the extent to which this upper layer is trapped is called c1.
For high shear strength and high fatigue strength s1 should be large.
For high peel, i.e. pull-out, strength and high static strength c1 should be large.
So, obviously we would like both s1 and c1 to be as large as possible, but as they are interlinked we have to strike a compromise.
Experience tells that such a good compromise is c1 = s1 = half the thickness of the punch side layer.
Depending on the loading conditions and what type of strength is the most important, we can adapt the parameters c1 and s1 to fit the task.
As is shown on the figure to the right, we can for example increase c1 if we are looking for a high pull-out strength, while we would increase s1 if we were looking for a high fatigue strength.
The ST-value gives us information about the relation between s1 and c1.
The starting point for tuning the parameters is the following:
for a rectangular clinch joint ST = 1/2 of total thickness
for a round clinch joint ST = 1/3 of total thickness
With this setting we get roughly s1 = c1
If we now want to favor say pull-out strength, we should make a clinch joint where the ST-value is smaller than for the above base setting, i.e. we want to squeeze the clinch point more.
If, conversely, we want to have a higher shear strength, we should make a joint where the ST-value is larger than for the base setting.
Obviously, these guidelines are intended just to serve as a starting point. If the overlapping sheets have very different thickness or if we have a multi-layer situation, then confirming tests are necessary.
