Effect of Different Tool Geometries on the Mechanical Properties of Al-Al Clinch Joints

Abstract

The use of clinch joints, e.g. vehicle structures, is determined by the reliability of the joint and its strength properties - in particular the fatigue strength. Clinch connections offer the advantage over form-closure and force-closure processes that they can also be used for hybrid material combinations. In order to be able to evaluate the influence of the geometry parameters such as e.g. undercut, neck thickness or also base thickness on the fatigue behavior, three clinch connections (in optimum and compromise design) with different tool parameters were designed and examined using the example of a joining task with aluminum sheet material. For this purpose, fatigue curves (F-N curves) in the range of high to very high numbers of load cycles (N = 105 to 107) were determined. In this load cycle range, a so-called "neck fracture" is mainly to be expected as the type of failure, whereas for quasi-static tests, a “buckling” is more likely to occur. The tests were carried out on single-cut overlapping shear tensile specimens. Metallographic and scanning electron microscopic examinations of the joints and the fracture surfaces served to identify the crack initiation site and to clarify the respective type of failure. Significant differences in the damage behaviour of the three clinching variants could be shown. This observation enables one step into the direction of fully understanding the relationship along the causal chain "joint requirements - joining process - fatigue strength". Thus the adaptability of the clinching process can be improved.

Design of clinched joints on the basis of binding mechanisms

“…1 Distribution of residual strain (a) [12] and contact force (b) [13] at the joint be qualified and quantified via metallographic cross-sections with subsequent measurement of the quality parameters. For the qualification of a force-closure component, a torsional load can be applied to rotationally symmetrical clinch joints [4]. This can also be used to distinguish the effect of different surface conditions on the tribological system.…”

“…The hardness at the homogeneous conductor is, caused by the precipitation hardening process increasing from 66.5 ± 0.8 HB5 to 96.1 ± 0.8 HB5 [4]. That means a harder surface can realize a higher surface pressure between the joining partners.…”

“…The measured values documented were the applied torque in the EC tightening spindle and via an external torque angle encoder with a resolution of 0.25°. The specimen geometry and arrangement were taken from [4].…”

“…Clinched joints are designed according to geometric features such as neck thickness t n and undercut f [3]. These values correlate with the non-destructively measurable quality criterion of bottom thickness t b if the tool geometries are known [4]. Clinched joints are based on the binding mechanisms form-and force-closure as shown by Groche in [5] and Salamati in [6].…”

Numerical Investigation of the Corrosion and Fatigue Behavior of Clinched Joints

“…Under static loading unbuttoning occurs, where the upper and lower sheets separate from each other. Making general assumptions on the fatigue behavior of a clinched joints is difficult because small changes in the geometry, caused, e.g., by using different tool geometries [6], have a large influence on the joint's load-bearing capabilities.…”