Experimental and theoretical investigations of the mechanical strength of clinching

Abstract

`Mechanical Clinching' or `Press Joining' is a novel technique for applying a structural connection between two or more sheets of material. Commercial cold-formed steel framing systems have been developed using the clinch as the primary method of structural connection. Design guidance for the use of mechanical clinching in structural applications is currently limited to recommendations from the work of past research programmes.

In this research the database on clinching shear resistance is extended by shear testing of mechanical clinches using an Instron tensile testing machine. S-type and H-type clinches are tested in layers of two and three with variable steel thicknesses and at different angles of applied load. The cyclic shear resistance of single clinches is investigated by applying variable loading over 10,000 cycles. A clinch design factor of safety is proposed based on the static and cyclic clinch test results and analysis. Pop rivets, self-piercing rivets and self-tapping screws are also tested in shear and comparison is made with clinch shear resistance characteristics.

Rotational shear resistance of groups of clinches is investigated in experimental and finite element tests by applying in-plane moment to groups of clinches in a range of steel thicknesses and at different group spacings. Moment resistance of clinch groups is also investigated in cantilever and H-frame cross-beam full-scale tests where groups of 4,6 and 8 clinches are applied to connect cold-formed steel components.

Full-scale tests are carried out on 10 truss beams up to 6m in length, joined by clinching struts and ties to folded parallel chords. Strain readings allow forces in the clinches over the course of each test to be recorded. Simplified clinch shear deformation characteristics are applied in finite element truss tests modelling the experimental truss tests. In a theoretical finite element model for each test a truss with no shear deformation at the connection nodes is analysed. Comparison is made with a finite element model allowing shear deformation at the connections to isolate the effect of clinch shear deformation on truss elastic stiffness and peak load.