A model to describe the high rate performance of self-piercing riveted joints in sheet aluminium

This paper investigates the performance of self-piercing riveted joints in aluminium sheet (A5754) at typical automotive crash speeds. Tension, shear and peel joint specimens have been tested over the speed range from low rate (10−3 m/s) to high rate (5 m/s), using a high rate servo-hydraulic machine. A heat treatment was applied to some of the material and joint specimens to simulate the paint bake conditioning applied to a car body in industry. The aluminium sheet material used to form the substrates was tested over the strain rate range from quasi-static (∼10−3 s−1) to near 500 s−1, and strain rate dependency on strength was not observed. Although the sheet material tested showed no strain rate dependency, this paper reports a measurable reduction in performance for the tension and shear joint test results at higher rate, which has not been previously reported. Common to all joint types tested in this paper is interlock failure and this is a tension failure mode. A rate dependent empirical model which relates energy to the negative exponential of test speed is shown to be a good fit to the joint tension test results. The same model holds for the relationship between maximum displacement and test speed. After calibration, the model shows a transition in which performance decreases rapidly at higher rate for the joint interlock failure mode. An explanation of the cause is discussed. Critical to the findings reported in this paper is the design of the fixture and force transducer to test U shaped tension specimens over the speed range of interest. A finite element model of the fixture and test measurement system was developed to ensure a near optimal design. Sufficient details of these are given to reproduce the test results.