Behaviour and modelling of selfpiercing riveted connections using aluminium rivets

Abstract

The present thesis is concerned with the behaviour and modelling of self-piercing riveted connections using aluminium rivets. The objective has been to establish a database on the behaviour of self-piercing riveted connections using aluminium rivets with respect to the riveting process, the mechanical behaviour of single riveted connections and the structural behaviour of riveted joints. The objective has been met by a combination of laboratory tests, modelling and non-linear finite element numerical simulations.

The thesis consists of four parts, i.e. four independent journal articles. Two articles have been published and two articles are submitted for journal publication. The four parts are introduced by a synopsis, comprising a brief introduction to the problem and motivations for using aluminium rivets, as well as a summary of the main findings and conclusions.

Part I presents an initial study on the joining of two aluminium plates in alloy AA6060 in three tempers W, T4 and T6 by using aluminium rivets in alloys, i.e. AA6068-T6, AA7108-T5, and AA7278-T6. Tests have been carried out to understand the behaviour of the aluminium rivets during the self-piercing riveting process. Various defects in the aluminium rivets when joining were detected, e.g. rivet compression and rivet fracture. These defects were avoided by heating the plates to be joined into W temper. The mechanical strength of riveted connections using aluminium rivets was then tested under different loading conditions, and compared with that of a steel riveted connection. The data from the riveting process tests was used to validate a 2D-axisymmetric model generated in the finite element code LSDYNA for modelling the riveting process by using aluminium rivets. The model was able to capture with reasonable accuracy the overall deformation mode of the riveted connection as well as the force evolution during the riveting process. However, the model was not able to capture the fracture in the rivet since no failure criterion was considered for the aluminium rivets.

The main objective of Part II is to propose an adequate failure criterion which is able to predict the initiation of the rivet fracture during the riveting process. Here, a damage-based failure criterion, which was originally proposed by Lemaitre (1992), was chosen. A rivet compression test was proposed to better understand the behaviour of the rivet under compressive stresses. A simple calibration procedure for the failure model parameters was proposed based on inverse modelling of two tests, i.e. rivet compression test and uniaxial tension test. It was found that the model was able to predict the failure in the rivet when used for joining two plate materials with an appropriate friction value and rivet mesh size.

Part III evaluates how the riveting process and the subsequent natural aging of the plates to be joined influenced the final mechanical performance of a riveted joint using an aluminium rivet. Two U-shaped specimens in alloy AA6063-W, obtained by a solution heattreatment of the alloy in temper T4, were joined using an aluminium self-pierce rivet in alloy AA7278-T6. The mechanical behaviour was tested after 3 and 30 days of natural aging of these riveted connections. In order to evaluate the process effect on the mechanical properties of the riveted connection, a comprehensive material test programme was carried out. Test results revealed that there is an interaction between the pre-straining and natural aging which lowers the material properties in terms of the flow stress compared to the „virgin‟ material, i.e. the curve obtained after heat treatment and aging only. The process effect on the mechanical behaviour of the riveted connections was investigated more closely by using a 3D-numerical model. Numerical analyses showed that this lowering effect lowered the force level of the riveted joints by approximately 10%.

Part IV presents a study with the structural behaviour of self-piercing riveted joints based on aluminium and steel rivets respectively. Two T-components made of two open aluminium profiles in alloy AA6063 temper T4 joined by 6 and 12 rivets, respectively, were designed and tested under quasi-static loading conditions. A new test device was developed to perform the tests of the T-components under two different load cases. Experimental results of the T-components joined by using aluminium self-piercing rivets were then compared with the corresponding components joined by using steel rivets in terms of force-displacement curves, deformation modes of the components as well as rivet failure modes. The experimental results of the T-components based on aluminium rivets were used to validate a resultant-based point-connector model for self-piercing rivets proposed by Hanssen et al. (2011) using shell elements. It was found that the use of the model for predicting the structural behaviour of riveted joints based on the T-component tests by using shell elements gave acceptable results in most of the cases.