Residual stress and deformation in SPR joints of high strength materials

The first aim of this work was to study systematically force-displacement curve of self-piercing riveting (SPR) process in conjunction with specimen characterization. Carbon steel sheet with different thicknesses and hardnesses and steel rivets with different lengths and hardness levels were used to examine the influence of material properties on the force-displacement characteristic curve. Flat dies with three specific diameters and five types of depth also have been used to observe their influence on the joint formation and force-displacement curve. This study shows that the force-displacement curve can be used to monitor the quality of a joint within the process parameters (rivet length and hardness, total stack thickness and die geometry) studied. A simple and effective model was developed to determine rivet flaring in selfpiercing riveted (SPR) joints by taking into account the effect of C-frame deflection on the force-displacement curve. Both interrupted and un-interrupted SPR experiments were performed to define the start of rivet flaring and the subsequent path of the flaring rivet tip. Two key point events of the forcedisplacement curves were identified, d0 and dmax for start and end of rivet flaring respectively. A relationship was established for interrupted SPR samples, and was then validated for complete (un-interrupted) joints. The study shows that the characteristic force-displacement curve can be used to predict the rivet flaring inside an SPR joint without characterising the cross-section of a joint.