Numerical modelling of high-speed nailing process to join dissimilar materials: Metal sheet formulation to simulate nail insertion stage

Lightweigthing structures using mixed material components have become one of the main target of automotive industry future. In most cases, introduction of dissimilar materials leads to joining issues. Either materials can not be welded, due to thermal incompatibilities, neither joined by traditional plastic deformation-based processes (clinching, riveting,…) due to extended mechanical properties. New generations of Advanced High Strength Steels (AHSS) as well as boron steel have brought these joining processes to its limits. In this context, High-Speed Nailing is a very promising technology since a dynamic punch – working velocities up to 37 m/s - enables a wide range of materials to be joined – inclunding AHSS - in a short cycle time (10 ms) and is adapted to high production rate. However car manufacturers remain sceptical since joining mechanisms are not clearly understood and joint strength can not be predicted yet by engineering simulations. These considerations have driven the present work which proposes an insight into one of the main current issue of high-speed joining modelling: requirements on the material constitutive and damage laws to model fastener insertion stage. A step-by-step model construction methodology has been chosen here and applied to mixed materials configurations of dual-phase steel (1.5 mm-thick) and cast aluminium (2.5 mm-thick) joined in the two following superpositions: cast aluminium/steel and steel/cast aluminium. Strain and strain-rate dependant constitutive laws are calibrated on the basis of quasi-static and dynamic tests until strain rates up to 6000 s−1. Two coupled-damage approaches are calibrated on both materials and applied to nailing simulations. Results are twofold: (1) whatever is the damage formulation, sheet reaction forces and fracture modes are well predicted by joining simulations; (2) upper and lower sheets fracture locations exhibit a distinct loading path, close to bi-axial state on the plane-stress locus for lower-sheet and mixed shear-tension state, out of the plane stress locus, for the upper sheet. Beyond the differences between damage formulations, simulations have established that critical damage parameter at fracture Dc has a major influence on nail insertion predictibility, driving material ductility, therefore kinetic energy tranformed into plastic work.