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Resistance spot welding is perhaps the most frequently encountered joining method for steel sheet in the automotive industry. It is accomplished by passing an electrical current through metal sheets via electrodes. The sheets are held together under the pressure exerted by the electrodes and heat is induced by the electrical current which generates a molten nugget between the sheets. The molten nugget then solidifies to form a bond. During the spot welding process, important changes occur in mechanical and metallurgical properties of the spot welded areas and heat affected zones appear. Although routinely used by the industry, the physics involved in the process are far from trivial, and generally involve a combination of electrical, mechanical, thermal, and metallurgical fields. In particular, the contact area between electrode and workpiece generates an additional electric contact resistance dependent on the models parameters. This contact resistance will have a decisive impact on the shape and size of the nugget and therefore the welds quality. In keeping with LS-DYNA's philosophy of offering flexible and robust tools, the approach adopted by the EM solver consists in using a DEFINE_FUNCTION to define this contact resistance allowing a precise model calibration when combining experimental and numerical results and thus enabling the engineer or researcher to get a clearer picture and better understanding of how the different physics are involved.