NASA invention entitled, The Hydraulic Controlled Auto-Adjustable Pin Tool for Friction Stir Welding (U.S. Patent 5,893,507), also known as the Retractable Pin-Tool (RPT) has been instrumented with a load detecting devise which allows the forces placed upon the welding pin to be detected during welding. The forces can now be characterized during both the plunge phase and subsequent welding. Of particular interest are those forces experienced as the welding probe comes within close proximity to the back-up anvil. For a given material, it is believed that unique forces are generated relative to the distance between the welding probe and the anvil. The forces have been measured and characterized using aluminum lithium alloy 2195, and correlations have been made between these forces and the pin's position relative to the backside of the weld material.
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