The Research Of In-Situ Meacurement Of Local Zones' Stress-strain Curves On Welded Joints By Using Instrumented Double Holes Micro-shear Test

Welded joints exhibit heterogeneity of mechanical properties. The standard tensile test provides the global response of the whole joint but no information occurring locally within the different constituents. This information is necessary to improve the material processing, and to numerically evaluate the component performance. Therefore, it is quite necessary and very important to determine the local mechanical properties for welded joints. Estimation method of the local mechanical properties for welded joints, based mainly micro-tensile method, an automated ball indentation technique. However, in-situ measurement of local zones'stress-strain curves cannot be achieved in welded joints.An in-situ test instrument, that relates to the double holes micro-shear test approach, was presented for determining the local mechanical properties of welded joints. Its major components include:load unit that consists of DC motor,screw,slider with the bears and the lever where a indenter is mounted; load-displacement measured unit that is be comprised of the load cell and the displacement sensor; and fixing mechanism whose major parts are the U-shaped mechanical frame and grips. The great advantages of the technique are ability to be in-situ measurement of stress-strain curve and provide localized properties, and the technique has emerged as a nondestructive method for measuring the yield and tensile strength values of in-service welded joints, and without any service interruption. In this method, require no the removal of samples from the in-service welded joints for testing, drill only the double holes whose diameter are 6 millimeters in the two sides of the point to be determined in welded joints, in-situ test instrument was fixed by the double holes, load material bridge by an indenter until it is broken, record the response between applied load and indenter displacement by the Labview soft systerm. Hence, the incremental values of indenter load and displacement are converted to the load-displacement curve from the double holes micro-shear test, the incremental stress-strain curve is obtained according to elasticity and plasticity theories. Then yield strength and tensile strength of the different zones in welded joints were obtained, and the local mechanical properties of welded joint was evaluated.In order to obtain respectively the relations between the shear yield stress and yield strength, between the ultimate shear stress and the ultimate strength from the double-hole micro-shear test and the uniaxial tensile test. The two test methods are operated in 6063 aluminium alloy extrusion,red copper and x70 pipe line steel rolled plate. The true stress-true strain curve in 6063 aluminium alloy were drawed with making use of the relations between the double-hole micro-shear test and the uniaxial tensile test. Finally the double hole micro-shear test have carried on the local area of the x70 piple line steel welded joints and 6063 aluminium alloy welded joints, the distributions drawing are similar from the double hole micro-shear test results and hardness in welded joints.