Theory And Method Research On Planestress Field Measurement Based On Acoustoelastic Effect Of Critically Refracted Longitudinal Wave

Due to the shortage of comman residual stress measurement as destructive, operating complex, low interspace accessibility, a nondestructive plane strees measurement system is developed based on the acoustoelasticity theory of critically refracted longitudinal (Lcr) wave.With the advantage in its propagation along subsurface of material, it is choosen as measuring wave for surface stress measurement. Stress state of the solid with Lcr wave spreading is analysed, its acoustoelasticity equation on the complex stress condition is derived base on the nonlinearity stress-strain relationship. According the stress along the thinkness is zero on the surface, acoustoelasticity equation of Lcr wave in the complex stress state is simplified to the plane stress state. Two polarized acoustoelasticity coefficients of aluminium alloy7N01are demarcate by uniaxial tensile experiment. And the acoustoelasticity of Lcr wave is verification in static plane stress field: velocity of Lcr wave measured is shown to be in good agreement with it calculated.Irrationality of Lcr wave stress measurement based on uniaxial stress state acoustoelasticity is be analyzed. The neglct of acoustoelasticity effect of stress perpendicular to the Lcr wave propagation direction will cause large deviation in the measurement of plane stress field. To the purpose of conveniently engineering application, the relationship of Lcr wave velocity and stress is transformed to spreading time and stress. A new subsurface stress measurement named three-dimensional method base on Lcr wave is proposed, which can detect the value and direction of principal stress. And this method can be simplified to orthogonal method, which can detect the stress along the test direction directly. A stress measurement system is established by pulse signal generator, oscilloscope, Lcr wave transducer and computer. It’s supporting software programmed by NI Labview. The rapid and real time measurement process with high accuracy is achieved, its measurement accuracy of Lcr wave propagation time is±0.5ns. Known stress state static stress field based on Aluminium alloy7N01is measured by this system, the measurement error of three-dimensional method is8.80MPa, and orthogonal method is5.49MPa. Butted welded plate residual stress are measured by this system, and the results are verified by calculating and hole drill method separately to prove its reliability and applicabitity. Analysis the shortage of ultrasonic stress measurement based on Lcr wave in low spatial resolution. Point out it is inapplicable in measuring gradient stress field like welding residual stress field. In this paper, propagation path extension method is proposed to solve the low spatial resolution problem on Lcr wave stress measurement. This method can raise the spatial resolution from tens of millimeter to a several millimeter level with out decline the stress resolution. A static stress field of7N01aluminum alloy is measured by this method, and its result match well with it calculated. It is seen that propagation path extension method is applicable in plane stress field measurement.The Lcr wave stress measurement system is put into monitoring the stress state of high speed train in the flied in manufacturing process, servce and detect the machining of new technology application. It is also used in evaluating the oil pipeline butt joint weld residual stress and residual stress elimination of titanium alloy. The contribution of this system are confirmed by clients. The perspect of this Lcr wave stress measurement system is bright.