Ultrasonic velocity variation (UVV) along different directions induced by material weak anisotropy will lead to residual stress measurement error using ultrasonic inevitably. In this research, the influence of weak anisotropy on residual stress measurement is analyzed theoretically and experimentally. Firstly, the in-plane acoustoelastic wave equation is modified by considering material anisotropy, which can be characterized by two in-plane anisotropic parameters. Further, an analysis model is also established to express the linear superposition of weak anisotropy and acoustoelastic effect on UVV. Based on this, a plane residual stress measurement method has been developed, which was verified using the rolling aluminum alloy plate after friction stir welding. From the comparison with hole-drilling method, it was adequate and capable to measure the plane residual stress by the relative UVV along three particular directions.

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Plane residual stress measurement using ultrasonic considering weak anisotropy of metallic material

When an ultrasonic wave is applied to the end of a magnetic rod and a standing wave is generated, high frequency stress will be generated at the standing wave nodes according to the frequency of the ultrasonic wave. The change in magnetic moment generated by this high frequency stress can be easily observed with a detector coil. On the other hand, since stress is not generated at the positions of the antinodes, an induced electromotive force is not produced in the detector coil. Therefore, observation of the stress distribution in the sample rod with the detector coil is possible. Since this phenomenon is reversible, a standing wave may also be generated by applying high frequency voltage, using a detector coil as an excitation coil. In that case, the impedance of the excitation coil changes from the node to the antinode positions. Therefore, it is possible to measure the stress distribution in a sample using only an excitation coil. This paper is a report of research into the effect that the standing wave in a magnetic rod exerts on the impedance of an excitation coil.

The Effect of Ultrasonic Resonance on Impedance in Magnetic Rods

This paper explores the possibility to measure changes in internal stress in ferromagnetic steels using magnetostatic methods. The device consists of a permanent magnet placed close to the steel piece and a magnetic probe between both elements. The magnetostatic field measured by using the probe is partially due to the magnetization of the steel piece. Internal stress variations in the steel alter its magnetic permeability due to the magnetoelastic effect, varying the magnetostatic field measured by using the probe. For the device we present here, the relative variation of the measured magnetic field is of the order of 1.7 × 10−5/MPa of internal stress.

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Magnetostatic determination of variations of internal stress in magnetic steels

In magnetic substances under oscillating stress due to the application of acoustic waves (such as ultrasonic waves), an oscillating anisotropic field may be generated by the inverse effect of magnetostriction. If this magnetic field oscillation can be detected, the stress state inside the magnetic substance can be determined. Based upon this principle, we have set up an apparatus for stress measurements, applied it to Ni plates with different residual stresses, and have obtained output signals corresponding to different stresses. This measuring technique makes it possible to know the stress state in structures before the formation of cracks, and hence offers a very promising nondestructive testing device.

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Takumi Yamaguchi

Papers

Measurement of residual stress of magnetic substance by surface acoustic waves