The electromagnetic acoustic resonance (EMAR) is applied for noncontact stress measurement of electric-resistance-welded steel pipes subjected to pure bending. The EMAR is a combination of the resonance method and electromagnetic acoustic transducers (EMATs). At a resonance, many reflection echoes are coherently overlapped each other, which serves to provide an easily measurable signal intensity, compensating in excess for the weak coupling of EMATS. The EMAT used here generates and detects the shear waves propagating in the thickness direction. It has the active area of 7x6mm2. The steel pipes were bent in a four-point bending configuration. The bending stress is measured by moving the EMAT in the circumferential direction around the outer surface of the central part of the bent pipe. At each measuring point, the shear-wave birefringence is measured, which is the normalized difference of the resonance frequencies of the shear waves polarized in the axial and circumferential directions. The birefringence is generally the sum of the texture-induced anisotropy and the acoustoelastic effect. They have to be separated for the stress measurement. In the present case, the bending stress was determined by fitting the measured birefringence to a sinusoidal function of the circumferential angle, thereby eliminating the texture contribution. Thus evaluated stress showed a good agreement with that estimated by the strain gauge method. This technique is a highly practical one because of the noncontact aspect of the EMAT; that is, it generally avoids the surface preparation for removing the roughness, the paintings, the rust and so on.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 875 - 878 Publisher:Ashgate Publishing Company
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