Subharmonic phased array for crack evaluation using surface acoustic wave

Abstract

To accurately measure closed crack length, we proposed an imaging method using a subharmonic phased array for crack evaluation using surface acoustic waves (SAW SPACE) with water immersion. We applied SAW SPACE to the hole specimen in a fundamental array (FA) image. The hole was imaged with high resolution. Subsequently, SAW SPACE was applied to fatigue crack and stress corrosion crack (SCC) specimens. A fatigue crack was imaged in FA and subharmonic array (SA) images, and the length of this particular fatigue crack measured in the images was almost the same as that measured by optical observation. The SCC was imaged and its length was accurately measured in the SA image, whereas it was underestimated in the FA image and by optical observation. Thus, we demonstrated that SAW SPACE with water immersion is useful for the accurate measurement of closed crack length and for imaging the distribution of open and closed parts of cracks with high resolution.

Ultrasonic phased array with surface acoustic wave for imaging cracks

“…On the other hand, the closure of the crack at the root due to the high-temperature environment, which can occur in thermal power plants has been explicitly observed in eddy current testing (ECT). 32 In such a case, it is necessary to apply a nonlinear ultrasonic method to SAW PA, 23,24 such as by using subharmonic waves 3 and/or thermal stress. 10 Furthermore, from a physical viewpoint, the comparison between UT and ECT is useful for clarifying the physical mechanism of crack closure.…”

“…Furthermore, the array transducers used in situ for crack depth measurement can also be used for SAW PA by preparing a wedge with the third critical angle for the generation of the Rayleigh wave and using the delay laws formulated for electronic scanning. 24 This can markedly reduce the cost of introducing SAW PA to in situ inspection.…”

“…23,24 SAW PA was first implemented for contact testing, where a wedge with an array transducer was placed in contact with a specimen through a liquid couplant. 9 However, the sensitivity was low.…”

“…Strictly speaking, leaky Rayleigh and Rayleigh waves propagate beneath and outside the wedge, respectively. By exciting each element of the array transducer in accordance with a delay law, 24 a Rayleigh wave is focused at an arbitrary point on the surface of the specimen. When a surface defect is in the vicinity of the focal point, the Rayleigh wave is scattered at the defect, and thereafter, the scattered waves are received by the array transducer through mode conversion from the Rayleigh wave into a longitudinal wave at the interface.…”

Characterizing stress corrosion cracking in stainless steel using nonlinear resonant ultrasound spectroscopy

“…Ohara et al developed the Subharmonic Phased Array for the detection of SCC on the opposing surface of a metal sample and on the same side as the array using surface acoustic waves. 13,14 Dynamic Acousto-Elastic Testing is a low frequency pump and higher frequency probe technique that was used by Rivière et al to extract and additional nonlinear parameters. 15 Ohara et al used NRUS to locate SCC by analyzing the measured from a couple different resonance modes based on whether the mode had a strain node or antinode at the location of the SCC.…”