Improvement of Closed Crack Selectivity in Nonlinear Ultrasonic Imaging Using Fundamental Wave Amplitude Difference

Abstract

To realize the high selectivity of closed cracks, we propose a fundamental wave amplitude difference (FAD) method based on the threshold behavior of fundamental waves caused by the contact vibration of closed cracks. This is realized by the subtraction of a fundamental array (FA) image at a small input amplitude multiplied by the amplification factor from that at a large input amplitude. The formulation clarified that FAD can selectively image closed cracks while eliminating other linear scatterers, which cannot be completely eliminated by the subharmonic wave amplitude difference (SAD) method. Furthermore, FAD was experimentally verified in a closed fatigue crack specimen using the closed-crack-imaging method of subharmonic phased array for crack evaluation (SPACE). Thus, we demonstrated that FAD is useful for achieving the higher selectivity of closed cracks against other linear scatterers than previous amplitude difference methods without filtering out the subharmonic or superharmonic waves.

Application of high-polymer adhesive tape to closed-crack detection using second-harmonic ultrasonic pulse waves

“…Conventional methods are useful for finding open cracks but cannot detect closed cracks. Recently, nonlinear ultrasonic (second-harmonic or subharmonic) pulse waves have been studied for use in NDE [1][2][3][4]. The secondharmonic frequency component 2 f 0 is generated by the nonlinear vibrations at the contact of surface solids such as closed cracks when the finite-amplitude ultrasonic waves with a fundamental frequency component f 0 are transmitted; this phenomenon is known as the contact acoustic nonlinearity (CAN) [5].…”

“…However, it is difficult to distinguish the source of the secondharmonic components, so that flawless samples are apt to be misevaluated as defective. Recently, to prevent CAN at the interface between a transducer and a sample, noncontact ultrasonic wave excitation methods, such as those using an electromagnetic acoustic transducer (EMAT) [2], and novel methods using a subharmonic ultrasonic wave that is not generated from a coupling agent have been proposed [3]. However, the use of EMAT is confined to conductive samples, and high-voltage electrical source for the excitation of subharmonic ultrasonic components are required.…”

Sol-gel composite film fabrication by paint stencil printing

“…In various industries such as power plants, ultrasonic nondestructive testing (NDT) is widely used because of subsurface defect/flaw detection and cost effectiveness [1][2][3]. Solgel composite films were found to be suitable for ultrasonic transducer because of high temperature durability, curved surface suitability, no necessity of couplant nor backing material, reasonable signal strength, relatively low center frequency such as 2-50 MHz, and high signal-to-noise ratio (SNR).…”

Ultrasonic phased array with surface acoustic wave for imaging cracks

“…Here the crack depth is the size of a crack in the thickness direction of structures and the crack length is its size in the direction perpendicular to the crack depth. Thus far, measurement of the crack depth by ultrasonic testing (UT) with bulk waves has been widely carried out since ultrasound is scattered at the crack tip in the depth direction, [1][2][3][4][5][6][7][8][9][10][11][12][13] where phased array with bulk waves has been intensively studied recently [14][15][16][17] and most UT has been applied from the side opposite to the crack opening. On the other hand, accurate measurement of the crack length is not easy since the edge part of a crack in the length direction is generally shallow, resulting in the low sensitivity in UT with bulk waves.…”

“…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. Therefore, we proposed SAW PA for water immersion testing.…”