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RESEARCH PAPERS

Thermal Opening Technique for Nondestructive Evaluation of Closed Cracks

[+] Author and Article Information
Hironori Tohmyoh

Department of Nanomechanics,  Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japantohmyoh@ism.mech.tohoku.ac.jp

Masumi Saka, Yuichi Kondo

Department of Nanomechanics,  Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan

J. Pressure Vessel Technol 129(1), 103-108 (Apr 11, 2006) (6 pages) doi:10.1115/1.2389026 History: Received May 20, 2005; Revised April 11, 2006

This paper describes the “thermal opening technique” for enhancing the ultrasonic response of a tightly closed crack. In this approach, thermal stress is generated by cooling the part locally to reduce the crack closure stress in stainless steel plate, which improves sensitivity to detect the scattering wave from the crack surface; this has however not been successful by the usual ultrasonic inspection technique without introducing the thermal stress. It has been observed that the enhancement of the scattering wave continues for several hours after removing the thermal stress. This valuable and unique characteristic of the scattering wave is related to the change in the contact condition between the mating crack surfaces. Finally, a method is proposed for the evaluation of the depth of tightly closed crack from the difference in the time-of-flight between the scattering wave and the corner echo. The quantitative characteristic of the method is also verified experimentally.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Geometry of notched specimen. Units are in mm.

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Figure 2

Stress state (a), at which a crack is just opened by cooling a cracked part, expressed by superposing subproblems (b) and (c)

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Figure 3

Experimental setup for recording the scattering wave from the end surface of the tested plate. Units are in mm.

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Figure 4

Details of ultrasonic testing configuration with thermal opening technique for recording the scattering wave from the closed crack surface. Units are in mm.

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Figure 5

The changes in thermal stress in the tested plate obtained by the numerical analysis for various values of D

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Figure 6

Received signals without reflector (a), with reflector at L=25mm (b), and L=9mm (c)

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Figure 7

Received signals at T=0s (before starting cooling) (a), T=60s (during cooling) (b), T=210s (c), T=1h (d), and T=6h (e)

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Figure 8

Fatigue crack observed on the fractured surface

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Figure 9

Relationship between ζ and T. Upper (a) is the short time scale up to 600s, and below (b) is the long time scale up to 7h.

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Figure 10

Schematic of change in the contact condition between crack surfaces with change in the thermal stress (T.S.) in the tested plate

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Figure 11

Model of angled beam ultrasonic technique

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Figure 12

Path of the scattering wave for determining its time-of-flight

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