NDTnet 1998 Aug, Vol.3 No.8

Abstract

Fracture-based design of pressure vessels using high strength materials calls for detection of small planar defects in the nature of cracks. Nondestructive testing (NDT) is essential for the detection of such planar defects and the most suitable NDT method applied for the purpose is ultrasonic testing. It has been found that detection of these planer defects by ultrasonic shear waves is greatly influenced by the tightness of such defects, in addition to their orientation and other variables. Experimental studies have shown that a dispersion in the signal amplitude of the order of 12dB exists in the detection of a particular size of crack . This dispersion is less when the amount of tightness is less i.e the distance between the crack faces are wider. This phenomenon often results in wrong interpretation of the size of crack from the area - amplitude relationship resulting in the acceptance of a rejectable and rejection of an acceptable flaw indication. Welds may also contain defects like porosity, lack of fusion, lack of penetration etc. in addition to cracks. The behaviour of these defects will be different from cracks in terms of tightness. A study has been made to find out a solution to this problem in the case of tight crack type of planar defects. It essentially consists of stressing the crack and observing the response to the ultrasonic shear waves. The response is found to increase with stress, reach to maximum value and then remain constant till the failure . The stress at which the ultrasonic amplitude response of the crack reaches the maximum value holds the key to reliable detection and estimation of size of the crack. Once this stress value is determined, the method can be applied for on-site inspection. The study has been made in different materials. This paper highlights the practical issues like the pressurization required to get the required stress level, minimum width of the crack needed etc. for an increased reliability of defect detection by removing the effect of tightness. At this condition the behaviour of cracks and other weld defects will be similar towards the ultrasonic waves. Hence the acceptance of a rejectable flaw or rejection of an acceptable flaw will be eliminated .

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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