DEVELOPMENT OF CALIBRATION STANDARD FOR NON-DESTRUCTIVE TESTING OF SMALL DIAMETER TUBES
K. A. Gopal, N. Raghu, N. G. Muralidharan, P. V. Kumar and K. V. Kasiviswanathan Metallurgy and Materials Group Indira Gandhi Center for Atomic Research, Kalpakkam 603 102, India
Calibration of ultrasonic and eddycurrent test equipment using reference defect standards is a mandatory requirement as per inspection codes and procedures. The sizes and shapes of reference defects vary according to the size of the object examined and the sensitivity of detection desired. Inspection of small diameter tubes which are used in critical applications calls for machining of very small defects accurately. "U" notches of width 100 micrometers, depth 70 micrometers and notch length of 2 and 15 mm have been machined on the inside and outside surfaces of a stainless steel tube of inside diameter 3.2 mm. The notches were made in the circumferential and longitudinal directions. Microspark machining, which is one of the most versatile techniques used to machine standard reference defects for nondestructive examination, was used for machining these defects. Controlled erosion of the tube material takes place by striking a microspark discharge between the specially shaped electrode and the tube surface which are separated by a dielectric medium. The discharge current melts and vaporizes a tiny volume of the tube material. Precise machining is achieved by optimization of the variables. The profile of the notch generated and the notch dimensions are inspected using the replica method and special instruments. The replicas and the notch are compared by sequential metallographic polishing technique to ensure reliability of measurements. This paper describes the development work carried out for machining and inspection of small notches and the metallographic polishing technique used to establish the reliability of measurements.
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 663 - 666 Publisher:Ashgate Publishing Company
NDTnet
