NDTnet 1998 Aug, Vol.3 No.8

Abstract

The present paper addresses the problem of high accuracy measurement of stresses in installed generator press bolts. With the problem focused on 5 - 6 m long bolts made of Cr18/Mn5 steel, bolt length and material attenuation were beyond the capabilities of standard ultrasonic techniques for bolt stress measurement.

The use of longitudinal waves with frequencies 1, 2 and 4 MHz was not found to be sufficiently accurate, due to high attenuation of the direct part of the ultrasonic beam not affected by reflections at the cylindrical surface of the bolt.

Fig. 1. TOF as function of stress and temperature.

Transverse waves proved to be much less affected by attenuation. Therefore, this wave mode was preferred for TOF measurements, which were performed with a modern, digital instrument capable of zooming the screen image in several consecutive steps to achieve a very high resolution.Calibration of length versus time-of-flight at constant temperature gave reproducible results within .02 percent.

In order to be able to calculate bolt stresses based on measurements at elevated temperatures, correlation between stress, temperature and time-of-flight was established by measuring corresponding values of these parameters at temperatures up to 58°C. This correlation for the tested material was found to differ essentially from any other material described in available literature.

The developed technique allows accurate measurement and verification of bolt stresses in pre-service condition as well as during shut-down for in-service inspection. Compared with traditional ultrasonic bolt stress measurement, this technique is suitable for very long bolts and for bolts made of higher attenuating materials.

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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