NDTnet 1998 Aug, Vol.3 No.8

Abstract

The utilization of high strength bolts in the construction of bridges has spread during the past 20-25 years. Regulations about checking the state of bridges order the periodical examination of these bolts as well. This examination - in accordance with the referring DIN standard - is passed on the correlation between the degrees of further swivelling of the nut and the applied moment. Based on experience the following objections can be raised against this method:
• it is not non destructive, it brakes up the connections
• its correlation with the forces originating in the body of the bolt depends on the sliding-friction coefficient between the nut and the feed screw which changes considerably with time and shows wide scattering,
• it damages the bolt in many cases.

The applications of the measuring method to determine the forces originating in the body of the bolt is based on the following. During productions a certain residual stress state forms in the bolt. Our measurements show that it is fairly typical and there is little scattering in high bolts. The stresses originating during tightening add up vectorially to the residual stress.During tightening stress changes occur in the head of the bolt as well which are proportional to those in the body of the bolt, so performing the measurement in the proper parts of the head the forces in the body can be determined.

The measurement can be carried out in two ways:


a/ the MBN - body force correlation is determined by performing calibration on bolts used in the structure (5-10 pieces, 1-2 %). Following this, the measurement can be done on the structure itself
b/ very precise measurement of a small number of bolts can be done in the following way: the MBN is measured on the bolts in built-in state. Then the bolts are dismounted and it is examined what body force produces the same MBN.

The traditional method for the investigation of the forces originating in the body of the bolt, which is based on measuring the swivelling moment of the nut, can detect only the bolts with a very great lack of body force since the friction coefficient worsens with time.

The above described MBN measuring method investigates the stress state, so the obtained results are independent from the friction coefficient.

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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