Ultrasonic measurement of residual stress relaxation in welded steel plates using critically refracted longitudinal waves

Abstract

This study investigates whether or not there is a measurable amount of residual transverse stress relaxation in welded steel. This was determined by using two different methods of stress measurement. These methods involved strain gauges and ultrasonic testing. Specifically, critically refracted longitudinal (L[CR]) waves have been used to evaluate residual stresses and their dissipation in sample plates. The material investigated in this experiment was A.S.T.M. 1008/1010 steel. Each test specimen was approximately 30.5 cm by 40.6 cm and 1.3 cm thick (12 in by 16 in by 1/2 in). Each specimen was cut with T shaped slots cut from each edge of the plate. The geometry of the plate and the welds should provide for a uniaxial tension stress field in the area of investigation. After the plates were stress relieved by annealing, the transverse slots were then welded and allowed to cool. Strain gauges were then applied to the area of interest in the middle of the plate oriented parallel to the long side of the plate. Stress measurements would be made across the weld bead. Initial ultrasonic measurements were then performed. Both sides of the plates were investigated. At differing time intervals, a sample plate's welds were cut through. Strain gauge and ultrasonic measurements were then repeated. By comparing the before and after information, differential strain measurements and ultrasonic travel times were determined. A control plate determined zero stress travel-time data. The LCR travel-time and strain gauge measurements of the plates not only indicate that there is a measurable stress relaxation in the plates, but also that it occurs in a time dependent and predictable manner. Much of the stress relief occurred soon after the weld. The peak stress 375 minutes (6 1/4 hours) after the weld was approximately 250 MPa (35.5 ksi). Within 20 hours the stress had decayed to approximately 220 MPa (31.9 ksi). At 700 hours the stress was approximately 180 MPa (26 ksi).