Ultrasonic Measurement of Hoop Residual Stress in Stainless Steel Pipes

Abstract

The ultrasonic measurement of residual stress is based on the acoustoelastic law which is defined as the relation between the ultrasonic wave velocity and the material stress. The longitudinal critically refracted ( L CR ) waves are popular ultrasonic waves in the ultrasonic stress measuring application which are considered in this study to measure hoop residual stresses of welded pipes made from austenitic stainless steel. The stress measuring is applied on the inner and outer surface of pipes separately to evaluate variation of hoop residual stresses through the thickness of pipe. Furthermore, regarding to this theory that different ultrasonic wave frequencies could penetrate in different depths of material which leads to measuring the through thickness stress, four different testing frequencies of transducers are employed (1 Mhz, 2 Mhz, 4 Mhz and 5 Mhz transducers). It has been concluded that the difference in hoop residual stresses between inner and outer surfaces (which means that the variation of hoop stresses through the thickness) of pipes can be evaluated by L CR waves. These waves also showed good potential in measuring residual stresses in the welded zone and base metal of welded pipes.