EVALUATION OF THE ROLLING DIRECTION EFFECT IN THE ACOUSTOELASTIC PROPERTIES FOR API 5L X70 STEEL USED IN PIPELINES

摘要

Ultrasonic evaluation of stresses has experienced great development in the last decade, mostly because of the new electronic instruments and high-speed data acquisition systems now available. Past applications have been applied mostly using shear waves and the birefringence technique. Longitudinal critically refracted (skimming) waves have proved to be more sensitive to stress variation, and have been applied recently to oil steel pipelines. Oil pipe failures have caused a lot of environmental damage in Brazil. The environmental and economical costs are very high. This work describes part of a research effort to further apply longitudinal critically refracted waves to inspect the welded region in pipes in Brazil through a study of the acoustoelastic constants. Twenty-eight samples were cut from a flat plate of the oil industry class steel (API SL X70) which would be typically formed into pipe. The samples were prepared for tensile testing. Half of those were cut from what would be the longitudinal direction of the pipe, and the other half from the perpendicular direction. The samples are bars of 760x70xl0.8-mm~3 . Some bars were stress relieved before the test. The results show that there are significant differences between the acoustoelastic constants for the rolling direction and me perpendicular direction. It is clearly important to have the correct value for the acoustoelastic coefficient when applying the technique to stress measurement. There is a minor, but important influence of the stress relief process.