Influence of the Young’s Modulus on the Velocity of Ultrasonic Waves to Measure Mechanical Stresses

Abstract

The state of stress in mechanical components can be evaluated non-destructively using ultrasonic waves. The relation between stress and the velocity of elastic waves in a solid is given by the acoustoelastic theory. Among different types of ultrasonic waves, the longitudinal critically refracted (Lcr) wave is more sensible to stress variations. However, not only stress, but also other factors can influence the velocity of ultrasonic waves, as for example, the microstructure of the material under test. Since the Young’s modulus is affected by variations of the microstructure, a relation between wave velocity and the modulus would allow corrections of the values of wave velocity obtained during stress measurements. The aim of this work is to find a relation between the Young’s modulus and the velocity of Lcr waves for specimens of API 5LX70 steel. The material chosen in this work is used in the manufacture of pressure vessels and oil pipelines. Assessing the state of stress of such components in field can guarantee operational safety and economic gains by avoiding premature replacements. The time-of-flight (TOF) of the Lcr wave was measured in four samples of API 5LX70 steel and the results were related to the Young’s modulus measured in a tensile test and through ultrasonic tests. Since the distance traveled by the wave in the specimens was kept constant, the TOF was used instead of the wave velocity. The results showed a linear relation between the TOF measured and the Young’s modulus obtained by ultrasound and tensile test in the samples.