A comparison of the use of critically refracted longitudinal waves and magnetic Barkhausen noise for stress measurement in API 5L X70 steel. (November 2016) - Pereira, Paulo
According to acoustoelastic theory, ultrasonic wave velocity varies as a function of the state of stress in a body. In ferromagnetic materials, stress alters magnetic domain walls created by cyclic magnetic excitation and affects the amplitude of magnetic Barkhausen noise signals. In this work, both these properties are used to measure different levels of stress in specimens of API 5L X70 steel. Time of flight of ultrasonic critically refracted longitudinal (Lcr ) waves and magnetic Barkhausen noise are measured and compared using three parameters: sensitivity, linearity, and dispersion. The results show that while magnetic Barkhausen noise is much more sensitive to variations in stress, the time of flight values fit a linear curve better. Also, for the ultrasonic technique, dispersion between the curves for the different specimens is lower. For both techniques, measurement dispersion in the specimens when no stress is applied is high. The findings of this study can be used to indicate the limits within which each technique can be used and to help decide when it is more appropriate to use one technique rather than the other.