Ultrasonic monitoring of dislocations during fatigue of pearlitic rail steel

A computer-controlled narrowband ultrasonic pulser/receiver system was used to determine changes in the coefficient of attenuation and the acoustic velocity, with high precision, in pearlitic steel specimens during cyclic tensile fatigue. The specimens were interrogated through the fatigue-damaged zone in a direction normal to the loading and parallel to the crack formation planes, using shear wave bursts at 5 MHz. Attenuation and velocity are found to be sensitive to stress accumulation and dislocation pileup, but appear to be unaffected by the onset and growth of microcracks. Using the Granato and Lücke dislocation damping theory, the change in the coefficient of attenuation and the acoustic velocity was used to derive the relative change in dislocation density and loop length. The change these four parameters experience is more pronounced during the early stages of fatigue, when dislocations are mobile, than during the more advanced stages of fatigue. As fatigue damage accumulates, dislocations begin to multiply, tangle up and their mobility diminishes progressively. Consequently, the rate at which these four parameters change decreases with the diminishing mobility of dislocations.