Ultrasonic attenuation peak in steel and aluminum alloy during rotating bending fatigue
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I. INTRODUCTION FATIGUE is one of the major failure modes in metal structures. To evaluate the deterioration and the remaining life, many studies have appeared, which have employed ultrasonic, X-ray, magnetic, and mechanical measurements of properties as well as direct observation of the microstructure for defects such as dislocations, microcracks, macrocracks, and slip bands. In the ultrasonic approach, the main focus was on attenuation evolution.[1–10] Schneck et al.[1] measured attenuation at several stages of fatigue life using a bar-resonance method on a 0.22 pct C steel exposed to rotating bending fatigue. They varied the bending stress between 0.56 and 0.88 of the yield strength (fatigue limit/ yield strength 5 0.66). For all cases, the attenuation increased slightly at the beginning, and, after experiencing a long stable period, it increased remarkably just before failure. Bratina and Mills[2,3] studied attenuation recovery at several stages of cyclically stressed steels and indicated that recovery is caused by the interstitial point-defect diffusion to dislocations. Pawlowski[4] showed a correlation between the slope of early-stage attenuation increase and the remaining fatigue life. Many other studies showed similar results to those reported by Schenck et al. The initial increase was attributed to the increase of the dislocation density, and the final increase was attributed to the occurrence of microcracks and their subsequent growth. The present authors recently observed a novel behavior of the surface-skimming shear-wave attenuation during rotating bending fatigue of a 0.23 pct C steel pipe, including a sharp attenuation peak appearing at the final stage.[11] No study has reported such a peak, despite the long history of research. We developed a noncontact ultrasonic spectroscopy method to measure “pure” attenuation: electromagnetic acoustic resonance (EMAR).[12,13] This method, being free of interfering
transducers, can measure ultrasonic attenuation more swiftly and accurately than any other method. Measurement of pure attenuation can be made only by isolating the specimen from the transducers and the coupling materials. Here, we discuss the attenuation peak characteristics in more detail and the microstructural mechanism that produces the attenuation peak. For 0.45 pct C steel and a 5052 aluminum alloy, the peak appeared at approximately the same fractional remaining life, irrespective of the bending stress. We consider dislocation damping and scattering caused by cracks and grains as factors that change attenuation and sound velocity. To determine their contributions, we correlated the ultrasonic measurements with the presence of cracks, grainsize change, and recovery behavior after interrupting the cyclic stress. From these observations, we conclude that the attenuation peak is caused by a drastic change of dislocation mobility, not by the cracks themselves. II. MATERIALS Table I shows the chemical compositions and yield strengths of the materials. Figure 1 shows their microstructures. We anneale
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