Cyclic-Tension Fatigue Behavior in a SS400 Steel Plate Studied Using Ultrasonic Linear and Nonlinear Techniques
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SINCE Granato and Lu¨cke proposed the dislocation damping theory[1] in 1956 to express the consumption of elastic wave energy by dislocation line motion, ultrasonic waves have been applied in fatigue evaluation,[2–6] focusing on the variation in their attenuation and velocity. Conventional linear ultrasonic methods for evaluating materials have been limited primarily to longitudinal waves, due to their superior attenuation compared with shear waves, i.e., the inherent sensitivity limit in longitudinal waves to slight changes in materials.[7] An alternative nonlinear technique proposed in the 1960s uses harmonic amplitudes to study fatigue in materials. Hikata et al.[8–10] presented the dislocation harmonic theory, which led to the generation of second and third harmonics by dislocation line motion (the product of the dislocation monopole density and the fourth power of the dislocation loop length pinned at both ends); this theory has been used extensively in material testing.[11,12] Recent advances in dislocation theory have shown that the dislocation group is mobile and has a dipole-like dislocation structure. Cantrell and Yost argued that the second harmonic amplitude can be evaluated using the product of the dislocation dipole HIDEKI YAMAGISHI, Senior Research Engineer, is with the Institute for Manufacturing Research and Development, Toyama Industrial Technology Center, Takaoka 933-0981, Japan. MIKIO FUKUHARA, Professor, is with the New Industry Creation Hatchery Center, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: [email protected] Manuscript submitted February 23, 2015. Article published online September 8, 2015 5114—VOLUME 46A, NOVEMBER 2015
density and the third power of the dipole height as an elastic nonlinearity[13–16]; specifically, using longitudinal ultrasonic waves, they demonstrated that the amplitude of the second harmonic, normalized in terms of the absolute acoustic nonlinearity parameter b, increases several times in fatigued stainless steel 410Cb[17] and aluminum alloy A2024T4.[18] Recently, studies have demonstrated the potential of ultrasonic measurement devices based on harmonics to detect fatigue damage in structural steel,[19,20] nickel-based superalloy,[21–23] polycrystalline copper,[24] and aluminum alloy.[25,26] Furthermore, the lifetime in the very high-cycle fatigue (VHCF) regime (over 108 cycles) has been evaluated. For instance, in the VHCF of a magnesium alloy of which load is applied by an ultrasonic fatigue testing, it has been revealed that the nonlinearity rises sharply in the just before the fatigue failure.[27] In other development analyzing a nonlinear parameter, a method using acoustic emission (AE) focused on various harmonics has been demonstrated. In the low- and high-cycle fatigues of SCM440 steel, the 2nd harmonic increases gradually increasing with fatigue. On the other hand, the 3rd and sub-harmonics rise intensively just before the fatigue failure.[28] As described above, nondestructive evaluation of fatigue using nonlinear ultrasonic (NL
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