Acoustic Effects on Cyclic-Tension Fatigue of Al-4Cu-1Mg Alloy by Ultrasonic Shear Wave Methods
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FATIGUE is the most common of all causes of engineering failure,[1] which often brings serious damage to engineering systems or human society. To prevent this hateful breakdown, much work has been carried out on evaluation of actual fatigue level for materials. Evaluation techniques such as positron annihilation (PA),[2] electron backscattering diffraction (EBSD),[3] and X-ray diffraction (XRD)[4] have been used for nondestructive testing of fatigue, because they can evaluate imperceptible changes of lattice defect or crystal distortion associated with fatigue progress. However, these conventional methods have many severe restrictions such as specimen configuration, measuring conditions, and system costs for field application. Meanwhile, ultrasonic method has great advantages of mobility, facility, and costs for field application device. The ordinary linear ultrasonic methods for material evaluation have been mainly limited to usage of longitudinal waves because of its superior attenuation to shear waves. However, in the case of fatigue, the detection object is not an imperceptible change of crystal-lattice level but a microscopically obvious crack of stage II crack growth. Since most life of fatigue is gone for stage I crack growth, which is crack-initiation phase resulting from persistent slip bands (PSBs), it can be said that the ordinary ultrasonic method using longitudinal wave is not useful for damage evaluation of fatigue. Therefore, the effectiveness of nonlinear H. YAMAGISHI, Graduate Student, Institute for Materials Research, Tohoku University, is Senior Research Engineer, Central Research Institute, Toyama Industrial Technology Center, Takaoka, 933-0981, Japan. Contact e-mail: [email protected] M. FUKUHARA, Associate Professor, and A. CHIBA, Professor, are with Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan. Manuscript submitted February 13, 2008. Article published online January 7, 2009 486—VOLUME 40A, FEBRUARY 2009
methods using harmonic amplitudes has been studied since the 1960s.[5] As one of the factors which generate harmonic-frequency, an interaction with dislocation is considered. Assuming the motion of a dislocation group that has dipole-dislocation structure, Cantrell and Yost have concluded that the second harmonic amplitude can be evaluated by the product of the dislocation dipole density and the third power of the dipole height as elastic nonlinearity, and they have demonstrated that the amplitude of second harmonic increases several times in fatigued 410Cb stainless steel[6] and A2024T4.[7] The method has been showing promising potential. However, since the second harmonic also contains anharmonicity effects with crystal lattice, the interaction phenomenon has required close investigation for practical use. On the other hand, shear waves reveal sensitive response to even atomic or molecule change.[8] We can also use thermal- and electron-phonon interactions for material evaluation.[9,10] When shear waves are traveling in solids, a very subtle change
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