Accommodation of Plastic Deformation by Ultrasound-Induced Grain Rotation
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INTRODUCTION
HIGH frequency peening, and in particular, ultrasonic methods, such as ultrasonic peening and ultrasonic impact treatment are used more and more to alter the fatigue lifetime of welded components. Although reported results on fatigue life are very promising,[1,2] the detailed changes induced in the treated material and the mechanisms by which such changes occur are poorly understood. Ultrasonic impact treatment consists of an ultrasonic and a mechanical impact component. However, it is known that ultrasound absorption can lead to movement and creation of dislocations. The softening effect (reduction of the quasi-static stress) of superimposed ultrasonic vibrations on metals and alloys undergoing deformation is a well-known effect.[3–10] There are different hypotheses for the mechanism behind the changes in the material properties during ultrasonic excitation. Langenecker[5] proposed that ultrasound induces a change in the dislocation distribution as well as activation of new dislocation sources, occurring as a result of preferential energy absorption at defects in the crystalline lattice. An increase in dislocation mobility allows the metal to deform at a lower load. Other hypotheses include (i) the superposition of stresses[11–15] (ii) thermal softening of materials,[16] and (iii) the effect of a change in surface friction between the ultrasonic tool and the deformed material.[11] Existing theoretical models for stress superposition[12–14] assume that the intrinsic resistance to deformation of the metal is not R.K. DUTTA, PhD Researcher, is with the Materials ı¨ nnovation institute M2i, Mekelweg 2, 2628 CD Delft, The Netherlands, and also with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected], r.k.dutta@ tudelft.nl, [email protected] R.H. PETROV, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, and also with the Department of Materials Science and Engineering, Ghent University, Technologiepark 903, 9052 Ghent, Belgium. M.J.M. HERMANS, Assistant Professor, and I.M. RICHARDSON, Professor, are with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted July 29, 2014. Article published online June 13, 2015 3414—VOLUME 46A, AUGUST 2015
affected by the ultrasound, but with ultrasonic vibration the total stress applied to the sample is higher than the stress applied by the loading machine, because the ultrasonic vibrations also produce oscillatory stresses. The study by Daud et al.[17] showed that the reduction in mean stress is greater than the amplitude of oscillatory stress provided by the ultrasonic excitation, indicating that superposition of stress is inadequate to explain the stress reduction. As for thermal softening, previous studies[5,16–21] reported no or an insignificant specimen temperature rise during experiments, indicating that ultrasonic softening cannot merely be a thermal effec
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