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K.L. Yuan et alii, Frattura ed Integrità Strutturale, 34 (2015) 476-486; DOI: 10.3221/IGF-ESIS.34.53 478 velocity is considered. This average velocity is taken to be the maximum initial velocity of the waveguide. Considering that the sinusoidal harmonic signal delivered by the transducer is ( ) sin(2 ) x t A ft   (1) the maximum initial velocity is max 2 V fA   (2) where A is the maximum displacement amplitude at the waveguide output end and f represents ultrasonic oscillation frequency of UIT. a) b) Figure 2 : (a) Oscilloscope picture of transducer excitation (top) and ultrasonic impact excitation (bottom) with UIT [12] , (b) Model of ultrasonic impact, t 1 -ultrasonic impact length (reboundless), t 2 -pause between ultrasonic impacts (rebounding off), f im -impact frequency, f ul -ultrasonic oscillation frequency, and t 1 / T im =0.1~0.3 [12], f im =100~120Hz, f ul =27~44 kHz [1]. a) b) Figure 3 : Ultrasonic-assisted tension tests; (a) schematic diagram of test devices with different modes, (b) measured force-deformation data [12]. Acoustic softening effect The effect of ultrasonic energy on metal deformation behavior for a wide range of metals has been well known since early 1950’s [22]. It was observed that the apparent yield strength of the material is immediately reduced upon the application of ultrasonic energy and restores as the removal of ultrasonic energy. As for UIT, the relative motion between the specimen and pin complicates the acoustic softening process. Statnikov [12] performed an ultrasonic-assisted tension test, in order to confirm the effect of ultrasonic impact via an intermediate pin on the deformation resistance reduction in comparison with a direct contact of the ultrasonic transducer and the specimen, as shown in Fig.3. An obvious reduction about 24-