Hysteresis-like attenuation of ultrasound in severely plastically deformed nickel

Abstract

Ultrasound attenuation was measured in nickel specimens of about 30 mm diameter prepared using the high pressure torsion technique. The cold working process produced an equivalent shear strain increasing from zero at the center up to 1000% at the edge of the specimen. The fragmentation of the grains due to multiple dislocations led to an ultrafine microstructure with large angle grain boundaries. The mean value of the grain size distribution gradually decreased from $ 50 mm at the center to 0.2 mm at the edge. Laser pulses of 5 ns were employed for the excitation of broadband ultrasound pulses covering the spectral range of 0.1-150 MHz. The ultrasound pulses were measured from the opposite side of the specimen by means of an optical interferometer and a piezoelectric foil transducer in two experimental setups. The features of the detected signal forms are discussed. The absolute value of the attenuation decreases from the center to the edge of the specimen showing nearly linear frequency dependence. The variation of the phase velocity was measured in a 6 mm-thick high pressure torsion nickel sample, revealing a velocity increase from the center to the edge.

Propagation of flexural waves in inhomogeneous plates exhibiting hysteretic nonlinearity: Nonlinear acoustic black holes

“…(27) and (22) the condition for the validity of the description of the black holes in the regime of weak HQNL is obtained, f 3a=4 a=2À1 ½ 5a=4À1…”

“…However the question on a possible role of the nonlinear acoustic absorption in the black hole phenomena has not been studied yet to our knowledge. There are multiple classic [15][16][17][18] and modern [19][20][21][22][23][24][25][26][27][28] studies of the nonlinear acoustic absorption (nonlinear ''internal friction'' in classical terminology) in various kind of micro-inhomogeneous [20,26,29] materials, where the important role in the mechanical motion is played by dislocations, grain boundaries, inter-grain contacts, cracks, etc., i.e., by the mechanical elements that are significantly larger in dimensions than interatomic distances in the sample but, at the same time, are significantly smaller than the sample dimensions. These materials are also called mesoscopic [30,31] from the acoustic point of view, when they are tested by the acoustic waves which are significantly exceeding in length the dimensions of the mechanical elements responsible for their inhomogeneity.…”

“…It is well documented in the literature that the nonlinear absorption can dominate over the linear one in the acoustic resonance experiments, i.e., in standing waves [23,25,26,28,32,33], causing the increase in the width of the resonance significantly beyond the linear resonance width. However, important nonlinear absorption was also revealed in the travelling acoustic waves both in the microstructured and nanostructured materials [21,22,27,34,35]. Thus, the accumulated evidence of the nonlinear acoustic absorption in microinhomogeneous materials indicates that it is worth studying the nonlinear acoustic absorption of flexural waves in the microinhomogeneous plates in view of their potential applications for vibrations damping.…”