Acoustoplastic effect and the stress superimposition mechanism

The continuous stiffness measurement technique in nanoindentation intrinsically modifies the strength of the sample

“…In fact, earlier literature on acoustoplasticity [25][26][27][28][29][30] accounts for the effects of an oscillatory excitation as simply due to load superposition -the oscillatory load adds to the background load to give a higher resultant load. However, hardness is a measure of the mean pressure underneath the indenter [33], and for self-similar indenters this should be independent of the indentation force if the material does not exhibit intrinsic mechanisms that would destroy the self-similarity condition.…”

“…The phenomenon that a metal can be softened by the simultaneous application of a highfrequency vibratory load has been studied rather extensively as the topic of acoustoplasticity [22][23][24][25][26][27][28][29][30][31][32]. Such softening has been proposed to be due to an increase of dislocation mobility caused by the vibration excitation [23,24], or the higher peak stresses when an oscillatory stress is superimposed on a quasi-static one [27,28]. In Al, extensive subgrain formation was also observed in ultrasonic indentation tests on bulk samples when no subgrain formation was found in the quasi-static mode [31], and dislocation dynamics simulations revealed that an imposed oscillatory stress can lead to intrinsic enhancement of dislocation annihilation resulting in softening [32].…”

Synergetics of the interaction of mobile and immobile dislocations in the formation of dislocation structures in a shock wave. Effect of the stacking fault energy

“…In [7,14], the influence of the pressure, grain size, and volume density of precipitates on the transition from a cellular dislocation structure to a uniform dis tribution of dislocations behind the shock wave front was analyzed using the model equation for the disloca tion density in the following form [15,16]: (1) where ρ(y, t) is the density of mobile dislocations; y is the coordinate along the normal to the dislocation slip plane; t is the time; u is the dislocation velocity; D m is the coefficient of the diffusion of screw segments of dislocation loops through the double cross slip mech anism; λ m and δ f ρ 1/2 are the mean free paths of dislo cations between the events of their multiplication at obstacles of the nonstrain and strain (forest of disloca tions with density ρ f = ρ) origins, respectively; δ f ≈ 10 ⎯2 is the coefficient determining the intensity of the latter process; and h a is the characteristic distance of the annihilation of screw segments of dislocation loops through the cross slip mechanism [17]. In equation (1), the parameters ξ and β determine the type of dis location structure formed in the material.…”

“…Equation (1) is the result of the interaction between two kinetic processes developing in a plastically deformed crystal: multiplications of dislocations and their immobilizations and annihilations in dislocation dipoles [16] (2a)…”

Accommodation of Plastic Deformation by Ultrasound-Induced Grain Rotation

“…Other hypotheses include (i) the superposition of stresses [11][12][13][14][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][13][14] assume that the intrinsic resistance to deformation of the metal is not 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.…”