Minimum-wear characteristics for boundary friction between solids

A new parameter is proposed for characterizing boundary friction between solids—the probability Q that some contact spot will be converted to a wear particle as the real contact area shifts by one mean contact-spot diameter. A method is proposed for the phenomenological detection of optimum regimes of boundary friction, which are characterized by the minimum wear intensity of the sample material, and for measuring the corresponding quantity Q=Q ^opt. For babbitt, one of the most frictionless materials, the value of Q ^opt is found to be ≈2×10⁻¹⁰. When data on the linear wear intensity I _h , the contact pressure p _n , and the hardness H are available, Q can be calculated for the given test conditions. Deviations of Q from the value Q ^opt (for a given material) can serve as a criterion for how closely a prepared surface structure approaches the optimal. Zh. Tekh. Fiz. 68, 48–52 (April 1998)     

Pulse-shape distortion in transmission experiments with incoherent phonons in isotropic solids due to elastic scattering

Assuming homogeneously distributed elastic scattering centers in an isotropic dielectric plate, the transmission and distortion of phonon pulses, that are transmitted from a radiator element to a centered detector element at the opposite face, are investigated theoretically. Analytic expressions are derived for the singlescattered phonon signal due to three different phenomenological scattering characteristics: isotropic scattering, preferential forward scattering, and preferential forward/backward scattering. In particular, we find that the asymptotic decay of the scattered phonon signal depends on the associated scattering characteristic. Numerical results are given for the excitation by a delta pulse using all three scattering characteristics. As a special case, the pulse-shape distortion of a rectangular pulse of 100 ns duration is treated in more detail. Parameter is the ratio of phonon mean free path to plate thickness.     

Heat transfer between elastic solids with randomly rough surfaces

We study the heat transfer between elastic solids with randomly rough surfaces. We include both the heat transfer from the area of real contact, and the heat transfer between the surfaces in the non-contact regions. We apply a recently developed contact mechanics theory, which accounts for the hierarchical nature of the contact between solids with roughness on many different length scales. For elastic contact, at the highest (atomic) resolution the area of real contact typically consists of atomic (nanometer) sized regions, and we discuss the implications of this for the heat transfer. For solids with very smooth surfaces, as is typical in many modern engineering applications, the interfacial separation in the non-contact regions will be very small, and for this case we show the importance of the radiative heat transfer associated with the evanescent electromagnetic waves which exist outside of all bodies.     

Dry friction due to adsorbed molecules

Using an adiabatic approximation method, which searches for Tomlinson-model-like instabilities for a simple but still realistic model for two crystalline surfaces, with mobile molecules present at the interface, sliding relative to each other, we are able to account for the virtually universal occurrence of "dry friction." The model makes important predictions for the dependence of friction on the strength of the interaction of each surface with the mobile molecules.     

A new 1-D integral representation for dynamic response of anisotropic elastic solids due to a point force

A new 1-D integral is presented for calculating the transient response due to a suddenly applied point force in a general anisotropic solid. The integral is based on a 2-D solution for a line force. It is shown that the integral reduces to a simple expression for the static Green's function immediately after the passage of the last bulk wave. The computational efficiency and accuracy of the proposed formulation are demonstrated by numerical examples for zinc and copper.     

Decoherence due to elastic Rayleigh scattering

We present theoretical and experimental studies of the decoherence of hyperfine ground-state superpositions due to elastic Rayleigh scattering of light off resonant with higher lying excited states. We demonstrate that under appropriate conditions, elastic Rayleigh scattering can be the dominant source of decoherence, contrary to previous discussions in the literature. We show that the elastic-scattering decoherence rate of a two-level system is given by the square of the difference between the elastic-scattering amplitudes for the two levels, and that for certain detunings of the light, the amplitudes can interfere constructively even when the elastic-scattering rates from the two levels are equal. We confirm this prediction through calculations and measurements of the total decoherence rate for a superposition of the valence electron spin levels in the ground state of 9Be+ in a 4.5?T magnetic field.     

弹性体内平面波

由胡克定律直接得到弹性体内平面波波动方程.     

Dynamic elastic hysteretic solids and dislocations

Recently we showed that the quasistatic response of nonlinear mesoscopic elastic solids to stress can be explained by invoking the formation of dislocation-based incipient kink bands. In this Letter, using resonant ultrasound spectroscopy, we confirm that the dynamical behavior of these nonlinear elastic systems is due to the interaction of dislocations with the ultrasound waves, thus resolving a long-standing mystery.     

Cracks in random brittle solids:

Statistical models are essential to get a better understanding of the role of disorder in brittle disordered solids. Fiber bundle models play a special role as a paradigm, with a very good balance of simplicity and non-trivial effects. We introduce here a variant of the fiber bundle model where the load is transferred among the fibers through a very compliant membrane. This Soft Membrane fiber bundle mode reduces to the classical Local Load Sharing fiber bundle model in 1D. Highlighting the continuum limit of the model allows to compute an equivalent toughness for the fiber bundle and hence discuss nucleation of a critical defect. The computation of the toughness allows for drawing a simple connection with crack front propagation (depinning) models.     

Formation of a surface-layer substructure due to friction

The mechanism by which a fragmented microstructure is formed when there are large degrees of deformation which occur on the surface of a metal during friction, is considered. It is shown that size of the fragments under these conditions is a minimum and is related to the dislocation density, reaching a critical value in an extremely deformed state, above which the interdislocation interaction forces are greater than the internalfriction forces. Institute of Physics of the Strength and Properties of Materials, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–95, February, 1997.     

New friction force due to spontaneous light pressure

We have discovered a new friction force, acting on an atom in the field of two oppositely propagating elliptically polarized waves of low intensity. In contrast to the well-known friction forces, the new force does not vanish at zero detuning of the field from resonance, and the direction of the kinetic process (heating or cooling) is determined by the relative orientation and the ellipticity of the polarization vectors of the oppositely propagating waves. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 7, 439–444 (10 October 1999)     

Change of elastic constants due to interstitials

The elastic constants of an ideal crystal are changed due to point defects. The general formulas describing these changes are presented including the effects of the volume change due to the defects. The recently discovered resonance modes of interstitials in metals explain the experimentally observed large change of the elastic constants. For an 100-split-interstitial in Cu, these changes are calculated, partly analytically and by computer simulation. The results show the experimentally observed anisotropy of the shearmodulus-changes, which represents a further indication for the existence of the 100-dumbbell in Al and Cu.     

Correlation between laminar structure and crystal lattice in type-I superconductors due to elastic interaction

The effect of the anisotropic magnetoelastic energy on the correlation between the laminar structure and the crystal lattice in the intermediate state of the type-I superconductors is investigated.     

Rigorous field-theoretical approach to the contact mechanics of rough elastic solids

A statistical field theory is formulated, which allows one to calculate the pressure distribution Pr(p) in a contact formed by an elastic body and a rigid counter face of arbitrary topography. It is a cumulant expansion, which contains Persson's contact mechanics theory as the leading-order term. Our approach provides a framework with which corrections can now be derived systematically. As an example, Pr(p) is calculated to high accuracy for exponentially repulsive solids. Non-Gaussian tails in Pr(p) can be rationalized for surfaces whose height spectra differ from colored noise.     

Nonlinear solitary waves in nonlocal elastic solids

The possibility of new weakly nonlinear solitary waves in nonlocal elastic media is demonstrated. The properties of these waves are determined by the characteristic features of wave dispersion in the linear approximation, and their velocity and amplitude cannot exceed certain limiting values. In the case of small amplitudes and velocities close to the velocity of sound, the profile of the waves under consideration coincides with the profile of the soliton described by the Korteweg-de Vries equation. When the amplitude and velocity of the aforementioned waves reach their limiting values, the wave profile sharpens. It is concluded that the propagation of such waves in rocks and soils is possible.     

Indentation of elastic solids with rigid cones

Experimental investigations of the response of several elastic solids, carried out using an instrumented indentation machine, are described. These solids include a natural rubber compound, neoprene, and three different compositions of poly(dimethylsiloxane) (PDMS) when they were indented with tungsten carbide cones of included angles of 60°, 90°, 120° and 136°. It is shown that, except for the case of the 60° cone loading on to the blocks of PDMS (1?:?10) and PDMS (1?:?20), the indentation load versus indenter displacement behaviour of all the elastic solids corresponding to all the conical indenters is well fitted by the 1965 Sneddon theory for frictionless indentations. Video camera sequences of the process of indentation by cones of different included angles into the blocks of PDMS (1?:?10) and PDMS (1?:?20) have also been taken from which measurements of the elastic deformation of the indented surface have been made. It is shown that the magnitude of the elastic deformation of the indented surface closely agrees with the theory when the blocks are indented with cones of included angles of 90°, 120° and 136°. However, for the case of the 60° cone, there is again a discrepancy between experiment and theory; the magnitude of the deformation is significantly smaller than that predicted theoretically. It is suggested that the discrepancy between theory and experimental observations may be due to a non-zero coefficient of friction between the 60° conical indenter and the blocks of PDMS (1?:?10) and PDMS (1?:?20). It is also shown experimentally that, for indenter loading speeds in the range 0.2–200?mm?min^?1, no time-dependent effects were observed for the loading curves, thus justifying the use of the theory given by Love in and of the Sneddon theory of conical indentations of elastic solids. Finally, using the findings described in this work, a brief discussion is included to explain the discrepancy between the results obtained by Sabey in and those obtained by Greenwood and Tabor in the same year when tyre-tread rubber specimens obtained from a single source were indented with steel cones of different included angles.