Mechanism Of Internal Friction Research Articles

Dislocation internal friction in CsI single crystals

The dislocation internal friction dependency on sample crystallographic orientation, ultrasonic vibration amplitude, and plastic predeformation of the samples is studied for CsI single crystals using the composite piezoelectric vibrator method. The dislocations of the main slip system {110} 〈100〉 are shown to give the predominant contribution to the internal friction value determined at room temperature and in the kHz frequency range. The characteristic ranges of stress amplitude corresponding to different internal friction mechanisms are determined. The starting stress determination method making use of voltage-current characteristics of the composite vibrator is described. The starting stress numerical value for the easy slip system {110} 〈100〉 is found. [Russian Text Ignored]

Internal friction mechanism in improper ferroelectric-ferroelastic Gd2(MoO4)3

The low-frequency internal friction features of the improper ferroelectric-ferroelastic Gd2(MoO4)3(GMO) have been studied. The equilibrium state of phase transition parameters of z -orientation samples used in the experiment are not changed by the applied external mechanical stresses. The internal friction peak height is found to be proportional to the temperature rate and inversely proportional to the frequency and amplitude of mechanical stresses. A loss mechanism of mechanical energy has been proposed. According to this mechanism the internal friction peak appearing near the Curie point is due to the interaction of antiphase boundaries with domain boundaries and dislocations.

Piezoelectric properties of modified PZT ceramics

The influence of impurities on piezoelectric parameters such as the radial coupling factor kr and mechanical quality factor Qm have been measured. A mechanism of internal friction is proposed. The effects of the interaction of 90 degrees domain walls with defects introduced by impurities are explained. A-site vacancies reduce the local stress in a domain, making it become remarkably mobile. The easy motion of the domain wall causes the internal friction in the ceramics to increase and thus Qm to decrease. O-site vacancies act as pinning points for the domain walls, so that domain wall motion is reduced when O-site vacancies are dominant, and Qm is raised due to the reduced internal friction. The effect of poling conditions is also discussed.

Internal friction at first-order phase transitions in solids

Several internal-friction mechanisms in the first-order phase transition temperature region related to the fluctuation nucleation of a new phase and motion of the interphase boundaries affected by the external stresses are reviewed. Two different experimental conditions have been investigated: firstly measurements at constant heating rate, secondly after rather prolonged holding at each temperature. Elementary mechanical loss mechanisms have been proposed and the dependence of the internal friction on the frequency and amplitude of oscillation has been calculated for each mechanism. The predictions have then been compared with experimental data.

The Stabilizing Capacity of Bearings for Flexible Rotors With Hysteresis

The paper deals with the stability of a rotor subject to hysteretic forces and externally damped only through the shaft supports. Bearings with negligible cross-coupling are assumed, but asymmetry of support stiffness and damping is considered. The mechanism of internal friction is not defined specifically in the analysis; viscous Coulomb and standard linear hysteretic forces are considered as specific examples derived from a common general solution. It is concluded that stability limits may vary considerably with different hysteresis mechanisms and that irrespective of this, optima in bearing damping can be specified.

Internal friction data as explained on the basis of structural-energetic parameters of inorganic glasses

On the basis of a previously reported model the diffusive mechanism of internal friction in microheterogeneous inorganic glasses is discussed. A method of calculating the activation energy, the dimensions of the structure complexes and defects, as well as the masses of relaxing structure elements is proposed. The potentialities of the method for a series of binary and ternary alkali-silicate glasses are shown.

Relation of Alkali Mobility and Mechanical Relaxation in Mixed‐Alkali Silicate Glasses

Internal friction and Na and K self‐diffusion coefficients for (I–X)Na2OXK2O‐3SiO2 glasses were measured. Alkali diffusion was measured between 300° and 500°C using radioactive isotopes and a thin‐sectioning technique; internal friction was determined at room temperature to 500°C at 0.1 to 2000 Hz. Comparison of the data for alkali diffusion and internal friction in Na‐K glasses with those for other mixed‐alkali glasses shows that the mixed‐alkali peak is related to alkali mobility. It is concluded that the mechanism of mixed‐alkali internal friction is cooperative rearrangement of dissimilar alkali ions, with the slower‐moving ion controlling the rate of reorientation.

Internal friction mechanism that produces an attenuation in the Earth's crust proportional to the frequency

To explain an attenuation proportional to the frequency (internal friction Q−1 independent of the frequency) present for waves in the earth's crust, a nonlinear mechanism connected with the motion of dislocations is proposed. With the kink model for dislocation motion, it is shown that an energy loss occurs when the kink goes over the kink barrier and becomes unstable. This loss is proportional to the number of kink displacements, and for a given strain level the number of kink displacements per cycle, i.e. the Q−1 value, will be independent of the frequency. The energy loss measured is consistent with theoretical calculations of the dissipation stress, and the loop length lA and the number of dislocations found are consistent with the results for polycrystalline metals and rocks.

High-Temperature Internal Friction and Transport Mechanisms in Sodium Silicate Glasses

Recent experimental evidence in favor of a vacancy model of ionic transport in glasses is discussed. The model is applied to new internal friction data on binary sodium silicate glasses. The second, high-temperature peak in the internal friction spectra of these glasses is apparently due to the stress-induced diffusion of singly-bonded oxygen ions. These ions diffuse into vacancies or interstitial sites whose concentration is thermally activated. Approximately 1012 cm−3 free sites are frozen into the glass below 200°C.

Anelastic properties of the Earth

Known mechanisms of internal friction in crystalline materials are examined to find which are likely sources of seismic attenuation in the earth's mantle. Presently available laboratory data on rocks are not useful for this purpose because of the low temperatures and pressures at which they have been determined, but experiments on sintered oxides indicate the important damping mechanisms at high temperature. The sources of seismic attenuation in the order of their probable importance are viscous grain boundary damping, stress‐induced ordering, and dislocation damping. If any parts of the mantle are partially melted, stress‐induced flow of fluid through inter‐granular channels may also cause attenuation. These damping mechanisms characteristically result in internal friction showing a strong frequency dependence. Scattering is almost certainly not an important source of seismic attenuation. The variation with depth of internal friction due to a thermally activated relaxation mechanism is found, and it is shown that such a source of internal friction can account for existing data on the anelasticity of the earth. The low attenuation observed at depths below about 500 km can result from the effect of pressure in decreasing atomic mobility; the observed attenuation decrease is therefore not in itself evidence of a phase change. The relation between damping and the strength of the mantle is developed, and it is shown that seismic attenuation data do not put useful bounds on this quantity.

Decay of Elastic Waves in Solids with Dislocations

A nonlinear, time-dependent, integral stress—strain relationship is formulated. Response functions for various powers of the retarded stress have transforms which satisfy dispersion relations. Characteristics and characteristic equations for one-dimensional elastic—plastic wave propagation are developed. We assume that the mechanism for the decay of elastic waves is the same as the mechanism of ultrasonic internal friction. This permits us to evaluate the linear response function by comparison with frequency-dependent internal friction theory and experiment. The functional form of the nonlinear response function is found.

Recent Developments in Solid State Acoustics

Dislocations in metals are the source of internal friction and elastic dispersion. Depending on the strain amplitude, they can act by several different mechanisms. At the highest strain amplitudes, the Frank-Read source is the predominant mechanism and leads to an anelastic region and a fatigue plastic region. In the fatigue region dislocations can cut through other dislocations, producing jogs and vacancies. These vacancies deposited on incipient cracks can cause them to grow and produce the fatigue cracks observed in metals. At lower strains, breakaway from pinning atoms can cause an amplitude-dependent internal friction. This type of loss can be closely correlated to the density of dislocations, concentrations of impurity atoms, etc. For strains so small that breakaway cannot occur, an internal friction occurs which is amplitude independent. For zigzag dislocations not lying along minimum energy positions, the internal friction mechanism proposed by Koehler appears to account for this loss in the megacycle range but not in the kilocycle range. For moderately strained materials, dislocations can lie along minimum energy positions and give rise to Bordoni relaxations whose internal friction values peak at definite temperatures for definite frequencies. These appear to be connected with Peierls forces.

LXV. On the theory of the low-temperature internal friction peak observed in metals

Abstract Recent data by Niblett and Wilks on polycrystalline copper suggest that the low-temperature internal friction mechanism in metals first observed by Bordoni is determined by intrinsic properties of dislocations. This is contrary to an explanation by Mason according to which the activation energy of the process should depend on the impurity content of the material and the separation between dislocation nodes. A mechanism proposed elsewhere seems to account for all the major features of the experimental results. The relaxation phenomenon is thought to be due to dislocations which are confined by the Peierls stress to certain crystallographic directions. Under the combined action of thermal fluctuations and the applied stress they may form pairs of kinks. Internal friction peaks are observed if the frequency of the applied alternating shear stress is equal to the frequency of the formation of these kink pairs. The Peierls stresses deduced from the experimental results are rather large, being of the order of one thousandth of the shear modulus.

The Mechanism of Internal Friction due to the Motion of the Dislocation

The mechanism of internal friction due to the motion of dislocation was studied from the measurement on the variation of internal friction with cold-work. For copper, internal friction increases with the initial cold-work and decreases after showing a maximum. Since the features do not depend on the frequency used in the experiment at room temperatures, the internal friction due to dislocations is concluded to be originated from the static hysteresis which is proposed by Nowick.