Cavitation during diffusion creep of a magnesium alloy

Cavities associated with grain-boundary triple edges have been found in a Mg-0.55 Zr alloy after diffusion creep and the angular distribution of cavitated boundaries measured; cavities occur predominantly on boundaries normal to the stress axis and are rounded in form. Nucleation of cavities can, therefore, be associated with grain-boundary sliding and their growth with diffusion. It is suggested that similar cavities found in some alloys after superplastic deformation may also result from significant diffusion creep but this is not typical of all superplastic deformation.     

Cavitation Damage of the single-crystal nickel-base superalloy SRR99 during creep at 850°C

Various heat treatments of the single-crystal nickel-base superalloy SRR99 have been carried out to produce a wide range of initial γ′ phase morphology. The cavitation damage has been studied after creep fracture at 850°C and stresses between 400MPa and 650 Mpa. The crep fracture surfaces and longitudinal sections of crept specimens have been examined by image analysing system in order to determine the size distribution and the area fraction of (001)-planes cracks on fracture surface characterise the creep crack damage level. The cracking morphology in fractured material as a function of stress and γ′-phase size has also been investigated. The results show, that crack propagation occurred anisotropically on (001) crystallographic planes perpendicular to the aplied stress along the γ/γ′ interfaces. It was found, that creep cracks dependent on the shape and size of γ′-phases developing at the temperature and stress level.     

Cavitation inception

In this paper, some general aspects of cavitation inception in flowing situations are considered. Special attention is paid to the problem of the scale effects commonly encountered by design engineers in extrapolating results from model tests to prototype situations. Past experimental and theoretical investigations relevant to the problem are reviewed. Recent advances in the field, particularly the influence of viscous effects in cavitation inception on smooth bodies, are discussed. Several useful scaling laws which are primarily based on empirical correlations or physical observations are suggested. A specific criterion which could be used to assess the importance of thermodynamic effects in cavitation inception is indicated.     

Cavitation phenomena in soil-projectile interaction

An analytical criterion is developed for the contact area of a projectile nose penetrating through soil. The cavitation criterion is based on the disc penetration model developed earlier and defines the critical velocity at which separation starts as well as the relationship between any higher velocity and the area of contact. At very high velocities examples are studied from which both nose shape and soil failure criterion are found to have a pronounced effect on separation.     

Cavitation phenomena in magnetic liquids

Investigations of acoustic cavitation in magnetic liquids subjected to the action of inhomogeneous magnetic fields were carried out for the first time. Possible mechanisms of the effect of the fields on the dynamics of single bubbles, cavitation threshold, and cavitation-induced erosion are considered. The effect of intense ultrasound vibrations on samples of magnetic liquids with a kerosene base are studied experimentally with and without gradient magnetic fields. It is shown that for magnetic liquids prepared with the optimum content of stabilizing substance and subjected to centrifugation in fields of 6·10³g the action of powerful oscillations does not affect adversely their performance properties and structure, which supports the use of magnetic liquids as technological media. An increase-induced in cavitation-induced erosion of test samples after application of a gradient magnetic field is revealed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 1, pp. 103–111, January–February, 1996.     

Cavitation in model elastomeric composites

Layers of transparent silicone rubber were bonded between two steel spheres or between two parallel steel cylinders, to make simple mechanical models of particle-filled and fibre reinforced elastomers. When the steel end-pieces were pulled apart, visible cavities appeared suddenly in the rubber layer between them, at well-defined tensile loads and displacements. The critical conditions for cavity formation are shown to be in good agreement with a theoretical criterion for the unbounded elastic expansion of a microscopic precursor void within the rubber: that the local triaxial tensile stress attains a value of 5E/6, whereE is Young's modulus for the rubber. When the rubber layer was extremely thin, however, less than about 5% of the steel end-piece diameter, then the stress required to form a cavity was greater than this, and it increased rapidly as the rubber thickness was reduced further.     

Cavitation in Normal Liquid Helium 3

We have studied cavitation, i.e. bubble nucleation, by focusing ultrasound hurts in normal liquid helium 3 at temperatures down to 40 mK. As in helium 4, cavitation is found to be stochastic, with a cavitation probability 0.5 at a given value of the sound amplitude, which we define as a cavitation threshold. This threshold is found significantly lower in helium 3 than in helium 4, a result which agrees with theoretical predictions of a spinodal limit at - 3.1 bar in helium 3 instead of- 9.5 bar in helium 4- We also measured the temperature variation of this cavitation threshold; it decreases with temperature as expected for a thermally activated nucleation process. We have not yet found any evidence for a crossover toward cavitation by quantum tunneling below 120 mK as predicted by several authors; if confirmed, it might indicate that the superfluid coherence plays a role in quantum cavitation.     

Impression creep; a new creep test

A new indentation creep test is introduced in which the indenter is a circular cylinder with a flat end. Unlike conventional indentation tests, a steady-state velocity is observed in this new test shortly after a transient period during which the indenter makes a shallow impression on the surface of the specimen; hence the name impression creep. This steady-state velocity is found to have the same stress and temperature dependences as the conventional undirectional creep tests using bulk specimens. Three possible mechanisms are analysed in detail, bulk diffusion, surface diffusion, and dislocation creep. They have different stress and indenter-size (radius) dependences. Experimental results on succinonitrile crystals are reported.     

Cavitation in elastic and hyperelastic sheets

Plane stress, axially symmetric, cavitation patterns are examined for large sheets with an embedded central circular hole. Both (radially-uniform) remote tension and internal pressure loads are considered. Material behavior is modeled by finite strain Hookean-type elastic and hyperelastic constitutive relations with logarithmic strains.Governing field equations are reduced to a single ordinary differential equation with the principal stress difference as the independent variable. Calculations reveal that under internal pressure the usual definition of cavitation state does not apply as load increases monotonously along the deformation path.Introducing the specific cavitation energy, as a practical measure in cavitation analysis, provides a unified framework for assessment of cavitation phenomena. Comparison with related patterns of spherical and cylindrical cavitation fields supports the main findings of this study.     

引额济克工程软岩爆破成洞技术

引额济克引水隧道围岩软弱 ,爆破开挖难度大。采用预裂爆破技术、合混型掏槽方式及其相应的爆破参数 ,使隧道成洞获得了良好效果。     

Cumulative damage in creep

The effect of two step stress variation and intermittent loading on the creep behaviour of commercially pure aluminium and stainless steel has been investigated. In the two step stress variation, the first stress σ₁ was applied for a given time t₁ and the stress level was switched over to σ₂. The resultant creep rate ?_s2⁺ and the failure time t₂ have been observed. Under the intermittent loading programme, the stress cycle was applied in the order σ₁-zero-σ₁ and the average creep rate on each reloading has been observed. The experimental data appear to give a cumulative damage rule in the form

$\text{t}{}_1\text{t}{}_{\mathrm{r1}}\text{+}\text{t}{}_2\text{t}{}_{\mathrm{r2}}\text{e}\text{?}{}_{\mathrm{s2}}{}^{\mathrm{+}}\text{e}\text{?}{}_{\mathrm{s2}}\text{=1}$

where t_r is the creep rupture time corresponding to a given stress and ?_s2 is the creep rate under the second stress in normal creep.     

Failure time in creep

This brief note proposes a method to estimate the useful life of a component subjected to constant load creep, using the total displacement or strain at any instant and the elapsed time. In contrast to the conventional methods based on the minimum creep rate or the steady state creep rate, the present method gives a more realistic and less conservative estimate of the ‘failure time’ in creep of a material, and hence predicts a longer design life for the component or structure.     

Transitions in creep behaviour

Attempts to identify the mechanisms operating during creep are often made by examining plots which yield apparent activation energies, or the stress or grain size-dependences of creep-rate. The forms of such plots are here examined and the ambiguities which arise near transitions from one regime to another are noted. The ranges of temperature, stress and grain size commonly used are inadequate and serious errors in interpreting the results of creep tests will continue to be made until a better understanding of the interaction of the basic processes is developed, so as to enable the positions of transitions to be predicted.At Dept. of Metallurgy, University of British Columbia, Vancouver 8, BC, Canada until 31 March 1970.     

Mismatch effect in creep properties on creep crack growth behavior in welded joints

The finite element method based on ductility exhaustion model was used to systematically investigate the mismatch effect in creep properties on creep crack growth (CCG) behavior in welded joints. The crack-tip damage, stress states, CCG paths, CCG rate and rupture life were calculated for different configurations of creep properties between weldment constituents under the same load level, and the creep life assessment and design for welded joints were discussed. The results show that when the zone containing the crack is softer than at least one of the other two surrounding materials or both, the creep crack propagates straight along the initial crack plane. Otherwise, it will form a second crack in the soft material near interface. These simulation results were confirmed by the experimental observations in the literature, and the mechanism was analyzed. The harder surrounding materials can lead to higher CCG rate and shorter rupture life due to the higher constraint given from them. The early initiation and propagation of the second cracks increase CCG rate and reduce rupture life, and the incubation time of the second cracks in soft materials near interfaces should be accurately determined in the creep life assessment and design for the welded joints. A proper mismatch design with harder material containing crack and softer surrounding material can improve CCG properties of welded joints (decreasing CCG rate and prolong rupture life).     

Indentation creep

Abstract

From the hot hardness test, information can be obtained concerning the time dependent flow, or creep, of the material beneath the indenter. Analysis of these data for selfsimilar indentation (i.e. indentation using a pyramid or cone) leads to equations from which the power law creep exponent and activation energy for creep can be derived, within limits imposed by the approximations of the method. The technique is used to analyse hot hardness data for metals and ceramics, drawn from a number of sources.

MST/1484     

Cavitation Instabilities in Plastics and Rubber-Modified Plastics

Spherical void expansion in plastics and rubber-modified plastics is investigated under radial traction conditions. The plastics are modeled as elastic-plastic pressure-sensitive materials and the rubbers are modeled as nonlinearly elastic materials. First, the growth of a spherical void in an infinite plastic matrix is investigated under remote radial traction conditions. The results show that the cavitation stress of the plastic decreases significantly as the pressure sensitivity increases. Then, the growth of a spherical void located at the center of a spherical rubber particle in an infinite plastic matrix is investigated under remote radial traction conditions. The results indicate that without any failure criteria for the rubber, the cavitation stress does not exist when the void is small and the rubber is characterized by high-order strain energy functions. However, when a failure criterion for the rubber is considered at a finite stretch ratio, the results show that the cavitation stress for the plastic with the rubber particle becomes close to that for the plastic without the rubber particle. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cavitation of Nonlinear Hyperelasticity in Toroidal Body

Dealing with a class of isotropic material of hyperelasticity occupaying a * toroidal" body (see Fig.4) in the three-dimension, we give a proof of existence of the cavitating solutions.     

Cavitation bubble dynamics in a polar liquid

It is suggested to introduce a new term into the equation describing the cavitation bubble dynamics in a polar liquid that takes into account a finite relaxation rate of dipole molecules of the liquid during rapid collapse of the bubble. It is shown that solutions of the modified equation provide significantly lower values of the thermodynamic parameters of the gas at the end of the collapse, so that thermal mechanisms of the high-energy processes accompanying cavitation cannot be operative. A substantial difference between commonly accepted notions and the proposed physical mechanism of accumulation of the acoustic field energy is demonstrated, whereby the energy is concentrated in the electrostatic field of oriented dipoles rather than in the form of the energy of compressed gas in the cavitation bubble. Experimental data are presented that confirm the proposed model.     

Cavitation in alloy steels during superplastic deformation

A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.