Void Shrinkage Research Articles

粒状体のせん断過程における間隙の変化

Change of void ratio in granular medium during shear is examined through a trap-door test using aluminum rods with different radii. Displacement profile of circular disks and void ratio distribution of the assemblage of granules are revealed by image data processing based on the high-speed video record, while measuring the load acting on the door. Development of shear strain calculated by the displacement of disks is related to the expansion or shrinkage of voids within granular assemblage.

Ductile rupture at low triaxialities: influence of the inclusions upon void growth

When a ductile metal is subjected to some slightly tensile or compressive mean stress, it may occur that voids experience tensile stresses in some directions but compressive ones in other directions In this case void collapse in the latter directions can be prevented through contact with the inclusions which have generated the voids, and this greatly influences void growth and subsequent coalescence This paper is devoted to the analysis of this phenomenon. From numerical simulations, one derives an approximate model for ductile metals incorporating the effects of both void shape and possible hindering of void shrinkage by inclusions.

Development of Mechanical Stress in Cuni(mn) Films During Temperature Ramping: Related Mechanisms

ABSTRACTThis paper focusses on the development of biaxial stress in Cu0.57Nio.42Mno.ol thin films during annealing in Ar and, for comparison, in vacuum. Besides stress-temperature measurements also resistance-temperature investigations as well as chemical and microstructural characterization by Auger electron spectroscopy, scanning and transmission electron microscopy, and X-ray diffraction were carried out. To explain the stress evolution, atomic rearrangement (excessvacancy annihilation, grain-boundary relaxation, and shrinkage of grain-boundary voids) and oxidation were considered. Up to 250 - 300 °C grain-boundary relaxation was found to be the dominating process. A sharp transition from compressive to tensile stress between 300 °C and 380 °C is explained by the formation of a NiO surface layer

Staged curing of composite materials

In this paper the composites processing technique stage curing is developed. Stage curing is especially suited for the fabrication of thick composites in which thermal spiking and non-uniform consolidation are problematic. It is also useful for assessing the effects of lengthy lay-up times on mechanical integrity for large, complex composite structures. The effects of stage curing on the mode I interlaminar fracture toughness and interlaminar shear strength were investigated by double cantilever beam and interlaminar shear strength testing. From the results of these tests, no degradation was found for the staged cure cycles investigated. Additionally, the feasibility of reducing void content by stage curing was demonstrated. The interruption of the cure cycle with an intermediate cooldown under pressure before gelation results in shrinkage and collapse of voids. Subsequent gelation of the resin can give materials with reduced void content compared with conventional curing.

密着完了時間予測アルゴリズム 固相拡散接合予測支援システムに関する研究 第２報

The algorithm for predicting the intimate contact process in the solid state diffusion bonding has been developed. The numerical model proposed in our previous study (Part 1) was updated to predict the void shrinkage process in the final stage of bonding.The influence of dispersion of void spacing on the contacting process was examined based on the calculated results. The plural measurements of surface profiles was performed to estimate the variation of the bonding process. The distribution of void spacing estimated by the overlap method was also examined. It was useful to take into account the largest void spacing and the largest void volume for the prediction of the final stage. The algorithm was verified experimentally. It was indicated that the algorithm predicts the time taken to attain the full contact.

Effect of Surface Asperity on Interfacial Contact Process Controlled by Power Law Creep—Numerical Study of Viscoplastic Adhering Process

An interfacial contact process due to power law creep is studied using a finite element technique. The contact process is assumed to be produced by power law creep alone after initial intimate contact by instantaneous plastic deformation, i.e., no diffusional mechanisms for void shrinkage are taken into account. Also, the surface oxide film is not considered. If the bonded material is deformed, then the deformation is influenced by the initial faying surface wauiness with the asperity angle αo, and the contact process is achieved by two modes; surface folding at the bond-interface (type I) and interfacial expansion (type II), where the surface folding is the phenomenon that two faying surfaces are overlapped to each other. The surface folding phenomenon occurs preferentially when αo is less than 30 deg (as the surface wauiness height decreases). On the other hand, the interfacial expansion is dominant at αo > 45 deg. This can be explained in terms of the distribution of equivalent strain (stress) in the vicinity of the bond-interface.

The disappearance of voids during 180 keV Ni + bombardment of nickel

Ni +-ions with 180 keV energy promote void nucleation, growth and shrinkage as a result of impinging on nickel substrates held at temperatures between 600°C and 750°C. Growth proceeds to fluences near 3 × 10 21 ions/m 2 and is followed by shrinkage and, at 650°C and 700°C, increasing void number densities at higher fluences. Voids formed at 650°C, when ion bombarded at temperatures between 25°C and 550°C, decrease in size and number density, ultimately disappearing after a fluence of ∼ 2 × 10 21 ions/m 2 administered at the lower temperature. These observations are explained using rate theory modified to include the interstitials injected by the ion beam, the changing dislocation density in the irradiation damaged layer, and sputtering of the surface by the ion beam.

Formation and elimination of voids during the processing of thermoplastic of matrix composites

AbstractThe mechanisms of void formation and elimination during the stamping process of a polypropylene/glass fiber composite have been infestigated as a function of temperature, pressure, and time. Experiments were performed in a temperature and pressure controlled chamber, equipped with a rapid cooling facility. Samples of the composite as well as model polypropylene specimens containing calibrated voids were held at 200°C at different levels of hydrostatic pressure (1,10,100, and 300 bars), for a period of either 1 or 10 min. The speciments were subsequently characterized by denstiy measurements and morphologival observation. It was shown that: i) the expansion of the composite observed during the heating at 200°C under atmospheric pressure is largely induced by the gases previously dissolved in the polymeric matrix, ii) the rapid increase of the pressure during the stamping process leads to the closing of voids and, iii) the final holding under high calculation of void growth and shrinkage is in agreement with the morphological observations.

Identification of void shrinkage mechanisms

A method for identifying void shrinkage mechanism experimentally is presented. We treat the void shrinkage on the bonded interface during solid state diffusion bonding of similar metals. The voids are arranged at regular intervals. The experimental results were analysed by log( T/ t s) vs (1/ T) and log t s vs log P plots, where T is the absolute temperature, P the compressive stress, t s is the time taken to attain the void shrinkage of a certain volume ΔV. We obtained t s by measuring a certain growth of bonded area ΔS replaced by ΔV. Even if ΔS is adopted, we can identify the transition of the dominant mechanism by slopes of those plots. As a result, interface self-diffusion, volume self-diffusion and power law creep were experimentally identified as fundamental mechanisms which contribute to the void shrinkage process during diffusion bonding of metals.

Effect of Bulk Deformation on Viscoplastic Adhering Process—A Numerical Study of Solid State Pressure Welding

Viscoplastic intimate contact process of uneven surfaces is numerically studied by using the finite element model proposed in our previous paper. The model treats only the case that the interfacial contact is the rate determining step of the solid state bonding process. The distribution of the equivalent strain rate around the void surface is strongly influenced by the bulk constraint conditions, i.e., the interfacial deformation is greatly affected by the bulk deformation. The strain rate at the void tip is strikingly increased by the bulk deformation, which accelerates the void shrinkage on the bond interface. If the bulk is deformed, the contacting process is also affected by the asperity angle α0 due to surface waviness. When α0 < 30 deg, the bonded area growth is mainly produced by the folding phenomena of the faying surfaces.

Analysis of Thermal Stress Induced Void Growth During Thermal Cycling

AbstractRecent work in thermal stress induced voiding in passivated lines has focussed on the isothermal, quasi-steady state growth of existing voids. In this work, the transient growth and shrinkage of voids during the thermal cycling of narrow, passivated lines was studied. Analysis of experimental data for passivated lines demonstrates the quick buildup and evolution of a backstress at the grain boundary during heating and cooling. This backstress greatly reduces the driving force for void growth at higher temperatures and must be accounted for in stress induced void growth models for the case of appreciably varying temperatures in short time scales.

Analysis of Thermal Stress Induced Void Growth During Thermal Cycling

ABSTRACTRecent work in thermal stress induced voiding in passivated lines has focussed on the isothermal, quasi-steady state growth of existing voids. In this work, the transient growth and shrinkage of voids during the thermal cycling of narrow, passivated lines was studied. Analysis of experimental data for passivated lines demonstrates the quick buildup and evolution of a backstress at the grain boundary during heating and cooling. This backstress greatly reduces the driving force for void growth at higher temperatures and must be accounted for in stress induced void growth models for the case of appreciably varying temperatures in short time scales.

Recent void shrinkage models and their applicability to diffusion bonding

Recent void shrinkage models are reviewed and refined. The role of surface selfdiffusion is discussed for modelling void shrinkage by selfdiffusion along the void surface and bond interface. The refinement of the existing models makes it possible to discuss the effect of void crushing by power law creep, the validity of combining models of diffusion and creep, and the applicability of the existing models. The discrepancies between the various models are demonstrated by reference to the stress dependence of void shrinkage rate (log–log plots). The void crushing rate due to power law creep of the surrounding matrix is highly dependent on the degree of bulk deformation, i.e. the severity of bulk constraint. The applicability of the combined models of creep and diffusion are therefore limited by this condition.MST/1502

Recent void shrinkage models and their applicability to diffusion bonding

Recent void shrinkage models are reviewed and refined. The role of surface selfdiffusion is discussed for modelling void shrinkage by selfdiffusion along the void surface and bond interface. The refinement of the existing models makes it possible to discuss the effect of void crushing by power law creep, the validity of combining models of diffusion and creep, and the applicability of the existing models. The discrepancies between the various models are demonstrated by reference to the stress dependence of void shrinkage rate (log–log plots). The void crushing rate due to power law creep of the surrounding matrix is highly dependent on the degree of bulk deformation, i.e. the severity of bulk constraint. The applicability of the combined models of creep and diffusion are therefore limited by this condition.MST/1502

A numerical analysis of void shrinkage processes controlled by coupled surface and interface diffusion

The shrinkage behaviour of voids arrayed on a bond-interface (grain boundary) is analyzed by computer simulations. It is assumed that surface and interface self-diffusion operate in series. The junction between the void-surface and bond-interface is treated as a tip with a dihedral angle θ. Two numerical models are proposed to discuss the effect of interface and surface tensions on the void shrinkage. One of them always restricts θ to the value of the equilibrium balance (model 1). In the other (model 2), the equilibrium balance is ignored, i.e. the change in θ (free dihedral angle) is considered during the void shrinkage. For discussing the rate controlling step, model 2 is essentially the same as model 1. But, when the void shrinkage is controlled by the interface diffusion, they give void shapes quite different from each other, i.e. when the diffusivity ratio ƒ ⪢ 1 (ƒ = 2δ s D s /δ b D b , where δ s D s and δ b D b are produced by the thickness of the diffusion layer and the self-diffusion coefficient for the surface and interface, respectively), model 1 gives the equilibrium void shape but model 2 only exhibits the stationary void shape. The void morphology is not only influenced by the prescription of the equilibrium balance but also by the diffusivity ratio ƒ. The stress and temperature dependence of the void shrinkage can roughly be expressed by 2 δ s D s δ b D b/[2 δ s D s + δ b D b) P n ], where P is the bonding pressure and n the stress exponent. If θ is prescribed by the equilibrium balance, n increases from −1 to −0.3 with decreasing P. But, if θ changes, n is about −1. It is verified by experiments that the dihedral angle is not always constant and changes as increasing net stress is applied to the bond-interface. It is suggested that model 2 more exactly predicts the void-shrinkage process than model 1 as the pressure is increased.

Effect of collision cascade induced fluctuations in point defect concentrations on the void growth kinetics

Abstract The point defect production in collision cascades causes fluctuations of vacancy and interstitial concentrations. These fluctuations are shown to affect the void growth kinetics in irradiated metals. In the present paper the analysis of the void distribution behaviour as a function of void sizes is considered. Remarkable changes in void kinetics can exist when the dislocation density is comparatively small. Under conditions determined in the present paper, the cascade induced fluctuations of the void growth rate lead to some decrease in the void density in irradiated metals. Moreover, the spatially inhomogeneous void shrinkage prevails over the homogeneous one.

Spatially ordered void ensemble states at high irradiation temperatures

The physical mechanism of the spatially homogeneous void ensemble state instability with respect to small void size deviations induced by vacancy emission from voids is demonstrated by means of both the exact diffusion and chemical rate equations for the point defect concentrations in a volume containing voids. As it was shown in previous papers the corresponding instability conditions can be valid at rather high irradiation temperatures (higher than the swelling maximum). Here the nonlinear analysis of the void system response to small void size variations is carried out to obtain the qualitative characteristics of spatially ordered void distribution when the instability concerned takes place. Void ordering is the result of the spatially homogeneous void shrinkage after the completion of void nucleation process. Only “ordered” voids survive. The mathematical investigation is based on the conception of slowly and rapidly varying distributions, which is used to describe in unified fashion a spontaneous formation of ordered structures in a large class of nonequilibrium physical systems, whose properties depend on the parameters that differ in their spatial dispersion. In the case under consideration the spatial diffusion lengths of point defects and voids are such parameters. It is shown that the constructed periodical void distribution is stable in time with respect to small void size deviations. The roles of the anisotropic transport of self-interstitials in the void spatial ordering and the stochastic fluctuations of void growth rate in the expansion of the temperature range of the nonequilibrium phase transition from a spatially disordered to an ordered void system state are also discussed.

Theory of radiation-induced and thermal coarsening of the void ensemble in metals under irradiation

We propose a new physical mechanism of competition between vacancy voids, namely, the radiation coarsening mechanism which results in the growth of large voids at the expense of small ones due to the preferential absorption of radiation-produced interstitials rather than vacancies. The void preference is inversely proportional to their size, hence, small voids absorb extra interstitial atoms and consequently shrink, whereas large voids absorb extra vacancies and grow. Under typical radiation conditions the radiation coarsening rate is a factor of 10 2–10 4 higher than the rate of the well-known Ostwald coarsening induced by the void thermal emission of vacancies. The radiation coarsening is shown to limit the maximum number density of growing voids to the value dependent only on the dislocation density and material constants and independent of the void nucleation rate. A comparison with experimental data shows that it is this void number density that is formed under irradiation at fairly low temperatures where thermal vacancies may be neglected. The theory also explains the irradiation-induced void shrinkage observed in some experiments.

A numerical analysis of the void-shrinkage process controlled by surface-diffusion

Void-shrinkage is analyzed by computer simulations to visualize and predict the process of diffusion bonding. It is assumed that the atom-transport from boundary to void-surface is controlled by surface diffusion, that is, the coefficient of surface diffusion is much less than that of boundary diffusion. The shrinkage processes of isolated and arrayed voids on a boundary are respectively analyzed. For simplicity, the initial voids are assumed to be cylindrical with symmetrical cross sections. The computer simulations conducted to visualize the process of the void shrinkage indicate that a void shape changes its shape continuously. The void-shrinkage depends on geometrical parameters such as void-spacing, void-height and void-width as well as the external process parameters of temperature and pressure. It is found that the activation energy for the void-shrinkage can be obtained by ln ( T t v ) − 1 T plots, where t v is the time required to attain the void-shrinkage of a certain volume and T is the absolute temperature. It is also indicated that the stress-exponent n to t v gradually increases from −0.85 to −0.3 with decreasing bonding pressure.

拡散接合部の機械的性能に及ぼす接合雰囲気の影響

Diffusion-welded joints were made under welding atmosphere such as vacuum at 4 and 4×10-3 Pa, and Ar, H2 and air at 1×105 Pa. Better welding atmosphere for diffusion welding was investigated from the estimation of mechanical properties of diffusion-welded joints, defects such as void and inclusion, and residual gas in voids of diffusion-welded joints. The materials used were SUS304 stainless steel and Ti-6Al-4V alloy, and the results obtained are as follows:1) The best welding atmosphere is vacuum at 4×10-3 Pa for both materials in five kinds of welding atmosphere used.2) For the good selection of welding atmosphere, it is necessary to consider three items as follows.a) Welding atmosphere which does not have influence on quality of base metal.b) Welding atmosphere which does not oxidize the welding surface.c) Welding atmosphere which does not remain in voids.3) At diffusion-welded joints made under Ar atmosphere, Ar remains in voids of diffusion-welded joints. Ar does not diffuse into the base metal at all, and prevents the shrinkage of voids.