Showing papers on "Deformation (engineering) published in 1991"

Compressive behaviour of concrete at high strain rates

Abstract: Experimental techniques commonly used for high strain-rate testing of concrete in compression, together with the methods used for measurement and recording of stress and strain, are critically assessed in the first part of this paper. The physical capability of each loading method is discussed and some consideration is given to the definitions used for specifying the loading rate. The second part reviews the dynamic compressive strength (mostly uniaxial rather than bi- or triaxial) of plain concrete, while in the third part a review on deformation behaviour indicates that uncertainty and disagreement exist concerning changes in axial strain at high strain rates.

Prediction of steel flow stresses at high temperatures and strain rates

Abstract: The flow behavior of steels during deformation in the roll gap was simulated by means of single hit compression tests performed in the temperature range 800 °C to 1200 °C. Strain rates of 0.2 to 50 s−1 were employed on selected low-carbon steels containing various combinations of niobium, boron, and copper. The stress/strain curves determined at the higher strain rates were corrected for deformation heating so that constitutive equations pertaining to idealized isothermal conditions could be formulated. When dynamic recovery is the only softening mechanism, these involve a rate equation, consisting of a hyperbolic sine law, and an evolution equation with one internal variable, the latter being the dislocation density. When dynamic recrystallization takes place, the incorporation of the fractional softening by dynamic recrystallization in the evolution equation makes it possible to predict the flow stress after the peak. These expressions can be employed in computer models for on-line gage control during hot-rolling.

Aspects of fracture in particulate reinforced metal matrix composites

Abstract: The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.

Shape‐Memory Alloys as New Materials for Aseismic Isolation

Abstract: New results are presented in the area of hysteretic modeling and experimental characterization of shape memory alloys (SMAs). A stress‐induced micromechanical phase transition occurs in SMAs that causes inelastic deformation and gives rise to a large energy‐absorbing capacity. Because it is possible to achieve large hysteretic deformation in SMAs without incurring plastic deformation, SMAs have potential for use in earthquake‐engineering passive damping schemes. In order to represent such energy‐absorbing behavior, an existing one‐dimensional model of hysteresis is modified to include the macroscopic characteristics of SMAs. Also, the results of cyclic material‐characterization tests applied to a nickel‐titanium SMA known as Nitinol are presented. Hysteretic behavior closely resembling that of the superelastic material was obtained in the laboratory for cyclic strain levels up to 4.5%. The model of SMA behavior is also compared to the cyclic responses of Nitinol.

Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting

Abstract: The influence of composition and cooling rate on the hot ductility of steels has been reviewed. Models to predict hot ductility behaviour have been discussed and the parts of the trough which can be used to predict the likelihood of cracking occurring are highlighted. On tensile testing both deformation induced ferrite in sufficient quantity to improve ductility and dynamic recrystallisation occur but not when straightening during continuous casting; the strain being too low. This limits the use of the hot ductility curve in predicting cracking behaviour. The temperature range in which straightening of the continuously cast strand should be carried out is either 30°C below the Ar 3 when there is a large amount of ferrite (∼40%) present before deformation or above the T d, the temperature at which dynamic recrystallisation starts to take place in a tensile test; this being when the ferrite film no longer forms and precipitates are sufficiently coarse and few in number to influence the ducti...

Deformation twinning in h.c.p. metals and alloys

Abstract: The important role of deformation twinning in plastic deformation of h.c.p. metals and alloys is emphasized and the twin nucleation mechanisms are examined. A bifurcated homogeneous nucleation model is presented on the basis of the classical nucleation theory, the shape bifurcation theory and the elastic inclusion model. The activation energy for twin formation depends most sensitively on the twin-boundary energy. The structures and energies of (1122) and (1011) coherent twin boundaries are obtained by atomistic simulations using the Lennard-Jones potential for a model h.c.p. metal. The anisotropic coupling effect of normal stresses on twin disolocation mobility is determined over a wide temperature range (0-1200 K). The available experimental data on titanium and zirconium are consistent with the prediction from the proposed nucleation model and the temperature-dependent mobility of twin dislocations. Alloying effects on twinning and future research areas are discussed.

Calculated elastic constants and deformation potentials of cubic SiC.

Abstract: The full-potential linear-muffin-tin-orbital method in combination with the local-density-functional theory is used to calculate the equilibrium lattice constant, the cohesive energy, the bulk modulus and its pressure derivative, the elastic constants, the Kleinman internal displacement parameter \ensuremath{\zeta} the zone-center transverse-optical-phonon frequency, its Gr\"uneisen parameter and the corresponding energy-band strain, and optical-mode deformation potentials of cubic SiC. The results for equilibrium properties and the transverse-optical phonon at the center of the Brillouin zone are shown to be in good agreement with the available experimental data and previous first-principles calculations. The elastic constants of 3C-SiC are transformed to a trigonal symmetry tensor along the 〈111〉 direction, which allows a comparison to experimental data on hexagonal SiC. The agreement is found to be good. The elastic constants are also shown to be in good agreement with experimental data on the Young's and shear moduli and the Poisson ratio for polycrystalline SiC and with the Young's modulus of 3C-SiC whiskers along the 〈111〉 direction, which is related to the theoretical cleavage strength. Predictions are also made for the deformation potentials, which have not yet been measured. The discrepancies with previous atomic-sphere-approximation calculations for the trigonal strain and optical-mode deformation potentials indicate the importance of nonspherical terms in the potential for these deformations. The absolute deformation potential of the valence-band maximum is computed by means of a heterojunction calculation between strained and unstrained materials. This procedure is shown to give good agreement with previous calculations for Si.

Flow laws of polyphase aggregates from end-member flow laws

Abstract: An incompressible finite element model has been used to study the plane strain deformation of two-phase aggregates deformed by dislocation creep. Input for the model includes the power law flow laws of the two end-member phases and their volume fractions and configuration. The model calculates the overall flow law of the aggregate as well as the stress and strain rate variations within it. The input flow laws were experimentally determined for monomineralic aggregates of clinopyroxene and plagioclase. Results were calculated for a temperature of 1000°C, strain rates from 10−4 to 10−12S−1, and stresses of 1–1000 MPa. For these conditions, the end-member flow laws intersect on a log stress versus log strain rate plot at 10−8S−1. Some runs were made on finite element grids fit to an actual diabase texture (∼64% pyroxene, ∼ 36% plagioclase.) Other runs were made on idealized geometries to test the effects of varying the volume fraction of two phases, shape of inclusions, and relative strengths of inclusion and matrix. Important results include the following: (1) The model results satisfy the requirement that the aggregate strength must lie between the bounds set by the end-member flow laws and those set by assumptions of uniform stress and uniform strain rate. (2) The calculated diabase flow law matches well with that experimentally determined. (3) The aggregate strength within the uniform stress and uniform strain rate bounds is primarily affected by volume fraction, although certain phase geometries can also affect the strength. (4) Although the flow law for an aggregate of power law phases need not be a simple power law, we find it to be a good approximation. We have developed two simple methods of estimating the strength of an aggregate, given the end-member flow law parameters and volume fractions; both give results that agree with the finite element model calculations. (1) One method takes into account the phase geometry and gives a strength for the aggregate at any strain rate. (2) The other method can be used even if the phase geometry is unknown and gives expressions for the aggregate flow law parameters.

A framework to correlate a/W ratio effects on elastic-plastic fracture toughness (J c )

Abstract: Single edge-notched bend (SENB) specimens containing shallow cracks (a/W < 0.2) are commonly employed for fracture testing of ferritic materials in the lower-transition region where extensive plasticity (but no significant ductile crack growth) precedes unstable fracture. Critical J-values Jc) for shallow crack specimens are significantly larger (factor of 2–3) than the Jc)-values for corresponding deep crack specimens at identical temperatures. The increase of fracture toughness arises from the loss of constraint that occurs when the gross plastic zones of bending impinge on the otherwise autonomous crack-tip plastic zones. Consequently, SENB specimens with small and large a/W ratios loaded to the same J-value have markedly different crack-tip stresses under large-scale plasticity. Detailed, plane-strain finite-element analyses and a local stress-based criterion for cleavage fracture are combined to establish specimen size requirements (deformation limits) for testing in the transition region which assure a single parameter characterization of the crack-tip stress field. Moreover, these analyses provide a framework to correlate Jc)-values with a/W ratio once the deformation limits are exceeded. The correlation procedure is shown to remove the geometry dependence of fracture toughness values for an A36 steel in the transition region across a/W ratios and to reduce the scatter of toughness values for nominally identical specimens.

Plastic deformation and fracture of binary TiAl-base alloys

Abstract: The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α 2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.

Grain-size strengthening in terms of dislocation density measured by resistivity

Abstract: The effect of grain size on flow stress has been investigated in terms of dislocation density. The measurement of dislocation density was made for nickel having a high stacking fault energy, by means of electrical resistivity with which the dislocation density can be measured up to larger strains compared with transmission electron microscopy. It was found that the dislocation density for a given strain in specimens deformed in tension at 77 and 295 K increases in a linear manner with the reciprocal of grain size. It was also ascertained that the flow stress is proportional to the square root of dislocation density, irrespective of grain size, deformation temperature and the amount of plastic strain (ϵ). From the above two relationships, an equation between flow stress and grain size was obtained in a general form, which gives the Hall-Petch relation as the limited case at yield point or at small strains.

A theory for plastic deformation and textural evolution of olivine polycrystals

Abstract: Seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation. In order to understand better the origin of seismic anisotropy, we present a simple theory for plastic deformation and textural evolution of olivine polycrystals. Each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems, whose hardnesses and stress exponents are known from experiments. The evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local (crystal) deformation and the global (aggregate) deformation subject to the constraint of global strain compatibility. The axial compression texture predicted by our model agrees well to first order with that determined experimentally by Nicolas et al. (1973), although there are significant discrepancies in the details. Our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain, such that the crystallographic axes [100], [010], and [001] are concentrated around the finite strain axes a, c, and b, respectively (a > b > c). This result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy.

Ductile failure of a constrained metal foil

Abstract: A metal foil bonded between stiff ceramic blocks may fail in a variety of ways, including de-adhesion of interfaces, cracking in the ceramics and ductile rupture of the metal. If the interface bond is strong enough to allow the foil to undergo substantial plastic deformation dimples are usually present on fracture surfaces and the nominal fracture energy is enhanced. Ductile fracture mechanisms responsible for such morphology include (i) growth of near-tip voids nucleated at second-phase particles and or interface pores, (ii) cavitation and (iii) interfacial debonding at the site of maximum stress which develops at distances of several foil thicknesses ahead of the crack tip. For a crack in a low to moderately hardening bulk metal, it is known that the maximum mean stress which develops at a distance of several crack openings ahead of the tip does not exceed about three times the yield stress. In contrast, the maximum mean stress that develops at several foil thicknesses ahead of the crack tip in a constrained metal foil can increase continuously with the applied load. Mean stress and interfacial traction of about four to six times the yield of the metal foil can trigger cavitation and/or interfacial debonding. The mechanical fields which bear on the competition between failure mechanisms are obtained by a large deformation finite element analysis. Effort is made to formulate predictive criteria indicating, for a given material system, which one of the several mechanisms operates and the relevant parameters that govern the nominal fracture work. The shielding of the crack tip in the context of ductile adhesive joints, due to the non-proportional deformation in a region of the order of the foil thickness, is also discussed.

The role of equiaxed particles on the yield stress of composites

Abstract: Possible explanations are investigated for the yield strength enhancement of discontinuously reinforced Al alloy matrix MMCs, for the case of low temperature yield behavior where deformation occurs by dislocation slide. The Al alloys contain 0.1-10 micron diameter equiaxed particle discontinuous reinforcements of TiB2, Al2O3, and TiC. Attention is given to a single dislocation-particle interaction model, and both dislocation pile-up and forest-hardening multiple-dislocation particle interaction models.

Thermal-mechanical modelling of the flat rolling process

Abstract: 1 Introduction.- 2 Material Properties and Interfacial Friction.- 2.1 Resistance to Deformation.- 2.1.1 Empirical Constitutive Relations for Carbon Steels.- 2.1.2 High Strength Low Alloy Steels.- 2.1.3 Representation of the Results of Compression Tests.- 2.2 Friction in the Flat Rolling Process.- 2.2.1 Friction as a Function of Process and Material Parameters.- 2.2.2 Interfacial Forces in the Roll Gap During Flat Rolling.- 2.2.3 Discussion.- 3 One-Dimensional Models of Flat Rolling.- 3.1 Conventional Models.- 3.1.1 Orowan's Theory.- 3.1.2 Sims' Method.- 3.1.3 Bland and Ford's Technique.- 3.1.4 Ford and Alexander's Method.- 3.1.5 Tselikov's Solution.- 3.2 Refinement of the Conventional Models.- 3.2.1 Derivation of the Basic Equations.- 3.2.2 Roll Deformation.- 3.2.3 Roll Force and Roll Torque.- 3.3 Comparison of Mathematical Models of Flat Rolling.- 3.3.1 Comparison of Assumptions in Various Models.- 3.3.2 Statistical Analysis of Predictive Capabilities.- 3.4 Further Substantiation of the Predictive Capabilities of One- Dimensional Models.- 4 Thermal-Mechanical Finite-Element Modelling of Flat Rolling.- 4.1 Rigid-Plastic Finite-Element Method.- 4.1.1 Description of the Method.- 4.1.2 Boundary Conditions.- 4.1.3 Structure of the Computer Program.- 4.2 Heat Transfer.- 4.2.1 Numerical Solution of the Fourier Equation.- 4.2.2 Steady State Model With Convection.- 4.3 Results of Computations.- 4.3.1 Hot Rolling.- 4.3.2 Cold Rolling.- 4.4 Experimental Substantiation of the Model's Predictions.- 4.4.1 Hot Rolling of Steel.- 4.4.2 Warm Rolling of Aluminium.- 4.4.3 Cold Rolling of Aluminium.- 4.5. Role of the Heat Transfer Coefficient.- 5 The Role of the Shape Coefficient in Modelling of the Flat Rolling Process.- 5.1 Correlation between the Shape Coefficient and Load Parameters.- 5.1.1 The Effect of ? on the Friction Stresses.- 5.1.2 The Effect of ? on Forces and Torques.- 5.2 The Effect of the Shape Coefficient on Strain and Strain Rate Distributions.- 5.2.1 Finite-Element Analysis of Strain Rate Fields for Various Shape Coefficients.- 5.2.2 Efficiency of the Rolling Process.- 6 Conclusions.- References.- Author Index.

The physics of superplastic deformation

Abstract: Superplasticity is an important mode of deformation in metallic alloys with very small grain sizes (usually less than 10 μm). In general, high elongations are observed over a rather limited range of intermediate strain rates and there is a decrease in the superplastic effect at both high and low strain rates. The major experimental observations in superplastic metals are summarized and the physical mechanisms of flow are discussed with reference to the behavior at high, intermediate and low strain rates, respectively. Superplastic-like behavior has been reported recently in some ceramics but the experimental evidence suggests that the mechanism of flow in these materials in not the as in metals.

Plastic deformation of single crystals of Ti3Al with D019 structure

Abstract: Single crystals have been successfully grown by floating-zone melting of an intermetallic compound Ti3Al which has the β- to α-phase transformation at about 1453 K. Single-crystal specimens of three major orientations were compressed in a temperature range from 300 to 1273 K and examined for the deformation characteristics of the three slip modes, namely {1100}〈1 120〉 prism slip, (0001)〈1 120〉 basal slip and {1122}〈1 123〉 pyramidal slip. Deformation morphology and dislocation arrangements were studied by optical microscopy and transmission electron microscopy respectively. The deformation behaviour and dislocation arrangements, which are very different from those reported on polycrystalline materials, are presented and discussed.

The deformation and fracture of constrained metal sheets

Abstract: A brittle solid can be toughened by dispersing ductile inclusions in it. The degree of toughening depends on the properties, volume fraction and size of the ductile inclusions and on the strength of the interface between inclusion and matrix. Experiments and models are described which quantify the influence of interface strength. The experiments use a sandwich of lead bonded between two glass plates, in such a way that debonding can be controlled. Cracks are introduced into the glass, and the behaviour of the lead at and near the crack tip is examined. The way in which debond and crack-orientation influence the work of fracture is explored.

Deformation of rock: a pressure-sensitive, dilatant material

Abstract: Permanent (plastic) deformation of rock materials in the brittle regime (cataclastic flow) is modelled here in terms of Mohr-Coulomb behaviour in which all three of the parameters cohesion, friction angle and dilation angle follow hardening (or softening) evolution laws with both plastic straining and increases in confining pressure. The physical basis for such behaviour is provided by a sequence of uniaxial shortening experiments performed byEdmond andPaterson (1972) at confining pressures up to 800 MPa on a variety of materials including Gosford sandstone and Carrara marble. These triaxial compression experiments are important for the large range of confining pressures covered, and for the careful recording of data during deformation, particularly volume change of the specimens. Both materials are pressure-sensitive and dilatant. It is therefore possible to derive from these experiments a set of material parameters which allow a preliminary description of the deformation behaviour in terms of a non-associated, Mohr-Coulomb constitutive model, thus providing the first constitutive modelling of geological materials in the brittle-ductile regime. These parameters are used as input to a finite difference, numerical code (FLAC) with the aim of investigating how closely this numerical model simulates real material behaviour upon breakdown of homogeneous deformation. The mechanical and macrostructural behaviour exhibited by the numerical model is in close agreement with the physical results in that the stress-strain curves are duplicated together with localization behaviour. The results of the modelling illustrate how the strength of the upper-crust may be described by two different but still pressure-dependent models: the linear shear stress/normal stress relationship of Amontons (that is, Byerlee's Law), and a non-linear, Mohr-Coulomb constitutive model. Both include parameters of friction and both describe brittle deformation behaviour. Consideration of the non-linear bulk material model allows investigation of the geologic regimes which favour localization of the deformation over a continuing homogeneous deformation. More complex models are required to describe the deformation of rock with increasing depth and temperature as the behaviour becomes increasingly temperature-sensitive and rate-dependent.

A dynamic indentation technique for the characterization of the high strain rate plastic flow behaviour of ductile metals and alloys

Abstract: The objective of the paper is to describe a dynamic indentation (DI) technique suitable for the determination of the high strain rate flow behaviour of ductile metals and alloys and illustrate its use by characterizing the high strain rate flow behaviour of iron and OFHC copper. The DI technique is first described in detail and the dynamic hardness-strain data of iron and copper obtained using the technique is presented. It is also demonstrated that it is a suitable technique for characterizing the high strain rate flow behaviour as long as certain validity conditions are met. It is shown that these validity conditions are fully met in the case of copper and at low strain levels in iron. The reliability of the DI technique is finally demonstrated by comparing the present data with the literature data on similar materials and finally a critique of the DI technique is provided.

Fracture mechanisms of fiber-reinforced titanium alloy matrix composites Part III: Toughening behavior

Abstract: The damage mechanisms of several notched unidirectional SCS-6 fiber-reinforced titanium alloy matrix composites tested under three-point bending are evaluated. The key microstructure parameters which dominate the load-deflection curve, crack tip initiation, crack tip damage growth and fracture behavior of these composites are discussed. The role of the fiber-matrix interface in crack initiation and propagation is also examined. Results indicate that the crack initiation energy is affected by the fiber strength, matrix yield strength or shear strength. The crack propagation energy is controlled by matrix phase deformation, multiple fiber fracture and fiber pull-out.

Tensile deformation and fracture toughness of 2014 + 15 vol pct SiC particulate composite

Abstract: Tensile tests and fracture toughness experiments were conducted on 2014 aluminum alloy with 15 vol pct SiC particulate in a stage which fit within a scanning electron microscope. Strains associated with tensile deformation and the transition from slow to rapid crack growth were determined using the stereoimaging technique. Overall tensile elongations were measured at 1.6 to 2.4 pct, while localized strains were up to ≈50 pct but depended on the dispersion of SiC particles. Measured fracture toughness values ranged from 18.7 to 29:5 √m. Fractography revealed the virtual absence of dimpled rupture on both types of specimens. The fracture toughness values measured could be accounted for by computing the work done in forming new crack surfaces. To do this, the strain gradients determined during the tests were used in a previously developed model. Matrix ductility and particle dispersion are identified as the factors controlling toughness.

Theoretical model for the tensile work hardening behaviour of dual-phase steel

Abstract: The modified Crussard-Jaoul analysis was employed to describe the work hardening behaviour (the ln(dσ/dϵ) vs. ln σ curves) of a 1020 dual-phase steel with quenching and quenching plus tempering treatments and with various volume fractions of martensite ( V m ), which demonsrated that this dual-phase steel exhibits two stages of work hardening in the range of plastic deformation. The modified law of mixture was used to simulate the tensile stress-strain and the ln(dσ/dϵ) vs. ln σ curves for the steel. The simulations were divided in different ways in terms of the deformation state of martensite. For the steel with quenching treatment and with V m V m > 50% and the steel with quenching plus tempering treatments and with V m over the whole test range (33–85%), the work hardening behaviour in the first stage of deformation was well simulated with the assumption that martensite deforms elastically and then deforms partly elastically and partly plastically while ferrite deforms plastically. For the steel in all the above-mentioned cases, the work hardening behaviour in the second stage of plastic deformation was simulated with a model in which both phases deform plastically.

Localized deformation of SiCAl composites

Abstract: Tensile tests of SiC-6061 Al composites containing various volume fractions of whiskers or particles (20, 5 and 0 vol.%) showed that for samples containing a high volume fraction (20 vol.%) the fracture process was very localized, i.e. a very narrow neck. As the volume fraction of whiskers or particles decreased, the deformed region spread out. One might expect that the microstructure should correspond to the macroscale changes. In the highly deformed region the dislocation density is expected to be higher, in the less deformed regions the dislocation density should be lower, and if the deformation is very localized, then the high dislocation density should also be limited to a very narrow region. Overall, there is good agreement between the microstructure (dislocation density) change and the macroscale deformation of SiCAl composite tensile samples. The mechanism proposed to account for this change in deformation behavior as a function of volume fraction of SiC in aluminum is related to the expansion of the plastic zone (due to differences in thermal coefficients of expansion between SiC and aluminum) when the external stress is applied. Also, the localized deformation is related to localized clusters of SiC particles. There is a cooperative effect which leads to a region of very localized plastic deformation.

Role of surface morphology in wafer bonding

Abstract: The strain patterns detected by x‐ray topography in wafers bonded for silicon‐on‐insulator (SOI) technology were found related to the flatness nonuniformity of the original wafers Local stresses due to the bonding process are estimated to be about 1×108 dynes/cm2 The stress is reduced about 100 times for the thin (05 μm) SOI films Most of the wafer deformation occurs during room temperature mating of the wafers The deformation is purely elastic even at 1200 °C The magnitude of the stress appears insignificant for complimentary metal‐oxide‐semiconductor devices performance

An analysis of the effect of residual stresses on deformation and damage mechanisms in AlSiC composites

Abstract: The effect of thermally induced residual stresses on the mechanical properties and ductility of AlSiC composites was investigated numerically. The predicted behavior in uniaxial loading was calculated with and without the inclusion of the residual stresses which result from the mismatch in thermal expansion between aluminum and SiC. In this analysis, void nucleation by interfacial debonding at the whiskers' ends was assumed to be the limiting failure mechanism. Two cases, both with a fiber volume fraction of 20% and fiber aspect ratio of 4, but with different fiber spacings, were considered. The residual stresses had a small effect on the predicted ductility of the composite, even when a relatively weak interface strength was assumed. The residual stresses are shown to redistribute as interfacial failure is approached. A close end-to-end fiber spacing gives a greater flow strength in compression than in tension and the residual stresses which arise during thermomechanical processing tend to enhance this effect.