Showing papers on "Deformation (engineering) published in 1990"

Evolution of dislocation densities and the critical conditions for the Portevin-Le Châtelier effect

Abstract: A model is proposed for the critical strains associated with the Portevin-Le Châtelier effect (PLC) in terms of the strain dependence of the densities of mobile and forest dislocations. The classical critical condition for the onset of the PLC effect, viz. that of vanishing of the strain rate sensitivity of flow stress under the influence of dynamic strain aging is reexamined. The analysis takes into account the strain dependence of a key quantity: the elementary strain produced when all mobile dislocations perform a successful thermally activated step through the forest obstacles. This elementary strain is estimated by studying a system of coupled differential equations for the evolution of the two densities. Results are obtained in semi-quantitative form and compared with available data. It is shown that the following effects are consistently explained: the occurrence of critical strains for the onset and termination of jerky flow, occasional observation of two PLC regimes within the same deformation curve, the behaviour of the critical strains at high strain rates and low temperatures and, possibly, the particular behaviour exhibited by some alloys at low strain rates and high temperatures. Consequences for the “friction” and “forest” models of dynamic strain aging are discussed.

The brittle compressive fracture of ice

Abstract: The brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied. Measurements are reported of the fracture stress at temperatures from −10 to −50°C at strain rates of 10 −3 and 10 −1 s −1 for grain sizes from approximately 1 to 10 mm. Also a summary is reported of measurements by Jones et al . (unpublished) of the kinetic coefficient of friction for ice on ice at temperatures from −10 to −40°C at sliding velocities from 5 × 10 −7 m s −1 to 5 × 10 −2 ms −1 . Observations via high speed photography of internal cracking during loading are included. The strength, albeit scattered, increases with decreasing grain size, with decreasing temperature and at −10°C with decreasing strain rate. Similarly, the coefficient of friction increases with decreasing temperature and at −10°C with decreasing sliding velocity. Wing cracks were observed on some inclined cracks nucleated during loading. The results are explained in terms of the frictional crack sliding-wing crack model [as developed by Ashby and Hallam, Acta metall. 34, 497 (1986)] of compressive fracture. Finally, a simple model is presented for the transition from ductile to brittle behavior. It is based upon the competition between the building up and the relaxation of internal stresses within the vicinity of the internal cracks, and it leads to a transition strain rate which can be expressed in terms of the fracture toughness, the creep rate, the kinetic coefficient of friction and the microstructural scale of the material.

Structural changes during high-energy ball milling of iron-based amorphous alloys: Is high-energy ball milling equivalent to a thermal process?

Abstract: We report the first study of the effect of high-energy mechanical deformation on amorphous iron-based metallic alloys. The structural changes happening in amorphous iron-based materials containing Co or Ni during mechanical deformation show that the structural stability of an amorphous alloy against a thermal and a mechanical process are not related. Therefore, the concept of a high local effective temperature during the milling process cannot be singled out as the only reason for the observed structural transformations.

Tensile properties of short fiber-reinforced SiC/Al composites: Part II. Finite-element analysis

Abstract: The tensile properties of aluminum matrix composites containing SiC whiskers or particulate were investigated analytically and compared to experimental results. Two finite-element models were constructed and used for elastoplastic analysis. In both models, the SiC fibers are represented as longitudinally aligned cylinders in a three-dimensional array. The cylinder ends are transversely aligned in one model and staggered in the other. Using the models, the sensitivity of the predicted composite properties to the deformation characteristics of the matrix alloy was examined, and the general behavior of the models was validated. It was determined that both models are necessary to predict the overall composite stress-strain response accurately. The analytic results accurately predict: the observed composite stress-strain behavior; the experimentally observed increase in Young’s modulus and the work-hardening rate with increasing fiber volume content and aspect ratio; and the decrease and subsequent increase in proportional limit as the SiC volume fraction is increased. The models also predict that the transverse material properties should be insensitive to fiber aspect ratio. In addition, the model predicts the location of initial yielding and the propagation of the plastic region. These results offer insights into the deformation mechanisms of short fiber-reinforced composites.

Effects of particles on development of microstructure and texture during rolling and recrystallisation in fcc alloys

Abstract: Precipitation causes changes in the modes of plastic deformation and recrystallisation, as well as in the development of rolling and recrystallisation textures. The nature of these changes varies strongly with the size, shape, and density of the particles and, in particular, on whether the particles are cut by dislocations during deformation. This leads to numerous phenomena, some rather surprising, which are systematised, established through many new experimental results (mainly on aluminium alloys), and discussed.MST/1293

Deformation Processes in Minerals, Ceramics and Rocks

Abstract: An introduction.- References.- 1 Fracture and failure of brittle polycrystals: an overview.- 1.1 Introduction.- 1.2 Linear elastic fracture mechanics (LEFM).- 1.3 Quasi-static fracture and failure.- 1.4 Inelastic fracture processes.- 1.5 Fracture and failure prediction.- 1.6 Indirect monitoring of fracture.- 1.7 Concluding remarks.- Acknowledgements.- References.- 2 Single-crack behaviour and crack statistics.- 2.1 Introduction.- 2.2 Single-crack behaviour.- 2.3 Multiple-crack behaviour.- 2.4 Crack statistics: percolation models.- 2.5 Conclusions.- References.- 3 Fracture of polycrystalline ceramics.- 3.1 Introduction.- 3.2 Flaw size/grain size effects.- 3.3 Toughening mechanisms.- 3.4 Environmental effects.- 3.5 Implications of crack-interface tractions on failure predictions.- 3.6 Concluding remarks.- References.- 4 Compressive brittle fracture and the construction of multiaxial failure maps.- 4.1 Introduction.- 4.2 The isolated crack in an infinite plate.- 4.3 Crack interaction.- 4.4 Multi-axial failure maps.- 4.5 Summary and conclusions.- Acknowledgements.- References.- 5 Brittle-to-ductile transitions in polycrystalline non-metallic materials.- 5.1 Introduction.- 5.2 Effect of temperature (T) and strain rate ($${\dot \varepsilon }$$) on plastic flow and brittle cleavage.- 5.3 Effect of tension, compression, and confining pressure (stress triaxiality) on plastic flow and brittle cleavage.- 5.4 Transition from fracture to plastic flow.- 5.5 Discussion and conclusions.- References.- 6 Regimes of plastic deformation - processes and microstructures: an overview.- 6.1 Introduction.- 6.2 The phenomenology of creep.- 6.3 Creep mechanisms.- 6.4 The classification of deformation reAn introduction.- References.- 1 Fracture and failure of brittle polycrystals: an overview.- 1.1 Introduction.- 1.2 Linear elastic fracture mechanics (LEFM).- 1.3 Quasi-static fracture and failure.- 1.4 Inelastic fracture processes.- 1.5 Fracture and failure prediction.- 1.6 Indirect monitoring of fracture.- 1.7 Concluding remarks.- Acknowledgements.- References.- 2 Single-crack behaviour and crack statistics.- 2.1 Introduction.- 2.2 Single-crack behaviour.- 2.3 Multiple-crack behaviour.- 2.4 Crack statistics: percolation models.- 2.5 Conclusions.- References.- 3 Fracture of polycrystalline ceramics.- 3.1 Introduction.- 3.2 Flaw size/grain size effects.- 3.3 Toughening mechanisms.- 3.4 Environmental effects.- 3.5 Implications of crack-interface tractions on failure predictions.- 3.6 Concluding remarks.- References.- 4 Compressive brittle fracture and the construction of multiaxial failure maps.- 4.1 Introduction.- 4.2 The isolated crack in an infinite plate.- 4.3 Crack interaction.- 4.4 Multi-axial failure maps.- 4.5 Summary and conclusions.- Acknowledgements.- References.- 5 Brittle-to-ductile transitions in polycrystalline non-metallic materials.- 5.1 Introduction.- 5.2 Effect of temperature (T) and strain rate ($${\dot \varepsilon }$$) on plastic flow and brittle cleavage.- 5.3 Effect of tension, compression, and confining pressure (stress triaxiality) on plastic flow and brittle cleavage.- 5.4 Transition from fracture to plastic flow.- 5.5 Discussion and conclusions.- References.- 6 Regimes of plastic deformation - processes and microstructures: an overview.- 6.1 Introduction.- 6.2 The phenomenology of creep.- 6.3 Creep mechanisms.- 6.4 The classification of deformation regimes and mechanical properties.- 6.5 The structure of grain boundaries and interfaces between phases, and the nature of intergranular films.- 6.6 Microstructures, preferred orientation, and models of crystalline plasticity.- 6.7 Concluding remarks.- Acknowledgements.- References.- 7 Experimental deformation and data processing.- 7.1 Introduction.- 7.2 The traditional methods.- 7.3 A global inversion method.- 7.4 An example of creep data processing.- 7.5 Conclusion.- Acknowledgements.- References.- 8 Experimental studies of deformation mechanisms and microstructure in quartzo-feldspathic rocks.- 8.1 Introduction.- 8.2 Experimental techniques.- 8.3 Experimental deformation of quartz aggregates.- 8.4 Experimental deformation of feldspar aggregates.- 8.5 Experimental deformation of quartzo-feldspathic aggregates.- 8.6 Summary.- Acknowledgements.- References.- 9 Microstructural analysis and tectonic evolution in thrust systems: examples from the Assynt region of the Moine Thrust Zone, north-west Scotland.- 9.1 Introduction.- 9.2 Deformation histories in thrust systems.- 9.3 The Moine Thrust Zone of north-west Scotland.- 9.4 The Moine Thrust at the Stack of Glencoul.- 9.5 The Assynt Thrust sheet.- 9.6 The Sole Thrust System.- 9.7 Discussion.- 9.8 Conclusions.- Acknowledgements.- References.- 10 Mechanisms of reaction-enhanced deformability in minerals and rocks.- 10.1 Introduction.- 10.2 Experimental data.- 10.3 Field and petrographic data.- 10.4 Deformation mechanisms.- 10.5 Conclusions.- Acknowledgements.- References.- 11 Thermodynamics of rock deformation by pressure solution.- 11.1 Introduction.- 11.2 Microscale continuum theory of constituent behaviour.- 11.3 Thermodynamic framework for a macroscale theory of aggregate behaviour.- 11.4 Intergranular pressure solution as a stationary non-equilibrium process.- 11.5 Concluding summary.- Acknowledgements.- References.- 12 Densification of crystalline aggregates by fluid-phase diffusional creep.- 12.1 Introduction.- 12.2 The model.- 12.3 Experiments on sodium chloride.- 12.4 Discussion.- 12.5 Summary.- Acknowledgements.- References.- 13 Dynamic recrystallization and grain size.- 13.1 Introduction.- 13.2 Dynamic recrystallization by migration: mechanisms and models.- 13.3 Microstructural measurements as a function of stress.- References.- 14 Simulation of dislocation-assisted plastic deformation in olivine polycrystals.- 14.1 Introduction.- 14.2 Slip systems.- 14.3 Texture predictions from Taylor theory.- 14.4 Texture predictions from self-consistent theory.- 14.5 Discussion.- Acknowledgements.- References.- 15 On the slip systems in uranium dioxide.- 15.1 Introduction.- 15.2 Effects of non-stoichiometry on microhardness, flow stress, and preferred slip system.- 15.3 The structure of half-slipped dislocations.- 15.4 Questions for future work.- Acknowledgement.- References.- 16 A TEM study of dislocation reactions in experimentally deformed chalcopyrite single crystals.- 16.1 Introduction.- 16.2 Dislocation reactions in crystals strained at 200 C.- 16.3 Dislocation reactions in crystals strained at 400 C.- 16.4 Discussion.- 16.5 Summary and conclusions.- Acknowledgements.- References.

Shape Memory Alloys

Abstract: THE TERM SHAPE MEMORY ALLOYS (SMA) is applied to that group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory. Some materials also undergo a change in shape upon recooling. These materials have a two-way shape memory. Although a relat ively wide variety of alloys are known to exhibit the shape memory effect, only those that can recover substantial amounts of strain or that generate significant force upon changing shape are of commercial interest . To date, this has been the nickel-t i tanium alloys and copper-base al loys such as Cu-Zn-Al and Cu-AI-Ni. A shape memory alloy may be further defined as one that yields a thermoelastic martensite. In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent phase.

Large-Strain Bauschinger Effects in Fcc Metals and Alloys

Abstract: We have performed Bauschinger experiments on a variety of fcc metals and alloys, after large amounts of prestrain, using torsion and a short thin-walled tube geometry. The materials we studied were 99.99 pct Al, OFE copper, 70:30 brass, Al-1 pct Mg, Al-2 pct Mg, Al-0.17 pct Fe-0.07 pct Si, Al-0.8 pct Mn, and two Al-Cu alloys (Al-2.6 pct Cu and Al-4 pct Cu) given different heat treatments. For the material systems other than the Al-Cu alloys, the stress reversal was after a prestrain in shear of ≈3.0. Two stress reversals were performed on the Al-Cu alloys. The first was at γ = 0.3 and the second at γ = 1.2. Thus, for the Al-Cu, the prestrain and the final increment of deformation were in the same direction. The Bauschinger yield stress in these experiments was characterized by a very large offset shear strain of 0.05. This definition of reverse yield minimizes the effects of heterogeneous deformation and long-range internal elastic stresses that arise mainly from second-phase particles. We attributed the effects we observed to “isotropic hardening” associated with the dislocation substructures that developed in the different materials. We found that the behavior of these materials could be divided into two categories: those which deform by planar slip and those that form a “cell” structure and are characterized as having wavy slip. When the deformation was wavy in nature, we attributed the observed Bauschinger effects to be a result of the untangling of the “cells” formed during the prestrain. Different morphologies of cells had different behaviors when the stress was reversed. The behavior of the planar slip alloys depended on whether or not the barriers to dislocation activity were rigid or shearable. The θ′ precipitates in the Al-Cu alloys and the twin boundaries in the 70:30 brass constituted rigid barriers to dislocation motion, and a very large Bauschinger effect was observed. The solid solution Al-Cu material and that containing Guinier-Preston (GP) zones and θ″ had almost no Bauschinger effect when the yield stress in reverse deformation was considered. After yield, these materials hardened very rapidly and the flow stress in the reverse direction exceeded that for the equivalent amount of monotonie deformation.

Microscopic observations of adiabatic shear bands in three different steels

Abstract: Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈103/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described.

The axisymmetric deformation of a red blood cell in uniaxial straining Stokes flow

Abstract: The axisymmetric deformation of a red blood cell placed in a uniaxial straining Stokes flow is considered. The cell is modelled as a fluid capsule that contains a Newtonian fluid, and is bounded by an area-preserving membrane with negligible resistance to bending. First, it is theoretically demonstrated that spheroidal cells with isotropic membrane tension constitute stationary configurations. To compute transient cell deformations, a numerical procedure is developed based on the boundary-integral method for Stokes flow. Calculations show that initially prolate or oblate cells with isotropic membrane tension deform into stationary spheroids. Cells with a highly oblate initial shape may develop a persistent small pocket along their axis during the deformation. The shear elasticity of the membrane prevents folding, but may cause the formation of sharp corners and concave regions along the cell contour. A decrease in the membrane shear elasticity results in substantial increase in the magnitude of the transient and asymptotic membrane tensions. The maximum strain rate below which a red blood cell remains intact is estimated to be ex = 105 s−1.

The structure and deformation behaviour of poly( p -phenylene benzobisoxazole) fibres

Abstract: The relationship between structure and mechanical properties in as-spun and heat-treated high modulus poly (p-phenylene benzobisoxazole) (PBO) fibres has been examined using a combination of electron microscopy, mechanical testing, and Raman microscopy. The structure of the fibres has been determined by obtaining longitudinal sections, and electron diffraction has shown that skin regions are significantly more oriented than the fibre cores. Heat treatment of the fibres at elevated temperatures produces an improvement in the level of crystallinity especially in core regions. Heat treatment also produces an increase in fibre modulus but for fibres heat treated at 650° C there is a significant decrease in strength compared with ones heat treated at 600° C. Well-defined intense Raman spectra were obtained from individual fibres and three main bands at 1280, 1540 and 1615cm−1 have been identified. All three bands are sensitive to the level of applied strain with the 1280cm−1 being the most sensitive, shifting by-7.9cm−1% strain for PBO fibres heat treated at 600° C. The dependence of the sensitivity of the position of the 1615cm−1 band to strain upon fibre structure has been examined in detail. The rate of shift of band position with strain increases with fibre modulus. It is shown that these shifts in Raman bands are a direct reflection of molecular deformation within the fibres.

Compressional behaviour of carbon fibres

Abstract: Spectroscopic-mechanical studies have been conducted on a range of carbon fibres by bonding single filaments on the top surface of a cantilever beam. Such a loading configuration allows the acquisition of the Raman spectrum of carbon fibres and the derivation of the Raman frequency strain dependence in tension and compression. Strain hardening phenomena in tension and strain softening phenomena in compression were closely observed. The differences in the slopes of the Raman frequency versus applied strain curves in tension and compression respectively, have been used to obtain good estimates of the compression moduli. A method of converting the fibre Raman frequency versus strain data into stress-strain curves in both tension and compression, is demonstrated. Values of fibre stress and fibre modulus at failure in compression compare exceptionally well with corresponding estimates deduced from full composite data. The mode of failure in compression has been found to depend upon the carbon fibre structure. It is demonstrated that certain modifications in the manufacturing technology of PAN-based fibres can lead to fibres which show resistance to catastrophic compressive failure without significant losses in the fibre compressive modulus.

Internal Stresses, Dimensional Instabilities and Molecular Orientations in Plastics

Abstract: Part 1 Dimensional instabilities due to volume relaxation: general outline of volume relaxation volume-relaxation data for a number of polymers consequences for optical and electrical properties methods to reduce the volume-relaxation effects. Part 2 Dimensional instability due to the stresses acting during processing: linear viscoelastic theory viscoelastic behaviour of amorphous polymers the non-isothermal viscoelastic theory the recovery of large frozen-in deformation at temperatures below Tg theory of shrinkage stresses. Part 3 Residual thermal stresses due to rapid inhomogeneous cooling: the origin of cooling stresses elementary method for calulating cooling stresses in amorphous polymers (generalized Aggarwala-Saibel theory) refinements of the elementary theory test of the theory for chilled flat plates calculations for some other geometries. Part 4 Molecular orientations due to processing the definition of orientation the distinction with (frozen-in) strains discussion of the classical picture on anisotrophy effects in oriented solid polymers experiments on samples with a large frozen-in tensile strain experiemtns on samples with a large frozen-in simple shear the thermal shearing effect experiments on samples with a large frozen-in twist thermal twisting relation between deformation conditions and degree of frozen-in orientation frozen-in entropic stresses.

A molecular dynamics model of the orthogonal cutting process

Abstract: Material removal, as found in the cutting and grinding processes, is important in many areas of fabrication technology. Examples include high-speed machining by single point turning, surface milling and drilling. An idealized model that contains the essential physics of these processes is the two-dimensional orthogonal cutting geometry. Following the work of Shaw, an attempt is made to obtain a fundamental understanding of the commonly observed phenomena in the chip forming process: the atomistic mechanisms of plastic deformation within the primary shear zone and the observed shear angle ({phi}); the resultant energy flow from the tool doing work; the stress, strain and material flow fields; the morphology of the chip (smooth, segmented, crystalline or amorphous); the role of different depths of cut (t), rake angles ({alpha}), and clearance angles ({theta}); and the role of the tool tip radius, roughness (friction), and wear. The molecular dynamics computer simulation method is an ideal tool for the study of the atomistic mechanisms of these deformation processes. Simulations are performed for a model of copper at room temperature using 10{sup 3}--10{sup 6} atoms, depths of cut in the range l--50nm. and cutting speeds of 1--2500 meters per second (m/s). These simulations are strictly two dimensionalmore » (plane strain). Here, focus is on the chip forming process, the atomistic mechanisms of plastic deformation, the flow of energy into the chip and workpiece, and the material flow and stress fields for the orthogonal geometry ({alpha} = 0), sharp tools, and shallow depths of cut (t {approx} 5 nm). 5 refs., 3 figs.« less

The mechanical properties of laminated microscale composites of Al/Al2O3

Abstract: The tensile properties, at both room and elevated temperatures, of laminated thin films containing alternate layers of aluminium and aluminium oxide were investigated. At room temperature the strength of the films followed a Hall-Petch type relationship dependent on the interlamellar spacing, and the strength could be extrapolated from data for conventional grain size aluminium. At the finest interlayer spacing of 50 nm, the strength was equivalent toμ/70, whereμ is the shear strength of aluminium and the samples exhibited very extensive ductility. At elevated temperatures, cavitation became an important deformation mechanism but it occurred preferentially at Al/Al rather than Al/Al2O3 boundaries. The microstructure of the films was probed using transmission electron microscopy and fractography was used to investigate deformation and fracture mechanisms.

Structure, tensile deformation, and fracture of a Ti 3 Al-Nb alloy

Abstract: The development of Ti3Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α2 phase (based on Ti3Al, DO19) and βo, (based on Ti2NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D019 (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α2 phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α2 and βo phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the βo phase. A rationale incorporating the failure modes of the two phases—cleavage of α2 and slipband decohesion of βo—has been evolved to explain the trends in ductility with heat treatment.

The plasticity of particle-containing polycrystals

Abstract: The lattice misorientations adjacent to second-phase particles of silicon in a polycrystalline aluminium matrix deformed in compression have been measured by a TEM microtexture technique. The results have been analyzed in terms of a simple model which is based on a modification of Taylor's polycrystalline plasticity theory. A model in which independent deformation zones are formed in the vicinity of each particle for each active slip system, gives reasonable agreement with the experiments if overlap of the deformation zones is taken into account. The number of deformation zones formed at a particle and the size of the misorientation are found to be functions of particle size as well as strain.

Capture and Rebound of Small Particles Upon Impact with Solid Surfaces

Abstract: A new particle bounce model has been proposed and compared with the previously published experimental data. By considering the relationship between contact deformation mechanics and contact surface energy, it is possible to calculate both the energy required to break the contact surface and the unrecoverable energy contained in the bulk plastic deformation zone when the material yield point is exceeded. Energy spent to deform the local asperities between the contact surfaces was also found to be important when the incident kinetic energy is small. The present model considers the breaking of the contact surface, plastic deformation, and local asperity deformation, to be the major energy loss mechanisms during the impaction process. Both the elastic and plastic deformations have been treated, thus making it possible to calculate the coefficient of restitution over a wide range of particle incident speeds.

Inhomogeneous deformation in INCONEL 718 during monotonic and cyclic loadings

Abstract: The paper concentrates on the relation between microstructural observations of the dislocation structures and the macroscopic deformation responses of both aged and homogenized precipitate-hardened alloys at room temperature. The deformation responses are compared to the cyclic deformation response of an aged precipitate-hardened alloy. Early in the deformation, one deformation band per grain and little evidence of work hardening are observed; with increased deformation, work hardening begins, more bands nucleate, and their spacing becomes similar to that in the aged material. It is pointed out that the degree of coarseness of inhomogeneous deformation is not a result of a softening process within the bands due to precipitate shearing, but it is a function of the amount of work hardening within the bands.

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part I. The influence of phase stresses on the Bauschinger effect

Abstract: The Bauschinger effect (BE) in dual-phase steels has been computationally simulated, and the influence of phase stresses, developed due to nonhomogeneous deformation during preloading, on the BE has been investigated. Isotropic-and anisotropic-hardening models were used in finite-element method calculation to produce the reverse flow stress-strain curves (compression) of dual-phase steels from the reverse stress-strain curves of single-phase materials. Aspects of the Bauschinger effect, including the rounding of the reverse flow curve, yielding at low reverse stresses, high initial work-hardening rates, and the absence of permanent softening,etc., were elucidated by the variation in phase stresses in the constituent phase.

Strength and deformation characteristics of reinforced concrete walls under load reversals

Abstract: The effects of loding history and repair methods on the structural characteristics of reinforced concrete walls was investigated. Large scale slender wall models were tested to failure, then unloaded, repaired, and retested to destruction under various regimes of cyclic horizontal loading. It was found that, while repairing only the damaged regions of the compressive zone was sufficient to fully restore wall strength, the additional use of epoxy resins to heal major flexural and inclined web cracks led only to a marginal improvement of the structural characteristics, the latter being distinctly inferior to those of the original walls. Such results are in compliance with the concept of the compressive force path and demonstrate that, in contrast to widely held views, the compressive zone is the main contributor to shear resistance.

Review of the microscopic and macroscopic aspects of fracture of unmodified and modified epoxy resins

Abstract: Epoxy resins are useful as adhesives and as matrix materials in advanced composites. However, unmodified (or neat) epoxies are brittle and their low fracture energy causes serious concern in some applications. For this reason, modified epoxies have been developed by the addition of a rubbery or a rigid dispersion phase, or both, and considerable improvement in fracture properties has been achieved. In this paper, the deformation and failure processes involved in the fracture of unmodified epoxies have been discussed, and a review of the fracture behaviour of the diglycidyl ether of bisphenol A polymers modified with carboxyl-terminated butadiene acrylonitrile is presented together with illustrations of the microscopic aspects of localized deformation and their relationship to the macroscopic fracture behaviour. The degree of improvement in fracture properties in modified materials depends to a great extent on the unmodified epoxy. If the latter is capable of even small-scale plastic deformation at the crack tip, then this induces in the modified system a number of additional microscopic failure mechanisms such as cavitation of rubber particles, enhanced shear deformation of the matrix, debonding and tearing of rubber, crack pinning, and debonding and pull-out of fibres.

Differential stress‐induced melt migration: An experimental approach

Abstract: A gradient in the dilatational component of a differential state of stress will cause migration of the melt phase in a texturally (quasi)equilibrated partial melt. An experimental approach to image such deformation-induced melt migration is presented. Two-phase, solid-liquid aggregate beams (prepared by a glass-ceramic technique) having a primary crystalline phase of MgSiO3 (orthoenstatite with a limited amount of clinoenstatite intergrowths) in chemical and textural equilibrium with a sodium aluminosilicate glass are subjected to four-point flexure; a first-order thermodynamic analysis, based on the energy balance between grain boundaries (solid-solid interfaces) and solid-liquid interfaces, indicates that the melt phase flows from that side of the specimen under a compressive principal stress to the specimen side under a tensile principal stress. When the solid-liquid aggregate is characterized by a Newtonian rheology (i.e., the deformation occurs via a solution-precipitation-enhanced diffusional creep mechanism), the melt migration is easily observed as a large deformation transient accompanying the flexural flow of a specimen. The melt migration is thus characterized as a completely recoverable, anelastic strain in the two-phase system; the rheology of the partially molten beams is well modelled by eT(t)=e0[1−exp(−Bt)]+e˙sst where eT is the total inelastic strain, e0 is the total anelastic strain due to melt migration, e˙ss is the steady-state strain rate for the two-phase aggregate, t is time and B is a function of either the viscosity of the liquid phase or of the rheology (viscosity) of the two-phase aggregate. In the experiments reported here, the melt migration is shown to be rate limited by the kinetics of compaction and/or dilation of the crystalline residuum. The impact of the experimental approach on compaction-based models of melt transport and segregation is discussed.

Compressive behavior and deformation-mode map of an open cell alumina

Abstract: Compressive behavior of an open cell, porous ceramic has been examined and compared to a prior theoretical model. The study involved (i) microstructural characterization, (ii) crushing strength and Young's modulus measurements, and (iii) construction of a deformation-mode map. Initially, the crushing behavior was found to be different than predicted theoretically. Weaker struts throughout the material fractured during the loading and this damage was accumulated until a macroscopic crack or cracks propagated through the material at the crushing stress. Further work showed the discrepancy was related to the uniformity of loading in these porous materials. The use of compliant faces on the loading rams improved the loading uniformity, leading to a substantial reduction in the experimental scatter and increasing the likelihood of unstable crack propagation events rather than damage accumulation. Both crushing strength and Young's modulus were found to be dependent on cell size, but this was considered to be a result of strut cracking at the smallest cell size. A deformation-mode map was constructed using the average stress/strain values at critical points such as the onset of crushing, the minimum crushing stress, and the densification stress. Although some of the details of the deformation map were different from that expected theoretically, the map did appear to be a useful guide to the compressive behavior.