Showing papers on "Deformation (engineering) published in 1988"

Theoretical stress strain model for confined concrete

Abstract: A stress‐strain model is developed for concrete subjected to uniaxial compressive loading and confined by transverse reinforcement. The concrete section may contain any general type of confining steel: either spiral or circular hoops; or rectangular hoops with or without supplementary cross ties. These cross ties can have either equal or unequal confining stresses along each of the transverse axes. A single equation is used for the stress‐strain equation. The model allows for cyclic loading and includes the effect of strain rate. The influence of various types of confinement is taken into account by defining an effective lateral confining stress, which is dependent on the configuration of the transverse and longitudinal reinforcement. An energy balance approach is used to predict the longitudinal compressive strain in the concrete corresponding to first fracture of the transverse reinforcement by equating the strain energy capacity of the transverse reinforcement to the strain energy stored in the concret...

An experimental study of the formation process of adiabatic shear bands in a structural steel

Abstract: A series of experiments is described in which the local temperature and local strain are measured during the formation of an adiabatic shear band in a low alloy structural steel (HY-100). The specimen employed consists of a short thin-walled tube and the required rapid deformation rates are imposed by loading the specimen in a torsional Kolsky bar (split-Hopkinson bar). The local temperature is determined by measuring the infrared radiation emanating at twelve neighboring points on the specimen's surface, including the shear band area. Indium-antimonide elements are employed for this purpose to give the temperature history during deformation. In addition, high speed photographs are made of a grid pattern deposited on the specimen's surface, thus providing a measure of the strain distribution at various stages during shear band formation. By testing a number of specimens, it is possible to form a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequent loss in the load carrying capacity of the steel. It appears that plastic deformation follows a three stage process which begins with a homogeneous strain state, followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. It is estimated that the shear band propagates at a speed of about 510 m/s in the material tested. Results also include data on the stress-strain behavior of HY-100 steel over the temperature range —190°C to 250°C and at quasi-static as well as dynamic strain rates.

Two-dimensional thermal oxidation of silicon. II. Modeling stress effects in wet oxides

Abstract: For pt.I see ibid., vol.ED-34, p.1008-17 (May 1987). The authors propose that the stress from two-dimensional oxide deformation affects the kinetic parameter in the Deal-Grove model (1965). In particular, the viscous stress associated with the nonuniform deformation of the oxide is identified as the fundamental force of retardation. In this model, the stress normal to the Si-SiO/sub 2/ interface reduces the surface reaction rate in both convex and concave surfaces, whereas the stress in the bulk of the oxide (compressive for concave and tensile for convex surfaces) is responsible for the thinner oxides on the concave structures. The model is described by a simplified mathematical formulation made possible by the symmetry in cylindrical structures. Comparisons with experimental data, possible applications, and limitations of the model are also discussed. >

Experimental deformation of partially melted granitic aggregates

Abstract: The effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions (900°C, 1500 MPa, 10-4 to 10-6/s) where melt-free samples deform by dislocation creep, with microstructures approximately equivalent to those of upper greenschist facies. Experiments were performed on samples of various grain sizes, including an aplite (150 μm) and sintered aggregates of quartz-albitemicrocline (10–50 and 2–10 μm). Water was added to the samples to obtain various amounts of melt (1–15% in the aplite, 1–5% in the sintered aggregates). Optical and TEM observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels, while the melt in the aplites is inhomogeneously distributed. The effect of partial melt on deformation depends an melt amount and distribution, grain size and strain rate. For samples deformed with ˜ 1% melt, all grain sizes exhibit microstructures indicative of dislocation creep. For samples deformed with 3–5% melt, the 150 μm and 10–50 μm grain size samples also exhibit dislocation creep microstructures, but the 2–10 μm grain size samples exhibit abundant TEM-scale evidence of dissolution-precipitation and little evidence of dislocation activity, suggesting a switch in deformation mechanism to predominantly melt-enhanced diffusion creep. At natural strain rates melt-enhanced diffusion creep would predominate at larger grain sizes, although probably not for most coarse-grained granites. The effects of melt percentage and strain rate have been studied for the 150 μm aplites. For samples with ˜ 5 and 10% melt, deformation at 10–6/s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep. Conversely, deformation at 10-5/s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results. For samples with 15% melt, deformation at both strain rates produces cataclasis, presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains, which would produce high stress concentrations at point contacts. At natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate. However, if the melt is confined and cannot migrate, a high pore fluid pressure should promote brittle deformation.

The low-temperature plastic deformation of α-titanium and the core structure of a-type screw dislocations

Abstract: α-titanium single crystals, containing three different amounts of interstitial impurities, have been tested in compression at temperatures between 77 and 700 K. The mechanical behaviour is characterized by a strong temperature dependence of the yield stress for slip on the primary prismatic slip plane (1010)[1210], a strong dependence of the critical resolved shear stress on the orientation of crystal axis and the failure of the Schmid law, an anomaly at temperatures between 300 and 500 K which is associated with cross-slip into a first-order pyramidal plane, and the occurrence of two hardening stages on the deformation curves at low temperatures. Both conventional and in situ transmission electron microscopy examinations revealed that a large lattice friction opposes the motion of a-type screw dislocations at temperatures below 550 K. Since edge dislocations are found to be highly mobile, it appears that, at low temperatures, the yield stress of pure α-titanium is governed by a Peierls force act...

Buckling and Postbuckling of Delaminated Composites Under Compressive Loads Including Transverse Shear Effects

Abstract: The deformation of delaminated composites under axial compression is analyzed by a one-dimensional beam-plate model. In this model, a formulation that accounts for the transverse shear effects is also presented. With the perturbation technique, analytical solutions for the critical instability load and the postbuckling deflections are obtained. All possible instability modes, namely, local delamination buckling, global plate buckling, and coupled global and local (mixed) buckling, are considered. Specific emphasis is placed on studying the transverse shear effects on both the critical load and the postcritical characteristics, as well as the influence of the geometry such as that of the location of the delamination across the thickness. The postbuckling solution is used in conjunction with a /-integral formulation to study the postcritical characteristics with respect to possible quasistatic extension of the delamination and the energy absorption capacity of a beam.

Defects in plastically deformed 6H SiC single crystals studied by transmission electron microscopy

Abstract: Transmission electron microscopic studies have been performed on lattice defects in 6H silicon carbide single crystals; the crystals were subjected to microhardness indentation tests between room temperature and 1200°C and then to uniaxial compression at temperatures from 1100 to 1650°C. It was observed that their basal dislocations, the main defects introduced by deformation, were dissociated into Shockley partials separated by very wide stacking faults, the energy of which was evaluated to be 2.5 ± 0·9 mJ m−2. The lineshapes of the partials varied considerably, depending on the sign and angle (with the dislocation line) of the Burgers vector; this suggests that the mobility of the partials is affected by the geometry of dislocations. The fact that basal dislocations were aligned more or less in crystallographic 〈1120〉 directions indicates the presence of a high Peierls potential. Macroplastic deformation was observed above 1000°C. This critical temperature seems to be determined by the motion o...

Geometric non‐linear substructuring for dynamics of flexible mechanical systems

Abstract: A substructure synthesis formulation is presented that permits use of established flexible multibody dynamic analysis computer codes to account for structural geometric non-linear effects. Large relative displacement is permitted between points within bodies that undergo small strain elastic deformation. Components are divided into substructures, on each of which the theory of linear elasticity relative to a body reference frame is adequate to describe deformation and its coupling with system motion. Normal vibration and static correction deformation modes are used to account for elastic deformation within each substructure. Compatibility conditions are derived and imposed as constraint equations at boundary points between substructures. System equations of motion that include geometric non-linear effects of large rotation, in terms of generalized co-ordinates of a reference frame for each substructure and a set of deformation modes that are defined within the substructure, are assembled. The method is implemented in an industry standard flexible multibody dynamics code, with minimal modification. Use of the formulation is illustrated on the classical problem of a spinning beam with geometric stiffening and on a space structure that experiences large deformation.

Relationship between interfacial strain and the elastic response of multilayer metal films.

Abstract: We have investigated the structural and elastic properties of three distinct multilayer systems: Mo/Ni, Pt/Ni, and Ti/Ni. We demonstrate that the commonly observed lattice expansion perpendicular to the film plane is not a bulk effect, but is localized at the interface between the contacting metals. Incorporating the measured interfacial expansion into a universal binding relation, we show that the measured elastic softening versus compositional-repeat distance can result from the interfacial strain.

Transformation Plasticity of CeO2-Stabilized Tetragonal Zirconia Polycrystals: II, Pseudoelasticity and Shape Memory Effect

Abstract: A macroscopic shape memory effect is demonstrated by first deforming a CeO2-stabilized tetragonal zirconia polycrystal (Ce-TZP) between the Mb and As temperatures and then recovering the shape change by heating above Ab. Shape changes effected above Ab are immediately recoverable during unloading, giving rise to a pseudoelastic behavior. Deformation texture is reversible when the shape strain recovers. Both Mb and Ab, along with the associated temperature range for these effects, are depressed to lower temperatures by grain refinement. Prior deformation widens the gap between the transformation temperatures. These observations demonstrate that the shape accommodation in Ce-TZP arises from twinning and elastic distortions. The operation of pseudoelasticity and shape memory effect is rationalized in terms of martensitic nucleation statistics, the stability of thermoelastic martensite, and internal stresses at the martensitic interface. The implications on transformation plasticity and transformation toughening are explored.

Heteroepitaxial Growth and the Effect of Strain on the Luminescent Properties of GaN Films on (11\bar{2}0) and (0001) Sapphire Substrates

Abstract: GaN films grown on (11 0) and (0001) sapphire substrates are characterized by X-ray Bond's method and the low temperature photoluminescence measurement. The GaN films are found to be strained by the biaxial compressive stress. From the measured strain and the shift of PL peak energy, the deformation potential of GaN (the relation between the strain parallel to the c-axis and the band gap energy) is found to be 12 eV. The origin of this compressive stress is discussed.

Shape-memory effect and pseudoelasticity associated with the R-phase transition in Ti-50·5 at.% Ni single crystals

Abstract: The rhombohedral phase (R-phase) transition and associated deformation behaviour in Ti-50·5 at.% Ni single crystals have been investigated systematically by tensile testing age-treated specimens in various orientations over a wide temperature range. Shape-memory effects and pseudoelasticity were observed in the first stage as well as in the second stage of the stress-strain curves; here we attempt to clarify the characteristics of the transition and deformation behaviour in the first-stage yielding. The strain in the first stage shows strong temperature and orientation dependences as follows: (1) the strain increases with decreasing temperature below the TR point, which is the critical temperature for the R-phase transition; (2) the strain is largest in the [111]B2 orientation and smallest in the [001]B2 orientation. Both the temperature and orientation dependences show excellent quantitative agreement with the calculated results based on the lattice distortion associated with the R-phase transit...

Investigation of microstructural changes in a ferritic steel caused by high temperature fatigue

Abstract: The low cycle fatigue behavior of a ferritic steel, Fe9Cr1Mo modified by the addition of small amounts of V and Nb, was studied under total strain control at elevated temperatures. The effects of strain range, temperature, environment, and prior heat treatment were evaluated. The accompanying microstructural changes were observed by conventional and high voltage TEM and carbide coarsening was followed by small angle neutron scattering (SANS). Substantial softening was observed in all fatigue tests. The introduction of hold times at maximum strain decreases cycle life but extends time to failure. The stress relaxes markedly during these hold times. Fatigue life is considerably shorter in air than in vacuum, especially at low strain ranges. The presence of air appears to increase softening. Prior aging or raising the temperature decreases the stress range but, in the case of continuous cycling under vacuum, cycle life is determined primarily by the plastic strain amplitude. Microstructural changes responsible for the observed softening include change from the original lath structure to cells or subgrains; large decrease in the originally high dislocation density; carbide coarsening. Information obtained from SANS measurements is consistent with a rapid coarsening of the fine carbides during high temperature deformation.

Feldspar-influenced rock rheologies

Abstract: Progressive fracture, subsequent cataclasis, and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz-mica phyllonite during early Tertiary deformation in a 20-30-m-wide shear zone in southeastern Arizona. The ductile deformation proceeded at temperatures and pressures approximating, middle to upper crustal levels. Most of the deformation was taken up by the feldspar and its alteration products. Quartz deformed by both brittle fracture and crystal plastic mechanisms. These results show that under certain temperature-pressure-strain rate conditions feldspars are weaker than quartz. The observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature-activated plastic deformation in quartz nor of the frictional behavior of rocks, as inferred in the familiar strength vs. depth curves. It is more likely that the transition is complex and depends on a variety of parameters. One such factor delineated in this study is the fracture and fracture-facilitated syntectonic alteration of feldspar under hydrated conditions.

Nonlinear and nonlocal continuum model of transformation precursors in martensites

Abstract: An intrinsic mechanism for explaining the origin of the transformation precursors observed above the transition in the parent phase of many materials undergoing a martensitic transformation is proposed. It is based on a nonlinear and nonlocal elastic continuum model for the elastic displacement field describing the parent-product deformation for the two-dimensional analog of a cubic-tetragonal transformation. By minimizing the Landau-Ginzburg free energy functional for the total elastic (strain plus strain gradient) energy a static, possibly stable, continuous, periodically modulated {110}/〈110〉 strain pattern is obtained which corresponds to alternating layers of more and of less transformed material, consistent with experimental observations. This pattern is stabilized by the balance between nonlinear and nonlocal elastic effects. Numerical application to In1-x XT x , alloys gives the minimum period of the modulation in the order of nanometers, in agreement with experimental observations.

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

Abstract: Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(e = 4 × 10-6'to 4 × 10-3 s-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing e at 850 °C and with increasing temperature at e = 4 × 10-5 s-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (e = 4 × 10-5 s-1) and in the tests at higher e at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M23C6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates e≥4× 10-4 s-1. At 850 and 950 °C even at strain rates of 4 × 10-5 s-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries.