Showing papers on "Deformation (engineering) published in 1979"

Annihilation of dislocations during tensile and cyclic deformation and limits of dislocation densities

Abstract: It is proposed that, during dislocation glide at low temperatures, both screw and non-screw dislocations annihilate mutually with dislocations of opposite sign approaching on closely neighbouring glide planes. The experimental evidence is summarized and possible mechanisms are discussed. Phenomenological models of dislocation accumulation during deformation, taking into account the annihilation of dislocations, are formulated. The analysis of selected examples of tensile and cyclic deformation of f.c.c. and b.c.c. metal crystals demonstrates that annihilation of edge and/or screw dislocations occurs during strain hardening and can lead to steady state deformation. It follows that the dislocation densities introduced during deformation cannot exceed well-defined upper limits that are distinctly lower than the hypothetical upper limits estimated for the static case. The observations suggest that the critical spacings of dislocation pairs that annihilate are ∼.50–500 nm for screw and ∼ 1·6 nm for ed...

Adiabatic Plastic Deformation

Abstract: Adiabatic plastic deformation may play a role in such widely diverse areas as ballistic impact, explosive fragmentation, cryogenic behavior of materials, high velocity shaping and forming, machining, grinding 0), surface frictional effects, erosion (2), and seismic faulting. All of the adiabatic shearing phenomena are based on two facts: Approximately 90% of the work of plastic deformation is converted to heat, and the flow stress of most metals is quite sensitive to temperature, decreasin g as the temperature increases. That localized temperature increases and strain concentration play a major part in high speed deformation of metals was recognized by Zener & Hollomon (3) in 1944. The phenomenon is most clearly identified in most steels, in which heating above the transforma­ tion temperature causes the transformation of ferrite to austenite. On rapid cooling the austenite retransforms to a product that etches with difficulty and appears as a white band against the dark background of the remainder of the etched steel. These materials thus retain evidence for adiabaticity of the deformation, while in most other metals the evidence is significantly less definite. The use of the term "adiabatic deformation" is obviously an over­ simplification in the sense that some heat always transfers out of any deforming region. Moreover, to categorize one situation as "adiabatic" and another not is in many instances equivalent to labeling shades of gray as black or white. It

Effect of hydrostatic pressure on the deformation behavior of polyethylene and polycarbonate in tension and in compression

Abstract: The stress-strain response of crystalline high density polyethylene and of amorphous polycarbonate has been determined in tension and in compression at superimposed pressures up to 1104 MPa(160 ksi). Strain softening occurred in the polycarbonate at low pressures but was inhibited by pressure. Tensile necking occurred in both materials, but was promoted by pressure in polyethylene and inhibited in polycarbonate. The initial modulus, E, and the flow stress, σ, at a given offset strain varied linearly with the mean pressure, P, with essentially the same pressure coefficient, α. Thus, E = (1+αP)E0 and σ = (1+αP)σ0, where E0 and σ0 are values at zero mean pressure. In polyethylene, the coefficient, σ0, was the same in tension and compression, indicating that the strength differential between tension and compression was a simple manifestation of pressure-dependent yielding. In polycarbonate the coefficient, σ0, was different in tension and in compression, implying an effect due to the third stress invariant or to anisotropy. The results suggest a constitutive model for polymers in which the flow stress is linearly dependent on mean pressure, but in which inelastic volume change is negligible. The results also suggest that the pressure dependence of flow stress in polymers is the same as that of the initial modulus.

Constitutive Model for Short-Time Loading of Concrete

Abstract: A constitutive model based on nonlinear elasticity is proposed, where the secant values of Young’s modulus and Poisson’s ratio are changed appropriately. This alteration is obtained through the use of a nonlinearity index that relates the actual stress state to the failure surface. The model simulates the strain hardening before failure, the failure itself and the strain softening in the post-failure region. The dilation of the concrete and the influence of all three stress invariants are considered. All stress states including those where there are tensile stresses can be dealt with; however, the model is calibrated using experimental data obtained by a uniaxial compressive and tensile test only. The model predictions are demonstrated to be in good agreement with experimental results involving a wide range of stress states and different types of concrete.

Formability of sandwich sheet materials in plane strain compression and rolling

Abstract: The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming The observed failures were similar in both plane-strain indentation and rolling tests Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement

Earthquake precursory effects due to pore fluid stabilization of a weakening fault zone

Abstract: We report the analysis of two mechanisms by which pore fluids could partially stabilize the earthquake rupture process in natural rock masses. These mechanisms are based on dilatancy strengthening and on the increase of elastic stiffness for undrained as opposed to drained conditions. Both are studied in relation to an inclusion model in which a zone of strain weakening material, possibly representing a highly stressed seismic gap zone, is embedded in nominally elastic surroundings subjected to steadily increasing tectonic stress. Owing to the coupling between deformation and pore fluid diffusion, the inclusion does not exhibit an abrupt rupture instability; rather, a period of self-driven precursory creep occurs which ultimately accelerates to dynamic instability. The precursory time scale is reported for a wide range of constitutive parameters, including fluid diffusivity, ratio of undrained to drained stiffness, and factors expressive of strain softening and dilatancy. Our conclusions are that the precursory times for a spherical inclusion of 1-km radius are of the order of 15–240 days for a range of constitutive parameters that we suggest are representative. The predicted times are shorter by a factor of approximately 10 for a flattened ellipsoidal inclusion that we analyze with an 18 : 1 aspect ratio. It is suggested that perhaps only toward the latter part of the precursory period are the effects of accelerating inclusion strain detectable in terms of surface deformation or alteration of transport or seismic properties.

Deformation of sandwich sheet materials in uniaxial tension

Abstract: The stable and unstable plastic flow of stainless steel-clad aluminum and aluminum-clad stainless steel sandwich sheet materials deformed in uniaxial tension have been investigated. For the clad sheet materials studied experimentally, stable deformations were uniform in the component layers, and the assumption of isostrain was used in modeling the deformation behavior. The rule of mixtures, an average of component properties weighted by cross-sectional area fractions, was applied to determine sandwich uniaxial true stress-true strain curves from those of the components. In addition, measurements of residual stress distributions in deformed tensile specimens gave insight into states of stress during loading. A model to determine the magnitude of stresses which are generated by component normal plastic anisotropy differences was developed as well. With this knowledge of the stress state, predictions of uniform elongation of the clad sheet materials were made which compared favorably to experimental measurements. As for ductile monolithic sheet materials, stable flow of sandwich sheet materials in tension was limited by diffuse necking, which leads to local instability at higher strains. This local instability gives rise to a through-thickness localized thinning which terminates macroscopic deformation. Conditions for local instability in uniaxial tension have been developed for sandwich as well as monolithic sheet materials. Predictions from these models are in agreement with measurements.

Dynamic deformation of polycrystalline alumina

Abstract: Using one‐dimensional strain conditions, the dynamic stress‐wave response of polycrystalline Al2O3 was measured with interferometry in both stress‐wave loading and unloading to about 16 GPa and with slanted resistor gauges in loading to about 50 GPa. The stress‐wave loading and unloading measurements were of high resolution and showed a 9.1‐GPa elastic precursor wave (velocity 10.9 km/s) followed by a slower dispersive permanent deformation wave. Unloading was elastic in the stress range of these experiments. Both loading and unloading wave propagation were modeled well with a Maxwellian elastic‐stress‐relaxing model with a yield stress of 5.8 GPa and a relaxation time of 70 ns. The rate‐dependent model correctly predicts both the dispersion of the permanent deformation wave and the unloading‐wave behavior. The bulk pressure‐volume behavior of alumina is given by the shock‐velocity–particle‐velocity relationship of Us=8.14 +1.28up (km/s). Thermodynamic corrections to the dynamic bulk response yielded isot...

Deformation Mechanisms in Superplasticity

Abstract: The phenomenon of micro grain superplasticity, in which metals and alloys deform extensively at elevated temperatures under small stresses without risk of rupture, has been well documented over the past decade. Our under­ standing of the rate-controlling mechanisms for superplastic deformation processes in terms of range of stress, temperature, and strain rates is important not only for its intrinsic scientific value but also because of its significance in optimizing the processing parameters for forming opera­ tions (1). The strain rates for stiperplasticity are considerably lower than those for conventional hot-working processes. This factor has so far restricted the extensive adoption of superplastic forming operations. A sound theoretical understanding of the dependence of the superplastic strain rate on temperature, grain size, and flow stress will be of significant use in optimization of these variables, so that the material can be deformed at the highest possible forming rates and still benefit from the exceptionally large, neck-free ductility associated with superplasticity. Therefore, it is not surprising that the considerable attention given to this particular mode of deformation in recent years has resulted in two major advances: (a) several new models have been proposed for the per­ tinent deformation mechanism (2-6), and (b) more reliable new data (71 1, 17)have become available to examine the validity of the proposed models. Many of these advances were recently reviewed in excellent detail (12-15;

Flow stress model in metal cutting

Abstract: A model for the plastic deformation that occurs in metal cutting, based on dislocation mechanics, is presented. The model explains the fundamental deformation structure that develops during machining and is based on the well known Cottrell-Stokes Law, wherein the flow stress is partitioned into two parts; an athermal part which occurs in the shear fronts (or shear bands); and a thermal part which occurs in the lamella regions. The deformation envokes the presence of a cellular dislocation distribution which always exists in the material ahead of the shear process. This 'alien' dislocation distribution either exists in the metal prior to cutting or is produced by the compressive stress field which operates in front of the shear process. The magnitude of the flow stress and direction of the shear are shown to be correlated to the stacking fault energy of the metal being cut. The model is tested with respect to energy consumption rates and found to be consistent with observed values.

Interrelationship between creep deformation and creep rupture in 2¼Cr-1Mo steel

Abstract: Creep deformation and rupture have been studied in 2¼Cr-IMo steel over the stress range 60-210 MN m⁻² at 565°C. Creep damage accumulates by the initiation and growth of extensive cavitation at the prior austenite grain boundaries. Cavity formation predominates during the initial transient and individual cavities appear to nucleate on grain boundary carbides. Quantitative analysis of the cavitation kinetics in relation to the creep deformation processes suggests that cavity growth is directly related to deformation occurring at the grain boundaries. It is inferred that the vacancy diffusion mechanism is inhibited and growth is limited by the local creep process occurring at the grain boundaries. At low stresses the latter is controlled by intra-granular recovery creep such that the overall rate of cavitation is proportional to the macroscopic strain rate. However, results obtained under biaxial shear conditions imply that the rate of cavitation is additionally dependent upon the normal boundary str...

Flow and fracture behaviour of Ni3(Al·Ti) single crystals tested in tension

Abstract: The deformation and the fracture behaviour of the [0 0 1] orientated Ni3(Al.Ti) single crystals were investigated to determine the relation between the positive temperature dependence of the flow stress in the γ′-phase and the malleability of nickel-base superalloys. The positive temperature dependence of the flow stress is observed in the [0 0 1] orientation below about 1000 K (Tp) and the failure occurs in a catastrophic and brittle manner after considerable plastic deformation. The room temperature fracture stress increases with increase in the angle θ between the [0 0 1] orientation and the tensile axis at 290 K, and it is well expressed by a crack propagation criterion only by considering the effect of the normal stress on the {1 0 0} cleavage plane. The cleavage fracture stress for the [0 0 1] orientation is nearly independent of temperature below Tp, while the elongation decreases with temperature in contrast to the yield stress. The cleavage fracture of Ni3(Al.Ti) single crystals is explained by the rapid decrease of the mobile dislocation density due to a dislocation pinning mechanism based on the cube cross-slipping of the screw superdislocations which causes the positive temperature dependence of the flow stress. The insufficiency of the malleability of nickel-base superalloys seems to be attributed to that of the γ′-Ni3 (Al.Ti) phase, and the hot working of nickel-base superalloys near Tp in the γ′-phase should be avoided.

Neck formation and cavitation in the superplastic Zn-22% Al eutectoid

Abstract: The mechanical behaviour of the superplastic Zn-22% Al eutectoid is divisible into three distinct regions. Experiments show the deformation is quasi-uniform at intermediate strain rates in region II, but neck formation is important at low strain rates in region I. Extensive cavitation occurs in regions I and II, but fracture in region I is due to necking. The results provide strong evidence for a decrease in the true value of the strain rate sensitivity in region I.

Bond Failure of Deformed Reinforcing Bars Based on the Longitudinal Splitting Effect of the Bars

Abstract: The anchorage capacity of deformed reinforcing bars is restricted by one of the following failures: (1) bond slip failure where the bar is just pulled out and leaves the surrounding concrete without further damage, or (2) splitting failure where slipping between concrete and reinforcement causes splitting in concrete cover. In normal beams, with relatively thin concrete cover, the splitting failure is most common. Since splitting failure gives a much smaller anchorage capacity than bond slip failure does, it is very important to consider this failure. The test results indicate that the force developed along a deformed bar can be regarded as a straight line function of the length of the splitting lines in the section as well as of concrete tensile strength. Surrounding reinforcement has a great influence on the developed force depending on the amount of surrounding reinforcement area transversing the splitting surface. The influence of the shape of the bar surface deformations is studied by tests giving both types of failure. The deformation shape was varied with different diameter and different distances. Considering splitting failure, the distance between lugs had less importance than lug diameter. Both parameters studied are of less importance in case of splitting failure than in case of a pure bond slip failure. /Author/

Friction, Deformation and Fracture of Single-Crystal Silicon Carbide

Abstract: An investigation was conducted to determine the nature of the deformation and fracture of silicon carbide and its effects on friction properties. Friction experiments were conducted with hemispherical and conical diamond riders sliding on the basal plane of silicon carbide. The results indicate that, when deformation is primarily elastic, the friction does not depend on crystallographic orientation and there is no detectable fracture or crocking. When, however, plastic deformation occurs, silicon carbide exhibits anisotropic friction and deformation behavior. Surface fracture crack patterns surrounding wear tracks are observed to be or three types. The crack-geometries of two types are generally independent of orientation, the third crack, however, depends on the orientation. All surface cracks extend into subsurface. Anisotropic friction, deformation and fracture on the basal plane are primary controlled by the slip system {101¯0} ⟨112¯0⟩ and a cleavage of {101¯0}. Presented as an American Society of Lub...

Craze roles in the fatigue of polycarbonate

Abstract: Unnotched specimens of polycarbonate were deformed to failure in equal tension-compression, under load control. Fracture surfaces exhibited two distinctly different crack propagation regimes. In the first zone, the crack moves forward through an already existing craze. Craze ageing at the crack tip is necessary for propagation and several stress cycles are required before the crack tip can move from arrest site to arrest site. In the second zone, the crack tip must form and age its own craze(s) before moving forward. In the initial stage of Zone 2, the crack mobility is still restricted by the age of the craze. Later in this zone, the crack tip is able to create craze and move through it in the same cycle. Static tensile deformation from 90% to 95% of the yield point produced an increase in the low-load fatigue life. This increase is explained on the basis of the mechanics of coarse crazes.

Plastic deformation of oriented lamellae. II. Hot rolling of .BETA.-phase isotactic polypropylene.

Abstract: Isotactic polypropylene was crystallized under temperature gradient and an oriented β-phase sample was obtained. The β-phase film was rolled in three orthogonal directions at 68, 96, 122, and 142°C. The deformation behavior was investigated by wide angle and small angle X-ray diffraction methods. Anisotropy in deformation was observed in each case, including the rotation of lamellae, interlamellar slip, longitudinal and transversal chain slip. At high temperatures, the deformation proceeded mainly by interlamellar slip whereas chain slip could hardly be observed. During the hot roll deformation, the original β-phase crystal was destroyed and then recrystallized into the c-axis-oriented α-phase crystal. The amount of the β→α phase transition increased with increasing rolling temperature and draw-ratio. The spacing of the newly appeared α-phase lamellae changed, depending only on the rolling temperature. The final c-axis-oriented texture was composed of α-phase crystals. From these results, it is considered that the deformation proceeds, not by incorporation of the original lamellar blocks, but through the melting or unfolding of the original β-phase lamellae and subsequent recrystallization into the c-axis-oriented α-phase lamellae.