Showing papers on "Deformation (engineering) published in 1985"

Creep of crystals : high-temperature deformation processes in metals, ceramics, and minerals

Abstract: Preface 1 Mechanical background 2 The agents of deformation: lattice defects 3 Phenomenological and thermodynamical analysis of quasi-steady-state creep 4 Dislocation creep models 5 The effect of hydrostatic pressure on deformation 6 Creep polygonization and dynamic recrystallization 7 Diffusion creep, grain-boundary sliding and superplasticity 8 Transformation plasticity 9 Scaling and classification References Indexes

The role of water in the deformation of olivine single crystals

Abstract: Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100° to 1300°C and 300- and 1500-MPa confining pressure, with the oxygen fugacity around the specimen constrained near the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10−10 m2 s−1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300-MPa confining pressure. Crystals hydrothermally treated at 1300°C, 300-MPa confining pressure, and 10−5 s−1 strain rate are a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigations suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the “wet” experiments. First-order calculations suggest a stress exponent of n ∼ 2.5 and a water fugacity exponent of m ∼ 1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.

The inhomogeneity of plastic deformation

Abstract: Considerable local strain differences occur in single crystals as a consequence of slip bands. A significant contribution to this phenomenon is the effect of the deformation rate on the inhomogeneity of its distribution. In polycrystalline materials the plastic deformation is affected by grain boundaries and their vicinity as well as by various orientations of single grains in onephase metals and alloys, and by various component properties in multiphase materials. In some cases it has been possible to describe these phenomena by means of micromechanics. Valuable information has also been acquired by means of three-dimensional stereology.

Polycrystalline strengthening

Abstract: The strength of polycrystalline specimens can be related to interaction phenomena taking place during elastic and plastic deformation. Such phenomena are reviewed in term of macroscopic and microscopic strain accommodation processes required to maintain strain continuity across the grain boundaries. The strength-grain size relationships can be described in a number of empirical equations relating the yield stress and the flow stress in tension to various structural parameters. A number of such equations are reviewed and their predictive capability is discussed. Structural information of importance for the understanding of polycrystalline strengthening is obtained mainly from surface relief patterns and from bulk structures observed by transmission electron microscopy of thin foils. The results obtained by these methods are discussed and correlations are proposed. A number of features characterizing the deformed structure are summarized and the behavior of a number of metals and alloys is reviewed with emphasis on the structural changes in the interior of the grains and in the vicinity of the grain boundaries. The models for strain accommodation during deformation are discussed on the basis of the microstructures found, and this structural information is correlated with a number of strength structural equations. Finally, the flow stress of fcc and bcc polycrystalline specimens is related to the occurrence of microstructures formed by macroscopic and microscopic strain accommodation processes during plastic straining, and it is concluded that macroscopic processes may be strength determining at larger strains whereas microscopic effects may be of importance at small strains.

A Two Surface Model for Transient Nonproportional Cyclic Plasticity, Part 1: Development of Appropriate Equations

Abstract: A two surface stress space model is introduced with internal state variable repositories for fading memory of maximum plastic strain range and non-proportionality of loading Evolution equations for isotropic hardening variables are prescribed as a function of these internal variables and accumulated plastic strain, and reflect dislocation interactions that occur in real materials The hardening modulus is made a function of prior plastic deformation and the distance of the current stress point from the limit surface The kinematic hardening rules of Mroz and Prager are used for the yield and limit surfaces, respectively The structure of the model is capable of representing essential aspects of complex nonproportional deformation behavior, including direction of the plastic strain rate vector, memory of plastic strain range, cross-hardening effects, variation of hardening modulus, cyclic hardening or softening, cyclic racheting, and mean stress relaxation

Rubber‐toughening in polypropylene

Abstract: To deformation and fracture behavior of several polypropylene (PP) and rubber-modified PP materials have been investigated. Plastic deformation mechanisms of these systems depend upon the test rate and temperature with high rates and low temperatures being in favor of crazing. The ductility and toughness of these materials are explained in light of the competition between crack formation and the degree of plastic deformation through crazing and shear yielding. The second phase morphology with smaller average rubber particle diameter D appears to be more efficient than that with larger D in toughening PP. Theoretical calculations indicate that the stresses imposed upon the rubber particles due to volume shrinkage of PP during crystallization are sufficient to compensate for the stresses due to differential thermal contraction in cooling from solidification temperature to end-use temperature. The difference between these two is small, and therefore they provide very little contribution to interfacial adhesion between rubber particle and PP matrix, the adhesion being insufficient for the rubber particles to be effective in controlling craze propagation. The rubber particles, in addition to promoting crazing and shear yielding, can also improve the fracture resistance of PP by varying the crystalline structure of PP (e.g., reducing the spherulite dimensions).

Time-dependent deformation of metals

Abstract: The basic characteristics of timedependent deformation of metals are described in terms of dislocation properties. At high temperatures, diffusion controlled climb of edge dislocations is the rate limiting process, whereas at low temperatures, other forms of recovery involving cross-slip of screw dislocations operate. A composite model of plastic flow is used to describe the coupling between these recovery processes. The model is patterned after the persistent slip band structures observed in cyclically deformed fcc single crystals. Screw dislocations are allowed to move in the cell interiors and to deposit edge dislocations into the adjoining walls. Cross-slip and climb lead to dislocation rearrangement and annihilation in the two regions. These processes are coupled not only through the dislocation microstructure, but also through the mechanics of the composite structure. The model is used to describe various deformation properties of metals, including stage II, stage III, and stage IV strain hardening and saturation of the flow stress. The coupling of cross-slip and climb controlled recovery processes leads to gradual transitions in strain hardening and gives a natural account of the transition from low temperature deformation to high temperature creep. The model also leads to polarized dislocation structures, internal stresses, and anelastic creep properties.

A constitutive model for the stress–strain–time behaviour of ‘wet’ clays

Abstract: A constitutive model is developed to describe the stress–strain–time behaviour of ‘wet’ clays subjected to three-dimensional states of stress and strain. The model is based on Bjerrum's concept of total strain decomposition into an immediate (time-independent) part and a delayed (time-dependent) part generalized to three-dimensional situations. The classical theory of plasticity is employed to characterize the time-independent stress–strain behaviour of cohesive soils using the ellipsoidal yield surface of the modified Cam Clay model presented by Roscoe and Burland. The time-independent strain is divided into an elastic part and a plastic part. The plastic part is evaluated using the normality condition and the consistency requirement on the yield surface. The time-dependent (creep) component of the total strain is evaluated by employing the normality rule on the same yield surface as in the time-independent model and the consistency requirement which requires that the creep strain rate reduces to phenome...

On the formation of porosity and microchannels in growing scales

Abstract: It is well known that oxide scales develop porosity and microchannels that permit inward transport of molecular species from the ambient gas even under conditions when there is no evidence of cracking of the scales. It is proposed that such porosity and microchannels develop as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales. The presence of small amounts of impurities enriched at grain boundaries in the scales may greatly affect deformation and mechanical and transport properties in scales.

Elevated temperature creep-rupture behavior of the single crystal nickel-base superalloy NASAIR 100

Abstract: The creep and rupture behavior of [001] oriented single crystals of the nickel-base superalloy NASAIR 100 was investigated at temperatures of 925 and 1000 °C. In the stress and temperature ranges studied, the steady state creep rate, time to failure, time to the onset of secondary creep, and the time to the onset of tertiary creep all exhibited power law dependencies on the applied stress. The creep rate exponents for this alloy were between seven and eight, and the modulus-corrected activation energy for creep was approximately 350 kjoule/mole, which was comparable to the measured activa-tion energy for Ostwald ripening of the γ′ precipitates. Oriented γ′ coarsening to form lamellae perpendicular to the applied stress was very prominent during creep. At 1000 °C, the formation of a continuous γ-γ′ lamellar structure was completed during the primary creep stage. Shear through the γ-γ ' interface is considered to be the rate limiting step in the deformation process. Gradual thickening of the lamellae appeared to be the cause of the onset of tertiary creep. At 925 °C, the fully developed lamellar structure was not achieved until the secondary or tertiary creep stages. At this temperature, the γ-γ′ lamellar structure did not appear to be as beneficial for creep resistance as at the higher temperature.

Deformation and cracking behavior of protective oxide scales on heat-resistant steels under tensile strain

Abstract: The deformation and cracking behavior of oxide scales formed in air on four heat-resistant steels and on nickel 99.6 have been studied in constant-extension-rate tests at 800°C. The strain rates in the experiments ranged between 10−6 and 10−9 s−1. Acoustic emission (AE) was used as an instrument for detecting the beginning of scale cracking. Additionally, metallographic, SEM, and micro-probe investigations were performed which supported the results from the AE measurements. The strain-to-cracking of the scales did not exceed 0.5% except when lateral growth effects in the oxide scales occurred, leading to critical strains of up to nearly 2.5%. Also the crack distribution in the scales was measured. The deformation and cracking behavior of the scales investigated could be explained by model like considerations.

Localization of dilatancy in Ohshima granite under constant uniaxial stress

Abstract: The localization of dilatancy during creep of Ohshima granite under uniaxial compression was observed. Hypocenters of 3933 acoustic emission (AE) events were accurately located. It was found that the mechanical behavior of Ohshima granite was controlled by the localization of microcracks. During the stage of loading up to the creep stress, which is 83% of the average short-term fracture strength, the hypocenters of AE events were randomly distributed throughout the specimen. As soon as the primary creep began, abrupt migration and clustering of AE hypocenters into several near-surface zones were observed. AE events formed volumetric concentrations. This migration and clustering strongly suggested the rapid localized development of dilatancy at the very beginning of the primary creep stage. The distribution of AE hypocenters observed in this stage was unchanged until final faulting. By the end of the primary creep stage, AE events began to concentrate into one of these clusters, while the activities of other clusters gradually decayed. This change spread broadly and continuously in time during the creep. In the most active cluster, clustering of AE events by itself gave rise to more AE events. The shape of this cluster was spheroidal with the long axis parallel to the loading axis. No evidence directly related to planar focusing of dilatancy was found. Surface strains were mapped. The axial strain distributions in the loading interval showed that the state of stress within the sample was homogeneous. A large change in both axial and circumferential strain fields occurred during the early stage of the primary creep. After this drastic change, the pattern of strain distribution remained unchanged in the subsequent stage of the creep. The accelerated increase in one of the circumferential strain gauges during the tertiary creep stage showed strongly localized deformation preceding faulting. The development of localized dilatancy identified by hypocenter locations was confirmed by the surface strain mapping. The position where the anomalous acceleration in circumferential strain was observed was close to the active, remaining cluster. The migration and clustering of AE hypocenters to the circumferential surface at the beginning of primary creep in the uniaxial unjacketed specimen were explained in terms of stress corrosion. Since the surface is the most favorable for accessing atmospheric moisture, stress corrosion in the vicinity of the surface was facilitated.

Compressive Stability of Delaminated Random Short-Fiber Composites, Part I—Modeling and Methods of Analysis

Abstract: The compressive stability of delaminated random short-fiber composites is con sidered. A mechanistic model is introduced for the problem. Both buckling stability and crack stability (i.e., delamination growth) are investigated. Two analytical methods are developed for studying the buckling stability of the delaminated com posites. The first is formulated on the basis of the well-known Rayleigh-Ritz method, including transverse shear in the composite and distorsional deformation in the delaminated ligament. The second method employs a plane-elasticity, finite-element buckling analysis. Both local buckling and coupled global and local buckling are ex amined. A fracture mechanics approach is used to study the possible compressive crack growth at buckling. The validity of the model and the accuracy and efficiency of the methods of analysis are demonstrated and discussed.

Tensile stress-strain analysis of cold worked metals and steels and dual-phase steels

Abstract: A study has been made of the applicability of the differential Crussard-Jaoul (C-J) analysis that assumes the Ludwik power relation, the modified C-J analysis based on the Swift formula, and the Hollomon analysis to uniaxially prestrained metals and steels and high strength, formable, dual-phase steels. The pure aluminum and copper metals and a series of plain carbon steels with carbon ranging from 0.10 to 1.05 pct were uniaxially prestrained by a given amount of strain under ambient temperature. A plain carbon steel with carbon of 0.10 pct was utilized in manufacturing the dual-phase steels. An empirical analysis exhibited the limited applicability of the C-J analysis for the interpretation of the stress-strain relationship of uniaxially prestrained metals and steels. The C-J analysis was also less sensitive to changes in the deformation behavior of the dual-phase steels in which the ferrite matrix and the shape and distribution of the second phase martensite were altered by three heat treatments. The modified C-J analysis was most suitable for describing work-hardening of uniaxially prestrained metals and steels. This analysis revealed that the dual-phase steels deformed in two stages. The first stage was associated with deformation of the ferrite matrix, and the second stage was associated with uniform straining of ferrite and martensite. The more generally used Hollomon curves deviated from linearity over all the uniform strain range regardless of the uniaxially prestrained metals and steels and dual-phase steels. Thus, the Hollomon parameters could not be assigned to an entire curve.

The influence of cobalt, tantalum, and tungsten on the elevated temperature mechanical properties of single crystal nickel-base superalloys

Abstract: For alloys with the baseline refractory metal level of 3 percent Ta and 10 percent W, decreases in Co level from 10 to 0 percent resulted in increased tensile strength and creep resistance. Substitution of W for Ta resulted in decreased creep life at high stresses but improved life at low stresses. Substitution of Ni for Ta caused large reductions in tensile strength and creep resistance, and corresponding increases in ductility. For these alloys with low Ta plus W totals, strength was independent of Co level. The increases in tensile strength with increases in refractory metal content were related to the increases in gamma volume fraction and solid solution hardening. Increases in strength as Co level decreased were considered to be the result of coherency strain hardening from the increased lattice mismatch. Dislocation shear through the gamma-gamma interface is considered to be the rate limiting step in the deformation process.

Fabrication and Mechanical Properties of Lightweight Ceramics Produced by Sintering of Hollow Spheres

Abstract: Lightweight ceramics with densities in the range 8 to 24% of theoretical were fabricated by sintering hollow glass spheres. The densification behavior, which was complex, involved uniform shrinkage of the spheres, local densification, deformation of the spheres, and coarsening of the cells. A major flaw population was found to be relatively large areas which were devoid of spheres. There was a distinct difference in the mechanical behavior and fracture path for the low- and high-density materials. The behavior of the high-density materials appeared to agree well with existing micromechanical models in terms of Young's modulus, fracture toughness, and density. The fracture toughness of these cellular materials per unit weight is similar to that of bulk glass but is not translated into strength because of the relatively large flaw sizes in the final microstructure.

Temperature and strain rate dependence of stress-strain behavior in a nickel-base superalloy

Abstract: The temperature and strain rate dependence of yield and work hardening behavior in elevated temperature stress-strain testing was investigated in the nickel-base superalloy UDIMET 115. This alloy was heat treated to produce a bimodal distribution of hyperfine and coarse γ1 precipitates. Yield behavior is shown to be controlled primarily by dislocation pair cutting of the hyperfine precipitates. Stage II work hardening appears to be governed mainly by the dislocation networks generated on the coarse precipitates as these are bypassed under the Orowan mechanism. Analysis of Stage III work hardening using ϕvs σ plots shows that a steady state exists for the stress-strain deformation of this system. Results are also reported on extremely strong serrated flow at moderately elevated temperatures and a high temperature ductility minimum.

Analysis of necking and neck propagation in polymeric materials

Abstract: N ecking and neck propagation as observed in polymers which “cold draw” is analyzed numerically for a circular cylindrical tensile specimen. The entire load-deformation history of the bar is computed using the finite element method in conjunction with hill's (1958, 1959) variational principle. Rate-independent elastic-plastic material behaviour is assumed. Results are given for the overall load-elongation response of the bar, as well as for the evolution of the specimen profile and the stress and strain distributions in the bar at various stages of the deformation process. The implications of our results on conventional methods used to analyze tension data for polymers are also discussed.

Time-dependent Deformation of Composite Resins — Compositional Considerations:

Abstract: The compressive yield strength and creep of 21 dental composite resins were evaluated and correlated with filler volume percent and extent of cure in the resin. In general, yield and creep of composites were more dependent upon filler volume fraction than on extent of cure. However, the types of monomers and fillers used in the composite formulation appeared to play a major role in determining the compressive characteristics of the materials.