Showing papers on "Deformation (engineering) published in 1976"

The Effect of Hydrostatic Pressure on the Deformation Behavior of Maraging and HY-80 Steels and Its Implications for Plasticity Theory.

Abstract: Earlier results showed that the difference between the tensile and compressive strengths of tempered martensites is primarily a manifestation of the general pressure dependence of flow stress in these materials. However, the same results also showed that the volume expansion after deformation was much smaller than that predicted by the normality flow rule of plasticity theory for materials with such pressure dependence. Additional results now obtained on maraging and HY-80 steels support these conclusions. The results for all these materials exhibit a strong, but not perfect, correlation between pressure dependence, yield stress, and volume expansion. The volume expansion, however, which is believed to result primarily from the generation of new dislocations, is very small and does not appear to be essential to the pressure dependence. Most of the pressure dependence, the portion responsible for the discrepancy with the normality flow rule, may be an effect on dislocation motion. The results suggest that an appropriate plasticity model would be one in which the octahedral shear yield stress is linearly dependent on the mean pressure, but the volume change is negligible in violation of the normality flow rule. Such a model has been proposed previously for the plastic deformation of soils. However, unlike that model, the present theory includes strain hardening.

The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy

Abstract: As-quenched and tempered martensite in an Fe-0.2 pct C alloy were subjected to tensile testing and structural characterization by light and transmission electron microscopy. The light temper, 400°C-l min, did not change packet morphology, but did reduce dislocation density, coarsen lath size and cause the precipitation of carbides of a variety of sizes. The yield strength of the as-quenched martensite was strongly dependent upon packet size according to a Hall-Petch relationship, but tempering significantly diminished the packet size dependency, a result attributed to packet boundary carbide precipitation and the attendant elimination of carbon segregation present in the as-quenched martensite because of autotempering. Examination of thin foils from strained tensile specimens showed that a well-defined cell structure developed in the as-quenched martensite, but that the random distribution of jogged dislocations and carbide particles produced by tempering persisted on deformation of the tempered specimens.

Localization of plastic deformation

Abstract: The localization of plastic deformation into a shear band is discussed as an instability of plastic flow and a precursor to rupture. Experimental observations are reviewed, a general theoretical framework is presented, and specific calculations of critical conditions are carried out for a variety of material models. The interplay between features of inelastic constitutive description, especially deviations from normality and vertex-like yielding, and the onset of localization is emphasized.

Accelerated Evolution of Hydrogen from Metals During Plastic Deformation

Abstract: Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during Luder’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.

Comparison of the elongational behaviour of a polyethylene melt at constant stress and constant strain rate

Abstract: Two different apparatuses for measuring the elongational behaviour of polymer melts at constant tensile stress and constant elongational strain rate are described. Measurements on a low density polyethylene were carried out up to stretching ratios of 400. The homogeneity of sample deformation in both test methods was sufficient to reach a steady-state of elongational flow where the tensile stress and the strain rate as functions of time are constant. By unloading the molten rod the recoverable strain can be determined at any state of deformation. The recoverable strain increases with growing deformation and reaches a constant value in the steady-state. The elongational viscosity calculated from the rate of viscous flow agrees with the Trouton viscosity in the case of very small deformations only. With growing deformation the elongational viscosity increases up to a constant value in the steady-state which is greater than the Trouton viscosity by about a factor of six at measured strain rates of 0.03 s−1 and 0.1 s−1, respectively. The elongational viscosity and the recoverable strain in the steady-state measured with the two different test methods under the same experimental conditions are in good agreement.

Porous Rigid Plastic Materials Containing Rigid Inclusions; Yield Function, Plastic Potential and Void Nucleation

Abstract: In this paper, the effect on constitutive behavior of the presence of rigid particles, embedded in and bonded to a rigid-plastic porous matrix, is examined. It is shown that the yield function is altered, and that the familiar concept of the yield function as a plastic potential must be used more carefully. The results also show how a void nucleation mechanism could destabilize, causing rapid bulk softening and failure.

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

Abstract: The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.

Residual stresses in polymers and their effect on mechanical behavior

Abstract: Plates of bisphenol-A polycarbonate and poly(methyl methacrylate) have been quenched in ice water from temperatures slightly above their glass transition temperatures. Residual stresses are thus created, Measurement of these residual stresses has been accomplished by the “layer removal” method and the stress distributions through the thickness are presented. Compressive stresses, approximately 3000 psi, exist at the surface while tensile stresses-of at least 1000 psi exist in the interior. It is shown that these residual stresses can influence the notched Izod impact strengths for polycarbonates. The mechanism is thought to be suppression of craze initiation in advance of the notch due to the presence of residual compressive stresses for specimens notched prior to quenching. In the case of poly(methyl methacrylate), it is shown that compressive residual stresses at the surface can cause plastic yielding to occur in bending experiments resulting in permanent deformation and greater energy absorption.

Structure and properties of abs polymers. X. Influence of particle size and graft structure on loss modulus temperature dependence and deformation behavior

Abstract: ABS systems which differ distinctly in particle size and degree of grafting were prepared and investigated by dynamic mechanical measurements as function of temperature in the glass transition region of the rubber phase. Variation of rubber content within different sample series results in effects which were mainly referred to thermal stresses as consequence of phase interactions. Basic aspects of the deformation behavior of some of these ABS systems are studied by mechanical and morphological methods, and the operation modes of the component of a bimodal/bigraft-system concerning toughness are discussed.

Sorption of tritium by nickel during plastic deformation

Abstract: The effect of plastic deformation on the amount of tritium absorbed, the surface concentration, and the apparent diffusivity was determined by comparing elastically and plastically deformed sections of nickel tensile tubes that were filled with tritium during deformation. Plastic deformation increased the amount of tritium absorbed by the metal and adsorbed on the surface, but decreased the apparent diffusivity. These data for nickel can be explained by the trapping theory developed primarily to explain anomalous hydrogen diffusivity and solubility in iron, where the dislocations created during deformation are postulated to trap the tritium, thereby producing higher values for interior and surface tritium concentrations and lower values for tritium diffusivity.

The effect of grain size on the shock-loading response of 304-type stainless steel

Abstract: The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes—23, 55, 85 and 187 µm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both ɛ and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pet cold rolling, the strengthening efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation to α martensite during subsequent tensile deformation.

Deformation Structures in Minerals

Abstract: Although it is four decades since it was proposed (Orowan, 1934; Polanyi, 1934; Taylor, 1934) that linear defects or dislocations in crystals play a major role in the plastic deformation of crystalline solids, it is little more than two decades since the existence of dislocations in crystals was established and their motions clearly associated with deformation. Direct observation of dislocations and associated deformation structures in very thin specimens by transmission electron microscopy (TEM), initially achieved by Hirsch et al. (1956) and Bollman (1956), has proved to be the most fruitful method of study. It is interesting to note that mineral crystals figured significantly in these early developments; for example, the first observations of surface growth steps at the emergence of screw dislocations on crystal surfaces were made in natural beryl crystals (Griffin, 1950); and some of the classic early TEM studies of dislocations, partial dislocations and stacking faults in layer structures were made on graphite and talc, muscovite and chlorite (Amelinckx and Delavignette, 1960 a, b, 1961), in specimens produced by cleavage. However, largely because of difficulties in producing suitably thin “foils” of minerals with poorly-developed cleavage (Tighe, Chapter 3 of this volume) TEM studies of minerals proceeded slowly until late in the 1960’s.

Mechanisms of overload retardation during fatigue crack propagation. Discussion

Abstract: The effect of single overloads on room-temperature fatigue crack growth has been studied in two steels of markedly different yield stresses. The observed retardation effects have been presented and the evidence suggests that overload retardation is primarily due to residual compressive stresses generated in the crack tip region, and associated with crack closure effects. The results have been rationalized in terms of a fatigue crack growing through overload plastic zones of different shapes and sizes associated with plane stress and plane strain deformation.

The influence of stress distribution on the deformation and fracture behaviour of ceramic materials under compression creep conditions

Abstract: The stress distribution developed in test pieces during compression creep has been determine using the finite element method. The analyses are shown to account precisely for the inhomogeneous distribution of grain-boundary cracks developed during creep of polycrystalline magnesia and indicate that the accommodation of grain-boundary sliding by cavity formation is the rate-controlling process during high temperature creep of reaction-bonded silicon nitride.

Tensile deformation behaviour of ABS polymers

Abstract: Several grades of ABS polymer have been tested in uniaxial tension over a temperature range from 293 K (20° C) to 198 K (−80° C). Effects of strain-rate and temperature on the yield stress have been explored and the magnitude of the activation volumes and activation energies derived. Additionally, the volume strain and the longitudinal strain have been monitored simultaneously, from which data the contribution of crazing to the total deformation of the specimens has been obtained.

The shape memory effect and pseudoelasticity in polycrystalline Cu-Zn alloys

Abstract: The shape memory effect associated with the reverse transformation of deformed martensite, pseudoelastic behavior involved in stress-induced martensite formation and the reversion of strained martensite after an applied stress is relaxed aboveAf have been studied. Grain size and specimen geometry effects have been related to the above phenomena. Although recoverable strains as high as 10.85 pct were observed in coarse-grained (“bamboo” type) specimens, the shape memory effect is restricted in fine-grained specimens because of permanent grain boundary deformation and intergranular fracture which occurs at relatively low strains. A fine grain size also acts to suppress pseudoelastic behavior because permanent, localized deformation is generated concurrent with the formation of stress-induced martensite which inhibits reversion of the latter upon release of stress. The apparent plastic deformation of martensite belowMf can be restored by transforming back to the original parent phase by heating toAf (shape memory) or alternatively, can be recovered belowMf by applying a small stress of opposite sign. Martensite deformed belowMf with the same stress maintained while heating persists aboveAf, but reverts to the parent phase in a pseudoelastic manner when the stress is relieved. The athermal thermoelastic martensite, which forms in groups composed of four martensite plate variants, undergoes several morphology changes under deformation. One of the variants within a plate group cluster may grow with respect to the others, and eventually form a single crystalline martensitic region. At a later stage pink colored deformation bands form in the same area and join up with increasing stress, resulting in thermally irreversible kinks. The clusters of plate groups may expand like grain growth or contract as a whole during deformation, or act as immobile “subgrains” which lead to permanent deformation at their boundaries. Stress-induced martensite usually forms as one variant of parallel plates which join up with increasing stress to form single crystalline regions. Further stress leads to pink colored deformation bands, similar to those in the deformed athermal martensite. Other similarities and differences between the stress-induced and athermal martensite have been investigated and are discussed.

Strain‐rate dependence of acoustic‐emission power and spectra in aluminum alloys

Abstract: Strain‐rate dependence of acoustic‐emission power and spectra for deformation of aluminum alloys is investigated over a wide frequency range of 100 kHz to 4 MHz. In case of Al‐Cu‐Mg alloy (2024) and pure aluminum (1100), it is found that acoustic‐emission power is proportional to the first power of the plastic strain rate times the gauge length of the specimen. Normalized acoustic‐emission power spectra show no significant change at a given strain depending on the strain rate, though they vary with the strain in early stages of deformation. In case of Al‐Mg alloy (5056), repeated bursts of acoustic emission are recorded accompanied by repeated discontinuous yielding. The mode of inhomogeneous deformation and the pattern of acoustic emission vary with the strain rate.

Plastic deformation of polycrystalline zirconium carbide

Abstract: The compressive yield strength of arc-melted polycrystalline zirconium carbide has been found to vary from 77 kg per sq mm at 1200 C to 19 kg per sq mm at 1800 C. Yield drops were observed with plastic strain-rates greater than 0.003/sec but not with slower strain rates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameter m which range from 6.5 at 1500 C to 3.8 at 1800 C, and the product dislocation velocity stress exponent times T was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.

The temperature- and strain rate-dependent mechanical behavior of a polycrystalline galena ore

Abstract: Experiments on the compressive deformation of a polycrystalline galena ore (G2 galena) at temperatures from 20 degrees to 400 degrees C and at strain rates from about 10 (super -4) sec (super -1) to 10 (super -8) sec (super -1) have been performed in a triaxial test apparatus at a confining pressure of 1.5 kb. About 90 volume percent of the galena ore consists of lead sulfide with about 10 18 atoms/cc of sulfur in excess of that expected from the stoichiometric composition (PbS).The differential stress supported by G2 galena and its rate of work hardening decrease rapidly as either the temperature is raised or the strain rate is lowered. Differential stresses (Sigma ) measured at 12 percent strain range from 3,100 bars at 20 degrees C and strain rate (e) of approximately 10 (super -4) sec (super -1) , to 320 bars at 400 degrees C and strain rate of approximately 10 (super -8) sec (super -1) . At normalized differential stresses (given by Sigma /[radic]3mu ; where mu = average shear modulus) greater than about 2.2 X 10 (super -3) dislocation glide is the dominant deformation mechanism and e is related to Sigma by an "exponential equation"; e = A exp(-Q/RT) exp(BSigma ); where A and B are constants and Q is the activation energy for flow. At lower stresses dislocation glide plus polygonization (subgrain formation) occurs. Provided that strains greater than about 8 percent are achieved, a "power equation" holds under these conditions: e = C exp(-Q/RT) Sigma N ; where C and N are constants. N is about 7.3 and Q approximately 22.5 kcal mole (super -1) . Deformation involving subgrain formation is believed to proceed at a rate controlled by dislocation climb.Calculations have been made to determine whether Nabarro-Herring creep (i.e., diffusive mass transfer creep; e infinity Sigma ) is an important mechanism in the natural deformation of galena ores with the same composition as G2 galena. These show that at 300 degrees to 400 degrees C flow according to the "power equation" may give way to Nabarro-Herring creep at strain rates that are slower than those attained in the laboratory yet fast enough to be tectonically significant, provided that grain size does not exceed a few mm.The differential stresses that G2 galena (mean grain size = 0.07 mm) would support at the slow strain rates typical of folding and related tectonic deformation are estimated to range from about 154 bars at 300 degrees C and a strain rate of 10 (super -11) sec (super -1) , to about 4.5 X 10 (super -2) bars at 400 degrees C and a strain rate of 3 X 10 (super -14) sec (super -1) . If grain growth occurs so that Nabarro-Herring creep is precluded the stress supported under the latter conditions might be as high as 47 bars.

Deformation field of a misfitting inclusion

Abstract: J.D. Eshelby (1957, 1959) has calculated the deformation field associated with an ellipsoidal inclusion in a state of homogeneous strain within an infinite matrix. Since most real precipitates occur with facets, the strain within such an inclusion is not uniform. Thus, plate precipitates of θ ′ in Al-Cu and η in Al-Au have coherent broad faces with mismatches of 1.34 and 4.95 % respect- ively and semicoherent or disordered interfaces at the edges with residual mismatches of about −4.3 and −1.00% normal to the broad faces. The deformation field in the matrix around such precipitates has been calculated using Kelvin's (1848) result for the stress field due to a point force. The calculations show the existence of high stresses near the edges of the precipitates where they have an appreciable misfit. Unlike the case of an ellipsoidal inclusion, the stress fields of these precipitates have dilatational components which can affect the diffusion of solute atoms to them and, thus, the kinetics of interface migration. The behavior of alloys containing these precipitates indicates that the moduli of the precipitates are somewhat greater than those of the matrices. The present calculations, based on the assumption that the two moduli are the same, underestimate the actual deformation field in the matrix. In real systems, therefore, the effects of the deformation field on misfit dislocation nucleation and kinetics of interface migration are likely to be somewhat greater in general.