Showing papers on "Strain rate 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...

Observed Stress‐Strain Behavior of Confined Concrete

Abstract: Thirty‐one nearly full‐size reinforced concrete columns, of circular, square, or rectangular wall cross section, and containing various arrangements of reinforcement, were loaded concentrically with axial compressive strain rates of up to 0.0167/s. The circular sections contained longitudinal and spiral reinforcement, the square sections contained longitudinal reinforcement and square and octagonal transverse hoops, and the rectangular wall sections contained longitudinal reinforcement and rectangular hoops with or without supplementary cross ties. The longitudinal stress‐strain behavior of the confined concrete was measured and compared with that predicted by a previously derived stress‐strain model with allows for the effects of various configurations of transverse confining reinforcement, cyclic loading, and strain rate. The measured longitudinal concrete compressive strain when the transverse steel first fractured was also compared with that predicted by equating the strain energy capacity of the tran...

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.

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.

Plastic instabilities: Implications for the origin of intermediate and deep focus earthquakes

Abstract: Adiabatic or catastrophic plastic shear has been reported in metals, polymers, and metallic glasses. The phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner. The driving force for the plastic instability is the stored elastic strain energy of the loading system, and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding, although the precise mechanism is different. It is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones, the earthquake events being the result of instabilities in material undergoing plastic flow. It is argued that for a particular strain rate there exists a critical temperature, TC, which is depth dependent; for temperatures below TC the material is strain rate softening and, for a soft enough loading system, may undergo catastrophic plastic shear. For temperatures above TC the material is strain rate hardening and is always stable during plastic shear. The cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material, and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km. The precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the TC gradient. In particular, a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 MPa above 300 km to a maximum of ≈ 50 MPa below 300 km. The differences in foci distribution between subduction zones such as Tonga, New Hebrides, and Peru result from minor differences in the geothermal gradients within the slabs. The model predicts the development of triple seismic zones high in the slab, double seismic zones down to approximately 300 km, and single seismic zones down to ≈ 800 km. Such a distribution is to be expected of relatively young, cool slabs; as the slab heats up, the seismic activity retreats up the slab. The paper only proposes a deformation mechanism for earthquake generation, it does not address the stress field within the slab but only the distribution of strength. Thus the distribution of focal plane mechanisms is not considered, only the locations where earthquakes due to plastic instabilities are possible. The absence of earthquakes does not necessarily mean that the slab does not exist, it only means that the slab is too hot to undergo plastic instability. This means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab (i.e. > circa 3°C km−1).

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.

Microstructure effects on tensile properties of tungsten-Nickel-Iron composites

Abstract: Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studiedvia slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties.

Study on the substructure evolution and flow behaviour in type 316L stainless steel over the temperature range 21-900°C

Abstract: Tensile specimens of type 316L stainless steel with a grain size of 5.0 μm have been deformed at a constant strain rate of 10−3 s−1 over the temperature range 21–900°C and by differential strain-rate test technique over strain rates from about 10−5 to 10−3 s−1 at temperatures in the range 750–900°C. The normalized yield and flow stresses against temperature plots exhibit three regions. While in regions I and III the stresses decrease with increasing temperature, they increase with increasing temperature in region II. Transmission electron microscopy studies on deformed specimens show that at small strains the dislocations generated at grain boundaries have characteristic distributions: in region I the dislocations are confined to the vicinity of the grain boundary, in region II the dislocations are spread into the grain interior, and in region III the dislocations rearrange to form walls. The evolution of substructure and the work-hardening behaviour are explained by considering both intragranula...

A Flow-line Model for Calculating the Surface Profile and the Velocity, Strain-rate, and Stress Fields in an Ice Sheet

Abstract: A flow-line model is presented for calculating the surface profile and the velocity, strain-rate, and stress fields in an ice sheet with given base-elevation profile, ice thickness at the dome (divide), flow-law parameters, mass-balance distribution, and convergence/divergence conditions along the flow line. The model, which is based on a “quasi-similarity” hypothesis as regards the horizontal velocity-depth profiles, accounts for changes along the flow line in the depth distributions of temperature, normal stress deviators, and possible enhanced flow of deep ice of Wisconsin origin. A curvilinear coordinate system is applied with horizontal axes along flow lines and surface-elevation contours, respectively. The flow equations are reduced to two differential equations, one for the surface-elevation profile, and the other for a profile function that determines the depth distributions of velocities and strain-rates. The two equations are coupled through a profile parameter that communicates the influence of velocity-profile changes to the surface-profile equation. It is shown that the variation along the flow line of this parameter should also be considered when deriving flow-law parameters from ice-sheet flow-line data. For a symmetric dome, explicit expressions are derived for the depth distributions of the vertical velocity, strain-rates, and stresses. The strain-rate profiles display an inflection about half-way down the ice sheet, and, in the case of isothermal ice, have surface values 2.2 times their depth-averaged values. The depth distribution of the vertical velocity indicates that a relatively thick layer of almost stagnant ice is present at the ice-sheet base below a dome.

An analysis of the temperature and rate dependence of Charpy V-notch energies for a high nitrogen steel

Abstract: The brittle-ductile transition for a high nitrogen steel is investigated by numerical analyses of the Charpy impact test. The material is described in terms of an elastic-viscoplastic constitutive model that accounts for the nucleation and growth of micro-voids, leading to ductile fracture, as well as for cleavage failure by micro-crack nucleation. The temperature dependence of flow strength and strain hardening is included in the model, and this leads to the prediction of a transition from cleavage fracture to predominantly ductile fracture as the temperature increases. For the particular steel considered it is found that the variation of strain hardening with temperature has a strong effect on the failure mode transition. Both slow loading and impact loading of the Charpy specimen are analyzed. Most of the computations are based on a quasi-static formulation since, even at the strain rates encountered in the Charpy impact test, material strain rate sensitivity is the main time effect. The influence of material inertia is investigated in a few transient analyses.

Creep-fatigue interaction of inconel 617 at 950°C in simulated nuclear reactor helium

Abstract: Strain-controlled fatigue tests have been conducted in impure helium, simulating the primary-circuit coolant of a high temperature gas-cooled reactor to ascertain the influence of strain rate ( e = 4 × 10 −3 to 2 × 10 −5 s −1 ), hold condition at peak strains (tension-only, compression-only and tension-plus-compression holds) and hold time (up to 120 min) on the low cycle fatigue behaviour of Inconel 617. A strain range of 0.6% and a temperature of 950°C were employed for all the tests. Microstructural changes which occurred during fatigue deformation were evaluated and damage mechanisms which influence fatigue life identified. A small reduction in fatigue life was found with decreasing e. Irrespective of the position of hold at peak strain in a cycle, the hold time always reduced the fatigue life in comparison with continuously cycled tests at lower strain rates but of equal cycle duration. Tensile holds were found to be most damaging, followed by compression holds. Symmetrical tension-plus-compression holds led to fatigue lives which were very close to those of the continuously cycled tests. In the continuously cycled tests with strain rates down to e = 6.7 × 10 −5 s −1 , failure was always transgranular with no indication of creep damage. However, for tests with tensile holds, creep damage was evidenced by grain boundary cavitation and oxidation, which were responsible for a reduction in the fatigue life. The formation of thick oxide scales at the surface observed at longer hold times led to chromium-depleted surface zones in which the carbide precipitates were dissolved. The loss of carbides increased grain boundary sliding which enhanced the formation of grain boundary cracks. These cracks shortened the critical length of surface fatigue cracks which, during fracture of the specimen, linked with the intergranular creep cracks in tensile hold tests. The damaging effect of compression hold was attributed to increased inelastic strain and deformation ratcheting found in the cycle; failure occurred by local accumulation of tensile plastic strain which finally caused tensile necking.

Rate effects on the compression and recovery of dimensions of cork

Abstract: A study of the effect of strain rate on the compression behaviour of cork was carried out, which takes into account the anisotropy of the material Compression curves at three different rates were obtained for each of the three directions in cork (radial, axial and tangential) Strain-rate sensitivity coefficients,m, were also measured in experiments where the strain rate was suddenly changed during the tests The values ofm are fairly isotropic, around 006 For a given strain rate, the radial direction is stronger (ie larger stresses) than the other two, but these are not equivalent, the axial direction being slightly stronger for most of the strain interval between 0 and 80% The recovery of dimensions following compression in each direction was also studied The change in the three dimensions with time was monitored, following compression to 30% and to 80% strain in a given direction In the first case, recovery is almost total after ∼ 20 days, but for 80% compression the deformation is not completely recovered after unloading The recovery rate decreases appreciably with time and increases with the degree of deformation previously imposed An equation is proposed that describes the recovery behaviour with a reasonable accuracy

Crack-enhanced creep in polycrystalline material: strain-rate sensitive strength and deformation of ice

Abstract: A non-linear viscoelastic creep equation for polycrystalline material is presented. It incorporates the effect of cracking and is capable of describing primary, secondary and tertiary behaviour. The model predicts the formation of microcracks and thus the damage state due to the high-temperature grain-boundary embrittlement process. This paper describes its application in formulating crack-enhanced creep and material response under constant strain-rate loading conditions (theoretically the simplest case but actually the most difficult to maintain). The formulation makes it possible to define the rate effect on stress-strain response and the rate sensitivity of strength, failure time, failure strain, damage and damage rate, strain recovery, etc. Numerical correspondence between theory and experiment was observed when predictions were compared with available closed-loop, controlled, constant strain-rate strength and deformation data on pure ice. Calculations made use of material constants determined from independent constant-load creep tests.

Seismic slip and down-dip strain rates in wadati-benioff zones.

Abstract: The rate of accumulation of seismic moment in Wadati-Benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere. Between depths of 75 and 175 kilometers a typical down-dip strain rate is about 10 -15 per second, which implies that slabs in this depth range typically accumulate strain of order 10 -1 . This result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes. Mantle seismicity (repeated catastrophic shear failure) is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs. Slabs are penetratively deformed, and they have low flexural rigidity compared to oceanic plates at the earth9s surface.

Experimental determination of thermal and adhesion stress in particle filled thermoplasts

Abstract: The phenomena of dewetting in filled thermoplastics were studied by light microscopy and were correlated with variations in the slope of stress-strain diagrams in constant strain rate tests. In such diagrams, kinks in the plots were found to correspond to the dewetting stress. The corresponding local stress at a filler particle is then equal to the sum of the thermal compressive stress and the adhesion stress. It was shown that the adhesion stress was proportional to the reciprocal root of the particle radius. Also, values of dewetting stress predicted for inorganic particles with radii smaller than 2–4 micrometers are higher than the stresses at which crazes and shear-bands are formed near such particles, indicating that dewetting will not occur in those cases, and adhesion aids may be superfluous.

Sinter-Forging Characteristics of fine-Grained Zirconia

Abstract: Powder preforms of zirconia, containing 2.85 mol% yttria, were sinter-forged in simple uniaxial compression at 1400/sup 0/C by applying constant displacement rates to the specimens. Shear and densification strains and the uniaxial stress were measured as a function of time. In contrast with alumina and silicon nitride, zirconia appears to densify by a dislocation mechanism. As a consequence, the densification rate is linked to the applied strain rather than to the applied hydrostatic pressure: the powder compact requires a critical amount of compressive strain to consolidate to full density, irrespective of the strain rate or the stress at which that strain is applied.

Cyclic deformation of single crystals of iron–silicon alloys oriented for single slip

Abstract: Single crystals of iron and its alloys with 0·5, 0·9, 1·8 and 3 wt% Si oriented for single slip have been cyclically deformed in tension–compression at plastic strains between 10−4 and 10−2. The effects of cyclic hardening, changes of specimen cross-section, slip traces and dislocation arrangements have been studied at 295 and 524 K and a strain rate of 1·6 x 10−3 s−1. The cyclic deformation of b.c.c. Fe–Si alloy crystals at both temperatures and of f.c.c. metal crystals shows similarly shaped cyclic hardening and cyclic stress–strain curves, and similar strain localization in the persistent slip bands (PSBs) and development of the dislocation arrangement. Unlike in f.c.c. metals, in Fe–Si crystals there is appreciable secondary slip at the beginning of the saturation stage, leading to a cell structure in PSBs. The dislocation cell structure is typical of PSBs throughout their whole existence. The surface relief of emerging PSBs also differs from that in f.c.c. metals. Changes in specimen cross-s...

Test temperature and strain rate effects on the properties of a tungsten heavy alloy

Abstract: Liquid phase sintered tungsten heavy alloy specimens with a 90W-7Ni-3Fe composition were tested for temperature and strain rate effects on mechanical behavior Both fracture stress and strain were measured for samples tested at 20, 300, or 600 °C, with crosshead speeds ranging from 0004 to 400 mmJs in an argon atmosphere Fracture surface examinations showed a dramatic increase in tungsten cleavage as the ductility increased The effect of an increasing strain rate is a slight strength increase with a concomitant ductility decrease Alternatively, higher test temperatures degrade strength with a nonsystematic effect on ductility; maximum ductility occurs at 300 °C and a slow strain rate Surface oxidation at 600 °C greatly degrades ductility The results are mathematically modeled using classic strain rate dependent equations