Showing papers on "Strain hardening exponent published in 2001"

Tensile strain-hardening behavior of polyvinyl alcohol engineered cementitious composite (pva-ecc)

Abstract: A high-performance polyvinyl alcohol fiber-reinforced engineered cementitious composite (PVA-ECC) was developed for structural applications under the performance-driven design approach. Fiber, matrix, and fiber/matrix interfacial properties were tailored to micromechanics models to satisfy the pseudo strain-hardening condition. This research experimentally investigated the effects of fiber surface treatment and sand content on the composite performance. Results from uniaxial tensile tests show an ultimate strain exceeding 4%, as well as an ultimate strength of 4.5 MPa for the composites, with a moderate fiber volume fraction of 2%. The specimens reveal saturated multiple cracking with crack width at ultimate strain limited to below 100 nanometers. The underlying reason of the distinctly different tensile behavior between normal fiber-reinforced concrete and PVA-ECC is highlighted by the comparison of complementary energy from their fiber bridging stress and crack opening curves.

Deformation behavior and plastic instabilities of ultrafine-grained titanium

Abstract: Ultrafine-grained (UFG) Ti samples have been prepared using equal channel angular pressing followed by cold rolling and annealing. The deformation behavior of these materials, including strain hardening, strain rate dependence of flow stress, deformation/failure mode, and tensile necking instability, have been systematically characterized. The findings are compared with those for conventional coarse-grained Ti and used to explain the limited tensile ductility observed so far for UFG or nanocrystalline metals.

A computational model of viscoplasticity and ductile damage for impact and penetration

Abstract: A coupled constitutive model of viscoplasticity and ductile damage for penetration and impact related problems has been formulated and implemented in the explicit finite element code LS-DYNA. The model, which is based on the constitutive model and fracture strain model of Johnson and Cook, and on continuum damage mechanics as proposed by Lemaitre, includes linear thermoelasticity, the von Mises yield criterion, the associated flow rule, non-linear isotropic strain hardening, strain-rate hardening, temperature softening due to adiabatic heating, isotropic ductile damage and failure. For each of the physical phenomena included in the model, one or several material constants are required. However, all material constants can be identified from relatively simple uniaxial tensile tests without the use of numerical simulations. In this paper the constitutive model is described in detail. Then material tests for Weldox 460 E steel and the calibration procedure are presented and discussed. The calibrated model is finally verified and validated through numerical simulations of material and plate perforation tests investigated experimentally.

Dynamic Recrystallization in Pure Magnesium

Abstract: Microstructural evolution of commercial grade pure magnesium was studied during plastic deformation by torsion under high pressure at ambient temperature and by compression at temperatures ranging from 293 to 773 K and at a strain rate of 3 x 10 -3 s -1 . Grain refinement takes place by operation of dynamic recrystallization (DRX) at all examined temperatures. The mechanisms of DRX change with temperature and strain. As a result, unusual dependencies of recrystallized grain size against strain and recrystallized volume fraction against temperature are observed. In the temperature interval of 293-623 K the deformation twinning results in twin mechanism of DRX, which processes strain softening at an initial stage of deformation. At T ≤ 423 K the other mechanism of low temperature DRX takes place at high strains. Such DRX is accompanied by strain hardening. In contrast, continuous DRX (CDRX) yielding a steady-state flow operates frequently at temperatures ranging from 523 to 773 K. CDRX occurs mainly in overall recrystallization process at elevated temperatures. Discontinuous DRX (DDRX) takes place by bulging of boundaries of coarse recrystallized grains evolved from twins at T = 723 K. DDRX occurs repetitively, but gives an insignificant contribution into total recrystallization process. The present results suggest that the mechanisms of DRX and the deformation mechanisms are closely related.

Surface tension driven jet break up of strain-hardening polymer solutions

Abstract: Experimental studies attempting to ascertain the influence of viscoelasticity on the atomization of polymer solution are often hindered by the inability to decouple the effect of shear thinning from the effect of extensional hardening. Here, the influence of viscoelasticity on the jet break up of a series of non-shear-thinning viscoelastic fluids is quantified. Previous characterization using an opposed-nozzle rheometer identified the critical extensional rates for strain hardening of these model fluids. The strain hardening fluids exhibit a beads-on-string structure with reduction or elimination of satellite drops. Capillary instabilities grow on the filaments connecting the spheres and eventually break the filaments up into a string of very small drops about one order of magnitude smaller than the satellite drops formed by a Newtonian fluid with the same shear viscosity, surface tension, and density. These results confirm that strain hardening is the key rheological property in jet break up and that the critical extensional rate of a fluid is pertinent in determining the final characteristics of break up. Results suggest that the opposed-nozzle rheometer does probe extensional behavior in the range of extensional rates that are relevant to jet break up, providing a tool to roughly predict jet break up.

Precipitation Kinetics and Strengthening of a Fe-0.8wt%Cu Alloy

Abstract: Precipitation kinetics and strengthening have been investigated for a Fe-0.8wt%Cu alloy. Microstructure evolution during aging at 500°C has been studied by a combination of Transmission Electron Microscopy and Small-Angle X-ray Scattering to provide information on the nature and location of the precipitates as well as a quantitative estimate of their size and volume fraction. The associated mechanical properties have been studied by hardness and tensile tests. The precipitation kinetics measured in this study are fully compatible with results reported for alloys with higher Cu levels. Nucleation of Cu precipitates is promoted by the presence of dislocations whereas coarsening rates in the later stages of aging appear to be not affected by fast diffusion paths along dislocations The strength of individual precipitates increases with precipitate size based on the analysis of the mechanical test results. However, the strength of the largest precipitates observed remains approximately half of the strength required for the Orowan by-passing mechanism. The Russell-Brown model for modulus strengthening has successfully been applied to the current data. Study of the plastic behavior shows that the maximum initial hardening rate is related to the highest strength of the material. This unusual result may be explained by a dynamic strained-induced phase transformation of the precipitates from the bcc to the 9R structure. Consequently, the hardening potential of Fe-Cu alloys is associated with good plastic properties close to peak strength thereby indicating the excellent potential of copper as hardening element for the development of novel high strength interstitial free IF steels.

Indentation of a soft metal film on a hard substrate: Strain gradient hardening effects

Abstract: A new type of nanoindentation experiment showing the effect of a strain gradient on the flow strength of a crystalline material is conducted and analyzed. We show that by indenting a soft metal film (Al) on a hard substrate (glass) with a sharp diamond indenter a strong gradient of plastic strain is created. The true hardness of the film is observed to increase with increasing depth of indentation when the indenter tip approaches the hard substrate, in sharp contrast to the falling hardness with increasing depth in bulk materials. We associate this rise in hardness with the strong gradient of plastic strain created between the indenter and the hard substrate. We use the mechanism-based strain gradient (MSG) plasticity theory to model the observed indentation behavior. The modeling shows that the MSG plasticity theory is capable of describing not only the decreasing hardness with increasing depth of indentation at shallow indentations, as observed in bulk materials, but also the rise in hardness that occurs when the indenter tip approaches the film/substrate interface.

Cyclic behavior of ultrafine-grain titanium produced by severe plastic deformation

Abstract: The cyclic behavior is investigated of commercial purity massive ultrafine-grained titanium obtained by severe plastic deformation through equal channel angular pressing (ECAP). Both stress- and strain-controlled experiments were carried out to access the fatigue performance in terms of Wohler diagram, fatigue limit, Coffin–Manson plot, cyclic hardening curves and cyclic stress-strain curves in a range of plastic strain amplitudes from 7.5×10 −4 to 10 −2 . A significant enhancement of fatigue limit and fatigue life in the ultrafine-grained state is found under constant stress testing. No cyclic softening and degradation in the strain-controlled experiments is noticed in titanium contrary to wavy slip materials such as Cu subjected to ECAP. A simple one-parameter dislocation-based model is proposed to account for experimental results. It is shown that many cyclic properties of severely predeformed materials with fine grains can be rationalized in terms of Hall–Petch grain boundary hardening and dislocation hardening.

Melt Rheology of High l-Content Poly(lactic acid)

Abstract: The melt rheology of linear poly(lactic acid)s (PLA) characterized by a high content of the l-form of the monomer is comprehensively investigated. Measurements of dynamic, steady, and transient shear viscosities are presented. Extensional data on PLA are presented for the first time and show a strong strain hardening behavior. The Cox−Merz relationship is obeyed over a particularly wide range (roughly 3 decades of shear rate). Results for high molecular weight samples suggest that the plateau modulus is approximately 5 × 105 Pa. In addition, the zero shear viscosity, η0, for these materials is found to roughly scale with the expected 3.4 power vs molecular weight. The transient shear results are satisfactorily predicted using a truncated form of the K−BKZ constitutive equation and a set of Maxwell modes (Gk, λk) derived from the dynamic spectra. However, to capture the observed extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate...

An Exactly Solvable Phase-Field Theory of Dislocation Dynamics, Strain Hardening and Hysteresis in Ductile Single Crystals

Abstract: An exactly solvable phase-field theory of dislocation dynamics, strain hardeni ng and hysteresis in ductile single crystals is developed. The theory accounts f or: an arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations resulting from a piecewise qu adratic Peierls potential; the interaction between the dislocations and an applied resolved shear stress field; and the irreversible interactions with short-range obstacles and lattice friction, resulting in h ardening, path dependency and hysteresis. A chief advantage of the present theory is that it is analytically tr actable, in the sense that the complexity of the calculations may be reduced, with the aid of closed form analytical solutions, to the determination of the value of the phase field at point-obstacle sites. In particul ar, no numerical grid is required in calculations. The phase-field representation enables complex geometrica l and topological transitions in the dislocation ensemble, including dislocation loop nucleation, bow-out, pinching, and the formation of Orowan loops. The theory also permits the consideration of obstacles of varying strengths and dislocation lineenergy anisotropy. The theory predicts a range of behaviors which are in qualitative agreement with observation, including: hardening and dislocation multiplication in single slip under monotonic loading; the Bauschinger effect under reverse loading; the fading memory effect, whereby reverse yielding gradually eliminates the influence of previous loading; the evol ution of the dislocation density under cycling loading, leading to characteristic ‘butterfly’ curves; and others.

Shear Banding in True Triaxial Tests and Its Effect on Failure in Sand

Abstract: The occurrence of failure, mechanisms that create failure, and soil behavior in the vicinity of failure have been investigated. One mechanism is smooth peak failure, in which the soil continues to behave as a continuum with uniform strains, and smooth peak failure is followed by strain softening. Another mechanism is shear banding, whose occurrence in the plastic hardening regime limits the strength of the soil. True triaxial tests have been performed on tall prismatic specimens of Santa Monica Beach sand at three relative densities in a modified version of a cubical triaxial apparatus to study the effect of shear banding on failure in the full range of the intermediate principal stress. The experiments show that the strength increases as b [=(σ2 − σ3)/(σ1 − σ3)] increases from 0 to about 0.18, remains almost constant until b reaches 0.85, and then decreases slightly at b = 1.0. Shear banding initiates in the hardening regime for b-values of 0.18–0.85. Thus, peak failure is caused by shear banding in this...

Cyclic deformation behavior of single crystal NiTi

Abstract: Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.

Comparisons between three-dimensional and two-dimensional multi-particle unit cell models for particle reinforced metal matrix composites

Abstract: Three-dimensional and two-dimensional unit cell models for describing the mechanical behaviour of particle reinforced metal matrix composites (MMCs) are compared by assessing predictions obtained from microgeometries consisting of 20 randomly positioned elastic particles embedded in an elastoplastic matrix. The elastic response to uniaxial loading predicted by the three-dimensional unit cells is found to comply with the appropriate three-point bounds. Predictions for the elastoplastic regime are somewhat less satisfactory, indicating that configurations containing a higher number of particles will be required to resolve the regions of concentrated plastic strains that develop in inhomogeneous materials. This implies that in the nonlinear range the size of reference volume elements depends on material behaviour. Comparisons of results obtained from planar and three-dimensional multi-particle unit cells show clear differences in terms of both the overall stiffnesses and phase averages as well as the standard deviations of the microscale stress and strain fields. These differences are much more pronounced in the elastoplastic range, where planar analyses do not adequately describe the overall strain hardening behaviour of particle reinforced MMCs and tend to markedly underpredict the equivalent stresses and maximum principal stresses in the particles.

Microstructural Interpretation on Reliquefaction of Saturated Granular Soils under Cyclic Loading

Abstract: It is well known that the resistance to liquefaction of a saturated sand decreases sharply when it has been presheared, either cyclically or quasistatically, beyond a threshold value. The possible mechanism is discussed in light of recent findings on the microstructural anisotropy developed in preshearing (induced anisotropy). A column-like structure, through which applied stress is mainly transmitted, grows parallel to the major principal stress direction in the strain hardening process. Voids, randomly distributed at first, are also connected in series between the column-like structures. The anisotropic structure can carry the increasing stress as long as the major stress is applied parallel to the elongation direction of the structure. However, it becomes extremely unstable when the major stress is rotated. The excess porewater pressure increases markedly under undrained cyclic loading, particularly when the connected voids are stressed perpendicular to their elongation direction. This is the reason why once liquefied sand sharply loses liquefaction resistance in a subsequent reliquefaction test.

Identification of plastic hardening parameters of metals from spherical indentation tests

Abstract: Using the two-parameter power hardening rule σ=kepm, the parameters k and m are identified from spherical indentation loading–unloading tests which account for the variation of the indentation profile during elastic unloading and sphere deformation. The predicted and measured stress–strain curves are compared for several materials. Both experimental and actual data for 18G2A low-alloy steel are used to assess the accuracy of the identification procedure. Finally, identification of the stress–strain curve of an aluminium alloy is demonstrated.