Showing papers on "Strain hardening exponent published in 2011"

Strain-hardening UHP-FRC with low fiber contents

Abstract: This research work focuses on the optimization of strength and ductility of ultra high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading. An ultra high performance concrete (UHPC) with a compressive strength of 200 MPa (29 ksi) providing high bond strength between fiber and matrix was developed. In addition to the high strength smooth steel fibers, currently used for typical UHP-FRC, high strength deformed steel fibers were used in this study to enhance the mechanical bond and ductility. The study first shows that, with appropriate high strength steel fibers, a fiber volume fraction of 1% is sufficient to trigger strain hardening behavior accompanied by multiple cracking, a characteristic essential to achieve high ductility. By improving both the matrix and fiber parameters, an UHP-FRC with only 1.5% deformed steel fibers by volume resulted in an average tensile strength of 13 MPa (1.9 ksi) and a maximum post-cracking strain of 0.6%.

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Abstract: Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.

Hot deformation behavior of a medium carbon microalloyed steel

Abstract: The hot deformation behavior of a medium carbon microalloyed steel was studied using the hot compression flow curves corresponding to the temperature range of 850–1150 °C under strain rates from 0.0001 to 3 s −1 . A step-by-step procedure for data analysis in hot deformation was also given. The work hardening rate versus stress curves were used to reveal if dynamic recrystallization (DRX) occurred. The application of constitutive equations to determine the hot working constants of this material was critically discussed. Furthermore, the effect of Zener–Hollomon parameter ( Z ) on the characteristic points of flow curves was studied using the power law relation. The deformation activation energy of this steel was determined as 394 kJ/mol and the normalized critical stress and strain for initiation of DRX were found to be 0.89 and 0.62, respectively. Some behaviors were also compared to other steels.

A Lagrangian framework for analyzing microstructural level response of polymer-bonded explosives

Abstract: A cohesive finite element method (CFEM) framework for quantifying the thermomechanical response of polymer-bonded explosives (PBXs) at the microstructural level is developed. The analysis carried out concerns the impact loading of HMX/Estane at strain rates on the order of 104?105?s?1. Issues studied include large deformation, thermomechanical coupling, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating. The polymer matrix is described by a thermo-elasto-viscoelastic constitutive formulation, accounting for temperature dependence, strain rate sensitivity and strain hardening. The HMX crystals are assumed to be elastic. The CFEM framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be tracked and analyzed. Digitized micrographs of actual PBX materials and idealized microstructures with Gaussian distributions of grain sizes are used in the analysis. The formation of local hot spots as potential ignition sites is primarily due to the viscoelastic dissipation in the matrix in early stages of deformation and frictional heating along crack surfaces in later stages of deformation. The framework is a useful tool for the design of energetic composites and the results can be used to establish microstructure?response relations that can be used to assess the performance of energetic composites.

The growth and coalescence of ellipsoidal voids in plane strain under combined shear and tension

Abstract: New extensions of a model for the growth and coalescence of ellipsoidal voids based on the Gurson formalism are proposed in order to treat problems involving shear and/or voids axis not necessarily aligned with the main loading direction, under plane strain loading conditions. These extensions are motivated and validated using 3D finite element void cell calculations with overall plane strain enforced in one direction. The starting point is the Gologanu model dealing with spheroidal void shape. A void rotation law based on homogenization theory is coupled to this damage model. The predictions of the model closely agree with the 3D cell calculations, capturing the effect of the initial void shape and orientation on the void rotation rate. An empirical correction is also introduced for the change of the void aspect ratio in the plane transverse to the main axis of the void departing from its initially circular shape. This correction is needed for an accurate prediction of the onset of coalescence. Next, a new approach is proposed to take strain hardening into account within the Thomason criterion for internal necking, avoiding the use of strain hardening-dependent fitting parameters. The coalescence criterion is generalized to any possible direction of the coalescence plane and void orientation. Finally, the model is supplemented by a mathematical description of the final drop of the stress carrying capacity during coalescence. The entire model is developed for plane strain conditions, setting the path to a 3D extension. After validation of the model, a parametric study addresses the effect of shear on the ductility of metallic alloys for a range of microstructural and flow parameters, under different stress states. In general, the presence of shear, for identical stress triaxiality, decreases the ductility, partly explaining recent experimental results obtained in the low stress triaxiality regime.

Mechanical behaviour of strain hardening cement-based composites under impact loading

Abstract: This paper describes the material behaviour of a strain hardening cement-based composite (SHCC) at high strain rates. The results of highly dynamic spall experiments using a Hopkinson bar at strain rates 140–180 s −1 are arrayed against the results of quasi-static uniaxial tensile tests at strain rates of 0.001 s −1 . This comparison is based on the values of tensile strength, Young’s modulus, and fracture energy of the specimens. In addition, the experimental results of SHCC are related to the characteristic values of other concrete types. Differences in material behaviour are explained by the phenomena of crack formation and fibre pullout resistance.

Tensile deformation of a duplex Fe–20Mn–9Al–0.6C steel having the reduced specific weight

Abstract: The room temperature deformation characteristics of a duplex Fe–20Mn–9Al–0.6C steel with the reduced specific weight of 6.84 g/cm3 in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800–1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m2. Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.

Strain hardening behavior of a TRIP/TWIP steel with 18.8% Mn

Abstract: Tensile tests were carried out to study the strain hardening behavior of a TRIP/TWIP steel with 18.8% manganese. The results indicated that the true stress–strain curve can be divided into 4 stages in tension testing. Material is in an elastic region when the true strain is below 0.06. In the initial stage of the plastic deformation (ɛ = 0.06–0.14), ɛ-martensite was preliminarily formed, and that austenite transformed to α-martensite through the ɛ-martensite formation. When the true strain was between 0.14 and 0.35, the stacking fault energies were elevated by the increase of strain energy, deformation twinning occurred instead of the ɛ-martensite formation. The second derivative of the stress–strain curve satisfied the condition d2σ/dɛ2 > 0. Twinning induced plasticity dominated this stage. In the last plastic deformation stage (ɛ = 0.35–0.45), γ → α transformation occurred at the crossing of twins, and α-martensite grew along the thickness of the twinned regions.

Tensile and fatigue properties of a cast aluminum alloy with Ti, Zr and V additions

Abstract: The development of aluminum alloys for automotive powertrain applications is in high demand due to the required weight reduction and fuel efficiency. The aim of this study was to evaluate the microstructure and mechanical properties of a newly developed Al–7%Si–1%Cu–0.5%Mg cast alloy with further additions of Ti, Zr and V. The microstructure of the alloys consisted of Al dendrites surrounded by Al–Si eutectic structures with Mg/Cu/Fe-containing Si particles, and contained nano-sized trialuminide precipitates in the Ti/Zr/V added alloys. The alloys had a significantly (60–87%) higher yield strength but lower ductility than A356-T6 and 319-T6 alloys. With the addition of Ti/Zr/V both monotonic and cyclic yield strengths increased, but ductility and hardening capacity decreased due to reduced dislocation storage capacity caused by stronger interactions between dislocations and trialuminide precipitates. The Zr/V-modified alloy had a longer fatigue life, and all the alloys exhibited cyclic stabilization at low strain amplitudes and cyclic hardening at higher strain amplitudes. With increasing strain amplitude, the extent of cyclic hardening increased. Both cyclic yield strength and cyclic strain hardening exponent were higher than the corresponding monotonic yield strength and strain hardening exponent, indicating that a stronger cyclic hardening ability of the alloys developed. Fatigue cracks were observed to initiate at near-surface defects, and crack propagation was mainly characterized by the formation of fatigue striations together with secondary cracks.

Behaviour of Strain-Hardening Cement-Based Composites Under High Strain Rates

Abstract: In this research project the behaviour of strain-hardening cement-based composites (SHCC) subjected to low and high strain rates was studied. Uniaxial tension tests on dumbbell-shaped SHCC specimens were performed at rates ranging from 10 -5 s -1 to 50s -1 . For the tests performed at strain rates of 10 -2 s -1 and below, SHCC yielded a moderate increase in tensile strength and simultaneous decrease in strain capacity with increasing strain rate. When tested for higher strain rates from 10 to 50s -1 a considerable increase in tensile strain and strain capacity was measured. Microscopic investigation of the fracture surfaces showed that almost no fibre failure and an average pullout length of 2.5mm were found in the high strain rate test. This observation is in contrast to that of rapid quasi-static testing, where the average fibre pullout length of 300μm was much shorter. Furthermore, the fibres on the fracture surfaces produced in the high rate tests exhibited pronounced plastic deformations. Finally, quasi-static and high-speed tension tests on individual fibres and single fibre pullout tests were performed. While the increase in the tensile strength of the fibre was only moderate in the range of strain rates investigated, a considerable increase in bond strength between fibre and matrix was determined.