Showing papers on "Strain rate published in 1996"

The dynamic compressive behavior of beryllium bearing bulk metallic glasses

Abstract: In 1993, a new beryllium bearing bulk metallic glass with the nominal composition Zr41.25Ti13.75Cu12.5Ni10Be22.5 was discovered at Caltech. This metallic glass can be cast as cylindrical rods as large as 16 mm in diameter, which permitted specimens to be fabricated with geometries suitable for dynamic testing. For the first time, the dynamic compressive yield behavior of a metallic glass was characterized at strain rates of 102 to 104/s by using the split Hopkinson pressure bar. A high-speed infrared thermal detector was also used to determine if adiabatic heating occurred during dynamic deformation of the metallic glass. From these tests it appears that the yield stress of the metallic glass is insensitive to strain rate and no adiabatic heating occurs before yielding.

Topology of fine-scale motions in turbulent channel flow

Abstract: An investigation of topological features of the velocity gradient field of turbulent channel flow has been carried out using results from a direct numerical simulation for which the Reynolds number based on the channel half-width and the centreline velocity was 7860. Plots of the joint probability density functions of the invariants of the rate of strain and velocity gradient tensors indicated that away from the wall region, the fine-scale motions in the flow have many characteristics in common with a variety of other turbulent and transitional flows: the intermediate principal strain rate tended to be positive at sites of high viscous dissipation of kinetic energy, while the invariants of the velocity gradient tensor showed that a preference existed for stable focus/stretching and unstable node/saddle/saddle topologies. Visualization of regions in the flow with stable focus/stretching topologies revealed arrays of discrete downstream-leaning flow structures which originated near the wall and penetrated into the outer region of the flow. In all regions of the flow, there was a strong preference for the vorticity to be aligned with the intermediate principal strain rate direction, with the effect increasing near the walls in response to boundary conditions.

Constitutive behavior of tantalum and tantalum-tungsten alloys

Abstract: The effects of strain rate, temperature, and tungsten alloying on the yield stress and the strainhardening behavior of tantalum were investigated The yield and flow stresses of unalloyed Ta and tantalum-tungsten alloys were found to exhibit very high rate sensitivities, while the hardening rates in Ta and Ta-W alloys were found to be insensitive to strain rate and temperature at lower temperatures or at higher strain rates This behavior is consistent with the observation that overcoming the intrinsic Peierls stress is shown to be the rate-controlling mechanism in these materials at low temperatures The dependence of yield stress on temperature and strain rate was found to decrease, while the strain-hardening rate increased with tungsten alloying content The mechanical threshold stress (MTS) model was adopted to model the stress-strain behavior of unalloyed Ta and the Ta-W alloys Parameters for the constitutive relations for Ta and the Ta-W alloys were derived for the MTS model, the Johnson—Cook (JC), and the Zerilli-Armstrong (ZA) models The results of this study substantiate the applicability of these models for describing the high strain-rate deformation of Ta and Ta-W alloys The JC and ZA models, however, due to their use of a power strain-hardening law, were found to yield constitutive relations for Ta and Ta-W alloys that are strongly dependent on the range of strains for which the models were optimized

Deformation behavior of Zr65Al10Ni10Cu15 glassy alloy with wide supercooled liquid region

Abstract: We report the stress‐strain behavior for a Zr65Al10Ni10Cu15 (at.%) glassy alloy with a significant supercooled liquid region of 105 K. The transition temperature from inhomogeneous deformation to a homogeneous one increases with increasing strain rate. The alloy in the supercooled liquid state, however, exhibited the homogeneous deformation even at higher strain rates above 0.5 s−1. The strength was also dependent on the strain rate and independent of temperature in the inhomogeneous deformation region, and was sensitive to both strain rate and temperature in the homogeneous region. The stress‐strain curves in the homogeneous deformation mode were accompanied by a stress overshoot and its height increases with increasing temperature and strain rate.

Comparative study of strain rate effects on mechanical properties of glass fibre-reinforced thermoset matrix composite

Abstract: The tensile dynamic behaviour of glass fibre-reinforced phenolic and polyester resins has been determined in order to find the influence of strain rate on the mechanical properties of composite materials produced by the resin transfer moulding (RTM) and pultrusion processes Data and experimental test systems from the literature are analysed A new specimen design is created and validated using drop-weight dynamic tests The dynamic elastic modulus and strength tend to increase in an important ratio for the majority of the materials studied The shear modulus measured with off-axis and ±45° coupons produces different effects as a function of strain rate The influence of the reinforcement structure is emphasized and shown to be effective

Stress relaxation and elastic decohesion of viscoelastic polymer solutions in extensional flow

Abstract: The evolution of the transient extensional stresses in dilute and semi-dilute viscoelastic polymer solutions was measured with a filament stretching rheometer of a design similar to that first introduced by Sridhar et al. The solutions were polystyrene-based Boger fluids which were stretched at constant strain rates in the range 0.6 ⩽ ϵ 0 ⩽ 4 s−1 and to Hencky strains of ϵ > 4. The test fluids all strain-hardened and Trouton ratios exceeding 1000 were obtained at high strains. In addition to measuring the transient tensile stress growth, the decay of the tensile viscoelastic stress in the fluid column following cessation of uniaxial elongation was also monitored as a function of the total imposed Hencky strain and the strain rate. The measured relaxation functions were found to be significantly different from those observed following cessation of steady shear flow. The extensional stresses initially decayed very rapidly upon cessation of uniaxial elongation followed by a slower viscoelastic relaxation. For the most elastic fluids, partial decohesion of the fluid filament from the endplates of the rheometer was observed in tests conducted at high strain rates. This elastic instability is initiated near the rigid endplate fixtures of the device and it results in the progressive breakup of the fluid column into individual threads or ‘fibrils’ with a regular azimuthal spacing. These fibrils elongate and bifurcate as the fluid sample is elongated further. In tests conducted at the highest Deborah numbers, complete sample decohesion from the endplates and rapid elastic recoil were sometimes observed. The critical stress and strain at the onset of the instability were determined by monitoring the tensile force exerted by the filament, the sample radius, and were used to construct an approximate stability diagram. Flow visualization experiments using a modified stretching device showed that the instability develops as a consequence of an axisymmetry-breaking meniscus instability in the non-homogeneous region of highly deformed fluid near the rigid endplate.

Molecular dynamics simulations of the rheology of normal decane, hexadecane, and tetracosane

Abstract: Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain‐rate‐dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power‐law shear‐thinning behavior with a slope between about −0.40 and −0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45° at very low strain rate and is significantly smaller at high strain rate. This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian viscosity can be obtained from the plateau value of the shear viscosity at the lowest strain rates calculated from the nonequilibrium molecular dynamics simulation (NEMD). This is demonstrated by comparing the viscosity of decane obtained by extrapolating the NEMD simulation with an independent calculation using the standard Green–Kubo method. The transition from the non‐Newtonian regime to the Newtonian regime is also correlated with the disappearance of orientational order and with the longest relaxation time of the liquid alkanes simulated.

High-Strain-Rate Superplasticity in Metallic Materials and the Potential for Ceramic Materials

Abstract: High-strain-rate superplasticity (i.e., superplastic behavior at strain rates over 10-2s-1) has been observed in many meterials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained stucture of less than about 3 μm. Its deformation mechanism appears to be different from that in conventional superplastic materials. Experimental investigations showed that a maximum elongation was attained at a temperature close to the partial melting temperature in many superplastic materials exhibiting high-strain-rate superplasticity. Recently, a new model, which was considered from the viewpoint of the accommodation mechanism by an accommodatin helper such as a liquid or glassy phase, was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. The critical conditions of the quantity and distribution of a liquid phase for optimizing superplastic deformation was discussed and then applied to consider the possibility of attaining high-strain-rate superplasticity in ceramic materials.

Mechanical properties of mullite materials

Abstract: The mechanical behaviour of two 3Al2O3 · 2SiO2 dense mullite materials with the same level of impurities but different in nature has been studied Microstructure has been characterized by SEM and TEM Toughness, bend strength and Young's modulus have been determined from room temperature up to 1400 °C Dependence of toughness on strain rate has been investigated Special attention has been paid to correlate the trend of the mechanical parameters to fractographic observations by SEM

Compressibility of clays: fundamental and practical aspects

Abstract: The compressibility of natural clays is influenced by numerous factors: strain rate, temperature, sampling disturbance, stress path, and some restructuring factors. The first part of the paper reviews the effects of these factors, in particular of strain rate and temperature. The influence of drainage conditions on the effective stress-strain curves followed in various subelements of a consolidating clay layer is also discussed. In a second part, in-situ conditions are considered. In the overconsolidated range, and at the preconsolidation pressure, the behavior is influenced by most of the aforementioned factors, and can be compared with laboratory test results only on the basis of a semiempirical approach. In the normally consolidated range, major factors are strain rate and temperature, and their effect can be evaluated. In some cases, however, structuring phenomena can exist and decrease the viscous effects. Finally, practical conclusions concerning the evaluation of long-term settlements are given.

Characteristics of Static and Metadynamic Recrystallization and Strain Accumulation in Hot-deformed Austenite as Revealed by the Stress Relaxation Method

Abstract: The newly established technique of stress relaxation has been applied to measure the kinetics of static and metadynamic recrystallization of austenite in a low-carbon steel subsequent to compression executed at a strain rate of 0.1 or 0.01 s–1 at 900°C or 1000°C. The characteristics of static recrystallization were found to be consistent with those previously reported from double-stage deformation tests. Metadynamic recrystallization, contrary to static one, showed no dependence on strain and hardly any on temperature, but significant dependence on strain rate. The Avrami exponents were almost identical for the two processes, about 1.5-1.6 at 0.1 s–1 but decreased to 1.0-1.3 at 0.01 s–1. Metadynamic recrystallization resulted in complete softening except when relaxed after compression to a strain of 0.3 or beyond at a low strain rate of 0.01 s–1 The law of mixtures approach was found to be more accurate than the uniform softening model to describe recrystallization in partially recrystallized and subsequently deformed austenite. The results confirm the feasibility of the stress relaxation technique as an efficient method for investigating recrystallization kinetics in hot-deformed austenite.

The strain-rate and temperature dependence of the mechanical properties of polyetherketone and polyetheretherketone

Abstract: The mechanical properties of polyetheretherketone and polyetherketone were studied as a function of strain rate and temperature in the ranges 10−3 to 103s−1 and 20–200°C. At temperatures below the glass transition temperature, Tg, the strain-rate sensitivity of both polymers was found to be almost independent of temperature with a value of ∼4 MPa per decade of strain rate and the behaviour was well described by an Eyring relationship leading to an activation volume of ∼1 nm3. Above Tg, X-ray and differential scanning calorimetry studies show that cold-crystallization phenomena play a very important part in the polymer behaviour, leading to an increase of yield stress with increasing temperature. The crystallization was found to be highly strain-rate dependent with no increase in crystalline content occurring in quasi-static tests and increases of up to 20% in higher rate tests. Much of the data have been gathered using novel test equipment which is described in the paper.

Scalar imaging velocimetry measurements of the velocity gradient tensor field in turbulent flows. II. Experimental results

Abstract: Scalar imaging velocimetry is here applied to experimental turbulent flow scalar field data to yield the first fully resolved, non‐intrusive laboratory measurements of the spatio‐temporal structure and dynamics of the full nine‐component velocity gradient tensor field ∇u(x,t), as well as the pressure gradient field ∇p(x,t), in a turbulent flow. Results are from turbulent flows at outer scale Reynolds numbers in the range 3,000≤Reδ≤4,200, with Taylor scale Reynolds numbers Reλ≊45. These give a previously inaccessible level of detailed experimental access to the spatial structure in the velocity gradient tensor field at the small scales of turbulent flows, and through the much longer temporal dimension of these four‐dimensional data spaces allow access to the inertial range of scales as well. Sample spatio‐temporal data planes and probability distributions spanning more than 75 advection time scales (λν/U) are presented for various dynamical fields of interest, including the three components of the velocity field u(x,t), the nine components of the velocity gradient tensor field ∇u(x,t) through the full vector vorticity field ωi(x,t) and tensor strain rate field eij(x,t), the kinetic energy dissipation rate field Φ(x,t)≡2νe:e(x,t), the enstrophy field 1/2ω⋅ω(x,t), the enstrophy production rate field ω⋅e⋅ω(x,t), and the pressure gradient field ∇p(x,t). Continuity tests show agreement with the zero divergence requirement that exceeds the highest values reported from single‐point, invasive, multi‐probe measurements. Distributions of strain rate eigenvalues as well as alignments of the strain rate eigenvectors with both the vorticity and scalar gradient vectors are in agreement with DNS results, as are distributions of the measured helicity density fields u⋅ω(x,t). Results obtained for the true kinetic energy dissipation rate field show good agreement, up to 14th‐order, with previous inertial range structure function exponents measured by Anselmet et al. [J. Fluid Mech. 140, 63 (1984)] at much higher Reynolds numbers. In addition, probability distributions scaled on inner variables show good agreement among buoyant and non‐buoyant turbulent flow cases, further suggesting that these results are largely indicative of the high Reynolds number state of the inner scales of fully developed turbulent flows.

Magnetorheological fluid composites

Abstract: An interesting extension in the use of magnetic fluids has resulted from the development of magnetic fluid composites obtained by dispersing micrometre-sized non-magnetic particles in a magnetic fluid. The composites possess a yield stress in a magnetic field which can be described at sufficiently high strain rates by the Bingham relation , where is the shear stress perpendicular to the applied field, the extrapolated yield stress, the strain rate and the plastic viscosity. Thus, a composite, particle concentration , in a field 0.036 T with has a yield stress of 26 Pa. The yield stresses obtained experimentally for different and correspond well to values predicted theoretically by Rosensweig using a determination of based on a continuum concept of unsymmetric stress that develops in the deformed but unyielded anisotropic medium.

Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet

Abstract: Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation, which produced a tensile elongation of 671 pct at 525 °C.

Strain Rate and Preshear Effects in Cyclic Resistance of Soft Clay

Abstract: Cyclic constant-volume direct simple shear tests were performed on intact specimens of a sensitive clay reconsolidated to the in-situ stress state resulting in an overconsolidation ratio (OCR) of 2.2. The tests were conducted for different values of initial static undrained shear stress. The results confirmed that the high strain rate associated with cyclic loading has a significant effect on the cyclic resistance of soft clays. For up to 12 cycles, the strain-rate effect fully compensates the degradation of shear strength associated with cycling. The presence of an initial static shear stress decreases the cyclic resistance but increases the total undrained shear resistance, due to the partial or total disappearance of stress and strain reversal. The results indicate that for such a clay, the static undrained shear strength provides a conservative estimate in a pseudostatic analysis.