Showing papers on "Strain rate published in 2008"

A new material model for 2D numerical simulation of serrated chip formation when machining titanium alloy Ti–6Al–4V

Abstract: A new material constitutive law is implemented in a 2D finite element model to analyse the chip formation and shear localisation when machining titanium alloys. The numerical simulations use a commercial finite element software (FORGE 2005®) able to solve complex thermo-mechanical problems. One of the main machining characteristics of titanium alloys is to produce segmented chips for a wide range of cutting speeds and feeds. The present study assumes that the chip segmentation is only induced by adiabatic shear banding, without material failure in the primary shear zone. The new developed model takes into account the influence of strain, strain rate and temperature on the flow stress and also introduces a strain softening effect. The tool chip friction is managed by a combined Coulomb–Tresca friction law. The influence of two different strain softening levels and machining parameters on the cutting forces and chip morphology has been studied. Chip morphology, cutting and feed forces predicted by numerical simulations are compared with experimental results.

An improved computational constitutive model for brittle materials

Abstract: An improved computational constitutive model for brittle materials is presented. It is applicable for brittle materials subjected to large strains, high strain rates and high pressures, and is well‐suited for computations in both Lagrangian and Eulerian codes. The equivalent strength is dependent on the intact strength, fractured strength, strain rate, pressure, and damage. The pressure includes the effect of bulking, which is introduced through the transfer of internal energy from decreased shear and deviator stresses to potential internal energy associated with increased hydrostatic pressure. Examples are presented to illustrate the model.

High strain rate properties of metals and alloys

Abstract: The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to foll...

Room temperature formability of a magnesium AZ31 alloy: Examining the role of texture on the deformation mechanisms

Abstract: The deformation behavior, texture and microstructure evolution of six sample types of a commercial magnesium alloy AZ31 with different processing histories were investigated during plane strain compression at room temperature using a channel-die device. Although all the samples were deformed under the same conditions, i.e. temperature and strain rate, the initial state of the samples prior to deformation was responsible for the final texture and microstructure. Stress–strain curves showed a maximum ductility of 28% for the sample with a hot rolling history. EBSD analysis was carried out to give a better insight into the operating deformation mechanisms. Besides the expected { 1 0 1 ¯ 2 } -tensile twinning, { 1 0 1 ¯ 1 } -compression twinning and { 1 0 1 ¯ 1 } − { 1 0 1 ¯ 2 } -double twinning were also observed in some specimens and were correlated to microcrack formation, which caused an early shear failure.

An interacting micro-crack damage model for failure of brittle materials under compression

Abstract: A model is developed for brittle failure under compressive loading with an explicit accounting of micro-crack interactions. The model incorporates a pre-existing flaw distribution in the material. The macroscopic inelastic deformation is assumed to be due to the nucleation and growth of tensile “wing” micro-cracks associated with frictional sliding on these flaws. Interactions among the cracks are modeled by means of a crack-matrix-effective-medium approach in which each crack experiences a stress field different from that acting on isolated cracks. This yields an effective stress intensity factor at the crack tips which is utilized in the formulation of the crack growth dynamics. Load-induced damage in the material is defined in terms of a scalar crack density parameter, the evolution of which is a function of the existing flaw distribution and the crack growth dynamics. This methodology is applied for the case of uniaxial compression under constant strain rate loading. The model provides a natural prediction of a peak stress (defined as the compressive strength of the material) and also of a transition strain rate, beyond which the compressive strength increases dramatically with the imposed strain rate. The influences of the crack growth dynamics, the initial flaw distribution, and the imposed strain rate on the constitutive response and the damage evolution are studied. It is shown that different characteristics of the flaw distribution are dominant at different imposed strain rates: at low rates the spread of the distribution is critical, while at high strain rates the total flaw density is critical.

Flow behavior and microstructures of superalloy 718 during high temperature deformation

Abstract: Flow behavior and microstructures of superalloy 718 were investigated by hot compression tests performed at temperatures ranging from 950 to 1100 °C with strain rates of 10−3 to 1 s−1. The dependence of the peak stress on deformation temperature and strain rate can be expressed by a hyperbolic-sine type equation. The activation energy for superalloy 718 is determined to be 443.2 kJ mol−1. A power exponent relationship between the peak strain and the Z parameter is obtained. Microstructure analysis shows that the dynamically recrystallized grain size is inversely proportional to the Z parameter. The nucleation mechanisms of DRX are closely related to the value of Z parameter. Under low Z conditions, DRX nucleation and development are mainly assisted by the formation of twins near the original grain boundaries.

Recrystallization in AISI 304 austenitic stainless steel during and after hot deformation

Abstract: In order to improve the understanding of the dynamic and post-dynamic recrystallization behaviours of AISI 304 austenitic stainless steel, a series of hot torsion test have been performed under a range of deformation conditions. The mechanical and microstructural features of dynamic recrystallization (DRX) were characterized to compare and contrast them with those of the post-dynamic recrystallization. A necklace type of dynamically recrystallized microstructure was observed during hot deformation at 900 °C and at a strain rate of 0.01 s−1. Following deformation, the dependency of time for 50% recrystallization, t50, changed from “strain dependent” to “strain independent” at a transition strain (ɛ*), which is significantly beyond the peak. This transition strain was clearly linked to the strain for 50% dynamic recrystallization during deformation. The interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been established. The results also showed an important role of grain growth on softening of deformed austenite.

The Effect of Strain Rate on the Mechanical Properties of Human Cortical Bone

Abstract: Bone mechanical properties are typically evaluated at relatively low strain rates. However, the strain rate related to traumatic failure is likely to be orders of magnitude higher and this higher strain rate is likely to affect the mechanical properties. Previous work reporting on the effect of strain rate on the mechanical properties of bone predominantly used nonhuman bone. In the work reported here, the effect of strain rate on the tensile and compressive properties of human bone was investigated. Human femoral cortical bone was tested longitudinally at strain rates ranging between 0.14-29.1 s(-1) in compression and 0.08-17 s(-1) in tension. Young's modulus generally increased, across this strain rate range, for both tension and compression. Strength and strain (at maximum load) increased slightly in compression and decreased (for strain rates beyond 1 s(-1)) in tension. Stress and strain at yield decreased (for strain rates beyond 1 s(-1)) for both tension and compression. In general, there seemed to be a relatively simple linear relationship between yield properties and strain rate, but the relationships between postyield properties and strain rate were more complicated and indicated that strain rate has a stronger effect on postyield deformation than on initiation of yielding. The behavior seen in compression is broadly in agreement with past literature, while the behavior observed in tension may be explained by a ductile to brittle transition of bone at moderate to high strain rates.

Hot deformation and recrystallization of austenitic stainless steel. Part I: dynamic recrystallization

Abstract: The hot deformation behavior of a 304 austenitic stainless steel was investigated to characterize the evolution of the dynamically recrystallized structure as a starting point for studies of the postdeformation recrystallization behavior. The effect of different deformation parameters such as strain, strain rate, and temperature were investigated. The flow curves showed typical signs of dynamic recrystallization (DRX) over a wide range of temperatures and strain rates (i.e., different Zener–Hollomon (Z) values). However, under very high or very low Z values, the flow curves’ shapes changed toward those of the dynamic recovery and multiple peaks, respectively. The results showed that while DRX starts at a strain as low as 60 pct of the peak strain, a fully DRX microstructure needs a high strain of almost 4.5 times the initiation strain. The DRX average grain size showed power-law functions with both the Zener–Hollomon parameter and the peak stress, although power-law breakdown was observed at high Z values.

Hot deformation behaviour of Mg–3Al alloy—A study using processing map

Abstract: The hot deformation behaviour of Mg–3Al alloy has been studied using the processing-map technique. Compression tests were conducted in the temperature range 250–550 °C and strain rate range 3 × 10 −4 to 10 2 s −1 and the flow stress data obtained from the tests were used to develop the processing map. The various domains in the map corresponding to different dissipative characteristics have been identified as follows: (i) grain boundary sliding (GBS) domain accommodated by slip controlled by grain boundary diffusion at slow strain-rates ( −3 s −1 ) in the temperature range from 350 to 450 °C, (ii) two different dynamic recrystallization (DRX) domains with a peak efficiency of 42% at 550 °C/10 −1 s −1 and 425 °C/10 2 s −1 governed by stress-assisted cross-slip and thermally activated climb as the respective rate controlling mechanisms and (iii) dynamic recovery (DRV) domain below 300 °C in the intermediate strain rate range from 3 × 10 −2 to 3 × 10 −1 s −1 . The regimes of flow instability have also been delineated in the processing map using an instability criterion. Adiabatic shear banding at higher strain rates (>10 1 s −1 ) and solute drag by substitutional Al atoms at intermediate strain rates (3 × 10 −2 to 3 × 10 −1 s −1 ) in the temperature range (350–450 °C) are responsible for flow instability. The relevance of these mechanisms with reference to hot working practice of the material has been indicated. The processing maps of Mg–3Al alloy and as-cast Mg have been compared qualitatively to elucidate the effect of alloying with aluminum on the deformation behaviour of magnesium.

Preparation of C200 green reactive powder concrete and its static–dynamic behaviors

Abstract: In this paper, a new type of green reactive powder concrete (GRPC) with compressive strength of 200 MPa (C200 GRPC) is prepared by utilizing composite mineral admixtures, natural fine aggregates, short and fine steel fibers. The quasi-static mechanical properties (mechanical strength, fracture energy and fiber–matrix interfacial bonding strength) of GRPC specimens, cured in three different types of regimes (standard curing, steam curing and autoclave curing), are investigated. The experimental results show that the mechanical properties of the C200 GRPC made with the cementitious materials consisting of 40% of Portland cement, 25% of ultra fine slag, 25% of ultra fine fly ash and 10% of silica fume, 4% volume fraction of steel fiber are higher than the others. The corresponding compressive strength, flexural strength, fracture energy and fiber–matrix interfacial bonding strength are more than 200 MPa, 60 MPa, 30,000 J/m2 and 14 MPa, respectively. The dynamic tensile behavior of the C200 GRPC is also investigated through the Split Hopkinson Pressure Bar (SHPB) according to the spalling phenomena. The dynamic testing results demonstrate that strain rate has an important effect on the dynamic tensile behavior of C200 GRPC. With an increase of strain rate, the peak stress rapidly increases in the dynamic tensile stress–time curves. The C200 GRPC exhibits an obvious strain rate stiffening effect in the case of high strain rate. Finally, the mechanism of excellent static and dynamic properties gains of C200 GRPC is also discussed.

Strain hardening behaviour and the Taylor factor of pure magnesium

Abstract: Taylor orientation factors for strain hardening in textured and random polycrystals of magnesium were derived from the ratio of the strain hardening rates of polycrystals to that of single crystals deforming by equivalent polyslip. For polycrystals with textures that inhibit basal and prismatic slip while favouring pyramidal polyslip, the Taylor factor is estimated to be between 2.1 and 2.5, increasing to about 4.5 for randomly textured polycrystals. The micromechanics of strain hardening in polycrystals are discussed.

Measurement of Epoxy Resin Tension, Compression, and Shear Stress–Strain Curves over a Wide Range of Strain Rates Using Small Test Specimens

Abstract: The next generation aircraft engines are designed to be lighter and stronger than engines currently in use by using carbon fiber composites. In order to certify these engines, ballistic impact tests and computational analyses must be completed, which will simulate a “blade out” event in a catastrophic engine failure In order to computationally simulate the engine failure, properties of the carbon fiber and resin matrix must be known. When conducting computer simulations using a micromechanics approach, experimental tensile, compressive, and shear data are needed for constitutive modeling of the resin matrix material. The material properties of an Epon E862 epoxy resin will be investigated because it is a commercial 176°C (350°F) cure resin currently being used in these aircraft engines. These properties will be measured using optical measurement techniques. The epoxy specimens will be tested in tension, compression and torsional loadings under various strain rates ranging from 10−5 to 10−1 s−1 and tempera...

Processing maps for hot deformation of rolled AZ31 magnesium alloy plate: Anisotropy of hot workability

Abstract: Processing maps on rolled AZ31 magnesium plate have been developed in the range 300–550 °C and 0.0003–10 s −1 by hot compression of specimens parallel to the rolling direction (RD), the transverse direction (TD), or the normal direction (ND) with a view to examine whether the hot workability is anisotropic. The processing map for RD specimens exhibited a single wide domain of workability in the temperature range 350–550 °C and strain rate range 0.0003–0.3 s −1 in which dynamic recrystallization (DRX) occurs. The apparent activation energy in this domain is estimated to be 143 kJ/mole, which is close to that for self-diffusion in magnesium. On the other hand, the maps for the TD and ND specimens exhibited two DRX domains, one at lower strain rates ( −1 ) and the other at higher strain rates (>1 s −1 ). The apparent activation energies estimated in the lower strain rate DRX domain of TD and ND maps are 180 and 168 kJ/mole, respectively, suggesting cross-slip occurs in this domain. In the higher strain rate domain, the apparent activation energy estimated in both TD and ND specimens is 105 kJ/mole, which is close to that for grain-boundary diffusion. The results suggest that the hot deformation behavior is anisotropic and a simple correlation with the known primary rolling texture ( { 0 0 0 2 } 〈 1 0 1 ¯ 0 〉 ) revealed that both the first order and second order pyramidal slip systems are favored in the RD orientation resulting in higher workability. The other two orientations may be considered “harder” as regards hot workability since either the first order or the second order pyramidal slip is likely to dominate. In all the three orientations and within the DRX domains, the grain size varies linearly with the Zener-Hollomon parameter.

Constitutive relationships for 22MnB5 boron steel deformed isothermally at high temperatures

Abstract: The strain, strain rate and temperature dependency of a boron steel, which was isothermally deformed under uniaxial compression tests, has been investigated at temperatures between 600 and 900°C, and at strain rates of 0.1, 1.0 and 10.0 s −1 . Two constitutive models were used to correlate the plastic behavior: the Voce constitutive relation in combination with the kinetic model proposed by Kocks and the phenomenological model proposed by Molinari–Ravichandran. The Kocks model has been introduced in the Voce formulation to describe the temperature and the strain rate dependency of the saturation stress and of the yield stress. The Molinari–Ravichandran model is based on a single internal variable that can be viewed as being related to a characteristic length scale of the microstructure that develops during deformation. It has been shown that the plastic behavior of the boron steel can be well described using these two models.

The high strain rate response of PVC foams and end-grain balsa wood

Abstract: The uniaxial compressive responses of two polymeric foams (Divinycell H100 and H250) and balsa wood (ProBalsa LD7) have been measured over a wide range of strain rates, ranging from 10−4 s−1 to 4000 s−1. These materials are widely used as cores for composite sandwich structures. The high strain rate compression tests were performed using a Split-Hopkinson Pressure Bar made from AZM magnesium alloy, with semi-conductor strain gauges used to measure the low levels of stress in the specimens. The experimental data for compressive strength as a function of strain rate are adequately approximated by power-law fits. The compressive yield strength of the H250 PVC foam and balsa wood doubles when the strain rate is increased from quasi-static rates (10−4 s−1) to rates on the order of 103 s−1. In contrast, the H100 PVC foam displays only a small elevation in uniaxial compressive strength (about 30%) for the same increase in strain rate.

Nanoindentation identifications of mechanical properties of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds derived by diffusion couples

Abstract: We report in this paper Young's moduli and hardness of Cu 6 Sn 5 , Cu 3 Sn, and Ni 3 Sn 4 intermetallic compounds (IMCs) measured by nanoindentation. The samples were prepared by annealing Sn–Cu and Sn–Ni diffusion couples. Indentations performed along the directions lateral and perpendicular to the IMC layers show statistically indistinguishable Young's moduli and hardness for each of the three IMCs, implying that these polycrystalline IMC aggregates are rather isotropic. Nanomechanical responses of the IMCs were shown to depend greatly on the strain rate during loading while independent of the strain rate during unloading. Multiple pop-in events were observed for Cu 6 Sn 5 during loading at a strain rate lower than about 0.1–0.5 s −1 . Topographies of the residual impressions were quantitatively measured and the pile-up features were apparent for the three IMCs.