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M A Lebyodkin - One of the best experts on this subject based on the ideXlab platform.
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on the mechanism of unstable plastic flow in an austenitic femnc twip steel
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2009Co-Authors: Tatiana Lebedkina, M A Lebyodkin, Ph J Chateau, Alain Jacques, Sebastien AllainAbstract:Abstract The complex character of plastic deformation of the austenitic steel Fe22Mn0.6C is studied at room temperature with the aid of high-frequency local extensometry. It is shown that the plastic flow instability, associated with fluctuations of the flow stress, results from quasi-continuous propagation of deformation bands along the specimen axis. This propagation mode is dominant in the entire range of the Applied Strain-Rate from 2.1 × 10 −5 s −1 to 10 −1 s −1 . Such behavior differs from that of various alloys deforming via dislocation glide under conditions of dynamic Strain ageing (Portevin-Le Chatelier effect), which is characterized by a transition from a repetitive occurrence of static deformation bands at lower Strain Rates to a relay-race and, finally, quasi-continuous deformation band propagation at higher Strain Rates. The unusual behavior of the deformation bands bears evidence to a particular kind of instability in the investigated steel. A possible role of deformation twins in the observed dynamics of plastic instability is discussed.
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geometrically non linear modeling of the portevin le chatelier effect
Computational Materials Science, 2009Co-Authors: Thomas Bohlke, Yuri Estrin, G Bondar, M A LebyodkinAbstract:Abstract In this work we investigate the plastic instabilities associated with the Portevin–Le Chatelier (PLC) effect in Al alloy 2024. A semi-phenomenological approach is taken. A simple geometrically non-linear elastic–viscoplastic constitutive model is proposed for simulation of material response under various Applied Strain Rates. Using the model we determine numerically the relation between the critical Strain for the onset of discontinuous yielding and the Applied Strain Rate. The results obtained are in very good quantitative agreement with the available experimental data (collected from tests at room temperature) and cover both the normal and the inverse behavior of the critical Strain. The simulations are performed using non-linear finite element method. Additional verification of the proposed constitutive framework was carried out using statistical analysis of the simulated stress–time series. A transition from a non-linear chaotic regime to self-organized critical behavior of the localized Strain bands were predicted in terms of the temporal two-point correlation function of the stress–time series. Finally we investigated the influence of different factors, such as the geometry of the specimen, its orientation with respect to the rolling direction and loading conditions (Strain Rate), on the type of PLC instabilities and the critical conditions for their onset.
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a finite element model for the portevin le chatelier effect based on polycrystal plasticity
Modelling and Simulation in Materials Science and Engineering, 2002Co-Authors: Schalk Kok, Armand Joseph Beaudoin, Daniel A Tortorelli, M A LebyodkinAbstract:A polycrystal plasticity model, embedded in a finite element framework, is used to describe the Portevin–Le Chatelier effect in a velocity controlled tension test. The transition from continuous to discontinuous band propagation is captured as the Applied Strain Rate is decreased. Statistical analysis of the stress drops also indicate power law distributions for continuous propagation shifting to peaked distributions at lower Strain Rates, consistent with experimental observations. The model requires no `artificial' initial inhomogeneity since spatial gradients exist naturally due to grain incompatibilities.
Tri Thuong Ngo - One of the best experts on this subject based on the ideXlab platform.
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loading Rate effect on crack velocity in ultra high performance fiber reinforced concrete
Construction and Building Materials, 2019Co-Authors: Tri Thuong Ngo, Jun Kil Park, Dongjoo KimAbstract:Abstract Loading Rate effect on crack propagation in ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a pre-notched three-point bending specimen in an improved-Strain energy frame impact machine (I-SEFIM) and image processing techniques. The crack velocity of up to 984 m/s and the crack initiation Strain Rate of up to 271 s−1 were observed. Crack velocity in UHPFRCs increased as the Applied Strain Rate increased. Fiber reinforcements significantly affected on the crack velocity in the UHPFRC at static Rates, but slightly did at high Strain Rates. There is a strong correlation between the Strain-Rate sensitivity and the dynamic crack growth characteristics of UHPFRCs.
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Strain Rate dependent shear failure surfaces of ultra high performance fiber reinforced concretes
Construction and Building Materials, 2018Co-Authors: Tri Thuong Ngo, Dongjoo Kim, Jae Heum Moon, Sungwook KimAbstract:Abstract Strain Rate-dependent shear behavior of ultra-high-performance fiber-reinforced concretes (UHPFRCs) with confining pressure was investigated using a new shear test setup in an improved Strain energy impact machine (I-SEFIM). Different confining pressures were Applied to the specimens prior to shear testing at both static and high Strain Rates, and were maintained during testing. The shear strength of UHPFRCs was highly sensitive to the Applied Strain Rate and confining pressure. The effect of confining pressure on the shear strength was more pronounced at a static Rate rather than at high Strain Rates. Strain Rate-dependent shear failure surfaces of UHPFRCs were proposed.
Dongjoo Kim - One of the best experts on this subject based on the ideXlab platform.
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loading Rate effect on crack velocity in ultra high performance fiber reinforced concrete
Construction and Building Materials, 2019Co-Authors: Tri Thuong Ngo, Jun Kil Park, Dongjoo KimAbstract:Abstract Loading Rate effect on crack propagation in ultra-high-performance fiber-reinforced concrete (UHPFRCs) was investigated using a pre-notched three-point bending specimen in an improved-Strain energy frame impact machine (I-SEFIM) and image processing techniques. The crack velocity of up to 984 m/s and the crack initiation Strain Rate of up to 271 s−1 were observed. Crack velocity in UHPFRCs increased as the Applied Strain Rate increased. Fiber reinforcements significantly affected on the crack velocity in the UHPFRC at static Rates, but slightly did at high Strain Rates. There is a strong correlation between the Strain-Rate sensitivity and the dynamic crack growth characteristics of UHPFRCs.
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Strain Rate dependent shear failure surfaces of ultra high performance fiber reinforced concretes
Construction and Building Materials, 2018Co-Authors: Tri Thuong Ngo, Dongjoo Kim, Jae Heum Moon, Sungwook KimAbstract:Abstract Strain Rate-dependent shear behavior of ultra-high-performance fiber-reinforced concretes (UHPFRCs) with confining pressure was investigated using a new shear test setup in an improved Strain energy impact machine (I-SEFIM). Different confining pressures were Applied to the specimens prior to shear testing at both static and high Strain Rates, and were maintained during testing. The shear strength of UHPFRCs was highly sensitive to the Applied Strain Rate and confining pressure. The effect of confining pressure on the shear strength was more pronounced at a static Rate rather than at high Strain Rates. Strain Rate-dependent shear failure surfaces of UHPFRCs were proposed.
T G Nieh - One of the best experts on this subject based on the ideXlab platform.
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Flow serration and shear-band propagation in bulk metallic glasses
Applied Physics Letters, 2009Co-Authors: Hongxia Chen, T G Nieh, S X Song, J.c. Huang, Jason S.c. JangAbstract:Flow serration in bulk metallic glasses (BMGs) was analyzed using high-sensitivity Strain gauges. Based on the displacement-time profile for one serration, shear-band propagating speed was determined and found to be insensitive to the Applied Strain Rates. The disappearance of serration at high Strain Rates is a result that the signal of displacement burst was overwhelmed by the Applied Strain Rate. In comparison with the ductile Pd-based and brittle Mg-based BMGs, the ductility of BMGs appears to be closely related to the dynamics during shear-band propagation.
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a nanoindentation study of serRated flow in bulk metallic glasses
Acta Materialia, 2003Co-Authors: Christopher A Schuh, T G NiehAbstract:Abstract Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serRated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete “pop-in” events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading Rate; slower indentation Rates promote more conspicuous serrations, and rapid indentations suppress serRated flow. Analysis of the experimental data reveals a critical Applied Strain Rate, above which serRated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation Rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest Rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high Rates, a single shear band cannot accommodate the imposed Strain rapidly enough, and consequently multiple shear bands must opeRate simultaneously.
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superplasticity in a bulk amorphous pd 40ni 20p alloy a compression study
Intermetallics, 2002Co-Authors: C L Chiang, T G Nieh, Yoshihito KawamuraAbstract:Abstract Compressive deformation behavior of a cast Pd 40 Ni 40 P 20 bulk metallic glass in the supercooled liquid region (589–670 K) was investigated at Strain Rates ranging from 10 −4 to 10 −2 s −1 . The material exhibited excellent mechanical formability in the supercooled liquid region. However, in contrast to a Newtonian behavior generally observed in oxide glasses, the present alloy also showed a non-Newtonian behavior, depending upon the temperature and Applied Strain Rate. Specifically, the alloy is like a Newtonian fluid at high temperatures, but becomes non-Newtonian at low temperatures and high Strain Rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution transmission electron microscopy. The non-Newtonian behavior is proposed to be associated with the glass instability during deformation.
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effect of Strain Rate on compressive behavior of a pd40ni40p20 bulk metallic glass
Intermetallics, 2002Co-Authors: Toshiji Mukai, T G Nieh, Yoshihito Kawamura, Akihisa Inoue, Kenji HigashiAbstract:Abstract Mechanical deformation of Pd 40 Ni 40 P 20 was characterized in compression over a wide Strain Rate range (3.3×10 −5 to 2×10 3 s −1 ) at room temperature. The compression sample fractured with a shear plane inclined ∼42 degree with respect to the loading axis, in contrast to ∼56 degree for the case of tension. This suggests the yielding of the material deviates from the classical von Mises yield criterion, but follows the Mohr-Coulomb yield criterion. Fracture stress as well as Strain was found to decrease with increasing Applied Strain Rate. The compressive stress (∼1.74 GPa) was also found to be higher than the tensile fracture stress at a quasi-static Strain Rate. Close examination of the stress–Strain curves revealed that localized shear might have occurred at a compressive stress of about ∼1.4 GPa, much lower than the “apparent” yield stress of 1.74 GPa. However, the stress of 1.4 GPa for shear band initiation is almost the same as the fracture stress measured at a dynamic Strain Rate of 5×10 2 s −1 . These results suggested that the fracture of a bulk metallic glass is sensitive to the Applied loading Rate.
Leon M Keer - One of the best experts on this subject based on the ideXlab platform.
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Strain Rate sensitivity of sintered silver nanoparticles using Rate jump indentation
International Journal of Mechanical Sciences, 2018Co-Authors: Xu Long, Wenbin Tang, Yihui Feng, Chao Chang, Leon M Keer, Yao YaoAbstract:Abstract Nanoindentation experiments were performed at room temperature on pressure-less sintered silver nanoparticles (AgNP) samples. Two representative die attach solder materials: sintered electrically conductive silver adhesive and conventional Sn-3.0Ag-0.5Cu solder were investigated. A novel technique of multiple Strain-Rate jumps is adopted accompanied by the continuous stiffness measurement, which can effectively determine the Strain Rate sensitivity (SRS) with good accuracy. Different Strain Rates and indentation depths are considered to obtain the nanomechanical properties such as hardness and Young's modulus. Compared with Young's modulus of sintered AgNP, the effect of Applied Strain Rate is more influential to the hardness. In the loading stage, the SRS exponent decreases due to a smaller variation of hardness at a greater indentation depth. In the holding stage prior to the unloading of the Applied indentation force, the creep displacement is relatively insensitive to the Applied Strain Rate, however, the creep Strain Rate decays exponentially and the corresponding stress exponents are determined.
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Strain Rate sensitivity of sintered silver nanoparticles using Rate jump indentation
International Journal of Mechanical Sciences, 2018Co-Authors: Xu Long, Wenbin Tang, Yihui Feng, Chao Chang, Leon M KeerAbstract:Nanoindentation experiments were performed at room temperature on pressure-less sintered silver nanoparticles (AgNP) samples. Two representative die attach solder materials: sintered electrically conductive silver adhesive and conventional Sn-3.0Ag center dot 0.5Cu solder were investigated. A novel technique of multiple Strain-Rate jumps is adopted accompanied by the continuous stiffness measurement, which can effectively determine the Strain Rate sensitivity (SRS) with good accuracy. Different Strain Rates and indentation depths are considered to obtain the nanomechanical properties such as hardness and Young's modulus. Compared with Young's modulus of sintered AgNP, the effect of Applied Strain Rate is more influential to the hardness. In the loading stage, the SRS exponent decreases due to a smaller variation of hardness at a greater indentation depth. In the holding stage prior to the unloading of the Applied indentation force, the creep displacement is relatively insensitive to the Applied Strain Rate, however, the creep Strain Rate decays exponentially and the corresponding stress exponents are determined. (C) 2018 Elsevier Ltd. All rights reserved.