A study on the determination of plastic properties of metals by instrumented indentation using two sharp indenters

An earlier optimisation approach proposed by Luo et al. [Luo, J., Lin, J., Dean, T. A., 2006. A Study on the Determination of Mechanical Properties of a power-law Material by Its Indentation Force-Depth Curve. Philosophical Magazine, 86(19), 2881-2905], which is based on the assumption that the instrumented indentation force-depth response of an elastic–plastic material is a linear combination of the corresponding elastic and elastic–perfect plastic materials, is extended in this work to extract mechanical properties of a power-law material from two given experimental indentation P–h curves for conical indenters of half included angles of 60° and 70.3°. It was found that the non-uniqueness problem encountered in the single P–h curve optimisation approach is effectively removed by the two P–h curves optimisation. The appropriateness of the use of second half included angle of 60° is discussed. For the five representative materials Al, Ti, Fe, Ni and steel, it was found that the maximum relative prediction errors for E, σ_y and n are 2%, 10.4% and 11.3%, respectively. The prediction accuracy of mechanical properties E, σ_y and n is generally better than other methods reported in the literature.     

Pressure dependency,strength-differential effect,and plastic volume expansion in metals

Within the framework of a purely mechanical rate-type theory of finitely deforming elastic-plastic materials, a special set of constitutive equations is introduced. These equations include a dependence both on mean normal stress and on plastic strain, and involve non-normality of plastic strain-rate. They accommodate both the strength-differential effect and plastic volume expansion. Values of the material coefficients in the constitutive equations are approximately determined with the help of published experimental data for AISI 4330 steel. The plastic strain-rate is shown to have a large non-normal component, resulting in a small plastic volume expansion. The theory is in reasonable agreement with experiment for plastic strains up to about 2% in magnitude. Further experiments are suggested so that the constitutive coefficients can be completely and accurately determined.     

Photoelastic technique to investigate the effect of temperature and strain rate on plastic flow in mild steel

Dynamic photoelastic-coating technique was used to observe successive developments of plastic flow in tension at a temperature ranging from ?157°C to 20°C. A type of plastic flow occurred which was determined by a combination of temperature and strain rate. A correlation was found to exist between photoelastic observations and the equation of thermal activation.     

Multiscale modelling of the induced plastic anisotropy in bcc metals

This paper presents a new framework to predict the qualitative and quantitative variation in local plastic anisotropy due to crystallographic texture in body-centered cubic polycrystals. A multiscale model was developed to examine the contribution of mesoscopic and local microscopic behaviour to the macroscopic constitutive response of bcc metals during deformation. The model integrated a dislocation-based hardening scheme and a Taylor-based crystal plasticity formulation into the subroutine of an explicit dynamic FEM code (LS-DYNA). Numerical analyses using this model were able to predict not only correct grain rotation during deformation, but variations in plastic anisotropy due to initial crystallographic orientation. Optimal results were obtained when {1 1 0}〈1 1 1〉, {1 1 2}〈1 1 1〉, and {1 2 3}〈1 1 1〉 slip systems were considered to be potentially active. The predicted material heterogeneity can be utilised for research involving any texture-dependent work hardening behaviour, such as surface roughening.     

Effect of plastic deformation on the acoustoelastic response of some materials

Acoustoelastic birefringence is measured with the acoustic polarimeter by transmitting ultrasonic shear waves at two perpendicular polarizations through the thickness of several uniaxial test specimens. The results are availablefor the following materials: SAE 1010 and SAE 4118 steel, pure titanium, 2024 aluminum and 60\2-40 brass, but are only presented here for SAE 4118 steel and pure titanium. The uniaxial test specimens have been subjected to plastic deformation followed by complete unloading. It is shown that the assumption that the plastic flow leading to the residual-stress state does not change the acoustic response of the material, does not hold for all materials and that further characterization development is required for general quantitative residual-stress determination.     

Experiments on the effect of acceleration on the drag of tapwater bubbles

Three-dimensional (3D) trajectories of spherical air bubbles passing through a converging part of a rectangular channel have been measured. Bubble diameters, d _b, were less than 1 mm and the Reynolds numbers, Re _b, for stagnant tapwater and for mean liquid velocity vˉ _L=0.25 m/s were in about same range. Received: 15 January 2001 / Accepted: 12 June 2001     

FCC金属塑性屈服的尺度效应和应变率响应

对纳米尺度单晶铜的剪切变形进行了分子动力学（MD）模拟．模拟结果表明,单晶铜的剪切屈服应力随模型几何尺度的增大而降低,而随着应变率的增大而升高．基于位错形核理论,建立了一个修正的指数法则来描述面心立方（FCC）金属的尺度效应,该法则与较大尺度范围内（从纳米到毫米以上）的数值模拟结果以及实验数据都符合得比较好．另外,MD模拟中发现单晶铜存在一个临界应变率,当施加的应变率小于该值,剪切屈服应力几乎不随应变率变化而变化;当大于该值,剪切屈服应力会随着应变率的增加迅速升高．最后根据模拟的结果建立了单晶铜和单晶镍塑性屈服强度的应变率响应模型．     

A two-dimensional dislocation dynamics model of the plastic deformation of polycrystalline metals

Two-dimensional dislocation dynamics (2D-DD) simulations under fully periodic boundary conditions are employed to study the relation between microstructure and strength of a material. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a preexisting dislocation population, dislocation sources, and grain boundaries. The mechanical response of such a material is tested by uniaxially loading it up to a certain stress and allowing it to relax until the strain rate falls below a threshold. The total plastic strain obtained for a certain stress level yields the quasi-static stress-strain curve of the material. Besides assuming Frank-Read-like dislocation sources, we also investigate the influence of a pre-existing dislocation density on the flow stress of the model material. Our results show that - despite its inherent simplifications - the 2D-DD model yields material behavior that is consistent with the classical theories of Taylor and Hall-Petch. Consequently, if set up in a proper way, these models are suited to study plastic deformation of polycrystalline materials.     

The effect of dynamic impact and water potential on some surface soil properties

A direct shear test with a superimposed impact was used to simulate the action of a track on the soil surface and to study the effect on soil surface properties. Results showed that impact increased bulk density, reduced saturated hydraulic conductivity and decreased cone penetrometer resistance. An impact plus shear treatment reduced the residual shear strength to approximately 60 kPa compared with 85 kPa for a shear only treatment. Water tension also greatly influenced the changes measured with the order of greatest change being −5>−10>−60>t-100>−300 kPa. The results are discussed with respect to soil trafficability and soil structural change with vehicle passage.     

Hydrogen induced shear localization of the plastic flow in metals and alloys

Hydrogen enhanced localized plasticity (HELP) is a viable mechanism for hydrogen embrittlement supported by experimental observations. According to the HELP mechanism, hydrogen induced premature failures result from hydrogen induced plastic instability which leads to hydrogen assisted localized ductile processes. The objective of this work is to reveal the role of hydrogen in possibly localizing the macroscopic deformation into bands of intense shear using solid mechanics methodology. The hydrogen effect on material deformation is modeled through the hydrogen induced volume dilatation and the reduction in the local flow stress upon hydrogen dissolution into the lattice. Hydrogen in assumed to reside in both normal interstitial lattice sites (NILS) and reversible traps associated with the plastic deformation. The analysis of the plastic deformation and the conditions for plastic flow localization are carried out in plane strain uniaxial tension. For a given initial hydrogen concentration in the unstressed specimen, a critical macroscopic strain is identified at which shear localization commences.     

QUASI－FLOW CORNER THEORY ON LARGE PLASTIC DEFORMATION OF DUCTILE METALS AND ITS APPLICATIONS

ＱＵＡＳＩ－ＦＬＯＷＣＯＲＮＥＲＴＨＥＯＲＹＯＮＬＡＲＧＥＰＬＡＳＴＩＣＤＥＦＯＲＭＡＴＩＯＮＯＦＤＵＣＴＩＬＥＭＥＴＡＬＳＡＮＤＩＴＳＡＰＰＬＩＣＡＴＩＯＮＳＨｕＰｉｎｇ（胡平）ＬｉｕＹｕｑｉ（柳玉启）ＧｕｏＷｅｉ（郭威）ＴａｉＦｅｎｇ（台风）（Ｒ...     

Effect of rate and rate history on plastic deformation: Experiments and constitutive modelling

A series of uniaxial and biaxial cyclic tests with proportional or nonproportional loading path and with different strain-rate histories are conducted on thin-walled circular cylindrical specimens of type 304 stainless steel. The results of these tests show that once a material is stabilized under a lower strain rate, the stress-strain response is not appreciably affected by a jump to a higher strain rate. A rate-dependent constitutive model recently proposed by and [1991] has been extended to predict the above-mentioned strain-rate history effect. A comparison between the predictions of the extended model and the test results shows that most of the rate-dependent features of the material can be simulated by the model.     

Experimental examination of the hypothesis of local determinability in the plastic deformation of metals

Ilyushin's general plasticity theory expressed in a vector space corresponding to the deviatoric tensor space is available in formulating precise stress-strain relations of inelastic deformations by incorporating the deformation history of metals under complex loadings. In this case, the hypothesis of local determinability proposed by Lensky (1960) is useful for determining the explicit form of the stress-strain relation. In the present paper, the hypothesis is discussed by using not only data obtained by an experiment in which the history effect on the succeeding deformation is clearly estimated, but also data described in previous work. As the result, the hypothesis of local determinability has been confirmed to hold with high accuracy in every case.     

The effect of temperature gradient on the correlation of temperature fluctuations

In this paper we present the experiment results of the space correlations of temperature fluctuations when there is and is not temperature gradient. From those results we can see clearly that the temperature fluctuation field is isotropic without temperature gradient, and it is obviously anisotropic with temperature gradient. The temperature correlation along gradient direction is obviously larger than the vertical to direction. Our experiment results agree with the theory results of D.W. Dunn and W.H. Rend^[1].     

Effect of plastic properties of fluids on phase permeabilities

The linearity of Darcy's law is known to be disturbed at both high and low flow velocities [1–3]. In the first case, this is associated with the increase in the inertial component of the hydraulic losses in the presence of large pressure gradients. The effect was theoretically investigated, for example, in [4]. In the second case, the nonlinearity is associated with the interphase interaction of the fluids and the skeleton of the porous material on the contact surface [5]. Here, within the context of the percolation approach [6, 7], the behavior of the phase permeabilities is analyzed for low flow velocities, when on the microlevel (flow in an individual pore channel) the fluids display plasticity properties [8].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 110–115, March–April, 1991.