Analysis of indentation loading curves obtained using conical indenters

Using dimensional analysis and finite-element calculations we determine the functional form of indentation loading curves for a rigid conical indenter indenting into elastic-perfectly plastic solids. The new results are compared with the existing theories of indentation using conical indenters, including the slip-line theory for rigid-plastic solids, Sneddon's result for elastic solids, and Johnson's model for elastic-perfectly plastic solids. In the limit of small ratio of yield strength (Y) to Young's modulus (E), both the new results and Johnson's model approach that predicted by slip-line theory for rigid-plastic solids. In the limit of large Y/E, the new results agree with that for elastic solids. For a wide range of Y/E, some difference is found between Johnson's model-and the present result. This study also demonstrates the possibilities and limitations of using indentation loading curves to extract fundamental mechanical properties of solids.

Analysis of instability of a cylindrical soap film of finite dimensions

By means of experiments of instability of a uniform cylindrical soap film, Boys had showed that the bubble molded by the film is unstable when its length is greater than its circumference. Recently that is generally called the Rayleigh Criterion. In this paper, a linear theory in hydrodynamics is applied to analyze the stability of the cylindrical soap film supported by two equal size disks; all conditions of the stationary wave on the end plates of two disks are given. From here we get that the Rayleigh Criterion on the stability of the cylindrical soap film is proved.

Instability from steady and axisymmetric to steady and asymmetric floating half zone convection in a fat liquid bridge of larger Prandtl number

The linear instability analysis of the present paper shows that the thermocapillary convection in a half floating zone of larger Prandtl number has a steady instability mode w(i) = 0 and m = 1 for a fat liquid bridge V = 1.2 with small geometrical aspect ratio A = 0.6. This conclusion is different from the usual idea of hydrothermal instability, and implies that the instability of the system may excite a steady and axial asymmetric state before the onset of oscillation in the ease of large Prandtl number.

Relationships Between Initial Unloading Slope, Contact Depth, and Mechanical Properties for Conical Indentation in Linear Viscoelastic Solids

Using analytical and finite element modeling, we examine the relationships between initial unloading slope, contact depth, and mechanical properties for spherical indentation in viscoelastic solids with either displacement or load as the independent variable. We then investigate whether the Oliver-Pharr method for determining the contact depth and contact radius, originally proposed for indentation in elastic and elastic-plastic solids, is applicable to spherical indentation in viscoelastic solids. Finally, the analytical and numerical results are used to answer questions raised in recent literature about measuring viscoelastic properties from instrumented spherical indentation experiments.

Continuous spectrum growth and modal instability in swirling duct flow

We present results on the stability of compressible inviscid swirling flows in an annular duct. Such flows are present in aeroengines, for example in the by-pass duct, and there are also similar flows in many aeroacoustic or aeronautical applications. The linearised Euler equations have a ('critical layer') singularity associated with pure convection of the unsteady disturbance by the mean flow, and we focus our attention on this region of the spectrum. By considering the critical layer singularity, we identify the continuous spectrum of the problem and describe how it contributes to the unsteady field. We find a very generic family of instability modes near to the continuous spectrum, whose eigenvalue wavenumbers form an infinite set and accumulate to a point in the complex plane. We study this accumulation process asymptotically, and find conditions on the flow to support such instabilities. It is also found that the continuous spectrum can cause a new type of instability, leading to algebraic growth with an exponent determined by the mean flow, given in the analysis. The exponent of algebraic growth can be arbitrarily large. Numerical demonstrations of the continuous spectrum instability, and also the modal instabilities are presented.

Onset of Adiabatic Shear Instability in Strain Gradient Dependent Metal Matrix Composites

In this paper, effect of strain gradient on adiabatic shear instability in particle reinforced metal matrix composites is investigated by making use of the strain gradient dependent constitutive equation developed by Dai et al. [9] and the linear perturbation analysis presented by Bai [10]. The results have shown that the onset of adiabatic shear instability in metal matrix composites reinforced with small particles is more prone to occur than in the composites reinforced with large particles. This means that the strain gradient provides a strong deriving force for onset of adiabatic shear instability in metal matrix composites.

Influence of liquid bridge volume on instability of floating half zone convection

Thermocapillary instabilities on floating half zone convection in microgravity environment were investigated by linear instability analysis method. The critical Marangoni numbers were obtained and compared with the experimental ones. The influences of the liquid bridge volume and the aspect ratio on the critical Marangoni number were analyzed. It is found that the liquid bridge volume and the aspect ratio have great influence on the critical Marangoni number. There was a gap region where the oscillatory convection will not be observed in present analyses and in experiments in the curve of the critical Marangoni number vs the liquid bridge volume for the case of large Prandtl number and small aspect ratio.

Physical modeling of untrenched submarine pipeline instability

Wave-induced instability of untrenched pipeline on sandy seabed is a `wave-soil-pipeline' coupling dynamic problem. To explore the mechanism of the pipeline instability, the hydrodynamic loading with U-shaped oscillatory flow tunnel is adopted, which is quite different from the previous experiment system. Based on dimensional analysis, the critical conditions for pipeline instability are investigated by altering pipeline submerged weight, diameter, soil parameters, etc. Based on the experimental results, different linear relationships between Froude number (Fr) and non-dimensional pipeline weight (G) are obtained for two constraint conditions. Moreover, the effects of loading history on the pipeline stability are also studied. Unlike previous experiments, sand scouring during the process of pipe's losing stability is detected in the present experiments. In addition, the experiment results are compared with the previous experiments, based on Wake II model for the calculation of wave-induced forces upon pipeline. It shows that the results of two kinds of experiments are comparable, but the present experiments provide better physical insight of the wave-soil-pipeline coupling effects.

Instability in threshold voltage and subthreshold behavior in Hf-In-Zn-O thin film transistors induced by bias-and light-stress

Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC). In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (Δ VT), while under light stress, VT consistently shifts negatively. In all cases, there was no significant change in field-effect mobility. Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days. We observe that the PPC becomes more pronounced at shorter wavelengths. © 2010 American Institute of Physics.

Hill Instability Analysis Of Tlp Tether Subjected To Combined Platform Surge And Heave Motions

This paper presents the Hill instability analysis of Tension Leg Platform (TLP) tether it, deep sea. The 2-D nonlinear beam model which is Undergoing Coupled axial and transverse vibrations, is applied. The governing equations are reduced to nonlinear Hill equation by use of the Galerkin's method and the modes superposition principle. The Hill instability charted Lip to large parameters is obtained. An important parameter M is defined and can he expressed as the functions of tether length, the platform surge and heave motion amplitudes. Some example studies are performed for various environmental conditions. The results demonstrate that the nonlinear coupling between the axial and transverse vibrations has a significant effect on the response of structure.. It needs to be considered for the accurate dynamic analysis of long TLP tether subjected to the combined platform surge and heave motions.