Stiffness of clays and silts: Normalizing shear modulus and shear strain

An analysis is presented of a database of 67 tests on 21 clays and silts of undrained shear stress-strain data of fine-grained soils. Normalizations of secant G in terms of initial mean effective stress p9 (i.e., G=p9 versus log g) or undrained shear strength cu (i.e., G=cu versus log g) are shown to be much less successful in reducing the scatter between different clays than the approach that uses the maximum shear modulus,Gmax, a technique still not universally adopted by geotechnical researchers and constitutive modelers. Analysis of semiempirical expressions forGmax is presented and a simple expression that uses only a void-ratio function and a confining-stress function is proposed. This is shown to be superior to a Hardin-style equation, and the void ratio function is demonstrated as an alternative to an overconsolidation ratio (OCR) function. To derive correlations that offer reliable estimates of secant stiffness at any required magnitude of working strain, secant shear modulus G is normalized with respect to its small-strain value Gmax, and shear strain g is normalized with respect to a reference strain gref at which this stiffness has halved. The data are corrected to two standard strain rates to reduce the discrepancy between data obtained from static and cyclic testing. The reference strain gref is approximated as a function of the plasticity index.Aunique normalized shear modulus reduction curve in the shape of a modified hyperbola is fitted to all the available data up to shear strains of the order of 1%. As a result, good estimates can be made of the modulus reduction G/Gmax ±30% across all strain levels in approximately 90% of the cases studied. New design charts are proposed to update the commonly used design curves. © 2013 American Society of Civil Engineers.

Numerical analysis of shallow foundations under combined horizontal and torsional loading

Foundations of subsea infrastructure in deep water subjected to asymmetric environmental loads have underscored the importance of combined torsional and horizontal loading effects on the bearing capacity of rectangular shallow foundations. The purpose of this study is to investigate the undrained sliding and torsional bearing capacity of rectangular and square shallow foundations together with the interaction response under combined loading using three-dimensional finite element (3D-FE) analysis. Upper bound plastic limit analysis is employed to establish a reference value for horizontal and torsional bearing capacity, and an interaction relationship for the combined loading condition. Satisfactory agreement of plastic limit analysis (PLA) and 3D-FE results for ultimate capacity and interaction curves ensures that simple PLA solution could be used to evaluate the bearing capacity problem of foundation under combined sliding and torsion.

Functionally different pads on the same foot allow control of attachment: stick insects have load-sensitive "heel" pads for friction and shear-sensitive "toe" pads for adhesion.

Stick insects (Carausius morosus) have two distinct types of attachment pad per leg, tarsal "heel" pads (euplantulae) and a pre-tarsal "toe" pad (arolium). Here we show that these two pad types are specialised for fundamentally different functions. When standing upright, stick insects rested on their proximal euplantulae, while arolia were the only pads in surface contact when hanging upside down. Single-pad force measurements showed that the adhesion of euplantulae was extremely small, but friction forces strongly increased with normal load and coefficients of friction were [Formula: see text] 1. The pre-tarsal arolium, in contrast, generated adhesion that strongly increased with pulling forces, allowing adhesion to be activated and deactivated by shear forces, which can be produced actively, or passively as a result of the insects' sprawled posture. The shear-sensitivity of the arolium was present even when corrected for contact area, and was independent of normal preloads covering nearly an order of magnitude. Attachment of both heel and toe pads is thus activated partly by the forces that arise passively in the situations in which they are used by the insects, ensuring safe attachment. Our results suggest that stick insect euplantulae are specialised "friction pads" that produce traction when pressed against the substrate, while arolia are "true" adhesive pads that stick to the substrate when activated by pulling forces.

Static analysis of Euler-Bernoulli beams with multiple unilateral cracks under combined axial and transverse loads

The paper deals with the static analysis of pre-damaged Euler-Bernoulli beams with any number of unilateral cracks and subjected to tensile or compression forces combined with arbitrary transverse loads. The mathematical representation of cracks with a bilateral behaviour (i.e. always open) via Dirac delta functions is extended by introducing a convenient switching variable, which allows each crack to be open or closed depending on the sign of the axial strain at the crack centre. The proposed model leads to analytical solutions, which depend on four integration constants (to be computed by enforcing the boundary conditions) along with the Boolean switching variables associated with the cracks (whose role is to turn on and off the additional flexibility due to the presence of the cracks). An efficient computational procedure is also presented and numerically validated. For this purpose, the proposed approach is applied to two pre-damaged beams, with different damage and loading conditions, and the results so obtained are compared against those given by a standard finite element code (in which the correct opening of the cracks is pre-assigned), always showing a perfect agreement. © 2013 Elsevier Ltd. All rights reserved.

The structural and photoluminescence character of InAs quantum dots grown on a combined InAlAs and GaAs strained buffer

The structural and photoluminescence (PL) properties of the InAs quantum dots (QDs) grown on a combined InAlAs and GaAs strained buffer layer have been investigated by AFM and PL measurements. The dependence of the critical thickness for the transition from 2D to 3D on the thickness of GaAs layer is demonstrated directly by RHEED. The effects of the introduced-InAlAs layer on the density and the aspect ratio of QDs have been discussed.

The effect of a combined InAlAs and GaAs strained buffer layer on the structure and optical property of InAs quantum dots

The character of InAs quantum dots (QD) directly deposited on a combined InAlAs-GaAs (XML) strained buffer layer (SBL) has been investigated. This growth technique realizes high-density QD (5.88 x 10(10) cm(-2)) by changing the thickness of GaAs in InAlAs-GaAs SBL. The dependence of the density and the aspect ratio of QD on the GaAs thickness has been discussed in detail. The photoluminescence (PL) measurements demonstrate an obvious redshift with the increase of GaAs thickness. In addition, the deposition of InAs QDs grown on the combined InAlAs-GaAs SBL has an important effect of the QD properties. The ordered QD array can be observed from the sample deposited by atomic layer epitaxy, of which the PL peak shows an obvious redshift in comparison to the molecular beam epitaxy (MBE) QDs when the GaAs thicknesses are equal. (c) 2004 Elsevier B.V. All rights reserved.

Influence of combined InAlAs and InGaAs strain-reducing laser on luminescence properties of InAs/GaAs quantum dots

We have fabricated self-organized InAs/GaAs quantum dots (QDs) capped by 1 nm In0.2Al0.8As and 5 nm In0.2Ga0.8As strain-reducing layer (SRL). The luminescence emission at a long wavelength of 1.33 mum with narrower half width is realized. A wider energy separation between the ground and first excited radiative transitions of up to 102meV was observed at room temperature. Furthermore, the comparative study proves that luminescence properties of InAs/GaAs QDs overgrown with combined InAlAs and InGaAs SRLs are much better than that of one capped with InGaAs or InAlAs SRL. (C) 2002 Elsevier Science B.V. All rights reserved.

Influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy

The influence of band bending and polarization on the valence band offset measured by x-ray photoelectron spectroscopy (XPS) is discussed, and a modification method based on a modified self-consistent calculation is proposed to eliminate the influence and thus increasing the precision of XPS. Considering the spontaneous polarization at the surfaces and interfaces and the different positions of Fermi levels at the surfaces, we compare the energy band structures of Al/Ga-polar AlN/GaN and N-polar GaN/AlN heterojunctions, and give corrections to the XPS-measured valence band offsets. Other AlN/GaN heterojunctions and the piezoelectric polarization are also introduced and discussed in this paper.

In situ analysis of electropolymerization of aniline by combined electrochemistry and surface plasmon resonance

The combination of electrochemistry with surface plasmon resonance (SPR) has been used to characterize the growth of polyaniline (PAn) on a gold electrode surface during potential cycling. Potential-modulated SPR characteristics of the PAn film were also revealed. The potential switch between the oxidized and reduced states of PAn can lead to a large change of SPR response due to the variation in the imaginary part of the dielectric constant of PAn film resulting from the transition of the film in conductivity. The redox transition of the PAn film during potential cycling is very profitable to the SPR measurements. Two modes of SPR measurement, SPR angular scan (R-theta) and the time evolution of the reflectivity change at a fixed angle (R-t), were displayed to study the growth process of the PAn film. The angle shift of the resonance minimum recorded at each cathodic limit of cyclic potential scanning allows for the unambiguous measurement of the film growth. During cyclic potential scanning, the R-t curve was repeatedly modulated with the direction of the potential ramp as a result of the redox switch of the PAn film, and the amplitude of potential-modulated reflectivity change was well correlated with the cyclic number. The time differential of the R-t curve permits continuous monitoring of the film growth process. These results illustrate that the combined technique is suitable for studying the electropolymerization process of a conducting polymer.

Evaluation of a combined wavelet and a combined principal component analysis classification system for BCG diagnostic problem

Heart disease is one of the main factor causing death in the developed countries. Over several decades, variety of electronic and computer technology have been developed to assist clinical practices for cardiac performance monitoring and heart disease diagnosis. Among these methods, Ballistocardiography (BCG) has an interesting feature that no electrodes are needed to be attached to the body during the measurement. Thus, it is provides a potential application to asses the patients heart condition in the home. In this paper, a comparison is made for two neural networks based BCG signal classification models. One system uses a principal component analysis (PCA) method, and the other a discrete wavelet transform, to reduce the input dimensionality. It is indicated that the combined wavelet transform and neural network has a more reliable performance than the combined PCA and neural network system. Moreover, the wavelet transform requires no prior knowledge of the statistical distribution of data samples and the computation complexity and training time are reduced.

A combined DFT and SCRF study of solvent effects on 4-methyl-3-penten-2-one

The IR spectrum of 4-methyl-3-penten-2-one is interpreted with the aid of normal coordinate calculations within the Onsager self-consistent reaction field (SCRF) model, using a density functional theory (DFT) method at the Becke3LYP/6-31G* level. The solvent effects on the geometry, energy, dipole moment, and vibrational frequencies of 4-methyl-3-penten-2-one in the solution and in the liquid phase are calculated using the Onsager SCRF model. The calculated vibrational frequencies in the liquid-phase are in good agreement with the experimental values. The solvent reaction field has generally weak influence. For the two main bands of C=C and C=O mixed vibrational modes, small frequency shifts (5-6 cm(-1)), but relatively large changes in IR intensities (up to 101 km mol(-1) in the liquid phase) are found. (C) 1999 Elsevier Science BV. All rights reserved.