Stress and displacement discontinuity element method for undrained analysis

Stress (SD) and displacement (DD) discontinuity methods have been developed for problems where seepage and deformation are not coupled. This is an important limitation in geomechanics where the ground is usually saturated. Of particular importance are problems that fall within the “short term analysis” concept, where a phenomenon occurs over a period of time that is very short compared to the permeability of the medium. In this paper we present an analytical formulation to solve undrained conditions and deformation problems in a poroelastic saturated medium in plane strain conditions, with the assumption of short term analysis (i.e. no change in volume of the medium). Explicit functions for SD and DD elements are obtained and are incorporated into the code FROCK. The new code is verified by a number of comparisons with theoretical solutions, also presented in the paper, and with solutions from the Finite Element Method. In undrained compression, the compression field around the tip of an open flaw decreases and the tension field increases compared to drained compression loading, while the shear stresses remain unchanged. For a closed frictional flaw undrained compression decreases the compression and tension fields around the tip. The shear stresses however have the same distribution as in drained loading but are larger. This is caused by the decrease of the effective normal stresses due to the excess pore pressures which in turn decrease the shear resistance of the flaw.     

Phase Engineering of 2D Tin Sulfides

Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase‐selective growth of both hexagonal tin (IV) sulfide SnS₂ and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO₂ substrates through atmospheric pressure vapor‐phase method in a conventional horizontal quartz tube furnace with SnO₂ and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations.     

Temperature and magnetic field effect on oscillations observed in GaInNAs/GaAs multiple quantum wells structures

The photoconductivity of p-i-n GaInNAs/GaAs multiple quantum well (MQW) mesa structures is investigated. When illuminated with photons at energy greater than the GaAs bandgap, a number of oscillations are observed in the current–voltage I–V characteristics. The amplitude and position of the oscillations is shown to depend upon the temperature, as well as upon the exciting wavelength and intensity. Due to the absence of the oscillations in the dark I–V and at temperatures above T = 200 K, we explain them in terms of photogenerated electrons escaping from quantum wells via tunnelling or thermionic emission.     

Modelling 3D spatial enclosure of urban open spaces

Existing urban open space typologies within dense urban fabrics cannot meet society’s open space requirements in developing countries’ metropolitan cities, such as Istanbul. Because of high building densities, it is a challenging task to create new open spaces within urban cores. Developing new tools that work with the existing built environment is crucial to reveal ‘opportunity spaces’ that can act as breathing points within dense urban fabrics. In this research, a new model is developed to evaluate the 3D spatial enclosure of open spaces using basic geometrical properties and geographic information system (GIS) tools. As a case study, Istanbul’s changing spatial organization is analyzed using this model.     

Deep Convolutional Generalized Classifier Neural Network

Neural Processing Letters - Up to date technological implementations of deep convolutional neural networks are at the forefront of many issues, such as autonomous device control, effective image...     

A Subtractive Photoresist Platform for Micro‐ and Macroscopic 3D Printed Structures

The selective removal of structural elements plays a decisive role in 3D printing applications enabling complex geometries. To date, the fabrication of complex structures on the microscale is severely limited by multistep processes. Herein, a subtractive photoresist platform technology that is transferable from microscopic 3D printing via direct laser writing to macroscopic structures via stereolithography is reported. All resist components are readily accessible and exchangeable, offering fast adaptation of the resist's property profile. The micro‐ and macroprinted structures can be removed in a facile fashion, without affecting objects based on standard photoresists. The cleavage is analyzed by time‐lapse optical microscopy as well as via in‐depth spectroscopic assessment. The mechanical properties of the printed materials are investigated by nanoindentation. Critically, the power of the subtractive resist platform is demonstrated by constructing complex 3D objects with flying features on the microscale.     

Slip propagation along frictional discontinuities

Slip initiation and propagation along non-homogeneous frictional surfaces are investigated by loading specimens of gypsum in biaxial compression. The specimens used in the tests are composed of two individual blocks with perfectly mated contact surfaces. The contact surfaces have on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. Four test series are performed using specimens with different surface characteristics along the contact surface. The experiments are conducted by applying first a normal stress across the frictional surface and then increasing the shear stress until final debonding and slip along the strong surface occur. The magnitude of the normal stress used in the experiments ranges from 0.7 to 15 MPa (about 50% of the unconfined compression strength of the material). Slip starts on the weak area and, as the shear stress is increased, propagates towards the strong area. Full slip along the weak area introduces a sharp transition between the area of the discontinuity that has slipped and the area that has not. This transition creates a large concentration of stresses which can be treated within the framework of fracture mechanics as a mode II frictional crack. With further loading, rupture occurs through the strong area as an unstable process that coincides with failure. The results show that the critical energy release rate G_IIC is a good indicator of the rupture. However, G_IIC is not a material property. It depends on the normal stress applied, on the frictional characteristics of the interface, and on the critical slip required for the transition from peak to residual. A slip initiation model is proposed based on experimental observations and on fracture mechanics theory and is incorporated into a finite element method code. Predictions with the model compare very well with experiments.     

Flow control valves for analytical microfluidic chips without mechanical parts based on thermally responsive monolithic polymers

Monolithic plugs of poly(N-isopropylacrylamide) cross-linked with 5% methylenebisacrylamide have been prepared by photoinitiated polymerization within the channel of a microfluidic device. The volume change associated with the polymer phase transition at its lower critical solution temperature of 32 degrees C allows both the rapid swelling and the deswelling of the monoliths enabling the polymer to close or open the channel as it functions as a nonmechanical valve. Thermoelectric elements capable of changing the temperature of the system between 17 and 57 degrees C were used to actuate the valve. Flow through the device was monitored by fluorescence measurements via the laser-triggered photobleaching of a dye contained in the liquid phase. Photobleaching occurs quickly once the flow is stopped, and the time required to open and close the valve was 3.5 and 5.0 s, respectively. No changes in function were observed even after 120 open-close cycles. Although the 2-mm-long valve was prepared from a polymerization mixture consisting of only a 5% aqueous solution of monomers, it resists pressures of up to 1.38 MPa (200 psi) without observable structural damage.