Image based simulation of one-dimensional compression tests on carbonate sand

High factors of safety and conservative methods are commonly used on foundation design on shelly carbonate soils. A better understanding of the behavior of this material is, thus, critical for more sustainable approaches for the design of a number of offshore structures and submarine pipelines. In particular, understanding the physical phenomena taking place at the microscale has the potential to spur the development of robust computational methods. In this study, a one-dimensional compression test was performed inside an X-ray scanner to obtain 3D images of the evolving internal structure of a shelly carbonate sand. A preliminary inspection of the images through five loading increments has shown that the grains rearrange under loading and in some cases cracks develop at the contacts. In order to replicate of the experiments in the numerical domain, the 3D image of the soil prior to loading was imported into a micro Finite Element (µFE) framework. This image-based modelling tool enables measurements of the contact force and stress map inside the grains while making use of the real microstructure of the soil. The potential of the µFE model to contribute insights into yield initiation within the grain is demonstrated here. This is of particular interest to better understand the breakage of shelly grains underpinning their highly compressive behavior

Risk and sensitivity quantification of fracture failure employing cohesive zone elements

Many structures are subjected to the risk of fatigue failure. For their reliability-based design, it is thus important to calculate the probability of fatigue failure and assess the relative importance of the involved parameters. Although various studies have analyzed the fatigue failure, the stage of fracture failure has been less focused. In particular, the risk analysis of fracture failure needs to be conducted considering its importance in actual structures. This article proposes a new probabilistic framework for the risk and sensitivity analysis of structural fatigue failure employing cohesive zone elements. The proposed framework comprises three steps, namely finite element analysis using cohesive zone elements, response surface construction, and risk and sensitivity analysis of fatigue failure, which require several mathematical techniques and algorithms. The proposed framework is tested by applying it to an illustrative example, and the corresponding analysis results of fracture failure probability with different threshold values of a limit-state function are presented. In addition, the sensitivities of failure risk with respect to the statistical parameters of random variables are presented and their relative importance is discussed

Computational study of reservoir sand production mechanisms

A numerical model is developed to simulate fluid flow conditions around a wellbore and to evaluate mechanisms governing fluid flow, pressure gradients, rock failure and the ensuing sand production. The rock material behaviour matches sandstone described by the Drucker–Prager material failure model. Conditions for erosion are governed through two criteria: a material failure criterion described by the Drucker–Prager model and a sanding criterion expressed by an eroded solid mass generation model. The interplay between controlling operating and reservoir conditions is assessed. In addition, contributions of the following key factors to interstitial fluid velocity, plastic strain, pore pressure variation and sand production are appraised: drawdown, wellbore perforation depth, mud pressure and erosion criteria. Despite a decrease in pore fluid velocity at the vicinity of the wellbore at increasing depth, sand production increases with wellbore/perforation depth. Likewise, at constant drawdown, sand production is aggravated as wellbore/perforation depth increases. The rate of increase in the plastic zone following the onset of sand production is inconstant. Furthermore, mud pressure is demonstrated as an effective tool for attenuating sand production. An understanding of interactions between key parameters governing reservoir responses and the effect on sanding during oil/gas production is imperative if extraction operations are to be optimised.Published onlin

07.15: Experimental and numerical investigation of cold‐formed steel built‐up stub columns

This paper describes a numerical and experimental programme carried out at the University of Sheffield on built‐up stub columns fabricated from cold‐formed steel. A total of 20 built‐up columns with four different cross‐sectional geometries were tested between fixed end conditions. Two of the cross‐sectional geometries were assembled using M6 bolts and the other two using M5.5 self‐drilling sheet metal screws. The connector spacing was varied among specimens of the same cross‐sectional geometry. The cross‐sections were assembled from flat plate, plain channels and lipped channels with nominal thicknesses ranging from 1.2 mm to 2.4 mm. The initial geometric imperfections of each specimen were recorded prior to testing and their material properties were determined by means of tensile coupon tests. Single lap shear tests were also carried out in order to study the connector behaviour of the bolts and the screws used to assemble the specimens. As part of the numerical part of the study, the test specimens were modelled using the commercially available ABAQUS software package. The recorded geometric imperfections, the measured material properties and the connector behaviour data obtained from the lap shear tests were incorporated into the FE models. Special attention was paid to the connector modelling in order to find an effective and simple way to represent their actual behaviour. The FE models were further used to quantify the effect of the connector behaviour on the buckling response of cold‐formed steel built‐up stub columns

Crystal and Molecular Structure and DFT Calculations of the Steroidal Oxime 6E-Hydroximino-androst-4-ene-3,17-dione (C₁₉H₂₅NO₃) a Molecule with Antiproliferative Activity

The single crystal X-ray structure of the novel steroid derivative, 6E-hydroximino-androst-4-ene-3,17-dione ( C19H25NO3) (code name RB-499), possessing antiproliferative activity against various cell lines is presented. The analysis produced the following results: chemical formula C19H25NO3; Mr = 315.40; crystals are orthorhombic space group P212121 with Z = 4 molecules per unit cell with a = 6.2609(2), b = 12.5711(4), c = 20.0517(4) Å,Vc = 1578.18(7) Å3, crystal density Dc = 1.327 g/cm³. Structure determination was performed by direct methods, Fourier and full-matrix least-squares refinement. Hydrogens were located in the electron density and refined in position with isotropic thermal parameters. The final R-index was 0.0324for 3140 reflections with I > 2σ and 308 parameters. The Absolute Structure Parameter − 0.07(5) confirms the correct allocation of the absolute configuration. The presence of the double bond C=O at position 3 in Ring A has caused a distortion from the usual chair conformation and created an unusual distorted sofa conformation folded across an approximate m-plane through C(1)–C(4). Ring B is a distorted chair, its conformation being influenced by the presence of the C(6)=N(6)–O(6)H group in position 6. Ring C is a symmetrical chair. Ring D exhibits both a distorted mirror symmetry conformation [influenced by the C(17)=O(17) group] and a distorted twofold conformation. DFT calculations indicated some degree of flexibility in rings A, C and D with ring A showing the greatest variation in torsion angles. The crystal packing is governed by H-bonds involving O(3), O(6) and O(17). DFT calculations of bond distances and angles, optimized at the B3LYP/6–31++G(d,p) level, were in good agreement with the X-ray structure