Analysis of the Stability of Openings Excavated in Anisotropic Rocks

Openings excavated in rocks with anisotropic strength are often affected by serious instability, related to slip along the weakness planes. The Jaeger criterion, which is a discontinuous approach, is widely used in the mining and oil and gas industry, because is based on well-known rock strength parameters. However, this model cannot capture features related to the stability of openings drilled in some anisotropic rocks with the combined effect of the in situ state of stress. The Hoek & Brown criterion, adapted to anisotropic rocks, is a continuous criterion that can describe the complex behavior of different types of anisotropy exhibited by rock material. Here we interpreted the results of triaxial tests carried out on a shale and we defined the parameters of the Jaeger criterion and the modified Hoek & Brown criterion. We investigated the stability of boreholes drilled in this shale by varying the in situ state of stress and we compared the results of the two criteria. We found that the Hoek & Brown criterion can appropriately describe the behavior of this shale and can predict more accurately the width of the instability of openings excavated in different conditions

The Importance of Viscoplastic Strain Rate in the Formation of Center Cracks during the Start-Up Phase of Direct-Chill Cast Aluminum Extrusion Ingots

A comparison of experimental observations and computer simulations shows that trends in the occurrence and severity of center cracks in direct-chill (DC) cast ingots due to different initial casting speed histories may best be explained by the changes in viscoplastic strain rate close to the center of the base of the ingot. The thermomechanical histories of five ingots were simulated and correlations between stresses, strains, strain rates, and liquid pressure drops due to feeding restrictions were considered

Experimental validation of finite element codes for welding deformations

International audienceA single pass metal inert gas welding on an austenitic steel plate has been presented for the purpose of providing controlled experimental data against which numerical codes quantifying welding stresses can be validated. It includes a moving heat source with material deposit, and completes thus existing validation data. The experiment has been addressed by a numerical code, WeldSimS, reproducing qualitatively the distortion during welding quite well. Quantitative differences between the numerical and experimental results, however, indicate the need for more accurate modelling tools than those presently available, which are all based on commonly accepted modelling principles and input data

Deep Gray Matter Demyelination Detected by Magnetization Transfer Ratio in the Cuprizone Model

Abstract In multiple sclerosis (MS), the correlation between lesion load on conventional magnetic resonance imaging (MRI) and clinical disability is weak. This clinico-radiological paradox might partly be due to the low sensitivity of conventional MRI to detect gray matter demyelination. Magnetization transfer ratio (MTR) has previously been shown to detect white matter demyelination in mice. In this study, we investigated whether MTR can detect gray matter demyelination in cuprizone exposed mice. A total of 54 female C57BL/6 mice were split into one control group () and eight cuprizone exposed groups (Ns~6). The mice were exposed to 0:2% (w/w) cuprizone for up to six weeks. MTR images were obtained at a 7 Tesla Bruker MR-scanner before cuprizone exposure, weekly for six weeks during cuprizone exposure, and once two weeks after termination of cuprizone exposure. Immunohistochemistry staining for myelin (anti-Proteolopid Protein) and oligodendrocytes (anti-Neurite Outgrowth Inhibitor Protein A) was obtained after each weekly scanning. Rates of MTR change and correlations between MTR values and histological findings were calculated in five brain regions. In the corpus callosum and the deep gray matter a significant rate of MTR value decrease was found, 0:96% per week (pv:0001) and 0:39% per week (pv:0001) respectively. The MTR values correlated to myelin loss as evaluated by immunohistochemistry (Corpus callosum: R 2~: 43, pv:0001. Deep gray matter: R 2~: 21, pv:001), but did not correlate to oligodendrocyte density. Significant results were not found in the cerebellum, the olfactory bulb or the cerebral cortex. This study shows that MTR can be used to detect demyelination in the deep gray matter, which is of particular interest for imaging of patients with MS, as deep gray matter demyelination is common in MS, and is not easily detected on conventional clinical MRI

Nonlinear Vehicle Velocity Observer with Road-Tire Friction Adaptation

Abstract-A nonlinear observer for lateral velocity of an automotive vehicle is extended for robustness with respect to unknown road surface conditions. The observer uses a friction model parametrized with the maximum road-tire friction coefficient, and an adaptive parameter update law is designed for estimation of this coefficient. The adaptive nonlinear observer is proven to be uniformly globally asymptotically stable under a uniform δ -persistency-of-excitation condition, and a set of additional technical assumptions, using results related to Matrosov's Theorem. The adaptive observer is validated using experimental data from a car

Modeling of Stress, Distortion, and Hot Tearing

International audienceComputational modeling of mechanical behavior during solidification is becoming more important. Thermal and microstructural simulations alone are insufficient to predict the quality of the final product that is desired by the casting industry. Accurate calculation of displacements, strains, and stresses during the casting process is needed to predict residual stress and distortion and defects such as the formation of cracks such as hot tears. It also helps predict porosity and segregation. As computing power and software tools for computational mechanics advance, it is becoming increasingly possible to perform useful mechanical analysis of castings and these important related behaviors

HOT TEARING SUSCEPTIBILITY IN DC CAST ALUMINUM ALLOYS

Hot tearing and loss of dimensional stability are two key defects related to industrial aluminum alloy casting processes. In order to investigate their occurrence, a new semi-solid constitutive law [20] for AA5182 that takes into account cooling rate, grain size and porosity has been implemented into a Direct Chill casting process model for round billets. The semi-solid stressstrain predictions provided by this new constitutive law as well as the effects of processing parameters were then examined to demonstrate the relationships between defect formation, microstructure, and processing variables

An analytical model to predict the volume of sand during drilling and production

© 2016 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Sand production is an undesired phenomenon occurring in unconsolidated formations due to shear failure and hydrodynamic forces. There have been many approaches developed to predict sand production and prevent it by changing drilling or production strategies. However, assumptions involved in these approaches have limited their applications to very specific scenarios. In this paper, an elliptical model based on the borehole shape is presented to predict the volume of sand produced during the drilling and depletion stages of oil and gas reservoirs. A shape factor parameter is introduced to estimate the changes in the geometry of the borehole as a result of shear failure. A carbonate reservoir from the south of Iran with a solid production history is used to show the application of the developed methodology. Deriving mathematical equations for determination of the shape factor based on different failure criteria indicate that the effect of the intermediate principal stress should be taken into account to achieve an accurate result. However, it should be noticed that the methodology presented can only be used when geomechanical parameters are accurately estimated prior to the production stage when using wells and field data

Porosity and permeability development in compacting chalks during flooding of nonequilibrium brines: Insights from long-term experiment

We report the complete chemical alteration of a Liège outcrop chalk core resulting from a 1072 flow-through experiment performed during mechanical compaction at 130°C. Chemical rock-fluid interactions alter the volumetric strain, porosity, and permeability in a nontrivial way. The porosity reduced only from 41.32% to 40.14%, even though the plug compacted more than 25%. We present a novel analysis of the experimental data, which demonstrates that the geochemical alteration does not conserve the volume of the solids, and therefore, the strain is partitioned additively into a pore volume and solid volume component. At stresses beyond yield, the observed deformation can be explained by grain reorganization reducing the pore space between grains and solid volume changes fromthe rock-fluid interactions. Themechanical and chemical effects are discussed in relation to the observed permeability development.publishedVersio