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Non-Schmid Effects and Criteria for Dislocation Nucleation on Different Slip Systems at Grain BoundariesWyman, Richard Durtschi 01 June 2016 (has links)
Criteria for grain boundary dislocation nucleation are developed. A bicrystal containing two grain boundaries is placed under varying triaxial stress states using molecular dynamics. The local resolved shear, normal, and co-slip stresses needed for grain boundary dislocation nucleation are found. A framework is developed to detect the slip system grain boundary dislocation nucleation occurs on. A survey of the different ways grain boundary dislocation nucleation occurs in the sample shows a single grain boundary can nucleate dislocations in a rich variety of ways. Using the nucleation system and resolved stress values, criteria for grain boundary dislocation nucleation on different slip systems are developed. The proposed form of nucleation criterion suggests the activation stress has a linear dependence one the resolved shear, normal, and co-slip stresses. A residual analysis largely validates the efficacy of the proposed linear model. We show that the nucleation slip system cannot be predicted by a maximum Schmid factor analysis due to the non-Schmid resolved normal and co-slip terms. We show that a system's global pressure generally fails to predict nucleation; a local stress in the grain being nucleated into should be used. Using the nucleation criteria for each slip system, a yield surface for dislocation nucleation is built for the grain boundary used in this work.
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Punching Shear Failure Analysis of Reinforced Concrete Flat Plates Using Simplified Ust Failure CriterionZhang, Xuesong, n/a January 2003 (has links)
Failure criteria play a vital role in the numerical analysis of reinforced concrete structures. The current failure criteria can be classified into two types, namely the empirical and theoretical failure criteria. Empirical failure criteria normally lack reasonable theoretical backgrounds, while theoretical ones either involve too many parameters or ignore the effects of intermediate principal stress on the concrete strength. Based on the octahedral shear stress model and the concrete tensile strength under the state of triaxial and uniaxial stress, a new failure criterion, that is, the simplified unified strength theory (UST), is developed by simplifiing the five-parameter UST for the analysis of reinforced concrete structures. According to the simplified UST failure criterion, the concrete strength is influenced by the maximum and intermediate principal shear stresses together with the corresponding normal stresses. Moreover, the effect of hydrostatic pressure on the concrete strength is also taken into account. The failure criterion involves three concrete strengths, namely the uniaxial tensile and compressive strengths and the equal biaxial compressive strength. In the numerical analysis, a degenerated shell element with the layered approach is adopted for the simulation of concrete structures. In the layered approach, concrete is divided into several layers over the thickness of the elements and reinforcing steel is smeared into the corresponding number of layers of equivalent thickness. In each concrete layer, three-dimensional stresses are calculated at the integration points. For the material modelling, concrete is treated as isotropic material until cracking occurs. Cracked concrete is treated as an orthotropic material incorporating tension stiffening and the reduction of cracked shear stiffness. Meanwhile, the smeared craclc model is employed. The bending reinforcements and the stirrups are simulated using a trilinear material model. To verify the correctness of the simplified UST failure criterion, comparisons are made with concrete triaxial empirical results as well as with the Kupfer and the Ottosen failure criteria. Finally, the proposed failure criterion is used for the flexural analysis of simply supported reinforced concrete beams. Also conducted are the punching shear analyses of single- and multi-column-slab connections and of half-scale flat plate models. In view of its accuracy and capabilities, the simplified UST failure criterion may be used to analyse beam- and slab-type reinforced concrete structures.
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[en] EXPERIMENTAL STUDY OF THE STRESS INFLUENCE ON PERMEABILITY OF PRODUCING OIL ROCKS / [pt] ESTUDO EXPERIMENTAL DA INFLUÊNCIA DE TENSÕES NA PERMEABILIDADE DE ROCHAS PRODUTORAS DE PETRÓLEOFLAVIO DA SILVA AZEVEDO 20 July 2005 (has links)
[pt] Reservatórios de hidrocarbonetos são sistemas dinâmicos que
estão
constantemente mudando durante a história da produção
(depleção). A produção
de fluidos, a partir de reservas de hidrocarbonetos, reduz
a poro-pressão do
reservatório, podendo levar à compactação das rochas devido
ao aumento das
tensões efetivas. Por outro lado, a injeção de água em um
reservatório pode
aumentar a poro-pressão e, com isso, diminuir a tensão
efetiva. O conhecimento
de mudanças de tensão e poro-pressão é essencial para uma
boa gestão do
reservatório, porque a alteração da tensão in situ durante
a produção pode ter um
impacto significante na performance do reservatório,
variando a permeabilidade
da rocha. O objetivo da atual pesquisa é estudar
experimentalmente a variação, a
anisotropia e a histerese de permeabilidade de rochas
produtoras de petróleo
(arenitos) sob variação dos estados de tensão hidrostático
e triaxial verdadeiro.
Para realização dos ensaios foi utilizado um novo
equipamento triaxial verdadeiro,
que aplica de forma independente as três tensões principais
em corpos de prova
cúbicos, atingindo, desta maneira, um estado de tensão mais
realista para o estudo
das propriedades relevantes das rochas. Os resultados dos
ensaios apresentaram
evidências que estimativas de produção e de reserva de
hidrocarbonetos podem
ser significativamente melhoradas, quando a permeabilidade
é considerada uma
variável dinâmica. A permeabilidade dos arenitos Berea, Rio
Bonito e Botucatú se
mostrou altamente influenciada, tanto pelo estado de tensão
hidrostático quanto
pelo triaxial verdadeiro. Contudo, o estado hidrostático
apresentou maior
influência na permeabilidade. / [en] Hydrocarbons reservoirs are dynamic systems that constantly
changes
during depletion. The production of fluids from a reservoir
often reduces pore
pressure such that there is an increase in its effective
stresses. This may cause
compaction which, in turn, may impact matrix permeability.
On the other hand,
during water injection on reservoirs, the pore pressure
increases and effective
stress decreases. An understanding of these changes is of
fundamental importance
to performance predictions and management of the stress-
sensitive reservoirs. The
main objectives of this work is to experimentally
investigate the stress dependence
of rock producing oil permeability, its anisotropy and
hysteresis under hydrostatic
and true triaxial stress conditions. In the present
investigation a new true triaxial
equipment, one that is able to apply the three principal
boundary stresses
independently using cubic samples was utilized. This
equipment can apply a
realistic stress state to the rock samples for measuring of
the relevant properties
under stress states that mimic the in situ condition. The
experimental results have
produced evidence that hydrocarbon production and reserve
estimates may
significantly improve when permeability is considered as a
dynamic variable.
Permeability in all three formations (Berea, Rio Bonito and
Botucatú sandstones)
was shown to be strongly stress-dependent both under a
hydrostatic stress state
and under a true triaxial stress state. Nevertheless, the
effect of stress states on
permeability has clearly shown that permeability reduction
under true triaxial
stresses was less than that under hydrostatic stresses.
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