A Damage Mechanics Approach to Life Prediction for a Salt Structure

Excavated rooms in natural bedded salt formations are being considered for use as repositories for nuclear waste. It is presumed that deformation of the rooms by creep will lead to loss of structural integrity and affect room life history and seal efficiency. At projected repository temperatures, two possible fracture mechanisms in salt are creep-induced microcracking in triaxial compression and cleavage in tension. Thus, an accurate prediction of room life and seal degradation requires a reliable description of the creep and damage processes. While several constitutive models that treat either creep or fracture in salt are available in the literature, very few models have considered creep and damage in a coupled manner. Previously, Munson and Dawson formulated a set of creep equations for salt based on the consideration of dislocation mechanisms in the creep process. This set of creep equations has been generalized to include continuum, isotropic damage as a fully coupled variable in the response equation. The extended model has been referred to as the Multimechanism Deformation Coupled Fracture (MDCF) model. A set of material constants for the creep and damage terms was deduced based on test data for both clean and argillaceous salt. In this paper, the use of the MDCF model for establishing the failure criteria and for analyzing the creep response of a salt structure is demonstrated. The paper is divided into three parts. A summary of the MDCF model is presented first, which is followed by an evaluation of the MDCF model against laboratory data. Finally, finite-element calculations of the creep and damage response of a salt structure are presented and compared against in-situ field measurements

Optical gain of interdiffused InGaAs-As and AlGaAs-GaAs quantum wells

We have analyzed theoretically the effects of interdiffusion on the gain, differential gain, linewidth enhancement factor, and the injection current density of In0.2Ga0.8As-GaAs and Al0.3Ga0.7As-GaAs quantum-well (QW) lasers. We have calculated the electron and hole subband structures including the effects of valence band mixing and strains. The optical gain is then calculated using the density matrix approach. Our results show that the gain spectrum can be blue-shifted without an enormous increase in the injected current density. Imposing an upper limit (416 A·cm-2) on the injection current density for a typical laser structure, we find that the InGaAs-GaAs and AlGaAs-GaAs QW lasers can be blue-shifted by 24 and 54 mn, respectively. Our theoretical results compare well with the tuning ranges of 53 and 66 meV found for AlGaAs-GaAs QWs in some experiments. This indicates that the interdiffusion technique is useful for the tuning of laser operation wavelength for multiwavelength applications.published_or_final_versio

Electro-absorption and refraction at 1.5 μm in InGaAs/AlGaAssuperlattice growth on GaAs substrate

High indium concentration In0.65Ga0.35As/Al 0.33Ga0.67As superlattices on GaAs substrates are useful for modulators and optical communication applications. This is due to the lowest loss 1.55 μm optimum wavelength for operation of fiber optic systems. The optical parameters such as absorption coefficient and change in refractive index with applied electric field are investigated.published_or_final_versio

Interdiffusion effect on the gain of InGaAs/InP quantum well laser

Lattice-matched In0.53Ga0.47As/InP quantum well (QW) structures are of considerable interest in photonic application since they enabled device operation in the 1.3micrometers to 1.55micrometers wavelength range which is of importance for optical communication systems. The process of interdiffusion modifies the as-grown square QW to a graded QW which alter the subband structure and optical properties of the QW. Thus it provides a useful tool for bandstructure engineering. The interdiffusion process of InGaAs/InP QW provides more degrees of freedom than AlGaAs/GaAs QW system since interdiffusion can occur for group-III, group-V, and groups III plus V together. These are determined by the temperature and chemical environment used during annealing of the QW structure. The effect of interdiffusion on the laser performances of InGaAs/InP QWs is also studied based on these different types of diffusion processes. It is found that the operating wavelength shows both a red shift and a blue shift depending on the types of diffusion process. It is also found that group-III interdiffusion gives the best performance of InGaAs/InP QW laser when comparing to the other tow types of interdiffusion in terms of a smaller threshold carrier density.published_or_final_versio

Analysis of three types of interdiffusion process in InGaAs/InP quantum-well and their devices implications

The optical properties of Ino 53Gao 47AS/I11P single quantum well (QW) (with an as-grown well width of 60Å structures) interdiffused with different cation and anion interdiffusion rates have been theoretically analyzed for applications in optoelectronics. The interdiffusion of InGaAs/InP QW structures is complicated as interdiffusion can occur for either (i) only group-Ill (In,Ga), (ii) group-V (As,P), or (iii) both group-Ill and group-V sublattices. Depending on the resulting composition profiles, the shifts (blue or red) of the transition energies can be tuned to wavelengths between 1.3µm to 1.55µm for device applications. The results show that the control of the rates of cation and anion interdiffusion offers interesting possibilities for designing optoelectronic devices such as modulators and lasers.published_or_final_versio

Theoretical Analysis of Diffused Quantum-well Lasers and Optical Amplifiers

Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.published_or_final_versio

Performance analysis of burst segmentation schemes supporting multiple traffic classes

Optical burst switching (OBS) with the support of multiple traffic classes is an important topic. Assuming traffic classes are maintained via choosing different offset times, a new OBS scheme called optical burst switching with burst splitting (OBSS) is proposed in this paper. OBSS provides high system throughput by taking advantages of the idle gaps on output wavelength channels when the Just-Enough-Time (JET) reservation scheme is used. Then an analytical model that supports N traffic classes is constructed and verified by simulations. The proposed model can be applied to the conventional OBS schemes as well as the OBSS scheme we proposed. Numerical results show that in our OBSS scheme, traffic classes can be effectively maintained via different offset times, and the packet loss probability is also much smaller than the conventional OBS. It is interesting to find that the choice of offset time for a specific traffic class affects the packet loss probabilities of classes higher than it, while the impact to classes below it is much smaller. © 2005 IEEE.published_or_final_versio

Design and implementation of multiplier-less tunable 2-D FIR filters using McClellan transformation

This paper proposes new structures for realizing tunable 2-D fan and elliptical filters with different spectral characteristics using McClellan transformation. The 1-D prototype is a variable digital filter obtained from the interpolation of a set of desirable impulse responses and is implemented using the Farrow structure. The coefficients of the sub-filters and the transformation parameters in the Farrow structure are represented in SOPOT, which can be easily implemented as simple shift-and-add operations. Furthermore, the transformation part can be shared between the sub-filters leading to significant saving in hardware complexity. Several design examples are given to demonstrate the effectiveness and feasibility of the proposed approach.published_or_final_versio

An efficient NC tool path planning approach

One major problem encountered in NC machining is that cutting load fluctuates in twisty tool paths and increases momentarily when milling concave corners where stock material concentrates. Much research has been conducted on controlling milling forces but the problem remains unsolved. This paper presents a solution to reduce and stabilise cutting load in NC milling operation by applying computational techniques to isolate the concave corners of a machining region, thereby forming a major region and a number of disjoint sub-regions. Due to the suppression of concave corners, the boundary profile of the major region will become smoother. Contour-parallel offset tool paths can then be used to remove the material in the major region. For the isolated sub-regions, special corner removing tool paths with the use of different cutter sizes will be employed. By using this approach, the adverse effects caused by excessive undulation of milling tool path can be reduced.published_or_final_versio