Direct comparison of high voltage breakdown measurements in liquid argon and liquid xenon

As noble liquid time projection chambers grow in size their high voltage requirements increase, and detailed, reproducible studies of dielectric breakdown and the onset of electroluminescence are needed to inform their design. The Xenon Breakdown Apparatus (XeBrA) is a 5-liter cryogenic chamber built to characterize the DC high voltage breakdown behavior of liquid xenon and liquid argon. Electrodes with areas up to 33 cm2 were tested while varying the cathode-anode separation from 1 to 6 mm with a voltage difference up to 75 kV. A power-law relationship between breakdown field and electrode area was observed. The breakdown behavior of liquid argon and liquid xenon within the same experimental apparatus was comparable

Failure Processes in Elastic Fiber Bundles

The fiber bundle model describes a collection of elastic fibers under load. the fibers fail successively and for each failure, the load distribution among the surviving fibers change. Even though very simple, the model captures the essentials of failure processes in a large number of materials and settings. We present here a review of fiber bundle model with different load redistribution mechanism from the point of view of statistics and statistical physics rather than materials science, with a focus on concepts such as criticality, universality and fluctuations. We discuss the fiber bundle model as a tool for understanding phenomena such as creep, and fatigue, how it is used to describe the behavior of fiber reinforced composites as well as modelling e.g. network failure, traffic jams and earthquake dynamics.Comment: This article has been Editorially approved for publication in Reviews of Modern Physic

Implications of alternative operational risk modeling techniques

Quantification of operational risk has received increased attention with the inclusion of an explicit capital charge for operational risk under the new Basle proposal. The proposal provides significant flexibility for banks to use internal models to estimate their operational risk, and the associated capital needed for unexpected losses. Most banks have used variants of value at risk models that estimate frequency, severity, and loss distributions. This paper examines the empirical regularities in operational loss data. Using loss data from six large internationally active banking institutions, we find that loss data by event types are quite similar across institutions. Furthermore, our results are consistent with economic capital numbers disclosed by some large banks, and also with the results of studies modeling losses using publicly available “external” loss data.Bank capital ; Risk management ; Basel capital accord

Investigations of Material Response to Fatigue Phenomena in Contacting Bodies

Investigating the fatigue performance of machine components has been of significant interest to improve reliability and reduce the maintenance costs. In the current work, analytical as well as experimental approaches are used to investigate material response to contact fatigue damage. In particular, two fatigue phenomena namely; fretting fatigue and rolling contact fatigue (RCF) are studied. Fretting fatigue is a damage mechanism observed in machine components subjected to fretting in tandem with fluctuating bulk stresses. A fretting test fixture was developed to investigate fretting fatigue behavior of AISI 4140 vs. Ti-6Al-4V in a cylinder-on-flat contact configuration. The critical damage value for AISI 4140 was extracted using the method of variation of elasticity modulus. The fretting fatigue lives obtained from the proposed computational fatigue damage model were found to be in good agreement with the experimental results. The RCF investigation focuses on developing a modified 2D numerical model to simulate RCF damage in line contact configuration. First, a new computationally efficient approach is developed to investigate sub-surface initiated spalling in large bearings. Previously developed continuum damage mechanics based 2D fatigue model was modified to incorporate stress mapping procedure and dynamic remeshing tool to make the model computationally efficient. The new approach was validated against the previous numerical model for small rolling contacts. The scatter in the RCF lives and the progression of fatigue spalling for large bearings obtained from the model show good agreement with experimental results available in open literature. The ratio of L10 lives for different sized bearings computed from the model correlate well with the formula derived from the basic life rating for radial roller bearing as per ISO 281. Furthermore, the RCF model was extended to incorporate elastic-plastic material in order to investigate RCF of case carburized steels. A series of micro-indentation tests were conducted to obtain the hardness gradient in the case carburized 8620 steel. The hardness gradient in the material was modeled by changing the yield strength as a function of depth. The residual stress distribution due to carburization process was modeled by modifying the damage evolution law. The model was used to compare the rolling contact fatigue (RCF) lives of through hardened and case carburized bearing steel with different case depths. Based on the model results, the optimum case depths to maximize the RCF lives of the case carburized bearings at different loading conditions were obtained. This model was then modified to investigate RCF in refurbished case carburized bearings. Refurbishing process was simulated by removing a layer of material from the original surface after a set number of fatigue cycles. The original material properties, residual stresses and the fatigue damage accumulated prior to refurbishing in the remaining material were preserved. The refurbished geometry was then subjected to additional fatigue cycles until damage was detected. According to model results, more fatigue cycles prior to refurbishing enhance the total fatigue life of refurbished bearings. It was also found that beneficial impact of refurbishing on RCF lives of case carburized bearings depends on the relative values of case depth, contact half width, refurbishing depth

Stochastic Stick - Slip Model Linking Crustal Shear Strength and Earthquake Interevent Times

The current understanding of the earthquake interevent times distribution (ITD) is incomplete. The Weibull distribution is often used to model the earthquake ITD. We link the earthquake ITD on single faults with the Earth's crustal shear strength distribution by means of a phenomenological stick - slip model. We obtain Weibull ITD for power-law stress accumulation, i.e., $\sigma(t) = \alpha t^{\beta}$, where $\beta >0$ for single faults or systems with homogeneous strength statistics. We show that logarithmic stress accumulation leads to the log-Weibull ITD. For the Weibull ITD, we prove that (i) $m= \beta m_s$, where $m$ and $m_s$ are, respectively, the ITD and crustal shear strength Weibull moduli and (ii) the time scale $\tau_s = (S_s/\alpha)^{1/\beta}$ where $S_s$ is the scale of crustal shear strength. We generalize the ITD model for fault systems. We investigate deviations of the ITD tails from the Weibull due to sampling bias, magnitude selection, and non-homogeneous strength parameters. Assuming the Gutenberg - Richter law and independence of $m$ on the magnitude threshold, $M_{L,c},$ we deduce that $\tau_s \propto e^{- \rho_{M} M_{L,c}},$ where $\rho_M \in [1.15, 3.45]$ for seismically active regions. We demonstrate that a microearthquake sequence conforms reasonably well to the Weibull model. The stochastic stick - slip model justifies the Weibull ITD for single faults and homogeneous fault systems, while it suggests mixtures of Weibull distributions for heterogeneous fault systems. Non-universal deviations from Weibull statistics are possible, even for single faults, due to magnitude thresholds and non-uniform parameter values.Comment: 32 pages, 11 figures Version 2; minor correction

Doctor of Philosophy

dissertationGranular diamond composites are particulate reinforced composites, where the particulate phase is a grade of high-hardness polycrystalline diamond, embedded in a tougher, hard-material matrix. Granular diamond composites are hierarchically-structured materials. In addition to the macrostructure and microstructure, granular diamond composites have a mesostructure that encompasses the morphology of the matrix and granules and is characterized by parameters such as component volume fraction, granular sphericity, and matrix uniformity. The mesostructure is functionally designed to improve the performance of the composite in petroleum well-drilling applications by increasing the fracture resistance while maintaining the wear resistance. The impact of the mesostructure on the flexural strength, wear resistance, impact resistance, and in-field performance was measured. Physical testing showed that volume fraction of the tougher, matrix phase can be increased significantly before the wear resistance of the composite decreases appreciably. But the testing also showed that the method developed to produce the composites resulted in component materials with inferior properties. The flexural strength of a polycrystalline-diamond/tungsten-carbide material system was explored using several modeling techniques, including analytic, two-dimensional numeric, and three-dimensional numeric models. Residual stresses, arising from the change in conditions after the material is formed in a high-temperature, high-pressure sintering process, have a significant impact on the calculated strength of the composite. Dilatational residual stresses have never been treated in a rigorous manner in the literature and are often neglected completely. In this study, the thermal and dilatational residual stresses were modeled. Stresses from externally applied loads preferentially concentrate in the stiffer diamond phase. Thermal residual stresses strengthen the stiffer and weaker diamond phase through residual compression and weaken the carbide phase through residual tension. The dilatational residual stresses partially counteract the thermal residual stresses. Without thermal residual stresses the composite would have lower strength due to premature failure in the diamond phase. Without dilatational residual stresses the composite would have lower strength due to premature failure in the carbide phase. The strengths predicted by the enhanced models match the measured strengths quite well, despite significant uncertainty in the material properties and process parameters

Effect of Roller Geometry on Roller Bearing Load-Life Relation

Cylindrical roller bearings typically employ roller profile modification to equalize load distribution, minimize stress concentration at roller ends and allow for a small amount of misalignment. The 1947 Lundberg-Palmgren analysis reported an inverse fourth power relation between load and life for roller bearings with line contact. In 1952, Lundberg and Palmgren changed their load-life exponent to 10/3 for roller bearings, assuming mixed line and point contact. The effect of roller-crown profile was reanalyzed in this paper to determine the actual load-life relation for modified roller profiles. For uncrowned rollers (line contact), the load-life exponent is p = 4, in agreement with the 1947 Lundberg-Palmgren value but crowning reduces the value of the exponent, p. The lives of modern roller bearings made from vacuum-processed steels significantly exceed those predicted by the Lundberg-Palmgren theory. The Zaretsky rolling-element bearing life model of 1996 produces a load-life exponent of p = 5 for flat rollers, which is more consistent with test data. For the Zaretsky model with fully crowned rollers p = 4.3. For an aerospace profile and chamfered rollers, p = 4.6. Using the 1952 Lundberg-Palmgren value p = 10/3, the value incorporated in ANSI/ABMA and ISO bearing standards, can create significant life calculation errors for roller bearings

Catastrophic disruptions revisited

We use a smooth particle hydrodynamics method (SPH) to simulate colliding rocky and icy bodies from cm-scale to hundreds of km in diameter, in an effort to define self-consistently the threshold for catastrophic disruption. Unlike previous efforts, this analysis incorporates the combined effects of material strength (using a brittle fragmentation model) and self-gravitation, thereby providing results in the ``strength regime'' and the ``gravity regime'', and in between. In each case, the structural properties of the largest remnant are examined.Comment: To appear in Icaru

Lifetime Assessment of Electrical Insulation

In this chapter, a review of the Weibull probability distribution, probability ranking, and the Weibull graphical estimation technique is presented. A review of single-stress and multiple-stress life models of electrical insulation is also introduced. The chapter also describes the graphical, linear and multiple linear regression techniques used in estimating the parameters of the aging models. The application of maximum likelihood estimation technique for estimating the parameters of combined life models of electrical insulation is illustrated