No / Representations for the stress fields created around an infinite row of collinear, antiplane shear cracks moving within one-dimensional hexagonal quasicrystals, and the resulting stress intensity factors and the J-integral, are determined in closed-form and discussed, using an extended method of dislocation layers. The solutions for a finite quasicrystalline plate containing a single moving crack and a plate with a moving edge crack are also provided by this analysis. (C) 2014 Elsevier Ltd. All rights reserved.

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The life of railway wheels and rails has been decreasing in recent years. This is mainly caused by more traffic and running at higher vehicle speed. A higher speed usually generates higher forces, unless compensated by improved track and vehicle designs, in the wheel-rail contact, resulting in more wear and rolling contact fatigue (RCF) damage to the wheels and rails. As recently as 15 years ago, RCF was not recognised as a serious problem. Nowadays it is a serious problem in many countries and ''artificial wear'' is being used to control the growth of cracks by preventive re-profiling and grinding of, respectively, the wheels and rails.  This can be used because a competition exists between wear and surface initiated RCF: At a high wear rate, RCF does not have the opportunity to develop further. Initiated cracks are in this case worn off and will not be able to propagate deep beneath the surface of the rail or wheel. When wheel-rail damage in terms of wear and RCF can be predicted, measures can be taken to decrease it. For example, the combination of wheel and rail profiles, or the combination of vehicle and track, can be optimised to control the damage. Not only can this lead to lower maintenance costs, but also to a safer system since high potential risks can be detected in advance. This thesis describes the development of a wheel-rail life prediction tool with regard to both wear and surface-initiated RCF. The main goal of this PhD work was to develop such a tool where vehicle-track dynamics simulations are implemented. This way, many different wheel-rail contact conditions which a wheel or a rail will encounter in reality can be taken into account. The wear prediction part of the tool had already been successfully developed by others to be used in combination with multibody simulations. The crack prediction part, however, was more difficult to be used in combination with multibody simulations since crack propagation models are time-consuming. Therefore, more concessions had to be made in the crack propagation part of the tool, since time-consuming detailed modelling of the crack, for example in Finite Elements models, was not an option. The use of simple and fast, but less accurate, crack propagation models is the first step in the development of a wheel-rail life prediction model. Another goal of this work was to verify the wheel-rail prediction tool against measurements of profile and crack development. For this purpose, the wheel profiles of trains running on the Stockholm commuter network have been measured together with the crack development on these wheels. Three train units were selected and their wheels have been measured over a period of more than a year. The maximum running distance for these wheels was 230,000 km. A chosen fatigue model was calibrated against crack and wear measurements of rails to determine two unknown parameters.  The verification of the prediction tool against the wheel measurements, however, showed that one of the calibrated parameters was not valid to predict RCF on wheels. It could be concluded that wheels experience relatively less RCF damage than rails. Once the two parameters were calibrated against the wheel measurements, the prediction tool showed promising results for predicting both wear and RCF and their trade-off. The predicted position of the damage on the tread of the wheel also agreed well with the position found in the measurements. / <p>QC 20150526</p>

Diagnosis and treatment of teeth with longitudinal fractures is challenging. Cracks are usually not visible radiographically; they require a thorough evaluation to aid in diagnosis. Patients may be asymptomatic, demanding the dentist rely on clinical findings to make a diagnosis. Early diagnosis of the presence and extent of a crack is essential for the successful management of a cracked tooth. There is limited information in the literature regarding the prevalence or predictive factors of cracks extending into the pulp chamber of teeth. The purpose of this study was to determine the prevalence of intrapulpal cracks in maxillary molars and to identify factors that may aid in diagnosing the existence of a crack. All maxillary molar teeth requiring non- surgical root canal therapy or retreatment at the Virginia Commonwealth University graduate endodontic clinic from June 2016 through December 2016 were included in the study after obtaining patient consent. Teeth were examined visually, transilluminated, stained, and examined microscopically for the presence of an intrapulpal crack. Demographic information, subjective data associated with the chief complaint, objective results of diagnostic testing (percussion, palpation, bite stick test, transillumination, probing depths, existing restorations, and diagnosis) were analyzed using chi-square and logistic regression (p<0.05) to identify associations of these findings with the existence of a crack. A total of 18% (15/82 teeth) of maxillary molars that were evaluated and had endodontic treatment initiated were cracked. There was a significant association between cracked teeth and pain on biting (P<.0001) and with probing depths greater than 4 millimeters (mm) (P < 0.003). Those positive on a tooth slooth test were more likely to have an intrapulpal crack (P<.001) and teeth with a positive transillumination test were also found to be associated with the presence of a crack(P < .001).

This thesis investigates a novel way to simulate cracks as an extension of the Mate- rial Point Method (MPM). Previous methods, like CRAMP (CRAcks with Material Points), often use an explicit crack representation to define the material crack. We use an implicit crack representation defined as the intersection between pieces of the original specimen created by a pre-fracture process. Material chunks are there- after forced together using massless particle constraints. The method has proven successful in tearing scenarios, and the main benefits are: (1) minor computational overhead compared to the initial MPM algorithm; (2) simple to implement and scales well in 3 dimensions; (3) gives easy and controllable setup phase for desired material failure mode. The development of the crack extension has required a fully general MPM solver that can handle arbitrarily many distinct bodies connected in the same simulation. Current collision schemes for MPM exists, however these are often focused on two-body collisions and does not scale well for additional objects due to inaccuracies in contact normal calculations. We present a method that uses an iterative pair-wise comparison scheme to resolve grid collisions that extends to any number of collision objects.

Few studies have reported the incidence or prevalence of cracked teeth with pulpal involvement. No attempts have been made to evaluate the prevalence or clinical predictors for intrapulpal cracks. The purpose of this study was to investigate the prevalence of intrapulpal cracks in first and second mandibular molars and to determine if clinical findings are predictive for the existence of intrapulpal cracks. First and second mandibular molars (190) requiring non-surgical root canal treatment at the VCU Graduate Endodontic Practice between February 15, 2013 and August 15, 2013 were analyzed retrospectively. Teeth were transilluminated, stained, and inspected for intrapulpal cracks using a dental microscope. Data gathered included: demographics, subjective and objective information regarding the chief complaint to include bite stick test, transillumination, probing depths greater than 4mm, existing restorations, and diagnosis. Chi-square and logistic regression were performed (p<0.05). The prevalence of intrapulpal cracks in first and second mandibular molars combined was 9% (17/190, 95%CI= 5.7% to 13.9%). The prevalence was 7% for 1st molars and 13% for 2nd molars. There was no statically significant difference in the prevalence between first and second mandibular molars. Individual characteristics predictive for the existence of intrapulpal cracks were age, probing depth greater than 4mm, transillumination and a positive Tooth Slooth™ test (all p<0.05). Staining of the pulp chamber after access identified only one of 17 intrapulpal cracks. Staining of the pulp chamber did not significantly increase the ability to identify intrapulpal cracks. Neither diagnosis, sex, nor existing restorations were significant predictors for intrapulpal cracks. Probing depth greater than 4mm, age over 40 and a positive Tooth Slooth™ test were conjointly significant for predicting intrapulpal cracks.

There is no universal protocol for diagnosing, treating and managing cracked teeth. The purpose of this survey was to investigate the use of traditional methods of crack detection and to explore how treatment decisions were made using an intrapulpal crack classification. The electronic survey was sent to 1115 active members of the American Association of Endodontists (AAE) and The Digital Office (TDO™) community. Comparisons were assessed using logistic or repeated-measures regression. The most often used diagnostic method was probing. When the crack involved one wall, 85% of respondents would complete root canal therapy > 50% of the time or always. For two or more walls, the percentage dropped to 44%. When the crack involved the floor or orifices, 60% would not complete treatment. For necrotic teeth, 36% of respondents preferred extraction as opposed to 3% if vital. This survey illustrated the anecdotal nature of detection, diagnosis and management of cracked teeth.

The aim of this study was to compare the incidence of root canal dentinal micro-cracks after canal instrumentation using reciprocating files (WaveOne Gold® and Twisted Adaptive®) and continuous rotation files (Edge Evolve® and EndoSequence®) in an ex-vivo benchtop study. This project used a novel methodology of finding dentinal defects using the “K-cube”, which allows evaluators to visualize sectioned root surfaces before instrumentation and after instrumentation. Mesial roots from 40 human mandibular first molars were divided into 4 groups of 10 for each file type. Root section pictures were taken with a Zeiss Discovery V20 stereomicroscope before and after canal instrumentation. Each of the pre-instrumentation and post-instrumentation images were evaluated for dentinal defects by four calibrated endodontists utilizing REDCap survey. Using a chi-square analysis, there was no statistically significant difference between dentinal defects created by continuous and reciprocating rotation (p=0.1924) and no difference between the four file types (p=0.2317).

The Swedish mining company LKAB is using limestone as additives for the production of its iron ore pellets. The company is considering five new proposals of limestones since the Nordkalk Company is soon no longer able to provide limestone from the Storugns quarry which is the one that is used today. The first purpose in this study was to characterize and compare these five new limestones in respect to their texture and their disintegration during the transport from the quarry to the final destination in Malmberget, considering the Storugns limestone as a reference material. The second aim was to focus on finding any link between texture and disintegration properties of the limestones. Another intention is that the methodology which is used in this study will help the company to consider new proposals of limestones in the future. Six limestones from the Orsa, Storugns, Stucks, Vasalemma, Verdal and Võhmuta quarries were analyzed by an optical microscopy analysis of the textures. Two types of physical tests were also included in the study: a tumbler test and a breakage test. Six different textures were identified by a qualitative thin section analysis which shows similarities between the limestones from different origins but also a high variability of texture within a same quarry. A more quantitative optical microscopy analysis led to the assumption of both the degree of lithification and the amount of initial micro-cracks for each sample. According to the physical test results, the limestones disintegrate as follows, from the less to the more disintegrated: Stucks (7,6%), Storugns (9,4%), Verdal (10,3%), Võhmuta (11,1%), Vasalemma (11,8%) and Orsa (17,6%). There is no evident textural parameter which is controlling directly the disintegration of limestone. However the samples with a rather high lithification and a rather low fracturing disintegrate less than samples with a rather low lithification and a rather high fracturing. It is assumed that the combination of degree of lithification together with the amount of initial micro-cracks is somehow controlling the disintegration of limestone. / Validerat; 20101217 (root)

Este trabalho disserta sobre os métodos usados para determinar a aceitabilidade de defeitos tipo trinca em um bocal de caldeira de geração de vapor. Inicia com uma avaliação sobre as tensões atuantes em bocais e introduz os conceitos de mecânica da fratura para aceitação de defeitos em estruturas. Propõe um método analítico para estimativa de tensões longitudinais em bocais e o compara ao cálculo de tensões por elementos finitos. Explora um caso de um defeito circunferencial e não aflorado em um bocal de uma caldeira real a qual fornece corpos de prova que são submetidos a ensaios de impacto a partir do quais é traçada sua curva de transição ductil-frágil. Analisa os resultados obtidos a partir destes ensaios e os confronta com os valores nominais e de referência. Esta análise embasa discussão sobre probabilidades de falha e fatores de segurança. / This work discusses the methods used to determine the acceptability of crack-like flaws in a boiler nozzle. It begins with an evaluation of the stresses acting on nozzles and introduces the concepts of fracture mechanics for the acceptance of defects in structures. An analytical method for estimating longitudinal stresses in nozzles is proposed and compared with numerical results obtained from the finite element method. It explores a case of a circumferential embedded flaw in a nozzle of a real boiler that provides specimens that are subjected to impact tests from which its ductile-brittle transition curve is drawn. It analyzes the results obtained from these tests and confront them with the nominal and reference values. This analysis bases discussion on probabilities of failure and safety factors.