The two-dimensional problem of a long crack in a non-magnetic, half-space conductor lying perpendicular to the flow of eddy-currents induced by a one-dimensional external current sheet is studied in the context of eddy-current non-destructive evaluation. Impedance changes due to closed, surface-breaking cracks and deep, subsurface cracks are calculated. The Wiener-Hopf technique is used to obtain an approximate solution for the magnetic field scattered by a subsurface crack and hence the impedance change. The solution is accurate to within 5% for cracks whose edges lie more than one electromagnetic skin depth (8) below the conductor surface. For the surface-breaking crack the Wiener-Hopf method yields a high-frequency asymptotic series solution for the magnetic field. The first term corresponds to the limit in which the field perturbations by the edge and corners of the crack are decoupled. The impedance change in this limit is found in closed form. Use of the Wiener-Hopf procedure in rigorously treating the open crack problem is investigated. The opening of a deep, subsurface crack whose width is much less than 0 is found to be undetectable to first order in the opening. A geometrical theory of eddy-current scattering is developed, based on the optical Geometrical Theory of Diffraction. The theory includes a procedure which accounts for multiple scattering of the fields between the edge of a crack and its image. The method is applied to subsurface and surface-breaking cracks, yielding solutions for a subsurface crack whose edge lies only 0.48 below the conductor surface and for a surface-breaking crack of depth 8 or more. Finally, perturbation theory is applied to the surface-breaking crack problem in the lowfrequency limit, giving the impedance change for a crack of depth up to 0.40

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Cracks in reinforced concrete structures can occur as a result of many phenomena such as fresh concrete bleeding, restrained shrinkage, thermal gradients, freeze-thaw cycles, alkali-aggregate reactions, and can also be induced by external loading. Thus, concrete becomes more vulnerable to the processes of deterioration by corrosion of reinforcement. The corrosion rate of cracked reinforced concrete in different exposure conditions has been studied by some researchers. However, it is not clear how the presence of cracks affects the corrosion-determining factors, which control the corrosion pattern at the crack. The objective of this project was to develop an understanding of the effects of cracking on the transport characteristics under wetting and drying cycles. In this project, flexural loading induced natural cracks, and parallel-wall artificial cracks were studied. The infiltration properties of those cracks were evaluated by the tension infiltrometry technique. The saturation conditions around the crack were monitored with the Time Domain Reflectometry (TDR) technique. A numerical simulation was carried out to model the evolution of saturation in the cracked beams; in the model two crack modeling approaches were employed and compared. The infiltration test showed that the presence of both artificial and natural cracks (0.3 mm and 1.0 mm) dramatically increased the permeability of concrete. The value of hydraulic conductivity was increased by up to 5 orders of magnitude at the location of the crack. The evolution of water saturation of the cracked concrete under wetting and drying conditions was analyzed as colour-scaled images and the water saturation contours were compared for different crack openings. For the artificial crack samples, a deviation from the expected “perfectly symmetric” flow regime around a straight crack was observed. This was probably caused by the micro cracks induced during the shim pull-out process or a non-uniform compaction around the shim insertion. For the natural cracks, in the drying phase, smaller cracks seemed to have better water storage. Hence, the water saturation decreased at a slightly slower rate. The crack behaved like an open surface that was exposed to the environment. Application of the same material properties to the open surface and the crack surface did not bring a large error for the water flow simulations. A hysteresis phenomenon has been found during the identification of the Van Genuchten material parameters using an inverse modelling approach, with Ks=5×10-10 m/s, α =4.33×10-4, for the wetting phase, n=1.32 and for the drying phase, n=2.0. The simulation results suggest that for the simple flexural crack, the 1D crack line averaged from the front and back crack lines is capable of representing the crack in the wetting and drying scenario. The crack could be modelled as “free surface” or “equivalent porous medium”.

This dissertation reports investigations on shear in cracked reinforced concrete (RC) elements including the development and implementation of a material subroutine for the commercial finite element (FE) program ABAQUS. The material subroutine UMAT is intended to substantially improve the shear behaviour of the standard concrete options of ABAQUS. At first the important shear theories are reviewed in detail and their advantages and drawbacks are summarised. The modified compression field theory (MCFT) is identified as a suitable shear theory worth being coded for its application in FE analysis. A comprehensive check on the MCFT confirms its suitability in a slightly modified form for the investigation of a variety of cracked structural RC elements. This check is conducted on a section analysis level by means of a developed program called LAYER which is coded according to the MCFT. The main part of the work is the implementation and testing of the material subroutine UMAT which is added to the source code of ABAQUS via an interface provided by the commercial FE program. Finally, the UMAT is utilised for examining the ductility of RC walls. It is concluded that shear deflections can influence the displacement and curvature ductility of squat structures in a substantial way, even though a flexural type of failure might prevail.

A time-domain model-based crack diagnostic methodology using vibration data is presented. Most of the damage detection methods proposed to date are based on modal parameters and are limited by the loss of information caused by data reduction and by the implicit assumption of linearity. The use of time domain information permits the direct inclusion of the nonlinear behavior due to crack opening-closure cycles. In addition, very little information is lost, since no signal processing or parameter identification steps are involved. The proposed method is based on a continuous model for the transverse vibrations of beams consisting of partial differential equations of motion with varying coefficients to account for the presence of damage. In order to provide accurate representation of the structure's behavior over a broader frequency range, a new continuous cracked beam model including shear effects and rotatory inertia is developed using the Hu-Washizu-Barr variational method. The resulting equations of motion are discretized by a Galerkin method using local B-splines as test functions. The crack is assumed to be either fully open or fully closed, resulting in a bilinear system. The simultaneous identification of crack location and depth is performed by minimizing the norm of the differences between the numerical and experimental time responses to multiple excitations. Impact, low frequency sinusoidal and Schroeder--phased multisine inputs are investigated as potential excitation methods. The cost function to be minimized presents several local minima that are shown to be related to the length of the response records. A genetic algorithm is used to overcome the multimodal nature of the objective function. The methodology is validated through simulated identifications of several damage scenarios. The importance of the inclusion of the nonlinear behavior is addressed, and the effects of model uncertainties and measurement noise are quantified in terms of minimum identifiable crack size. / Ph. D.

In this thesis, an attempt is made to summarize the previous archeological research associated with two feature types in Sweden made from fire-cracked stones: the fire-cracked stone embankment and the burnt mound. The ?lim is to study how trends in interpretation and focus have developed. Furthermore, a compilation of ten excavation reports of burnt mounds between 2002 and 2013 is presented with the aim of determining their purpose, their usage of methods in general and petrography. The result of the summary confirms that there are discernable trends present and the result from the compilation of reports show that the larger scope is prioritized and that there are a number of commonly used methods, but that petrography is not among them.

No description available.

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).

The body of knowledge that exists regarding cracked teeth is limited. The purpose of this study was to determine the prevalence of cracks among extracted teeth. This retrospective longitudinal cohort study included patients of the Virginia Commonwealth University School of Dentistry that underwent extraction procedures over a 6 year period. The sample consisted of 20,408 patients and 40,870 teeth. Statistical analysis software was used to identify diagnoses of a crack, fracture, or split tooth prior to extraction of the tooth by analyzing the Electronic Health Record (EHR) (axiUm™, Version 6.03.03.1035, Exan Corporation, Vancouver, BC, Canada). There were 3,228 teeth identified as cracked in the 40,870 extracted teeth—an overall prevalence of 7.90%. The percentage of cracked teeth were compared using a chi-square test of homogeneity. The prevalence of cracked teeth varied according to tooth type (chi-square = 95.5, df = 7, p < .0001). Tukey’s multiple-comparison procedure identified the groups of tooth types with a significantly different cracked prevalence. The mandibular 2nd molar had the highest prevalence (9.72%). Age and gender were also significantly correlated with cracked teeth.

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.