Evaluating the safety of unreinforced concrete structures, such as concrete dams, requires an accurate prediction of cracking. Developing a suitable constitutive material model and a reliable computational procedure for analysing cracking processes in concrete has been a challenging and demanding task. Although many analytical methods based on fracture mechanics have been proposed for concrete dams in the last few decades, they have not yet become part of standard design procedures. Few of the current research findings are being implemented by practising engineers when evaluating dam safety. This research is focused on the development of a suitable crack modelling and analysis method for the prediction and study of fracturing in concrete gravity dams, and consequently, for the evaluation of dam safety against cracking. The research aims to contribute to the continuing research efforts into mastering the mechanics of cracking in concrete dams. An analytical method for the purpose of establishing a crack constitutive model and implementing the model for the fracture analysis of concrete structures, in particular massive concrete gravity dams under static loading conditions, has been developed, verified and applied in the safety evaluation of a concrete gravity dam. The constitutive material model is based on non-linear fracture mechanics and assumes a bilinear softening response. The crack model has various improved features: (1) an enhanced mode I bilinear strain-softening approach has been put forward; (2) a new formula for bilinear softening parameters has been developed and their relation with linear softening has been outlined; (3) the influence of bilinear softening parameters on the cracking response has been studied; and (4) an enhanced modification to the shear retention factor which depends on the crack normal strain is included. The material model has been incorporated into a finite element analysis using a smeared crack approach. A sub-program was specially coded for this research. The validity of the proposed cracking model and the computational procedure developed for the purpose of analyzing the tensile fracture behaviour of concrete structures has been confirmed by verification on various concrete structures, including beams, a dam model and actual gravity dams. The crack modelling technique developed was successfully used in evaluating the safety of an existing concrete gravity dam in South Africa and adequately predicted the cracking response of the dam structure under static loadings. The main conclusions drawn are as follows: <ul><li>Both mode I and mode II fracture have been modelled successfully.</li> <li>The proposed bilinear softening model remains relatively simple to implement but significantly improves on predicting the softening response of “small-scale” concrete structures.</li> <li>Both plane stress and plane strain crack analyses have been considered and can be confidently adopted in two-dimensional applications.</li> <li>The proposed method is mesh objective.</li> <li>The crack modelling method developed can correctly predict the crack propagation trajectory and the structural behaviour with regard to fracturing in concrete structures.</li> <li>If not considering shear stress concentration near the tip of a crack, constitutive crack analysis normally indicates a higher safety factor and a higher Imminent Failure Flood (IFF) than the classical methods in the analysis of concrete gravity dams for safety evaluation.</li></ul> / Thesis (PhD(Civil Engineering))--University of Pretoria, 2007. / Civil Engineering / PhD / unrestricted

Roads are major public assets. The USA spends billions of dollars each year on road construction and maintenance. To keep these roads in a healthy condition and for better planning and allocation of maintenance budgets, knowledge of distressed locations is needed. Roads develop cracks when they are subjected to stresses that exceed their designed criteria or their materials properties. Early detection and repair of cracks has proven to be the most cost-effective strategy in limiting the damage to roads and reducing expenditures. Various methodologies of crack detection were developed and significant techniques were made in the last few years. One of the most important recent technologies is the infrared thermography, which allows the use of infrared waves for crack detection. Another important technology is the global navigation satellite system (GNSS) which currently includes the GPS and GLONASS constellations. With the help of these systems, accurate location coordinates (longitude, latitude and altitude) up to a few centimeters were located. The objective of this research is to test the combined use of GNSS and infrared thermography in an automated system for the detection of asphalt cracks and their locations. To achieve this goal, two tests have been conducted. The first one, regarding the location tagging, was done using two pairs of GPS receivers which can detect signals from both GPS and GLONASS navigation systems in single and dual frequencies (L1 and L2). Different modes have been set to the receiver and comparison graphs were developed to compare accuracies against modes. The second test involves an infrared camera mounted on a car and moving in speeds approaching highway speed limit. The images obtained from the camera were processed using cracks detection software to analyze cracks properties (length, width, density and severity). It was found that the images that were taken by a moving infrared camera were recognized by crack detection software for moving speeds up to 50 mph. At speeds higher than 50 mph, images were blurred. As for location test, The GLONASS combined by GPS receivers got slightly better results than GPS only in both dual and single frequencies. The GLONASS satellites are not always available in view and when they are there, the number of satellites that can be detected by receiver range from one to three satellites at the most and for only a short period of time. It is recommended that future research be conducted to investigate the effect of using different camera lenses on the clarity of the images obtained as well as the effect of raising the camera level above the pavement surface in such a way that the whole lane width (12 ft.) would be covered in one image. Also the total reliance on GPS only receivers in determining cracks location has proven to be enough for this application.

Many computational methodologies have been developed over the years with the goal of characterizing damage presence in structural components. In this thesis two approached were investigated. The first approach covered in this thesis is the modeling of damage presence through the reduction of the modulus of elasticity. This approach was mainly applied to a cantilevered beam of uniform cross-section. Various crack configurations/orientations were investigated, and from this data equations relating modulus as a function of crack length were derived. The second approach deals with modifying the local flexibility at the location of damage through modification of the element stiffness matrix. This approach has certain advantages in that it can adequately capture the global modal and displacement effects of damage without the added computational effort of mesh refinement that is required for physically modeled cracks.

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In addition to being useful for some nondestructive evaluation techniques, interactions between magnetic fields and defects in solids may also alter material properties. To explore this possibility, Maxwell's equations were coupled with a continuum mechanics model for elastic strain to formulate analytical expressions for the interaction of a magnetic field with several crack geometries. The influence of crack velocity and a realistic (nonlinear) magnetic susceptibility were included into a model of this type for the first time and shown to introduce unexpected trends in the magneto-elastic stress intensity. Singularities magneto-elastic stresses appear at different combinations of magnetic field strength and crack velocity, and the stresses at the crack tip switch sign. In a related study, the interaction of an alternating magnetic field with elastic stress through was explored through a coupling effect known as magneto-acoustic resonance. A model for the phenomena, in which magnetic waves excite elastic waves and vice versa, was formulated and used to explore the spin (magnon) and anti-plane elastic (phonon) interactions in piecewise homogeneous ferromagnetic spaces with two different sets of properties. The model suggests some combinations of magnetic field and frequency can produce a new kind of wave to appear. These new waves, which we call Accompanying Surface Magnetoelastic (ASM) waves, are localized at the interface between the two ferromagnetic media and they accompany reflection and transmission waves. It is shown that the amplitudes of the reflected, transmitted, and ASM waves depend strongly on magnetic field strength, frequency, and the angle of the incident wave, as well as on the physical properties of ferromagnetic media. / Ph. D.

Understanding failure mechanics of mechanical equipment is one of the most important aspects of structural and aerospace engineering. Crack growth being one of the major forms of failure in structural components has been studied for several decades to achieve greater reliability and guarantee higher safety standards. Conventional approaches using the finite element framework provides accurate solutions, yet they require extremely complicated numerical approaches or highly fine mesh densities which is computationally expensive and yet suffers from several numerical instabilities such as element entanglement or overly soften element behavior. The eXtended Finite Element Method (XFEM) is a relatively recent concept developed for modeling geometric discontinuities and singularities by introducing the addition of new terms to the classical shape functions in order to allow the finite element formulation to remain the same. XFEM does not require the necessity of computationally expensive numerical schemes such as active remeshing and allows for easier crack representation. In this work, verifies the validity of this new concept for quasi-static crack growth in tension with Abaqus' XFEM is employed. In the course of the work, the effect of various parameters that are involved in the modelling of the crack are parametrically analyzed. The load-displacement data and crack growth were used as the comparison criterion. It was found that XFEM is unable to accurately represent crack growth in the models in the elastic region without direct manipulation of the material properties. The crack growth in the plastic region is found to be affected by certain parameters allowing us to tailor the model to a small degree. This thesis attempts to provide a greater understanding into the parametric dependencies of XFEM crack growth. / Master of Science

The stress-strain state of ferromagnetic plane with a moving crack has been investigated in this study. The model considers a soft magnetic ferroelastic body and incorporates a realistic (nonlinear) susceptibility. A moving crack is present in the body and is propagating in a direction perpendicular to the magnetic field. Assuming that the processes in the moving coordinates are stationary, a Fourier transform method is used to reduce the mixed boundary value problem to the solutions of a pair of dual integral equations yielding to a closed form solution. As a result of this investigation, the magnetoelastic stress intensity factor is obtained and its dependency upon the crack velocity, material constants and nonlinear law of magnetization are highlighted. It has been shown that stress result around the crack essentially depend on external magnetic field, speed of the moving crack, nonlinear law of magnetization, and other physical parameters. The results presented in this work show that when cracked ferromagnetic structure is under the influence of magnetic field it is necessary to take into account the interaction effects between deformation of the body and magnetic field and that such interaction can bring to a new conditions for strengthening the materials. Closed form solutions for the stress-strain state are obtained, graphical representations are supplied and conclusions and prospects for further developments are outlined. / Master of Science