Rolling contact fatigue is a damage process that may arise in mechanical applications with repeated rolling contacts. Some examples are: gears; cams; bearings; rail/wheel contacts. The resulting damage is often visible with the naked eye as millimeter sized surface craters. The surface craters are here denoted spalls and the gear contact served as a case study. The work focused on the asperity point load mechanism for initiation of spalls. It was found that the stresses at asperity level may be large enough to initiate surface cracking, especially if the complete stress cycle was accounted for. The gear contact is often treated as a cylindrical contact. The thesis contains experimental and numerical results connected to rolling contact fatigue of cylindrical contacts. At the outset a stationary cylindrical contact was studied experimentally. The stationary test procedure was used instead of a rolling contact. In this way the number of contact parameters was minimized. The cylindrical contact resulted in four different contact fatigue cracks. The two cracks that appeared first initiated below the contact. The other two cracks developed at the contact surface when the number of load cycles and the contact load increased. The influence of a surface irregularity (asperity) was studied numerically with the Finite Element Method (FEM). Firstly, the stationary contact was modelled and investigated numerically. At the cylindrical contact boundary a single axisymmetric was included. The partially loaded asperity introduced a tensile surface stress, which seen from the asperity centre was radially directed. Secondly, FE simulations were performed where a single axisymmetric asperity was over-rolled by a cylindrical contact. The simulations were performed for pure rolling and rolling with slip. For both situations, tensile forward directed stresses in front of the asperity were found. The presence of slip and a surface traction greatly increased the stresses in front of the asperity. Finally, when rolling started from rest with applied slip, the distance to steady-state rolling was determined for elastic similar cylindrical rollers. / QC 20100702

Workpieces during cold forging fail basically due to ductile fracture. Ductile fracture can be predicted by damage models. In this study, various damage models such as Cockcroft &amp / Latham, McClintock, Freudenthal, Rice &amp / Tracy, Oyane, Ayada, Brozzo are investigated for their applicability to three workpiece materials: bearing steel (100Cr6), stainless steel (X5CrNiMo1810) and brass (CuZn39). The damage material parameters have been obtained by various tests such as tensile, standard compression, ring compression, compression with flanges and conical compression tests. The characterization has been assisted by finite element simulation of the various tests. It has been shown that the available damage models can predict the location of failure satisfactorily but are no able to predict the onset of failure quantitatively. Keywords: Formability Limit, Failure Criteria, Cold Forming, Surface Cracks, Finite Element Analysis

Slow speed rotating machines are the mainstay of several industrial applications worldwide. They can be found in paper and steel mills, rotating biological contractors, wind turbines etc. Operational experience of such machinery has not only revealed the early design problems but has also presented opportunities for further significant improvements in the technology and economics of the machines. Slow speed rotating machinery maintenance, mostly related to bearings, shafts and gearbox problems, represents the cause of extended outages. Rotating machinery components such as gearboxes, shafts and bearings degrade slowly with operating time. Such a slow degradation process can be identified if a robust on-line monitoring and predictive maintenance technology is used to detect impending problems and allow repairs to be scheduled. To keep machines functioning at optimal levels, failure detection of such vital components is important as any mechanical degradation or wear, if is not impeded in time, will often progress to more serious damage affecting the operational performance of the machine. This requires far more costly repairs than simply replacing a part. Over the last few years there have been many developments in the use of Acoustic Emission (AE) technology and its analysis for monitoring the condition of rotating machinery whilst in operation, particularly on slow speed rotating machinery. Unlike conventional technologies such as thermography, oil analysis, strain measurements and vibration, AE has been introduced due to its increased sensitivity in detecting the earliest stages of loss of mechanical integrity. This programme of research involves laboratory tests for monitoring slow speed rotating machinery components (shafts and bearings) using AE technology. To implement this objective, two test rigs have been designed to assess the capability of AE as an effective tool for detection of incipient defects within low speed machine components (e.g. shafts and bearings). The focus of the experimental work will be on the initiation and growth of natural defects. Further, this research work investigates the source characterizations of AE signals associated with such bearings whilst in operation. It is also hoped that at the end of this research program, a reliable on-line monitoring scheme used for slow speed rotating machinery components can be developed.

A local isotropic damage coupled hyperelastic-plastic framework is formulated in principal axes where thermo-mechanical extensions are also addressed. It is shown that, in a functional setting, treatment of many damage growth models, including ones originated from phenomenological models (with formal thermodynamical derivations), micro-mechanical models or fracture criteria, proposed in the literature, is possible. Quasi-unilateral damage evolutionary forms are given with special emphasis on the feasibility of formulations in principal axes. Local integration procedures are summarized starting from a full set of seven equations which are simplified step by step initially to two and finally to one where different operator split methodologies such as elastic predictor-plastic/damage corrector (simultaneous plastic-damage solution scheme) and elastic predictor-plastic corrector-damage deteriorator (staggered plasticdamage solution scheme) are given. For regularization of the post peak response with softening due to damage and temperature, Perzyna type viscosity is devised. Analytical forms accompanied with algorithmic expressions including the consistent material tangents are derived and the models are implemented as UMAT and UMATHT subroutines for ABAQUS/Standard, VUMAT subroutines for ABAQUS/Explicit and UFINITE subroutines for MSC.Marc. The subroutines are used in certain application problems including numerical modeling of discrete internal cracks, namely chevron cracks, in direct forward extrusion process where comparison with the experimental facts show the predicting capability of the model, isoerror map production for accuracy assessment of the local integration methods, and development two novel necking triggering methods in the context of a damage coupled environment.

A systematic procedure is followed to develop singularity-reduced integral equations for modeling cracks in two-dimensional, linear multi-field media. The class of media treated is quite general and includes, as special cases, anisotropic elasticity, piezoelectricity and magnetoelectroelasticity. Of particular interest is the development of a pair of weakly-singular, weak-form integral equations (IEs) for "generalized displacement" and "generalized stress"; these serve as the basis for the development of a Symmetric Galerkin Boundary Element Method (SGBEM). The implementation is carried out to allow treatment of general mixed boundary conditions, an arbitrary number of cracks, and multi-region domains (in which regions having different material properties are bonded together). Finally, a procedure for calculation of T-stress, the constant term in the asymptotic series expansion of crack-tip stress field, is developed for anisotropic elastic media. The pair of weak-form boundary IEs that is derived (one for generalized displacement and the other one for generalized stress) are completely regularized in the sense that all kernels that appear are (at most) weakly-singular. This feature allows standard Co elements to be utilized in the SGBEM, and such elements are employed everywhere except at the crack tip. A special crack-tip element is developed to properly model the asymptotic behavior of the relative crack-face displacements. This special element contains "extra" degrees of freedom that allow the generalized stress intensity factors to be directly obtained from the solution of the governing system of discretized equations. It should be noted that while the integral equations contain only weakly-singular kernels (and so are integrable in the usual sense) there remains a need to devise special integration techniques to accurately evaluate these integrals as part of the numerical implementation. Various examples for crack problems are treated to illustrate the accuracy and versatility of the proposed procedure for both unbounded and finite domains and for both single-region and multi-region problems. It is found that highly accurate fracture data can be obtained using relatively course meshes. Finally, this dissertation addresses the development of a numerical procedure to calculate T-stress for crack problems in general anisotropic elastic media. T-stress is obtained from the sum of crack-face displacements which are computed via a (regularized) integral equation of the boundary data. Two approaches for computing the derivative of the sum of crack-face displacements are proposed: one uses numerical differentiation, and the other one uses a weak-form integral equation. Various examples are examined to demonstrate that highly accurate results are obtained by means of both approaches. / text

Ascent of magma results in drastic drops of pressure and temperature during eruption. Exsolution or dissolution of water changes the physical and chemical properties of the magma and can promote or inhibit the formation of bubbles, crystals and cracks. The microstructural relations between bubbles, crystals and cracks are important records of processes immediately before and during volcanic eruptions and during deposition of volcanic products. This is an integrated study of analyses, conceptual and numerical models of textural relations, and water distribution patterns of natural and experimentally altered samples. Synchrotron Fourier transform infrared spectroscopy and focal plane array detectors open new possibilities for the analysis of the spatial distribution of volatiles in volcanic rocks. New ways of sample preparation, measurements and data analyses helped to create water distribution maps with spatial resolutions that are close to the diffraction limit (~3 μm). In order to constrain eruptive processes and mechanisms of lava emplacement, I describe textural features in volcanic glasses including bubbles, flow bands of crystals or bubbles, spherulites and different generations of cracks. In experiments, bubbles were grown under isobaric conditions, at one or two cooling steps, their textures were described and volume changes tracked. Water distribution patterns in the glass around the textures were described and categorized, and where possible, diffusion modeling was used to infer temperature- and timescales of formation. Rocks that are quenched within short periods of time after bubble growth preserve negative gradients of water toward the bubble margins. These gradients are generally not observed if the sample is kept at high temperatures for extended periods. If, however, a second step of cooling is added, water may be re-dissolved into the surrounding melt, which may lead to the complete resorption of bubbles. A conceptual of water redistribution during bubble resorption or collapse is used to interpret water heterogeneities across linear flow banding. These heterogeneities can be caused by shearing of bubbly magma, leading to collapse, degassing and resorption of water into the melt, creating a bubble free melt. Anhydrous spherulitic crystals grow both above and below the glass transition temperature (Tg) redistributiong water into the surrounding melt. Below Tg, cracks form and are successively hydrated by magmatic water from crystal growth or by meteoric water at temperatures far below Tg. The hydrated perlitic cracks in the samples of this study formed at elevated temperatures and are distinct from cracks formed at ambient temperatures without hydrated margins. This study shows that the heterogeneous distribution of water in volcanic rocks preserves the complex and non-linear degassing and cooling history of eruptive products. The timescales and temperatures discovered here provide new ways to interpret textural observations, water distribution patterns and signals of shallow volcanic unrest.

Bewehrungen mit einer hochmoduligen Polymertränkung erreichen im Betonbauteil Textilspannungen von über 1200 N/mm². Ab diesem Niveau wurden in Dehnkörperversuchen Längsrisse und Betonabplatzungen beobachtet. Für die Eingrenzung und Identifikation der elementaren, verantwortlichen Mechanismen weisen großformatige Dehnkörper störende Einflüsse von Randeffekten und Imperfektionen auf. Daher wurde ein neuer Versuchsaufbau entwickelt, bestehend aus einem in einen Betonquader eingebetteten Einzelroving. Die simple Herstellung und die Verlässlichkeit dieser Kategorie von Experimenten in Bezug auf die Reproduzierbarkeit und der geringen Streuung der Resultate machen sie zu einer robusten Grundlage für experimentelle Untersuchungen des Verhaltens von TRC. Die experimentell ermittelten Bruchlasten werden zunächst bezüglich ihrer Leistung diskutiert und dann die beobachteten Bruchvorgänge kategorisiert hinsichtlich kompatibler mechanischer Hypothesen für die Initiierung von Längsrissen und des Spaltversagen. / Reinforcements with a high-modulus, polymeric impregnation achieve textile tensions of 1200 N/mm² and more in concrete parts. Starting at this level longitudinal cracks and splitting were observed in tests. In order to isolate and identify the fundamental, responsible mechanisms large-scale specimen show an interfering impact of boundary effects and imperfections. Therefore a new test set-up was developed, consisting of a single roving embedded in a single concrete cuboid. The simplicity of production and reliability of this class of experiments in terms of repeatability and low scatter of results makes them a robust basis for experimental investigation of TRC behavior. The experimentally obtained strengths are discussed in terms of the performance, followed by the categorization of observed failure patterns with respect to the available mechanical hypotheses of the longitudinal crack initiation and splitting failure.

abstract: The primary objective of this study is to understand the effect of soil cracking on foundation performance for expansive soil profiles. Two major effects of cracks were studied to assess the effect of cracks on foundation performance. First, the effect of cracks on soil volume change response was studied. Second, the effect of cracks on unsaturated flow properties and extent and degree of wetting were evaluated. Multiple oedometer-type pressure plate tests were conducted to evaluate the effect of cracks on soil properties commonly used in volume change (heave) analyses, such as swell pressure, soil water characteristic curve (SWCC), and swell potential. Additionally, the effect of cracks on saturated and unsaturated hydraulic conductivity was studied experimentally to assess the impact of cracks on properties critical to evaluation of extent and degree of wetting. Laboratory experiments were performed on both intact and cracked specimen so that the effect of cracks on behavior could be benchmarked against intact soil response. Based on laboratory observations, the SWCC of a cracked soil is bimodal. However, this bimodal behavior is only observed in the very low suction ranges. Because the bimodal nature of the SWCC of cracked clays is only distinguishable at extremely low suctions, the bimodal behavior is unlikely to have engineering significance when soils remain unsaturated. A "lumped mass" parameter approach has been studied as a practical approach for modeling of cracked soils for both fluid flow and volume change determination. Laboratory unsaturated flow experiments were simulated using a saturated-unsaturated flow finite element code, SVFlux, to back-analyze unsaturated hydraulic conductivity functions for the subject soils. These back-analyzed results were compared to the results from traditionally-applied analyses of the laboratory instantaneous profile tests on intact and cracked specimens. Based on this comparison, empirical adjustments were suggested for modeling "lumped mass" cracked soil behavior in numerical codes for fluid flow through cracked soils. Using the empirically adjusted flow parameters for unsaturated flow modeling, example analyses were performed for slab-on-grade problems to demonstrate the impact of cracks on degree and extent of wetting under unsaturated and saturated flow conditions for different surface flux boundary conditions. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2011

[pt] O presente trabalho visa determinar, experimentalmente,Fatores de Intensidade de Tensões, KI e KII em barras e tubos, submetidos à tração, com trincas situadas: I) Junto ao pé do cordão de solda em barras planas soldadas ortogonalmente com relações comprimento de trinca, a, largura da barra, B, a/B igual a 0,1 e a/B igual a 0,2. II) Próximo à raiz da solda em tubos unidos a 90 graus com relações comprimento de trinca, espessura do tubo principal, t, a/t igual a 0,1; a/t igual a 0,2; a/t igual a 0,3. III) Barras e tubos simples (sem uniões soldadas) tendo as relações a/B igual a 0,1; 0,2; 0,3 e a/t igual a 0,1; 0,2; 0,25 e 0,3. Nos modelos planos, foi feito um estudo comparativo entre espécimes que possuíam entalhes usinados para simular as trincas e espécimes onde forma propagadas trincas de fadiga. Os resultados para as barras forma obtidos utilizando-se a técnica fotoelástica bidimensional e para os tubos, o método tridimensional de congelamento de tensões e corte em fatias. Nas barras os dados coletados foram submetidos aos programas CRACK e FLAW, e nos tubos, apenas ao programa FLAW, para determinação dos valores de KI e KII. / [en] This work aims to determine Stress Intensity Factors KI and KII for bars and tubes loaded in tension with the following geometry: I) T Join bars with cracks located near the weld toe. Relations between crack length, a, and bar width, B, were: a/B equal 0.05; a/B equal 0.1 a/B equal 0.2. II) Tubes with T joints with cracks located at the weld toe. The relations between crack length and tube thickness, t, were: a/t equal 0.1; a/t equal 0.2 and a/t equal 0.3. III) Plains bars with ratios a/B equal 0.05; a/B equal 0.1 and a/B equal 0.2. IV) Plains tubes with ratios a/t of 0.1, 0.2, 0.25 and 0.3. A comparative study was made between plane models that had artificial machined cracks and those that had natural fatigue cracks. Results for bars were obtained employing two-dimensional Photoelasticity technique and for tubes, the three-dimensional stress freezing and slicing method was used. To generate KI and KII, the collected data were submitted to FLAW and CRACK computer programs in the case of bars, while for tubes, only the FLAW program was used.

The fatigue of materials and structures is a phenomenon in which surface condition plays an important role; specially in initiating the micro-cracks. Moreover, the structural components are also subjected to aggressive environments and hence are susceptible to corrosion phenomenon. Therefore, it is necessary to protect them against the corrosion phenomenon. For aluminium alloys used in aeronautical applications, anodization is widely used surface treatment to increase the corrosion resistance. Despite the benefits obtained in terms of enhanced corrosion resistance, the anodizing process has a damaging effect on the fatigue performance of the base material. One of the reasons for this decrease in fatigue life is associated to the degradation of surface condition during the anodizing process. In this work, we have studied the influence of machining and anodization on fatigue behaviour of aluminium alloys 2214 and 7050. We showed experimentally that for 2214 alloy the process of anodization-sealing reduced the fatigue life considerably while for 7050 alloy the process of pickling is the major cause in decreasing the fatigue life. We have also developed life prediction model for anodized alloys. The model is based on determination of concentration of stress ‘Kt’ generated by the surface condition. The developed model integrates the aspects of multi-site initiation, coalescence and regime of short crack propagation.