Under certain loading conditions, surfaces topography coupled with materials degree of brittleness can significantly compromise the mechanical performance of structures. The foregoing remains valid even if roughness is intentionally introduced for engineering reasons. In either case, stress can concentrate. The case of the stress concentration in surfaces having randomly distributed pits is a problem that, although being very practical, yet it remains unsolved. The complexity of a random configuration renders difficult the problem of analytically finding relationships between surface parameters and markers indicative of mechanical failure. Another difficulty is the reproducibility of replicates of specimens possessing random rough surfaces, for destructive testing followed by statistical analysis. An experimental technique to produce highly controlled replicates of random rough surfaces (including modeling of degradation growth) was developed. This method was used to experimentally and statistically study the effects on fracture of early randomly degraded surfaces of poly methyl methacrylate (PMMA) versus topographical parameters. Growth of degradation was assumed to go from an engineering surface to one whose heights are normally distributed. (Early stage of degradation is meant to be that level of roughness which is in the neighborhood of the critical flaw size for a given material). Among other findings, it was found that neither stress nor strain alone can be used to predict fracture at this early stage of degradation. However, fracture location was found to be strongly correlated to the ratio of the root-mean square roughness (RMS) to auto correlation length (ACL), above some RMS threshold. This correlation decreases as the material becomes less brittle (i.e., decrease of Young’s modulus or increase of percent of elongation). Simultaneously, a boundary value problem involving traction-free random rough surfaces was solved using a perturbation method, assuming elastic and isotropic conditions. For small RMS/ACL ratio, the solution for the RMS stress concentration factor, kt was found to be: kt = 1 + 2*SQRT(2)*(RMS/ACL), which agrees very well with the experimental work. Finally, a generalization of stress concentration factor formulas for several geometrical configurations and loading conditions into the Modified Inglis Formula was proposed. Finite element analysis was carried out and comparison was made with both experimental and analytical results. Applications of these results are broad. In surface engineering, for example, our analytical solution can be coupled with Fick’s Law to find critical conditions under which a film could become unstable to random roughness. Additionally, in design and maintenance of surfaces in service, it can be used to preliminarily assess how stress concentrates in surfaces where well defined notches cannot be used as an approximation.

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.

Woven structures are steadily emerging as excellent reinforcing components in dualphase composite materials subjected to multiaxial loads, thermal shock, and aggressive reactants in the environment. Metallic woven wire mesh materials display good ductility and relatively high specific strength and specific resilience. While use of this class of materials is rapidly expanding, significant gaps in mechanical behavior classification remain. This thesis works to address the mechanics of material knowledge gap that exists for characterizing the behavior of a metallic woven structure, composed of stainless steel wires on the order of 25 microns in diameter, and subjected to various loading conditions and stress risers. Uniaxial and biaxial tensile experiments, employing Digital Image Correlation (DIC) as a strain measurement tool, are conducted on woven wire mesh specimens incised in various material orientations, and with various notch geometries. Experimental results, supported by an ample analytic modeling effort, indicate that an orthotropic elastic constitutive model is reasonably capable of governing the macro-scale elasticity of the subject material. Also, the Stress Concentration Factor (SCF) associated with various notch geometries is documented experimentally and analytically, and it is shown that the degree of stress concentration is dependent on both notch and material orientation. The Finite Element Method (FEM) is employed on the macro-scale to expand the experimental test matrix, and to judge the effects of a homogenization assumption when modeling metallic woven structures. Additionally, plasticity of the stainless steel woven wire mesh is considered through experimental determination of the yield surface, and a thorough analytic modeling effort resulting in a modified form of the Hill yield criterion. Finally, mesoscale plasticity of the woven structure is considered, and the form of a multi-scale failure criterion is proposed and exercised numerically.

Neste trabalho está apresentada uma alternativa para o cálculo do fator de concentração de tensões (FCT) em juntas tubulares soldadas do tipo Y. Redes Neurais Artificiais (RNA) foram utilizadas para representar a distribuição de tensões ao longo da junta tubular para os casos de carregamento força axial no plano e momento fletor no plano. As RNA podem aprender a partir de um conjunto de dados sem a necessidade de uma expressão matemática entre as variáveis dependentes e independentes; representa uma vantagem sobre o procedimento normalmente utilizado, ou seja, as equações paramétricas. O modelo proposto representa um avanço no projeto de juntas tubulares, uma vez que evita a necessidade de se conhecer uma expressão matemática para representar a distribuição de tensões na junta e fornece um método mais preciso para avaliar a distribuição de tensões ao longo da junta soldada. O conjunto de dados utilizado foi formado a partir de simulações numéricas das juntas soldadas através do MEF, nas quais foi considerada a geometria do cordão de solda. / An alternative approach to calculate stress concentration factors (SCF) in Y-type welded tubular joints is presented. Artificial Neural Networks (ANN) were used to represent the stress distribution along the tubular joints in both in-plane axial force and in-plane bending moment load cases. ANN can learn from a database without establishing a mathematical expression between dependent and independent variables, which is an advantage over the usual parametric equations approach. The proposed model represents an improvement in the tubular joints design, since it avoids the previous knowing of a mathematical expression to represent the stress distribution in the joint and provides an accurate method to evaluate the stress distribution along the welded fillet joint. The database herein used was completed with FE simulations of tubular joints which consider the geometry of the weld fillet.

Despite the developments in the last decades, field performance information for short aggregate pier improved ground is needed for future design and to develop a better understanding of the performance of the short (floating) aggregate piers. A full-scale field study was performed to investigate the floating aggregate pier behavior in a soft clayey soil. Site investigations included five boreholes and sampling, four CPT soundings, and SPT and laboratory testing. The soil profile consisted of 8m thick compressible clay overlying weathered rock. Four large plate load test stations were prepared. A rigid steel footing having plan dimensions of 3.0m by 3.5m were used for loading. Four 65cm diameter reaction piles and steel cross beams were used to load the soil in each station. First test comprised of loading the untreated soil up to 250 kPa with increments, and monitoring the surface settlements. Moreover, distribution of settlements with depth is recorded by means of deep settlement gages installed prior to loading. Other three tests were conducted on clay soil improved by rammed aggregate piers. In each station, seven stone columns were installed, having a diameter of 65cm, area ratio of 0.25, placed in a triangular pattern with a center to center spacing of 1.25m. The length of the columns were 3m, 5m in the two station resembling floating columns, and 8m in the last station to simulate end bearing columns to observe the level of the improvement in the floating columns. Field instrumentations included surface and deep settlement gages, and load cell placed on a aggregate pier to determine distribution of the applied vertical stress between the column and the natural soil , thus to find magnitude of the stress concentration factor, n , in end bearing and floating aggregate piers. It has been found that, the presence of floating aggregate piers reduce settlements, revealing that major improvement in the settlements takes place at relatively short column lengths. It has been also found that the stress concentration factor is not constant, but varies depending on the magnitude of the applied stress. The magnitude of stress concentration factor varies over a range from 2.1 to 5.6 showing a decreasing trend with increasing vertical stress.

The presence of notches or cracks causes stresses to amplify in nearby regions. This phenomenon is studied by estimating the Stress Concentration Factor (SCF) for notches, and the Stress Intensity Factor (SIF) for cracks. In the present work, a semi-analytical method under the framework of linear elasticity is developed to give an estimate of these factors, particularly for cracks and notches in finite domains. The solution technique consists of analytically deriving a characteristic equation based on the general solution and homogeneous boundary conditions, and then using the series form of the reduced solution involving the (possibly complex-valued) roots of this characteristic equation to satisfy the remaining non-homogeneous boundary conditions. This last step has to be carried out numerically using, say, a weighted residual method. In contrast to infinite domain problems where a fully analytical solution is often possible, the presence of more boundaries, and a variety in configurations, makes the solution of finite do-main problems much more challenging compared to infinite domain ones, and these challenges are addressed in this work. The method is demonstrated on several classical and new problems including the problems of a semi-circular edge notch in a semi-infinite and finite plate, an elliptical hole in a plate, an edge-crack in a finite plate etc.

Neste trabalho está apresentada uma alternativa para o cálculo do fator de concentração de tensões (FCT) em juntas tubulares soldadas do tipo Y. Redes Neurais Artificiais (RNA) foram utilizadas para representar a distribuição de tensões ao longo da junta tubular para os casos de carregamento força axial no plano e momento fletor no plano. As RNA podem aprender a partir de um conjunto de dados sem a necessidade de uma expressão matemática entre as variáveis dependentes e independentes; representa uma vantagem sobre o procedimento normalmente utilizado, ou seja, as equações paramétricas. O modelo proposto representa um avanço no projeto de juntas tubulares, uma vez que evita a necessidade de se conhecer uma expressão matemática para representar a distribuição de tensões na junta e fornece um método mais preciso para avaliar a distribuição de tensões ao longo da junta soldada. O conjunto de dados utilizado foi formado a partir de simulações numéricas das juntas soldadas através do MEF, nas quais foi considerada a geometria do cordão de solda. / An alternative approach to calculate stress concentration factors (SCF) in Y-type welded tubular joints is presented. Artificial Neural Networks (ANN) were used to represent the stress distribution along the tubular joints in both in-plane axial force and in-plane bending moment load cases. ANN can learn from a database without establishing a mathematical expression between dependent and independent variables, which is an advantage over the usual parametric equations approach. The proposed model represents an improvement in the tubular joints design, since it avoids the previous knowing of a mathematical expression to represent the stress distribution in the joint and provides an accurate method to evaluate the stress distribution along the welded fillet joint. The database herein used was completed with FE simulations of tubular joints which consider the geometry of the weld fillet.

Οι περιοχές συγκέντρωσης τάσης ή/και παραμόρφωσης στα σύνθετα υλικά, όπως και στα συμβατικά -μεταλλικά- υλικά, αποτελούν σημαντικά στοιχεία στο σχεδιασμό τμημάτων δομών και ολόκληρων κατασκευών. Ειδικότερα η μέθοδος σχεδιασμού με ανοχή στη βλάβη, η οποία έχει επικρατήσει, καθιστά την κατανόηση και μελέτη της συμπεριφοράς των περιοχών αυτών εκ των ων ουκ άνευ. Στο παρόν, γίνεται μια προσπάθεια για την καλύτερη κατανόηση της συμπεριφοράς των σύνθετων υλικών, όταν εμπεριέχουν περιοχές συγκέντρωσης τάσης. / Stress concentration or strain magnification locations on composite materials, similar to conventional-metallic materials, are a very important case in designing structural elements or structures in general. Damage tolerance design, particularly, which has become a dominant design method, makes the study and understanding of the behaviour of these locations necessary. In the present, an effort is presented for the better understanding of the composite materials behaviour in the presence of stress concentration locations.

Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-09T20:08:21Z (GMT). No. of bitstreams: 1 CamargoJunior_JoseAntoniode_M.pdf: 5956974 bytes, checksum: 439c5177de65f09b9855f932ec564537 (MD5) Previous issue date: 2007 / Resumo: Atualmente, praticamente todo o processo de desenvolvimento de componentes mecânicos que giram em alta rotação e que têm grande responsabilidade estrutural, principalmente quando fabricados em materiais de natureza frágil, como é o caso do ferro fundido, passa necessariamente por ensaios de centrifugação, que são realizados visando à determinação da rotação de ruptura. Além disso, os projetos atuais utilizam componentes com configurações geométricas que envolvem furos e outros tipos de concentradores de tensão. Em função disso, torna-se importante o conhecimento da influência de certos tipos de concentradores de tensão nesses componentes mecânicos. Este trabalho tem por objetivos determinar e analisar curvas de fator de concentração de tensão experimentais, por meio de ensaios de centrifugação, e numéricas, por meio de elementos finitos, em corpos anulares de espessura constante, submetidos à rotação e fabricados em ferro fundido cinzento perlítico GG25. Foram executados ensaios de centrifugação de cinco diferentes geometrias de corpos anulares: (a) sem concentrador de tensão; (b) com três concentradores de tensão do tipo semi-circular no raio interno; (c) com três concentradores de tensão do tipo semi-circular no raio externo; (d) com três furos passantes localizados nas proximidades do raio interno; (e) com três furos passantes localizados nas proximidades do raio externo. As tensões radial e tangencial foram determinadas por meio das equações clássicas para rotação de corpos anulares, sendo que a tensão equivalente foi determinada por meio do critério de von Mises, sendo esta comparada com o limite de resistência à tração do material determinada por meio de ensaios de tração. Foi também realizada uma análise simplificada por elementos finitos visando à determinação da tensão equivalente, também pelo critério de von Mises, para as cinco geometrias estudadas. Observou-se que cada geometria de descontinuidade geométrica apresenta um fator de concentração de tensão diferente a qual reduz a rotação de ruptura, quando comparada com o corpo anular sem concentrador de tensão. O corpo anular com concentrador de tensão do tipo semi-circular no raio interno possui os maiores níveis de fator de concentração de tensão médio, sendo este da ordem de 48% maior do que o sem concentrador. Por outro lado, o que apresentou menor nível de concentração de tensão foi o corpo anular com furos localizados nas proximidades do raio externo, sendo este da ordem de 5% maior do que o sem concentrador. Essas curvas poderão orientar o desenvolvimento de projetos de componentes mecânicos com relação aos efeitos dos concentradores de tensão analisados neste trabalho / Abstract: Currently, practically all the process of development of mechanical components working in high rotation and that they have great structural responsibility, mainly when manufactured in materials of brittle nature, as it is the case of the cast iron, passes necessarily for burst tests conducted in order to determine the burst speed. Moreover, the current projects use components with geometric configurations that involve holes and other types of stress raisers. As a function of this, the knowledge of the influence of certain types of stress raisers in these mechanical components becomes important. The objectives of this work is to determine and to analyze two types of curves of stress concentration factors, experimental, by means of the burst tests, and numerical, by means of finite element method, in annular specimens of constant thickness, submitted to rotation, and manufactured in perlitic gray iron GG25. Burst tests of five different shapes of annular specimens had been executed: (a) without stress raiser; (b) with three stress raisers of half-circular shape located in the internal radius; (c) with three stress raisers of halfcircular shape located in the external radius; (d) with three stress raisers of passing holes located near the internal radius; (e) with three stress raisers of passing holes located near the external radius. The radial and tangential stresses had been determined by means of the classic equations for rotation of annular bodies, the equivalent stress was determined by means of the von Mises criterion, and this was compared with the tensile strength of the material determined from tensile tests. Also a simplified analysis of finite element method was conducted in order to determine the equivalent stress, also by using von Mises criterion, for the five studied geometry. It was observed that each shape of geometry discontinuity presents a different stress concentration factor which reduces the rupture rotation, when compared with the annular specimen without stress raiser. The annular specimen with stress raiser of the half-circular type in the internal radius posses the biggest level of average stress concentration factor, about 48% greater of that without stress raiser. On the other hand, the annular specimen with three holes near the external radius presented minor level of stress concentration, about 5% greater of that without stress raiser. These curves will be able to guide the development of projects of mechanical components with regard to the effect of the analyzed stress concentrators in this work / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Wind power, especially offshore, is considered to be one of the most promising sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050. However, its popularity has always fluctuated with the price of fossil fuels since nowadays wind electricity production cannot compete with nuclear or coal electricity production. Support structures are thought to be one of the main drivers for reducing costs in order to make the wind industry more economically efficient. Foundations and towers should be fit for purpose, extending their effective service life but avoiding costs of oversizing. An exhaustive review of the background and state of the art of the Fatigue-Life assessment approaches has been carried out, combining analysis of the gathered experimental data and the development of Finite Element models based on contemporary 3D solid models with diverse Regression Analyses, in order to identify their weakness and evaluate their accuracy. This research shows that the guides and practices currently employed in the design and during the operation of the offshore wind turbine support structures are obsolete and not useful for optimisation, which generally leads to conservationism and an unnecessary increase in costs. The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N curves and the Stress Concentration Factors of Tubular Joints has been set out. Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint has been proposed.