No description available.

Determination of the fatigue life of a component requires knowledge of the local maximum fluctuation stress and the through-thickness stress distribution acting at the critical cross-section. This has traditionally been achieved through the use of stress concentration factors. More recently finite element methods have been used to determine the maximum stress acting on a weldment. Unfortunately, meshing large and complicated geometries properly requires the use of fine meshes and can be computationally intensive and time consuming. An alternative method for obtaining maximum stress values using coarse three-dimensional finite element meshes and the hot spot stress concept will be examined in this paper. Coarse mesh stress distributions were found to coincide with fine mesh stress distributions over the inboard 50% of a cross-section. It was also found that the moment generated by stress distribution over the inboard half of the cross-section accounted for roughly 10% of the total moment acting in all of the cases studied. As a result of this, the total moment acting on a cross-section may be predicted using knowledge of the stress distribution over the inboard 50% of a structure. Given the moment acting on a cross-section, the hot spot stress may be found. Using bending and membrane stress concentration factors, the maximum stress value may be found. Finally, given the maximum stress data, the fatigue life of a component may be determined using either the strain-life approach or fatigue crack growth methods.

Determination of the fatigue life of a component requires knowledge of the local maximum fluctuation stress and the through-thickness stress distribution acting at the critical cross-section. This has traditionally been achieved through the use of stress concentration factors. More recently finite element methods have been used to determine the maximum stress acting on a weldment. Unfortunately, meshing large and complicated geometries properly requires the use of fine meshes and can be computationally intensive and time consuming. An alternative method for obtaining maximum stress values using coarse three-dimensional finite element meshes and the hot spot stress concept will be examined in this paper. Coarse mesh stress distributions were found to coincide with fine mesh stress distributions over the inboard 50% of a cross-section. It was also found that the moment generated by stress distribution over the inboard half of the cross-section accounted for roughly 10% of the total moment acting in all of the cases studied. As a result of this, the total moment acting on a cross-section may be predicted using knowledge of the stress distribution over the inboard 50% of a structure. Given the moment acting on a cross-section, the hot spot stress may be found. Using bending and membrane stress concentration factors, the maximum stress value may be found. Finally, given the maximum stress data, the fatigue life of a component may be determined using either the strain-life approach or fatigue crack growth methods.

No description available.

The aim of the thesis was to study the susceptibility for brittle failures and the fracture process of duplex stainless steels at sub-zero temperatures (°C). In the first part of the thesis plates of hot-rolled duplex stainless steel with various thicknesses were used to study the influence of delamination (also known as splits) on the fracture toughness. The methods used were impact and fracture toughness testing. Light optical microscopy and scanning electron microscopy were used to investigate the microstructure and fracture surfaces. It was concluded that the delaminations caused a loss of constraint along the crack front which resulted in a stable fracture process despite the presence of cleavage cracks. These delaminations occurred when cleavage cracks are constrained by the elongated austenite lamellae. The pop-in phenomenon which is frequently observed in duplex stainless steels during fracture toughness testing was shown to occur due to these delaminations. The susceptibility for pop-in behaviour during testing increased with decreasing plate thickness. The toughness anisotropy was also explained by the delamination phenomenon.In the second part of the thesis duplex stainless steel weld metals from lean duplex and super duplex were investigated. For the lean duplex weldments with different nickel contents, tensile, impact and fracture toughness testing were conducted from room temperature to sub-zero temperatures. The result showed that increased nickel content decreased the susceptibility for critical cleavage initiation at sub-zero temperatures. The super duplex stainless steel weldment was post weld heat treated. The fracture sequence at low temperature was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture. Energy-dispersive X-ray spectroscopy investigation of the weld metals showed that substitutional element partitioning is small in the weld metal. However, for the post weld heat treated weldments element partitioning occurred which resulted in decreased nickel content in the ferrite. / <p>QC 20131108</p>

In this thesis, the heat affected zone (HAZ) of Gleeble simulated welds, the weldments and the creep specimens for several types of 9%-12% Cr ferritic steels were studied by focusing on the Type IV cracking in the fine grained zone (FZ). The field emission gun transmission electron microscopy (FEGTEM) and scanning electron microscopy (SEM) were used to measure the phosphorus segregation on the grain boundary (GB) and the creep fracture morphologies respectively. Meanwhile the well-developed grain boundary segregation and precipitation (GBSP) model was applied to simulate the experimental results. The experimental results have showed that the HAZ zone was characterised by softening and Type IV cracking. All the high Cr ferritic steel welds gave a microstructure of mainly tempered martensite and M23C6 precipitates after the post weld heat treatment (PWHT). There was no δ-ferrite observed in the HAZ. The Type IV cracking exhibited a mixed cracking mechanism in which the intergranular grain boundary separation is dominant due to the crack initiation by voids and the faster M23C6 growth with the service time. A new model on the mechanism of the Type IV cracking is established. The FEGTEM research has also showed obvious non-equilibrium phosphorus segregation at the grain boundaries, which is affected significantly by the quenching temperature. The phosphorus GB segregation deteriorates the weak grain boundaries. The experimental results were well in agreement with the GBSP modelling.

Welding, as one of the most effective joining methods for metals, has been extensively applied in engineering usage for a long time. When cracks occur in the vicinity of weldments, weld repairs are frequently considered for crack repair to extend service life. In order to evaluate to what extent the weld repair has improved the fatigue life of a cracked welded structure, it is necessary to be able to determine the residual life of the cracked welded joint, as well as the life of the weld repaired joint. Both these assessments require that the fatigue crack growth data be available. The determination of crack propagation rates of welded and weld repaired structures is thus of paramount importance to implement a damage tolerant approach to structural life extension. However, since most studies on welded joints so far have concentrated on fatigue life evaluation, at the present time only limited information is available on crack propagation rates in welded joints, and virtually none on fatigue behaviour and crack propagation in weld repaired joints. This thesis has focused on examination of fatigue and crack propagation behaviour in as welded and weld repaired aluminium alloy 5083, a weldable marine grade alloy extensively used in construction of high speed ferries and aerospace structures. Crack growth rates were measured during constant amplitude fatigue testing on unwelded, as-welded and weld repaired specimens of 5083-H321 aluminium alloy. A 3-D finite element analysis was conducted to determine the stress intensity factors for different lengths of crack taking into account the three-dimensional nature of the weld profile. The effects of crack closure due to weld residual stresses were evaluated by taking measurements of the crack opening displacements and utilised to determine the effective stress intensity factors for each condition. Metallurgical examinations and fractography of the fracture surface were conducted using an optical microscope and SEM. It was found that crack growth rates in welded plates are of the same order of magnitude as those of parent material when effective stress intensity factors were applied. However weld repaired plates exhibit higher crack growth rates compared to those of unwelded and once-only welded plates.

Este trabalho visa o refinamento do procedimento para estimativa dos parâmetros elasto-plásticos de tenacidade à fratura, integral J e CTOD, incorporando o efeito de dissimilaridade mecânica devido à introdução de juntas soldadas utilizando a metodologia eta, tal efeito não é previsto na formulação das atuais normas de avaliação de tenacidade à fratura, porém a dissimilaridade mecânica afeta fortemente a relação entre o carregamento global do espécime e as forças motrizes na ponta da trinca. Para o desenvolvimento das análises foi empregada a geometria normalizada C(T), compacta, pois esta pode apresentar potencias vantagens sobre a geometria SE(B), flexão três pontos, como menor consumo de material para a confecção dos espécimes, menor capacidade do aparato experimental e fácil manipulação na prática dos testes laboratoriais, porém carece de fatores eta quando comparados ao tradicional espécime SE(B). Os principais objetivos deste trabalho são gerar um compêndio de fatores eta e propor uma formulação robusta que incorpore os efeitos de dissimilaridade mecânica para o cálculo de integral J e CTOD. Para tal intento a matriz de análise deste trabalho abrange diferentes comprimentos de trinca, níveis de dissimilaridade mecânica e larguras de cordão de solda, 0.45a/W0.6 e 1M y 1.3, 10 mm2h20 mm respectivamente. As análises numéricas foram realizadas nas condições de estado plano de deformação (2D) e tridimensional (incluindo o efeito acoplado entre os campos de tensões e deformações no plano e fora do plano), os resultados fornecem um bom suporte ao uso da geometria C(T) com trinca central no cordão de solda para avaliação de tenacidade à fratura em juntas soldadas de materiais comumente aplicados na construção de dutos e vasos de pressão. / This work focuses on the evaluation procedure to determine the elastic-plastic J integral and CTOD fracture toughness based upon the eta-method for C(T) fracture specimens including overmatched weldments. Since fracture toughness test protocols do not address weld strength mismatch effect, this effect strongly alters the relationship between global loads and crack driving forces. The objectives of this investigation are to enlarge plastic eta-factor data base for C(T) specimen and to develop a robust formulation to address weld strength mismatch. The present analyses enable the introduction of a larger set of plastic eta-factors for a wide range of crack sizes (as measured by the a/W-ratio) and material properties, including different levels of weld strength mismatch, applicable to structural, pipeline and pressure vessel steels. Very detailed non-linear finite element analyses for plane-strain and full 3D models of standard C(T) fracture specimens provide the evolution of load with increased crack mouth opening displacement (and LLD) required for the estimation procedure. The results provide a strong support to use the plastic eta-factor in J integral and CTOD estimation procedures for center notch welded C(T) fracture specimen.

Este trabalho visa o refinamento do procedimento para estimativa dos parâmetros elasto-plásticos de tenacidade à fratura, integral J e CTOD, incorporando o efeito de dissimilaridade mecânica devido à introdução de juntas soldadas utilizando a metodologia eta, tal efeito não é previsto na formulação das atuais normas de avaliação de tenacidade à fratura, porém a dissimilaridade mecânica afeta fortemente a relação entre o carregamento global do espécime e as forças motrizes na ponta da trinca. Para o desenvolvimento das análises foi empregada a geometria normalizada C(T), compacta, pois esta pode apresentar potencias vantagens sobre a geometria SE(B), flexão três pontos, como menor consumo de material para a confecção dos espécimes, menor capacidade do aparato experimental e fácil manipulação na prática dos testes laboratoriais, porém carece de fatores eta quando comparados ao tradicional espécime SE(B). Os principais objetivos deste trabalho são gerar um compêndio de fatores eta e propor uma formulação robusta que incorpore os efeitos de dissimilaridade mecânica para o cálculo de integral J e CTOD. Para tal intento a matriz de análise deste trabalho abrange diferentes comprimentos de trinca, níveis de dissimilaridade mecânica e larguras de cordão de solda, 0.45a/W0.6 e 1M y 1.3, 10 mm2h20 mm respectivamente. As análises numéricas foram realizadas nas condições de estado plano de deformação (2D) e tridimensional (incluindo o efeito acoplado entre os campos de tensões e deformações no plano e fora do plano), os resultados fornecem um bom suporte ao uso da geometria C(T) com trinca central no cordão de solda para avaliação de tenacidade à fratura em juntas soldadas de materiais comumente aplicados na construção de dutos e vasos de pressão. / This work focuses on the evaluation procedure to determine the elastic-plastic J integral and CTOD fracture toughness based upon the eta-method for C(T) fracture specimens including overmatched weldments. Since fracture toughness test protocols do not address weld strength mismatch effect, this effect strongly alters the relationship between global loads and crack driving forces. The objectives of this investigation are to enlarge plastic eta-factor data base for C(T) specimen and to develop a robust formulation to address weld strength mismatch. The present analyses enable the introduction of a larger set of plastic eta-factors for a wide range of crack sizes (as measured by the a/W-ratio) and material properties, including different levels of weld strength mismatch, applicable to structural, pipeline and pressure vessel steels. Very detailed non-linear finite element analyses for plane-strain and full 3D models of standard C(T) fracture specimens provide the evolution of load with increased crack mouth opening displacement (and LLD) required for the estimation procedure. The results provide a strong support to use the plastic eta-factor in J integral and CTOD estimation procedures for center notch welded C(T) fracture specimen.