Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Depencence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc.) in the crack tip vicinity.

Fatigue-crack-growth-rate tests were conducted on compact tension specimens made of 7050-T7451 aluminum alloy and 9310 steel. Compact tension specimens were tested over a wide range of load ratios (0.1 less than, or equal to R less than, or equal to 0.9) to generate crack-growth-rate data from threshold to near fracture. Three methods were used to generate near threshold data. A crack-closure analysis was performed on both materials using the FASTRAN crack-closure model. The crack-growth-rate data for each material correlated very well and each collapsed onto a nearly unique curve in the low- and mid-rate regimes using the strip-yield model in the FASTRAN life-prediction code. For the 7050 alloy, a constraint factor of α = 1.8 was required, while for the 9310 steel α = 2.5 worked very well in correlating the test data over a very wide range in R values and rates from threshold to near fracture.

Surface-crack configurations are among the most important crack problems in the aerospace industry. The residual strength of a surface-cracked component is complicated by three-dimensional variation of the stress-intensity factor around the crack front and plastic deformations, which vary from plane stress at the free boundary, to nearly plane-strain behavior in the interior. In 1973, a two-parameter fracture criterion (TPFC) was developed to analyze fracture behavior of surface-crack configurations. Estimates were made around the crack front for fracture initiation—the critical parametric angle. Recently, NASA developed the Tool for Analysis of Surface Cracks (TASC) software that predicts critical location. This thesis is the application of the TPFC with the TASC critical angles using an equation developed from the TASC software. The TPFC was applied to three materials: a brittle titanium alloy, a ductile titanium alloy, and a ductile 301 stainless steel. The TPFC with the TASC critical angles correlated fracture behaviors well.

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Este estudo avaliou propriedades de fratura e seus parâmetros de superfície em composições de pasta de cimento de mesma porosidade capilar, variando o teor de filler calcário como substituinte ao cimento comercial. Ainda, para avaliação do efeito em composições de argamassa, foi avaliada a capacidade de redução de água que essa adição pode proporcionar e seu efeito na resistência mecânica. A avaliação dos parâmetros de fratura ocorreu por ensaio de flexão por carregamento em três pontos, com controle por taxa de deslocamento e as análises de superfície por ensaio de interferometria, em seções de fratura e entalhe. O aumento do teor de filler calcário proporcionou redução da demanda de água nas argamassas, para mesmo comportamento. A redução da demanda foi ainda maior para as composições dispersas com aditivo, em relação à referência com aditivo. As resistências à compressão das argamassas apresentaram boa correlação em relação ao ajuste com todas as composições, enquanto foi observado que argamassas de médio teor de filler podem apresentar mesma resistência à flexão que as argamassas de referência, mesmo que estas apresentem menor porosidade capilar. As pastas ensaiadas à flexão por carregamento três pontos não apresentaram comportamento quase-frágil, impedindo o cálculo da energia de fraturamento. Isto ocorreu devido ao método de ensaio adotado, com configuração inadequada, e que pode ser ajustada através da redução da taxa de deslocamento do ensaio, da geometria dos corpos de prova ou da geometria dos entalhes produzidos. Todas as composições apresentaram insensibilidade à profundidade de entalhe sob as condições de ensaio adotadas, possivelmente devido às falhas de configuração do método. As médias de resistência à flexão e do fator de intensidade de tensão crítico foram maiores para a composição de alto teor de filler, seguidos da referência e da composição de médio teor de filler, respectivamente. Em todas as análises e tipos de superfície estudadas, as composições de alto teor de filler apresentaram maior índice de rugosidade e amplitude entre picos e vales, enquanto não foi identificada diferença entre a referência e a composição de médio teor de filler. Estes dois parâmetros e a raiz quadrática da rugosidade indicaram que a rugosidade das superfícies fraturadas é maior que em superfícies cortadas com disco diamantado. / This study assesses fracture properties and their surface parameters in cement paste compositions of the same capillary porosity, varying the filler content of limestone as a substitute for commercial cement. Also, to evaluate the effect in mortar compositions, the water reduction capacity that this addition can provide and its effect on the mechanical resistance was evaluated. The evaluation of the fracture parameters was performed by three-point loading flexion test with displacement rate control and surface analysis by interferometry test in fracture and notch sections. The increase of limestone filler provided a reduction of the water demand in the mortars, in relation to the reference, for spreading of 265mm in table of consistency. The water demand reduction was even greater for compositions dispersed with admixture, relative to the reference with admixture. The compressive strength of the mortars presented a good correlation in the fit with all the compositions, while it was observed that mortars of medium filler content may have the same flexural strength as the reference mortars, even if they have lower capillary porosity. The pastes tested by three-point loading did not exhibit quasi-fragile behavior, preventing the calculation of fracturing energy. This was due to the inadequately configured test method adopted, which can be adjusted by reducing the test displacement rate, the geometry of the specimens or the geometry of the notches produced. All compositions presented insensitivity to the notch depth under the test conditions adopted, possibly due to method configuration failures. The averages of flexural strength and critical stress intensity factor were higher for the high filler composition, followed by the reference and medium filler composition, respectively. In all analyzes and surface types studied, high filler compositions presented higher roughness and amplitude index between peaks and valleys, while no difference between the reference and medium filler composition was identified. These two parameters and the quadratic root roughness indicated that the roughness of the fractured surfaces is greater than on surfaces cut with diamond disc.

In this study the plane linear elastic problem of a functionally graded layer which contains periodic cracks is considered. The main objective of this study is to determine the thermal stress intensity factors for edge cracks. In order to find an analytic solution, Young&rsquo / s modulus and thermal conductivity are assumed to be varying exponentially across the thickness, whereas Poisson ratio and thermal diffusivity are taken as constant. First, one dimensional transient and steady state conduction problems are solved (heat flux being across the thickness) to determine the temperature distribution and the thermal stresses in a crack free layer. Then, the thermal stress distributions at the locations of the cracks are applied as crack surface tractions in the elasticity problem to find the stress intensity factors. By defining an appropriate auxiliary variable, elasticity problem is reduced to a singular integral equation, which is solved numerically. The influence of such parameters as the grading, crack length and crack period on the stress intensity factors is investigated.

Semi-circular specimens (SCB) under three point-bending which are commonly used for fracture testing of rocks were used here for fracture mechanics tests. A total of 65 specimens were tested by using Ankara andesite rock. Investigations including the effects of initial notch thickness, different loading span ratios (S/R), flattened loading end, and little dimensional variations when preparing the specimens were carried out. Stress intensity factors for specimens with different geometries were computed individually by using a 3D finite element program ABAQUS. Specimens with a preliminary notch thickness varying from 0.84 to 3.66 mm were tested under three point bending. For a second group of specimens loading span was changed and fracture toughness variation was studied. Another change in the specimen geometry was made by machining a flat loading end at the upper load application point. Fracture toughness values were computed using the stress intensity values computed from numerical modeling and failure loads from the experiments. It was found that up to 2 mm fracture toughness was not affected by variations in the thickness of preliminary notches. Fracture toughness was not affected by changing the loading span. For specimens with flat loading ends, fracture toughness was about 16% lower than the value found from regular SCB type specimens loaded at a point at the top by a steel roller. As a result of about 46 experiments average fracture toughness of Ankara G&ouml / lbasi andesite was found as 1.36 MPa .

Modified Ring specimens are of the shape of discs having a hole inside and flattened ends. These specimens are used for determination of Mode I fracture toughness. Finite element program, named ABAQUS, is used for numerical modeling for finding stress intensity factors. Varying disc geometries were used for the experiments and numerical modeling in which size of the flat ends, radius of the hole inside, and external radius of the specimen were varied. Experiments were done by using pink Ankara andesite. Effects of internal hole radius, external disc radius and size of the flat ends on both stress intensity factor and fracture toughness were studied. In order to compare the results, fracture tests with semi-circular specimens under three point bending (SCB) were also performed. From a similar previous study, fracture toughness values of gray andesite were recalculated and compared to the fracture toughness values of pink andesite for varying geometrical factors. Size effect studies were performed as well for varying diameter of core specimens.Fracture toughness values of andesite were found to increase with increasing specimen size. Fracture toughness of 100 mm specimens was determined as 1.11&plusmn / 0.07 MPa&amp / #8730 / m, whereas fracture toughness of 75 mm specimens was 0.96&plusmn / 0.08 MPa&amp / #8730 / m. 100 mm or larger diameter specimens were suggested for the fracture toughness determination with the modified ring tests.