The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

Neste trabalho os resultados da metodologia de normalização de E. D. Reese e K. H. Schwalbe para a obtenção de curvas J-R, são comparadas com aquelas obtidas pela tradicional Técnica de Variação de Flexibilidade Elástica. Ambos os métodos foram aplicados à um aço de grau nuclear A508-CL3A, quando vários corpos-de-prova miniaturizados 0.394TC[S], foram testados em temperaturas típicas de operação de vasos de pressão nuclear. Corpos-de-prova miniaturizados formam tratados termicamente tentando simular o comportamento exibido pelos aços estruturais quando submetidos à diferentes níveis de exposição de nêutrons, sendo que outros foram ensaiados na condição de como-recebidos. Todos os procedimentos experimentais e analíticos empregados neste estudo foram totalmente computadorizados, incluindo aquisição e refinamento dos dados e o traçado da curva J-R. Esta última foi realizada conforme a mais recente versão das normas ASTM, denominada ASTM-E1820. As variáveis avaliadas influentes do comportamento das curvas de J-R, incluem o incremento de propagação de trinca e o número de pontos de dados de carga versus deslocamento. Também foi analisada a efetividade do método de normalização quando comparada com a técnica de variação de flexibilidade elástica. Os resultados desta pesquisa mostraram que a normalização é uma metodologia confiável e muito simples, tão logo a programação de software seja desenvolvida, para ser aplicada em uma vasta variação de comportamentos mecânicos como amostrados neste estudo. / In this research, results from Reese; Schwalbe (1993) linear normalization methodology for deriving J-R curves are compared to those obtained from the traditional unloading elastic compliance technique. Both methodologies were applied to a nuclear grade steel A508-CL3A. Several miniaturized 0.394TC[S] specimens, were tested at typical NPV operating temperatures. Small scale testpieces were heat treated aiming to simulate the embritlemment exhibited by the structural steel when submitted to different levels of neutron exposure, whereas other specimens were tested in the as-received condition. All experimental and analytical procedures employed in this study have been fully computerized, including data acquisition, data refinement and J-R curve plotting, when the guidelines supplied by most recent version of ASTM standard for J-R curves, namely ASTM-E1820, were followed. The assessed variables affecting J-R curves behavior comprised crack propagation increment, number of load-Ioad line displacement data points, and it was also analyzed the agreement of the normalization method when compared to the compliance technique. Research results have shown that linear normalization is a trustworthy and very simple methodology to be applied in a vast range of mechanical behavior as sampled in this study, as long as computer programming is employed.

Neste trabalho os resultados da metodologia de normalização de E. D. Reese e K. H. Schwalbe para a obtenção de curvas J-R, são comparadas com aquelas obtidas pela tradicional Técnica de Variação de Flexibilidade Elástica. Ambos os métodos foram aplicados à um aço de grau nuclear A508-CL3A, quando vários corpos-de-prova miniaturizados 0.394TC[S], foram testados em temperaturas típicas de operação de vasos de pressão nuclear. Corpos-de-prova miniaturizados formam tratados termicamente tentando simular o comportamento exibido pelos aços estruturais quando submetidos à diferentes níveis de exposição de nêutrons, sendo que outros foram ensaiados na condição de como-recebidos. Todos os procedimentos experimentais e analíticos empregados neste estudo foram totalmente computadorizados, incluindo aquisição e refinamento dos dados e o traçado da curva J-R. Esta última foi realizada conforme a mais recente versão das normas ASTM, denominada ASTM-E1820. As variáveis avaliadas influentes do comportamento das curvas de J-R, incluem o incremento de propagação de trinca e o número de pontos de dados de carga versus deslocamento. Também foi analisada a efetividade do método de normalização quando comparada com a técnica de variação de flexibilidade elástica. Os resultados desta pesquisa mostraram que a normalização é uma metodologia confiável e muito simples, tão logo a programação de software seja desenvolvida, para ser aplicada em uma vasta variação de comportamentos mecânicos como amostrados neste estudo. / In this research, results from Reese; Schwalbe (1993) linear normalization methodology for deriving J-R curves are compared to those obtained from the traditional unloading elastic compliance technique. Both methodologies were applied to a nuclear grade steel A508-CL3A. Several miniaturized 0.394TC[S] specimens, were tested at typical NPV operating temperatures. Small scale testpieces were heat treated aiming to simulate the embritlemment exhibited by the structural steel when submitted to different levels of neutron exposure, whereas other specimens were tested in the as-received condition. All experimental and analytical procedures employed in this study have been fully computerized, including data acquisition, data refinement and J-R curve plotting, when the guidelines supplied by most recent version of ASTM standard for J-R curves, namely ASTM-E1820, were followed. The assessed variables affecting J-R curves behavior comprised crack propagation increment, number of load-Ioad line displacement data points, and it was also analyzed the agreement of the normalization method when compared to the compliance technique. Research results have shown that linear normalization is a trustworthy and very simple methodology to be applied in a vast range of mechanical behavior as sampled in this study, as long as computer programming is employed.

The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.

This master´s thesis deals with the evaluation of fracture behavior of ODS steel MA956 at high temperature range. This behavior was tested by using miniaturized CT specimens, on which were performed experiments to measure of ductile crack growth resistance curves (J-R curves). The value of the fracture toughness was determined from these J-R curves. Fracture properties were consequently evaluated by using fractographic analysis of the fracture surfaces. Structural properties of material was identified by hardness measurement and analyzed by metallographic methods. Results of the measurements show drop of the fracture toughness with respect to the increasing temperature.

This thesis deals with a high temperature testing of fracture toughness and studies the size effect on measured values using miniature size CT specimen. Two types of specimen geometry were manufactured from P91 steel, the standard size and the quarter size specimen. J-R curves were obtained in the temperature range from 23°C to 600°C. No specimen size effect was observed at room temperature tests. The realized experiments together with fractography analysis demonstrated the drop of toughness at 400°C caused by the effect of dynamic strain aging.

The thesis deals with the determination of fracture toughness using compact tension (CT) test specimens at elevated and high temperatures. The experimental material steel P91 designated for application at temperatures 550650°C was used. The fracture toughness in the ductile fracture region of the steel was characterized by the R curve, which characterises the resistance against crack propagation depending on the crack length. The effect of temperature on the R curve at range 23600°C was evaluated. Next the specimen size effect using three sizes of CT specimens at 23°C and the effect of loading rate (2, 0,2 a 0,02 mm/min) at 600°C was examined. The results showed that the temperature has distinct effect on the R-curve, which yields minimal values at 400°C. Only the smallest test specimen size with thickness 6.25mm showed the specimen size effect giving about 10% lower values of toughness comparing to larger specimens. The effect of loading rate was clearly distinguishable. The values of toughness varied about 20% of the toughness value comparing individual loading rate.