This paper presents a numerical study of creep crack growth in a fracture mechanics specimen. The material properties used are representative of a carbon-manganese steel at 360 o C and the constitutive behaviour of the steel is described by a power law creep model. A damage-based approach is used to predict the crack propagation rate in a compact tension specimen and the data are correlated against an independently determined C* parameter. Elastic-creep and elastic-plastic-creep analyses are performed using two different crack growth criteria to predict crack extension under plane stress and plane strain conditions. The plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain upper bound of an existing ductility exhaustion model, for values of C* within the limits of the present creep crack growth testing standards. At low values of C* the predicted plane stress and plane strain crack growth rates differ by a factor between 5 and 30 depending on the creep ductility of the material. However, at higher loads and C* values, the plane strain crack growth rates, predicted using an elastic-plastic-creep material response, approach those for plane stress. These results are consistent with experimental data for the material and suggest that purely elastic-creep modelling is unrealistic for the carbon-manganese steel as plastic strains are significant at relevant loading levels.
This paper is concerned with assessing the integrity of cracked engineering components which operate at elevated temperatures. Fracture mechanics parameters are discussed for describing creep crack growth. A model is presented for expressing growth rate in terms of creep damage accumulation in a process zone ahead of the crack tip. Correlations are made with a broad range of materials exhibiting a wide spread of creep ductilities. It is found that individual propagation rates can be predicted with reasonable accuracy from a knowledge only of the material uni-axial creep ductility. An engineering creep crack growth assessment diagram is proposed which is independent of material properties but which is sensitive to the state of stress at the crack tip. Approximate bounds are presented for plane stress and plane strain situations and it is shown that crack growth rates about fifty times faster are expected under plane strain conditions than when plane stress prevails.
This paper presents a numerical study of creep crack growth in a fracture mechanics specimen. The material properties used are representative of a carbon-manganese steel at 360 o C and the constitutive behaviour of the steel is described by a power law creep model. A damage-based approach is used to predict the crack propagation rate in a compact tension specimen. Elastic-creep and elastic-plastic-creep analyses are performed using two different crack growth criteria to predict crack extension under plane stress and plane strain conditions. The plane strain crack growth rate predicted from the numerical analysis is found to be lower than that predicted from ductility exhaustion plane strain model (known as the NSW model), which uses the creep fracture mechanics parameter C* and the development of creep damage directly ahead of the crack tip to predict creep crack growth rates under plane strain/plane stress conditions. A modified NSW model (NSW-MOD) is presented in which the effect of the damage angle at the crack tip is considered in order to predict this difference. In the model it is assumed that fracture occurs first at the value of the crack tip angle, at which the creep strain, reaches its maximum value. It is found that the new NSW-MOD gives a better prediction of the plane strain upper-bound of the experimental data.
In this paper uniaxial tensile creep data are used in conjunction with fracture mechanics concepts to predict creep crack growth rates in materials having a wide range of creep ductilities. A model is proposed of creep damage accumulation in a process zone ahead of the crack tip. The model allows all stages of creep to be incorporated in an approximate manner and creep ductility to be stress and stress-state sensitive. Good agreement is obtained with experimental crack growth data on a range of low alloy steels, a stainless steel, an aluminium alloy and a nickel-base superalloy. It is found that cracking rate is insensitive to the creep process zone size but inversely proportional to creep ductility. Crack growth rates under plane strain conditions are shown to be about fifty times those for plane stress loading.
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