Theoretical and numerical modelling of creep crack growth in a carbon–manganese steel

Abstract

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.

A diffusion driven carburisation combined with a multiaxial continuum creep model to predict random multiple cracking in engineering alloys

“…The two mechanisms of damage are generally intergranular in nature and can run independently as creep needs an applied load whereas environmental attack such as oxidation or carburisation does not need load although applied stress will further assist in the damage process. As a simple first assumption a linear accumulation of these two mechanisms is proposed to give (3) This allows a method to predict failure under environmental creep conditions. The basic methodology for modelling creep damage and cracking based on the NSW remaining multiaxial ductility model [1][2][3] has been presented previously.…”

“…As a simple first assumption a linear accumulation of these two mechanisms is proposed to give (3) This allows a method to predict failure under environmental creep conditions. The basic methodology for modelling creep damage and cracking based on the NSW remaining multiaxial ductility model [1][2][3] has been presented previously. In addition a method for predicting time dependent surface damage and intergranular cracking due to oxidation, based on critical local damage rate criteria and a hardness distribution profile used to describe the oxide depth and profile has also been shown previously [4][5].…”

“…Thus using the NSW model [1,3] which takes into account the state of stress at the crack tip the damage increment for each element in multi-axial creep mode using the ductility criterion *…”

“…In the creep model [1][2][3] to derive the increments of local crack tip deformation can be summed and normalized with the multi-axial failure strain to quantify damage and crack extension under creep.…”

“…Mechanisms such as creep cracking [1][2][3][4][5] and oxidation/carburisation/nitriding [4][5][6][7][8][9][10][11][12][13] have in common a 'rate dependent' damage inducing component. It has been shown that three main categories of damage rate dependencies exist in the oxidation processes.…”