Damage-based Approach Research Articles

Modelling Crack Growth in the Life Assessment of Elevated Temperature Components

Modelling of Creep Crack Growth (CCG) using analytical and numerical methods is relevant to life assessment procedures of components operating at elevated temperatures. This paper compares an analytical crack prediction and a numerical based virtual CCG technique used in fracture mechanics components with sample experimental results. Two approaches are presented. First the well developed strain exhaustion model called the NSW and the modified NSW-MOD models which predict plane stress/strain bound crack initiation and growth rates for engineering alloys and the second a damage-based approach used to numerically predict the crack propagation rate in Finite Element models of fracture mechanics specimens. The results from both methods are correlated against an independently determined C* parameter. As an example the NSW and the extended NSW-MOD strain exhaustion models are applied to compare to the experimental data and FE predictions for two steels at Carbon-Manganese steel tested at 360 oC and a weld 316H stainless steel at 550 oC. For values of C* within the limits of the present creep crack growth data presented the plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain NSW model but more conservative than plane strain NSW-MOD model.

Creep Crack Growth Predictions in Component Using a Damage Based Approach

Abstract This paper presents a numerical study of simulated creep crack growth (CCG) prediction in a compact tension [C(T)] specimen and a pipe component containing cracks. The results are compared to experimental data of the same consisting of creep crack growth tests of a carbon-manganese steel (C-Mn) tested at 360°C. The constitutive behavior for this steel is described by a power law creep model and the uniaxial properties are used in a damage based model to predict CCG. For comparison between CCG properties of the pipe and C(T) specimens, different reference stress methods were used to derive appropriate C* values. It has been shown that the global solution for collapse in the cracked pipes gives the best correlation with the C(T) data. On this basis CCG rates for pipe are higher than those for C(T) specimens at the same value of C*. The damage-based approach combined with a crack tip constraint model based on void growth is also used to predict the crack propagation rates in the pipe component using a two-dimensional finite elements (FE) mesh. Elastic-plastic-creep analyses are performed using a node debonding criteria to simulate and predict crack extension under plane stress and plane strain conditions. It has been found that CCG against time predicted from the FE under plane stress conditions is almost the same as those for the experimental data. Comparing C* derived from different reference stress methods to C* from FE analysis it has been found that C* from FE under plane strain condition correlates best with C* from the global reference stress solution. Consequently the main CCG difference between the C(T) and the pipe component is found to be due to the method of estimating C* and not due to constraint effects.

Creep crack growth prediction using a damage based approach

This paper presents a numerical study of creep crack growth (CCG) in a fracture mechanics specimen. The material properties used are representative of a carbon–manganese steel at 360 °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 CCG 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.

Fracture in quasi-brittle materials: a review of continuum damage-based approaches

Starting from the early ideas on smeared-crack modelling of quasi-brittle materials (concrete, rock) as pioneered in the late 1960s various refinements are discussed such as the shear retention factor, the tension-stiffening concept and the tension-softening concept. Next, smeared-crack models are put in the unified context of damage mechanics together with the multiple-smeared crack approach and the (closely) related microplane damage models. The concept of fracture energy is introduced for tensile and compressive loadings and is also elaborated for reinforced concrete. A concise summary is given of recent finite element concepts for cohesive-zone models (fracture energy models).