Finite Element Analysis of the Fatigue Propagation of Bifurcated Cracks (original) (raw)

Evaluation of fatigue crack growth retardation and arrest in bifurcated cracks

2003

Fatigue crack kinking and bifurcation are well-known phenomena capable of inducing significant growth retardation or even crack arrest. However, symmetrically bifurcated crack models available in the literature cannot account for the propagation behavior observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in asymmetrically bifurcated cracks. The crack path and the associated stress intensity factors (SIF) of asymmetrically bifurcated cracks are numerically obtained for several bifurcation angles. A companion life assessment program is used to estimate the number of delay cycles associated with crack bifurcation, allowing for a better understanding of the influence of crack deflection in the propagation life of structural components.

F2004/26 Quantitative Evaluation of Fatigue Crack Growth Retardation Due

2014

Fatigue crack kinking and bifurcation are phenomena capable of inducing significant growth retardation or even crack arrest. However, bifurcated crack models available in the literature cannot account for the subsequent propagation behavior observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in kinked and bifurcated cracks. The crack path and associated stress intensity factors (SIF) of bifurcated cracks are numerically obtained for several bifurcation angles and branch lengths. From these results, empirical crack retardation equations are proposed to model the retardation factor along the crack path, allowing for a better understanding of the influence of crack deflection in the propagation life.

Bifurcated fatigue crack path and life predictions

The stress intensity factors (SIF) associated to branched fatigue cracks can be considerably smaller than that of a straight crack with the same projected length, causing crack growth retardation or even arrest. This mechanism can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e.g. in high R-ratio or in plane strain controlled fatigue crack growth. Analytical solutions have been obtained for the SIF of branched cracks, however numerical methods such as Finite Elements (FE) or Boundary Elements (BE) are the only means to predict the subsequent curved propagation behavior. In this work, a FE program is developed to calculate the path and associated SIF of branched cracks, validated through experiments on 4340 steel ESE(T) specimens. From these results, semi-empirical crack retardation equations are proposed to model the retardation factor along the crack path. The model also considers the possible interaction between crack branching and closure.

Quantitative Evaluation of Fatigue Crack Growth Retardation Due to Crack Branching

2004

Fatigue crack kinking and bifurcation are phenomena capable of inducing significant growth retardation or even crack arrest. However, bifurcated crack models available in the literature cannot account for the subsequent propagation behavior observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in kinked and bifurcated cracks. The crack path and associated stress intensity factors (SIF) of bifurcated cracks are numerically obtained for several bifurcation angles and branch lengths. From these results, empirical crack retardation equations are proposed to model the retardation factor along the crack path, allowing for a better understanding of the influence of crack deflection in the propagation life.

Finite element modeling of fatigue crack bifurcation

Computational Fluid and Solid Mechanics 2003, 2003

The influence of overload-induced crack deflections and bifurcations on the propagation behavior of mode I fatigue cracks is studied using specialized finite element (FE) software. The FE program is validated through comparisons between FE-calculated and analytical stress intensity factors (SIF) for a crack with a small kink at its tip. The SIF of bifurcated cracks are then obtained using the software. It is observed that such deviations of the crack path can cause significant growth retardation and even crack arrest.

PROPAGATION PATH AND FATIGUE LIFE PREDICTIONS OF BRANCHED CRACKS

2013

Fatigue cracks can significantly deviate from their Mode I growth direction due to the influence of overloads, multi-axial stresses, micro structural inhomogeneities such as grain boundaries and interfaces, or environmental effects, generating crack kinking or branching [1]. The stress intensity factors (SIF) associated to branched fatigue cracks can be considerably smaller than that of a straight crack with the same projected length, causing crack growth retardation or even arrest. This mechanism can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e.g. in high R-ratio fatigue problems under plane strain conditions. Analytical solutions have been obtained for the SIF of some branched cracks, however numerical methods are the only means to predict the subsequent curved propagation behaviour. In this work, a specialized Finite Element program is used to calculate the propagation path and associated SIF of bifurcated cracks. The numerical calculations are validated through experiments on 4340 steel ESE(T) specimens. A total of 6,250 FE calculations are used to fit empirical equations to the process zone size and crack retardation factor along the curved crack branches. The bifurcation simulations include several combinations of bifurcation angles, branch asymmetry ratios, crack growth exponents, and even considers interaction between crack branching and other retardation mechanisms such as crack closure, assuming the crack opening level is well known.

EVALUATION OF CRACK GROWTH RETARDATION IN BRANCHED FATIGUE CRACKS

Tecnologia em Metalurgia e Materiais, 2004

Overload-induced fatigue crack branching (or bifurcation) is a phenomenon capable of inducing significant growth retardation or even crack arrest. Such behavior can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e.g. in high R-ratio or in plane-strain controlled fatigue crack growth. However, the few analytical models available for branched cracks cannot be used to predict the subsequent crack growth nor account for the delays observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in bifurcated cracks. The crack path and the associated stress intensity factors (SIF) of asymmetrically bifurcated cracks are numerically obtained for several bifurcation angles. The companion life assessment program is used to estimate the number of delay cycles associated with crack bifurcation. The proposed approach is validated experimentally, allowing for a better understanding of the influence of crack deflection in the propagation life of structural components.

Propagation Path and Fatigue Life Predictions of Branched Cracks Under Plane Strain Conditions

Fracture of Nano and Engineering Materials and Structures, 2006

Fatigue cracks can significantly deviate from their Mode I growth direction due to the influence of overloads, multi-axial stresses, micro structural inhomogeneities such as grain boundaries and interfaces, or environmental effects, generating crack kinking or branching [1]. The stress intensity factors (SIF) associated to branched fatigue cracks can be considerably smaller than that of a straight crack with the same projected length, causing crack growth retardation or even arrest. This mechanism can quantitatively explain retardation effects even when plasticity induced crack closure cannot be applied, e.g. in high R-ratio fatigue problems under plane strain conditions. Analytical solutions have been obtained for the SIF of some branched cracks, however numerical methods are the only means to predict the subsequent curved propagation behaviour. In this work, a specialized Finite Element program is used to calculate the propagation path and associated SIF of bifurcated cracks. The numerical calculations are validated through experiments on 4340 steel ESE(T) specimens. A total of 6,250 FE calculations are used to fit empirical equations to the process zone size and crack retardation factor along the curved crack branches. The bifurcation simulations include several combinations of bifurcation angles, branch asymmetry ratios, crack growth exponents, and even considers interaction between crack branching and other retardation mechanisms such as crack closure, assuming the crack opening level is well known.

Crack Bifurcation as a Retardation Mechanism

In this work, a FE program is developed to calculate the path and associated SIF of branched cracks, validated through experiments on 4340 steel ESE(T) specimens. It is shown that very small differences between the lengths of the bifurcated branches are sufficient to cause the shorter one to eventually arrest as the longer branch returns to the pre-overload conditions. Crack retardation equations are proposed to predict the propagation behavior of branched cracks in an arbitrary structure, considering the possible interaction with other retardation mechanisms. These equations are fitted from a total of 6,250 FE calculations to the process zone size and crack retardation factor along the curved crack branches. From these quantitative results, it is shown that crack bifurcation may provide a sound alternative mechanistic explanation for overload-induced fatigue crack retardation on structural components when Elberian arguments cannot be used, in special for load interaction effects under closure-free conditions at high R-ratios, or when the closure load decreases after the overloads, as observed at low R-ratios under plane strain conditions.

Numerical prediction of the propagation of branched fatigue cracks

Computational Fluid and Solid Mechanics 2003, 2003

A specialized finite element (FE) program is used to predict the propagation behavior of asymmetrically bifurcated cracks, which can cause crack growth retardation and arrest. The branched crack propagation path and associated stress intensity factors (SIF) are obtained for several bifurcation angles. It is found that very small differences between the branch lengths are enough to cause the shorter one to eventually arrest due to shielding effects. The SIF of the longer crack branch is also reduced due to the deflections, but it returns to the original non-bifurcated value as the crack propagates away from the influence of the (arrested) shorter branch. It is verified that crack bifurcation may provide an alternate mechanistic explanation for overload-induced crack retardation.