Numerical evaluation of plasticity induced crack closure in 3D structures (original) (raw)
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A comparison of two and three-dimensional analyses of fatigue crack closure
International Journal of Fatigue, 2007
Plasticity-induced fatigue crack closure is an important mechanism in the reduction of the effective stress intensity factor range for a fatigue crack. A calculation of the level of reduction would allow more accurate predictions of fatigue crack growth rate. However, modelling plasticity-induced closure is not straightforward, particularly when the three-dimensional aspects of the problem are included. Some simplification is possible by reducing the problem to two dimensions, but it is not always clear how this can be achieved for practical crack geometries.In this work, two-dimensional plane stress and plane strain finite element analyses are used to predict crack opening in a centre-cracked plate. The results of these analyses are compared with those of a plane stress strip yield analysis and those of a three-dimensional finite element analysis. Results are obtained for different R-ratios and stress levels. Reasonable agreement is found between the plane stress finite element and strip yield results for higher levels of applied stress levels where an excessively high level of mesh refinement is not required. Plane stress finite element crack opening results agree with three-dimensional finite element results for the surface and plane strain finite element results agree with three-dimensional finite element results for the mid-thickness. The implications of the results for the behaviour of three-dimensional cracks are discussed.
Strategy of plasticity induced crack closure numerical evaluation
Theoretical and Applied Fracture Mechanics, 2019
A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack closure. The ability to accurately describe this phenomenon is of interest for the scientific and engineering community, because of its significant impact on the fatigue crack propagation rate. A strategy for numerical modelling of the most common closure mechanism the plasticity induced crack closureis presented in this paper. It was observed that the generally adopted suggestions for this type of simulations, such as the length of the crack growth or the number of substeps, are not necessarily valid in general, but require to be individually specified for particular conditions. The size of the elements in the vicinity of the crack front is also a widely discussed issue and it is shown here that even without convergence, the element size may be chosen as a fixed parameter leading to very reasonable closure values with low computational costs. A method of closure level determination based on change in specimen stiffness is described here and its performance is compared to the traditional first node displacement method with Load-Debond-Unload (LDU) and Load-Debond-Unload-Load-Unload (LDULU) loading schemes.
Finite element analysis of plasticity-induced fatigue crack closure: an overview
Engineering Fracture Mechanics, 2004
In this paper, a new methodology is presented to calculate crack opening values in planar geometries using the crack surface nodal force distribution under minimum loading as determined from finite element analyses (FEM). Finite element analyses are frequently used to model growing fatigue cracks and the associated plasticity-induced crack closure. Two-dimensional, elastic-perfectly plastic finite element analyses of middle-crack tension (MT) geometry were conducted to study fatigue crack closure and to calculate the crack opening values under plane-strain and plane-stress conditions. Mesh refinement studies were performed on geometry with various element types. Next, effect of a highly refined mesh on crack opening values was noted and significantly lower crack opening values than those reported in literature were found. The calculated crack opening values are compared with values obtained using finite element analysis and more conventional crack opening assessment methodologies. It is shown that the new method is independent of loading increment, integration method and crack opening assessment location. The compared opening values is exposed in good agreement with strip-yield models.
A numerical study of plasticity induced crack closure under plane strain conditions
International Journal of Fatigue, 2014
The level of plasticity induced crack closure (PICC) is greatly affected by stress state. Under plane strain conditions, however, the level and even the existence of PICC still are controversial. The objective here is to study the influence of the main numerical parameters on plane strain PICC, namely the total crack propagation, the number of load cycles between crack increments, the finite element mesh and the parameter used to quantify PICC. The PICC predictions were included in a parallel numerical study of crack propagation, in order to quantify the impact of plane strain values on fatigue life. The results indicate that literature may be overestimating plane strain PICC due to incorrect numerical parameters. The number of load cycles usually considered is unrealistically small, and its increase was found to vanish crack closure, particularly for kinematic hardening. This effect was linked to the ratcheting effect observed at the crack tip. The total crack increment, Da, must be large enough to obtain stabilized PICC values, but this may imply a huge numerical effort particularly for 3D models. The size of crack tip plastic zone may be overestimated in literature, which means that the meshes used may be too large. Additionally, the crack propagation study showed that the plane strain PICC has usually a dominant effect on fatigue life, and plane stress PICC is only relevant for relatively thin geometries.
International Journal of Solids and Structures, 2012
The numerical study of plasticity-induced crack closure using the node-release technique presents many difficulties widely studied in literature. For instance various rules, proposed for overcoming mesh sensitivity, are challenged by more recent studies. This paper intends to propose and evaluate a numerical method for the investigation of crack propagation under fatigue loading, and particularly for the assessment of plasticity-induced crack closure in three-dimension. The method is an extension of the ''steadystate method'' to cyclic loadings. The steady-state method allows a direct computation (on a fixed mesh, without releasing nodes) of stress and strain fields around the crack tip and in the wake for a steady crack growth. The method is extended to simulate crack propagation under fatigue loading. Therefore it constitutes a valuable numerical tool for gaining insight into the physics of crack propagation, as it provides accurate mechanical fields around the crack tip and their relation with crack growth rate, various loading modes and parameters. The proposed method is also compared with the classical node-release technique. A very good agreement between the two methods is found. However the steady-state method needs much less mesh refinement and computational time. Following an analysis of some features of the fatigue crack, a discussion on a crack closure criterion is opened, and a reliable criterion for the determination of local crack closure is proposed.
An Analysis of Crack Propagation and a Plasticity-Induced Closure Effect
In the present paper, a few computational models for the crack growth analysis are improved. The improvements consist of including the effect of the plasticity-induced crack closure, i.e. the effective stress intensity factor is computed through the finite element method in order to account the effect of plasticity-induced crack closure on fatigue crack growth. Thus, corrective factors for the effect of the plasticity-induced crack closure are determined here. The improved models are noticed to provide a fairly good correlation with available experimental data. Furthermore, the calculated results show that the plasticity-induced crack closure has a significant effect on fatigue crack growth and flawed structure life.
A numerical study of non-linear crack tip parameters.PDF
Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the Keff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid.
Engineering Fracture Mechanics, 2003
Two-dimensional, elastic-perfectly plastic finite element analyses of middle-crack tension (MT) and compact tension (CT) geometries were conducted to study fatigue crack closure and to calculate the crack opening values under plane-strain and planestress conditions. The behaviors of the CT and MT geometries were compared. The loading was selected to give the same maximum stress intensity factor in both geometries, and thus approximately similar initial forward plastic zone sizes. Mesh refinement studies were performed on both geometries with various element types. For the CT geometry, negligible crack opening loads under plane-strain conditions were observed. In contrast, for the MT specimen, the plane-strain crack opening stresses were found to be significantly larger. This difference was shown to be a consequence of inplane constraint. Under plane-stress conditions, it was found that the in-plane constraint has negligible effect, such that the opening values are approximately the same for both the CT and MT specimens.