A numerical study of non-linear crack tip parameters.PDF

Frattura ed Integrità Strutturale, 2015

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...

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.

Fatigue & Fracture of Engineering Materials & Structures, 2019

In this paper, we have extended our previous study on fatigue crack closure (Tong et al, FFEMS, 2018) to examine the phenomenon of crack opening displacement (COD) and its impact on the crack tip fields in both 2D and 3D specimen geometries using full-field experimental measurements and integrated finite element modelling. Digital image correlation (DIC) and digital volume correlation (DVC) were used to measure the near-tip material responses on the surfaces (DIC) and the interior (DVC) of the specimens. Materials with elastic-plastic and large plastic characteristics were chosen for the study, where plasticity-induced premature contact between the crack flanks is known to occur. Displacement maps around the cracks were obtained using DIC and DVC at selected load increments, and were introduced as boundary conditions into the finite element (FE) models to obtain the "effective" crack driving force in terms of J-integral, and the results were compared with those "nominal" from the standard FE analysis. Both visual observation and compliance curves were used to determine the "crack opening" levels; whilst the impacts of the crack opening on the crack driving force J and the normal strains ahead of the crack tip were evaluated in 2D and 3D. The results from the study indicate that, crack closure, although clearly identifiable in the compliance curves, does not appear to impact on global crack driving force, such as J-integral, or strains ahead of the crack tip, hence it may well be a misconception.

International Journal of Fatigue, 1986

In this paper an analytical crack closure model is developed, based on the Dugdale model, but modified to take into account the plastically deformed material left in the wake of an advancing crack. For specified maximum and minimum stress values expressed as fractions of the yield stress, the model predicts a crack opening stress from which an effective stress intensity factor may then be computed. Using this factor, the constant amplitude fatigue crack growth rate data for several stress ratios for each of three different aluminium alloys are shown to reduce to a single curve. From these data and the model, growth rates may be predicted without further tests for all steadystate cyclic conditions.

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.

2019

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.

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.

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.