A new method for analysis of part-elliptical surface cracks in structures subjected to fatigue loading (original) (raw)
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Variation in fatigue crack growth due to the geometrical and loading effects
2009
The problem of crack growth is a major issue in the prediction and maintenance of aerospace structures, as well as other structural elements in mechanical engineering. Fatigue crack growth as consequence of service loads depends on many different contributing factors. Due to the number and complexity of the mechanisms involved in the fatigue crack growth problem, no universal solution exists yet and there is no general agreement among researchers for any of the available models. Most of the results reported are dealing with geometry with some factors separately. This paper simulates the factors affecting the fatigue crack growth of metallic materials under cyclic loading. For the simulation purpose, three points bend (TPB) with span to width ratio 8:1 and compact tension (CT) specimen geometries were used. There are many factors affecting the fatigue crack growth in structures, such as initial crack length, stress ratio, aspect ratio and type of geometry. The behavior of such cases is shown using Forman model. The fatigue crack growth obtained from the two geometries was compared. Different values of these factors showed different effects on the fatigue crack growth. For further study need to validate the modelling procedure with experimental work as well as take into account the other factors such as; other types of geometries with fatigue crack models and environmental effects towards a universal solution.
Model for fatigue crack growth analysis
Procedia Structural Integrity, 2020
The application of damage tolerance to the design of components is based on the ability to predict fatigue crack growth (FCG) rate precisely. A literature review about analytical models showed a great number of models developed for specific materials and loading conditions. A numerical analysis of a CT specimen made of 304L stainless steel showed the complex influence of material parameters on FCG, which also depends on loading parameters, geometry and environmental conditions. Therefore an alternative to analytical models is proposed here, based on plastic CTOD, assuming that this is the crack driving force. A material law must be first obtained relating da/dN with plastic CTOD range, p, obtained numerically using the finite element method or experimentally using DIC. This law changes with material and includes all material parameters and also environmental conditions (temperature and atmosphere). The design of a specific cracked component is made using numerical tools in order to obtain p for different crack lengths. This second analysis includes the effect of geometrical and loading parameters.
Calculation of fatigue growth of internal cracks
Nuclear Engineering and Design, 1977
A method for analysis of internal cracks using finite elements is presented. The analysis is based on the potential energy release rate which is determined as a function of the crack shape. For an elliptic crack subjected to a cyclical loading the crack-growth is found by integrating the energy release rates associated with incremental extensions of the half axes. As examples, internal elliptical cracks located centricaUy and eccentrically in a thick plate are analyzed. The plate is modelled by 20-node isopara.metric solid elements. A condensation of degrees of freedom is performed so that only the freedoms necessary for defining the geometry of the growing crack are retained. Elliptic shapes with only one axis of symmetry are allowed to develop. The potential energy is calculated for different crack shapes and a least square smoothing technique is used for finding the energy release rates associated with growth of the half axes of the ellipse. The results obtained for a centric crack are compared with analytical expressions for a through centercrack and for an elliptic crack in an infinite body under uniaxial tension. In general good agreement between the different methods is observed. The energy release rates are integrated according to Paris's crack growth formula, and the geometry of the growing crack is visualized by plots of the crack periphery for prescribed numbers of load cycles.
Numerical Models for Fatigue Crack Evolution Study
Fatigue of Aircraft Structures, 2009
Numerical Models for Fatigue Crack Evolution StudyThe paper presents some considerations regarding to the numerical simulation of the behaviour of the riveted structures in fatigue loading conditions. In order to estimate the stress intensity factor, "k", different constitutive laws for the materials were considered. Choosing different contours for "J" integral calculation, some simplified models were studied. The final numerical results were analysed with respect to the physical tests.
Fatigue crack growth predictions based on damage accumulation calculations ahead of the crack tip
Computational Materials Science, 2009
Fatigue Crack growth modeling Critical damage model Strain-life method Hutchinson-Rice-Rosengren field Crack growth algorithm a b s t r a c t Models are proposed to predict the fatigue crack growth (FCG) process using crack initiation properties and critical damage concepts. The crack is modeled as a sharp notch with a very small but finite tip radius to remove its singularity, using a strain concentration rule. In this way, the damage caused by each load cycle and the effects of residual stresses can be calculated at each element ahead of the crack tip using the hysteresis loops caused by the loading, without the need for adjustable parameters. A computational algorithm is introduced to calculate cycle-by-cycle crack growth using the proposed methodology. A quite good agreement between the eN-based crack growth predictions and experiments is obtained both for constant and for variable amplitude load histories.
Crack Growth Model For Estimating The Fatigue Life Under Variable Loading
2011
This paper examines the fatigue short and long cracks behaviour in 2024 T 4 aluminum alloy under rotating bending loading and stress ratio R = -1. In the short cracks region, cracks grow initially at a fast rate but deceleration occurs quickly and, depending on the stress level, they either arrest or are temporarily halted at a critical length. This critical length is shown to conincide with the value of the microstructure parameter, grain size diameter, An empirical model which describes short and long cracks rates is developed and is seen in good agreement with the experimental observations in this alloy. Comparison of the empirical model lives results with cumulative fatigue results has shown encouraging experimentally agreement while liner rule gave a non – reasonable prediction.
A note on fatigue crack growth predictions based on damage accumulation ahead of the crack tip
Models are proposed to predict the fatigue crack growth (FCG) process using crack initiation properties and critical damage concepts. The crack is modelled as a sharp notch with a very small but finite tip radius to remove its singularity, using a strain concentration rule. In this way, the damage caused by each load cycle and the effects of residual stresses can be calculated at each element ahead of the crack tip using the correct hysteresis loops caused by the loading, without the need for adjustable parameters. A quite good agreement between the εN-based crack growth predictions and experiments is obtained both for constant and for variable amplitude load histories.
Materials Science and Engineering: A, 2004
Discrepancies in fatigue crack growth rate and threshold values observed in different specimen geometries are analyzed and discussed. To explain the discrepancies, a phenomenological approach is suggested going out from the assumption of linear elastic fracture mechanics. To this aim, two-parameter constraint-based fracture mechanics is used and the different levels of constraint in the vicinity of the fatigue crack tip are characterized by means of the T-stress. The results of the theoretical analyses correspond to the presented experimental data. It is concluded that under small scale yielding conditions (corresponding to high cycle loading) low level of the constraint (corresponding to negative values of the T-stress) substantially increases the rate of the fatigue crack propagation. The results presented make it possible to relate the experimentally measured data obtained on the specimens with different geometries and thereby contribute to more reliable estimates of the residual fatigue life of structures.
On the theoretical modeling of fatigue crack growth
Although fatigue is by far the most common mode of failure of structural materials, mech-anistic understanding is still lacking. For example, the fundamental Paris law which relates the crack growth rate to stress-intensity factor range is still phenomenological and no reliable mechanistic model has been established for a given material or class of materials despite numerous investigations over a half a century. This work is an attempt to theoretically model fatigue crack propagation induced by alternating crack-tip plastic blunting and re-sharpening in the mid-range of growth rates on the basis of inputs from experiments that measure macroscopic material behavior, e.g ., response to uniaxial cycling loading. In particular, we attempt to predict Paris law behavior by accounting for the material consti-tutive behavior in response to cyclic loading by modeling crack advance solely in terms of the underlying plastic dissipation. We obtain the steady-state condition for crack growth based on plastic dissipation, numerically using finite element analysis, which involves a methodology to address plastic closure upon unloading. For a given stress-intensity range, we calculate the crack propagation rate from the steady-state condition through each cycle of loading and unloading of a cracked compact-tension specimen, without resorting to any specific criterion for crack advance. Published by Elsevier Ltd.
Fatigue & Fracture of Engineering Materials & Structures, 2012
It is a difficult task to predict fatigue crack growth in engineering structures, because they are mostly subjected to variable amplitude loading histories in service. Many prediction models have been proposed, but no agreed model on fatigue life prediction adequately considering loading sequence effects exists. In our previous research, an improved crack growth rate model has been proposed under constant amplitude loading and its good applicability has been demonstrated in comparison with various experimental data. In this paper, the applicability of the improved crack growth rate model will be extended to variable amplitude loading by modifying crack closure level based on the concept of partial crack closure due to crack-tip plasticity. It is assumed in this model that the crack closure level can instantly go to the peak/valley due to a larger compression/tensile plastic zone resulted from the overload/underload effect, and gradually recovers to the level of constant amplitude loading with crack propagation. To denote the variation in the affected zone of overload/underload, a modified coefficient based on Wheeler model is introduced. The improved crack growth rate model can explain the phenomena of the retardation due to overload and the tiny acceleration due to underload, even the minor retardation due to overload followed by underload. The quantitative analysis will be executed to show the capability of the model, and the comparison between the prediction results and the experimental data under different types of loading history will be used to validate the model. The good agreement indicates that the proposed model is able to explain the load interaction effect under variable amplitude loading. Keywords crack closure; fatigue crack growth; load interaction effect; improved crack growth rate model; variable amplitude loading.