An algorithm for fatigue crack growth applied to mixed and biaxial mode loadings (original) (raw)
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Mixed-mode fatigue crack growth under biaxial loading
International Journal of Fatigue, 1984
Fatigue is still one of the main concerns when dealing with mechanical components failure. While it is fundamental to experimentally determine the fatigue material behavior using standard specimens, testing large and complex component geometries can be complicated. In these cases, the Finite Element Method can be a cost-effective solution but developing fatigue crack growth models is still a complicated task. In order to solve this problem, an algorithm for automatic crack propagation was developed. Using three different modules, the algorithm can generate a complex Finite Element Method model including a fatigue crack; solve this model considering complex loading conditions, by applying the superposition method; and calculate the fatigue crack propagation rate, using it to update the original model. In order to benchmark this solution two different problems were analyzed, a modified compact tension specimen and a cruciform specimen. By modifying the compact tension specimen hole location and simulating an initial crack, it was possible to understand how mixed mode conditions influence the fatigue crack path. Different load ratios and initial crack directions on the cruciform specimen were analyzed. Increasing the load ratio will increase the crack deflecting angle. The obtain solutions were compared with experimental results, showing good agreement. Therefore the developed algorithm can be used to predict the fatigue crack growth behavior on complex geometries and when different types of loads are applied to the component.
Computational Simulation of 3D Fatigue Crack Growth under Mixed-Mode Loading
Applied Sciences, 2021
The purpose of this research was to present a simulation modelling of a crack propagation trajectory in linear elastic material subjected to mixed-mode loadings and investigate the effects of the existence of a hole and geometrical thickness on fatigue crack growth and fatigue life under constant amplitude loading. For various geometry thickness, mixed-mode (I/II) fatigue crack growth studies were carried out to utilize a single edge cracked plate with three holes and compact tension shear specimens with various loading angles. Smart Crack Growth Technology, a new feature in ANSYS, was used in ANSYS Mechanical APDL 19.2 to predict the cracks’ propagation trajectory and their consequent fatigue life associated with evaluating the stress intensity factors. The maximum circumferential stress criterion is implemented as a direction criterion under linear elastic fracture mechanics (LEFM). According to the hole position, the results demonstrate that the fatigue crack grows towards the ho...
Analytical and experimental studies on fatigue crack path under complex multi-axial loading
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2006
In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Fractographical analyses of the failure surface and the crack orientation are helpful for identifying the effects of the non-proportional multi-axial loading. There are many factors that influence fatigue crack paths. This paper studies the effects of multi-axial loading path on the crack path. Two kinds of materials were studied and compared in this paper: AISI 303 stainless steel and 42CrMo4 steel. Experiments were conducted in a biaxial testing machine INSTRON 8800. Six different biaxial loading paths were selected and applied in the tests to observe the effects of multi-axial loading paths on the additional hardening, fatigue life and the crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It was shown that the two materials studied had different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were made using the critical plane approaches such as the Brown-Miller, the Findley, the Wang-Brown, the Fatemi-Socie, the Smith-Watson-Topper and the Liu's criteria. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the critical plane models give satisfactory predictions for the orientations of early crack growth of the 42CrMo4 steel, but less accurate predictions were obtained for the AISI 303 stainless steel. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multi-axial microstructure characteristics.
Fatigue crack propagation trajectories under mixed-mode cyclic loading
Engineering Structures, 2003
As the direction of the mixed-mode fatigue crack growth may not be known in advance, a new methodology is proposed to simulate the fatigue crack trajectories under general in-plane cyclic loading. The accumulated elastic strain energy is calculated in the vicinity of the crack tip for all possible prospective propagation directions. It is assumed that the fatigue crack extension takes place along the prospective direction which results to the minimum accumulated elastic strain energy in the vicinity of the crack tip. A new factor related to the accumulated elastic strain energy within a circular core around the crack tip seems to be a suitable parameter for predicting the mixed-mode fatigue crack growth path, as well as the fatigue crack growth rate. Using the proposed concept to perform the above predictions, only mode-I fatigue test data are required. Theoretical predictions are correlated well with test results taken by the literature.
Fatigue Crack Growth Analysis under Constant Amplitude Loading Using Finite Element Method
Materials
Damage tolerant design relies on accurately predicting the growth rate and path of fatigue cracks under constant and variable amplitude loading. ANSYS Mechanical R19.2 was used to perform a numerical analysis of fatigue crack growth assuming a linear elastic and isotropic material subjected to constant amplitude loading. A novel feature termed Separating Morphing and Adaptive Remeshing Technology (SMART) was used in conjunction with the Unstructured Mesh Method (UMM) to accomplish this goal. For the modified compact tension specimen with a varied pre-crack location, the crack propagation path, stress intensity factors, and fatigue life cycles were predicted for various stress ratio values. The influence of stress ratio on fatigue life cycles and equivalent stress intensity factor was investigated for stress ratios ranging from 0 to 0.8. It was found that fatigue life and von Mises stress distribution are substantially influenced by the stress ratio. The von Mises stress decreased as...
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.
Fatigue crack propagation in complex two-dimensional structural components under constant and variable amplitude loading is numerically predicted and experimentally verified. Cracks are fatigue propagated under constant and variable amplitude loading in standard CT specimens with holes specially positioned to attract or to deflect the crack. Therefore, the cracks do not follow a straight-line path, but curve toward the hole reaching it or not, depending on the hole positioning. A reliable and cost effective two-phase methodology is used in two pieces of software to numerically predict the fatigue crack propagation. First, the fatigue crack path and its stress intensity factor are calculated in a specialized finite-element software, using small crack increments. Numerical methods are used to calculate the crack propagation path, based on the computation of the crack incremental direction, and the stress-intensity factors K I , from the finite element response. Then, an analytical expression is fitted to the calculated K I (a) values, where a is the length along the crack path. This K I (a) expression is used as an input to a powerful general purpose fatigue design software based in the local approach, developed to predict both initiation and propagation fatigue lives under variable amplitude loading, considering interaction effects such as crack retardation or acceleration after overloads.
PREDICTION OF CRACK INITIATION DIRECTION AND FATIGUE CRACK GROWTH UNDER MIXED MODE LOADING
The present investigation has been under taken keeping in mind some of the problems concerning the crack propagation direction and growth under constant amplitude load in an inclined crack geometry. The present studies mainly focus on to develop and modified the crack growth criterion to account of the effect of biaxial factor, shearing loading and number of stress terms for different criterion. The effect of one, two and three solution on crack initiation angle determined on the basis of DET-criterion & MTS-criterion. It is seen that difference in crack initiation angle (α) obtained from two and three term solution of DET-criterion & MTS-criterion is significant and higher than one term solution. The comparisons of predicted result obtain from using DET-criterion and MTS-criterion has been made. When comparison is made between two terms solution, it is found that growth rate is higher in case of DET-criterion than MTS-criterion when α <45 0 , whereas fatigue growth rate obtain by MTS-criterion is found to be the higher than the DET-criterion for α >45 0. It is observed that only three terms are sufficient to determine the stress and displacement field ahead of the crack tip. Variation of crack initiation angle with crack angle for different biaxial load factor and shear load factor by DET-criterion and MTS-criterion for three term solution is compared. The crack initiation angle also depends upon biaxial factor and shear load factor. NOTATIONS: β = crack inclination angle 0 β = crack initiation angle 2a = Initial crack length r and θ = polar coordinate of crack tip II I K K , = mode I and mode II stress intensity factor xy y x τ σ σ , , = stress components b 1 = Biaxial factor b 2 = Shear or torsional load factor INTRODUCTION: Fracture mechanics has been established as an important principle dealing with the growth of fatigue crack. During recent years, the fracture mechanics has obtained a considerable importance for studding the crack growth behavior under static and fatigue loading. The majority of the research done in the area of fracture mechanics was accomplished in the opening mode only. Hence the practical application of the fracture mechanics is limited. The mixed mode crack growth problem has drawn comparably little attention, but is actually more realistic and possibly more dangerous than mode-I crack. The mixed mode growth occurs when a fatigue crack is nucleated along the inclusions or welded defects located making an angle with the axis of the applied load. The fatigue crack growth under multi axial stress is to be considering as mixed mode. The mixed mode cracks generally propagate in a non-self similar manner. Hence, in case of mixed mode loading condition, the study of the crack initiation angle and crack growth rate is of equal importance. A considerable amount of the work has been done to define governing mixed mode loading and crack growth behavior. Papadopoulos (1987) proposed the DET-criterion is based on determinants of the stress tensor. Using the stress as parameter, MTS-criterion proposed by Erdigan & Sih (1963) is based on the
Verification of FE model of fatigue crack propagation under mixed mode conditions
Meccanica, 2008
Results of FEM simulations of fatigue crack growth under mixed mode loading are presented in this paper. At Czech Technical University in Prague (CTU) the program code which uses commercial software ABAQUS for fatigue crack simulations has been developed. The program works on basis of gradual crack extending. In simulation are considered small scale yielding conditions. Both classic criteria (J-integral, maximum of hoop stress, Paris law) and two-parameter criteria incorporating T-stress (extended hoop stress criterion and modified Paris law) are used for crack behaviour description. Experiments were carried out on two sets of simple specimens. The results of numerical simulations are compared with carried out experiments. Computed crack growth rate corresponds to experiments for shorter cracks at which T-stress is not significant. Computed crack shapes are in very good agreement with experiments.