Effect of specimen thickness on fatigue crack growth behaviour under constant and variable amplitude loading (original) (raw)
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On the Specimen Thickness Effect on Fatigue Crack Growth
tecgraf.puc-rio.br
Crack closure is the most used mechanism to model thickness and load interaction effects on fatigue crack propagation. Based on it, the expected fatigue life of "thin" (plane-stress dominated) structures can be much higher than the life of "thick" (plane-strain dominated) ones, when both work under the same stress intensity range and load ratio. Therefore, if da/dN curves are measured under plane-stress conditions without considering crack closure, their use to predict the fatigue life of components working under plane-strain could lead to highly non-conservative errors. To avoid this error, it would be necessary to convert the measured crack growth constants associated with a given stress condition to the other using appropriate crack closure functions. However, crack closure cannot be used to explain some retardation effects after overloads on planestrain fatigue crack growth. In this work, experimental evidence show that ∆ ∆ ∆ ∆K eff does not control the crack growth rate of some representative fatigue tests. These results indicate that the dominant role of crack closure in the modeling of the fatigue crack growth problem should be reviewed.
Effect of Single Overload Ratio and Stress Ratio on Fatigue Crack Growth
World Academy of Science, Engineering and Technology, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering, 2013
In this investigation variation of cyclic loading effect on fatigue crack growth is the studied. This study is performed on 2024 T351 and 7050-T74 aluminum alloys, used in aeronautical structures. The propagation model used in this study is NASGRO model. In constant amplitude loading (CA), effect of stress ratio has been investigated. Fatigue life and fatigue crack growth rate were affected by this factor. Results showed an increasing in fatigue crack growth rates (FCGRs) with increasing stress ratio. Variable amplitude loading (VAL) can take many forms i.e. with a single overload, overload band... etc. The shape of these loads affects strongly the fracture life and FCGRs. The application of a single overload (ORL) decrease the FCGR and increase the delay crack length caused by the formation of a larger plastic zone compared to the plastic zone due without VAL. The fatigue behavior of the both material under single overload has been compared. Keywords—Fatigue crack growth, overload ...
Effect of specimen thickness on fatigue crack growth rate
Nuclear Engineering and Design, 2000
Fatigue tests were performed on the compact tension (CT) specimens of Type 304 stainless steel and Inconel 718. To investigate the effects of specimen thickness on crack tip deformation and fatigue crack growth rate (FCGR), specimens of different thickness were used. In the analysis, the elastic plastic fracture mechanics (EPFM) parameter known as the cyclic J-integral, DJ was adopted to observe the local plasticity at the crack tip and compared with the linear elastic fracture mechanics (LEFM) parameter known as the stress intensity factor range, DK. The results show that FCGR is a function of specimen thickness, the effect of which is accelerated as specimen thickness increases. Therefore, it is thought that not only applied stress level but also specimen thickness should be taken into account in the measurement of FCGR, which is not considered in ASTM E 647 (ASTM E 647, 1995. Standard test method for measurement of fatigue crack growth rates.).
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.
In this study, fatigue crack growth rate in mixed-mode overload (modes I and II) induced retardation zone has been predicted by using an ''Exponential model". The important parameter of this model is the specific growth rate. This has been correlated with various crack driving parameters such as stress intensity factor range, maximum stress intensity factor, equivalent stress intensity factor, and mode mixity, as well as material properties such as modulus of elasticity and yield stress. An equation has been formulated for specific growth rate which has been used to calculate crack growth rate under mixed-mode loading conditions. It has been observed that the crack growth rate predicted by the model is in good agreement with experimental results.
Engineering Fracture Mechanics, 1989
experimental and numerical study has been made of the mechanisms of fatigue crack growth and crack-closure behavior in an a//? titanium alloy Ti4A14Mo-2SnO.SSi (IMI 550), following both single and block tensile overloads. Closure immediately behind the crack front (near-tip closure) was found to be the main factor controlling load-interaction effects. Single tensile overloads were found to remove near-tip closure, and slightly reduce far-field closure along the crack length, resulting in an initial acceleration in fatigue crack growth rates. Subsequent delayed retardation of crack growth rates was accompanied by an increase in the near-rip closure load, due to the enlarged compressive residual stress in the overload plastic zone. High/low block overloads caused greater retardation than single overloads of the same magnitude, and this was attributed to changes in the degree of closure in the wake of the crack. Numerical predictions of such transient behavior, based on a modified Dugdale model, are found to be in close agreement with experimental results, both in terms of observed crack growth rates and crack opening displacements. Load-interaction effects were found to be most severe when the baseline stress intensity range (AK) was close to the fatigue threshold (AKrn), or, when the overload maximum stress intensity (K,,) approached the fracture toughness of the material. At low AK levels, the magnitude of the delay was sensitive to microstructure and found to be enhanced in coarse-grained P-heat-treated microstructures compared to standard fine-grained a //I microstructures. Based on these results, mechanistic sequences are suggested to explain the transient fatigue crack growth behavior following single and block tensile overload cycles.
Effect of the amplitude loading on fatigue crack growth
Procedia Engineering, 2010
The paper presents an experimental work of fatigue crack growth for aerospace 2024 T351 aluminum alloy under constant amplitude loading. At 10 Hz frequency, the effect of various amplitudes loading is examined for bending V-Charpy specimen. The results of the fatigue tests conducted at R=0.1, show the effect of the amplitude of loading on the fatigue life and fatigue crack growth rate. The fractography examinations were carried out on scanning electron microscopic SEM and show the presence of fatigue striations in Paris region.
On the fatigue crack growth prediction under variable amplitude loading
Computational and experimental analysis of damaged materials 2007, 2007
During the last decades, numerous papers have been published on fatigue life and fatigue crack growth prediction under variable amplitude loading. The fatigue crack growth prediction models are fracture mechanics based models that have been developed to support the damage tolerance concepts in metallic structures.
International Journal of Fatigue, 2007
A unified two-parameter fatigue crack growth driving force model was developed to account for the residual stress and subsequently the stress ratio effect on fatigue crack growth. It was found that the driving force should be expressed as a combination of the maximum stress intensity factor, K max , and the stress intensity range, DK, corrected for the presence of the residual stress. As a result, the effects of residual stresses manifest themselves in changes of the applied maximum stress intensity factor and the applied stress intensity range. A two-parameter function of the maximum total stress intensity factor, K max,tot , and the total stress intensity range, DK tot , was proposed to model the fatigue crack growth rate data obtained at various R-ratios. Based on the analysis, the unified two-parameter driving force, Dj ¼ K p max;tot DK ð1ÀpÞ tot , was derived accounting for the mean stress or the stress ratio effect on fatigue crack propagation. It was shown that the two-parameter driving force, Dj ¼ K p max;tot DK 0:5 tot , was capable of correlating fatigue crack growth data obtained under a wide range of load ratios and fatigue crack growth rates spanning from the near threshold to the high growth rate regime. The model was successfully verified using a wide range of fatigue crack growth data obtained for Al 2024-T351 aluminium alloy, St-4340 steel alloy and Ti-6Al-4V titanium alloy with load ratios, R, ranging from À1 to 0.7.
A new fatigue domain diagram, recently introduced by one of the authors, makes it possible to demonstrate the fatigue behavior of specimens under varying stress amplitude loading both qualitatively and quantitatively. The diagram is briefly reviewed and crack propagation and damage summation of steel specimens under two level and multi level tension-compression loading are simulated and discussed. Typical patterns of low-high and high-low sequence levels are explained, predicted and, with careful classification, shown to follow certain cumulative damage trends. Correlation with experimental results is shown and discussed. The main conclusion is that one can show repeatable trends in H-L and L-H two-step and multi-step loading sequences, only for cases where the local material properties are not drastically changed, and the failure pattern is similar (critical crack propagation or gross yielding) in all stages of the tests. Damage accumulation, expressed as additional crack length, is clearly shown on the general fatigue diagram. NOMENCLATURE a = crack length da/dN = crack propagation rate (da/dN),, (da/dN), = crack propagation rate in the short cracks and LEFM regimes C, rn = material constants E = modulus of elasticity F,, F, = parametric functions K = stress intensity factor (SIF) n = number of stress reversals Ni = number of stress reversals till separation R = stress ratio ( = umin/umax) S, = endurance limit S, = yield strength S, = ultimate tensile strength K,, = plane strain fracture toughness K, , , K,, = maximum and minimum SIF AK, = threshold SIF range AKen, AK,,, = effective and effective threshold SIF range ua, a , = stress amplitude and mean stress a = constant