On the Specimen Thickness Effect on Fatigue Crack Growth (original) (raw)
Related papers
On the dominant role of crack closure on fatigue crack growth modeling
International Journal of Fatigue, 2003
Crack closure is the most used mechanism to model thickness and load interaction effects on fatigue crack propagation. But assuming it is the only mechanism is equivalent to suppose that the rate of fatigue crack growth da/dN is primarily dependent on ⌬K eff = K max ϪK op , not on ⌬K. But this assumption would imply that the normal practice of using da/dN×⌬K curves measured under plane-stress conditions (without considering crack closure) to predict the fatigue life of components working under planestrain could lead to highly non-conservative errors, because the expected fatigue life of "thin" (plane-stress dominated) structures could be much higher than the life of "thick" (plane-strain dominated) ones, when both work under the same stress intensity range and load ratio. However, crack closure cannot be used to explain the overload-induced retardation effects found in this work under plane-strain, where both crack arrest and delays were associated to an increase in ⌬K eff . These results indicate that the dominant role of crack closure in the modeling of fatigue crack growth should be reviewed.
Mode I fatigue crack growth behaviour was experimentally investigated under constant and following the application of a single spike overload. Different specimen thicknesses were investigated at different overload ratios. A previously developed crack tip deformation model to correlate fatigue crack growth behaviour following k • a single overload cycle was used to correlate that behaviour for different specimen thicknesses. It was found that the effect of specimen thickness on the fatigue crack growth behaviour under constant amplitude of loading is marginal especially at growth rates greater than le mm/cycle. The retardation due to the application of a tensile single overload decreases with the increase in specimen thickness. Good agreement between relevant experimental results and those predicted using three dimensional crack tip deformation parameter was found.
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.
A strip model for fatigue crack growth predictions under general load conditions
Engineering Fracture Mechanics, 1991
Abstrac-A strip crack closure model, based on the Dugdal~Barenblatt model but mod&d to leave plastically deformed material in the wake of the crack tip, was investigated for various aspects of fatigue crack growth behaviow. A constraint factor was introduced into the model to account for the 3D effect at the crack tip to extend the applicable range of the original Dugdale-Barenblatt model to the plane strain condition. Comparisons with experimental data show that the constraint factor in the strip model results in a fairly good prediction for the fatigue crack closure under the plane strain condition. A variable constraint factor makes it possible for the model to account for the gradual change of stress state from plane strain to plane stress when the crack grows large. The change of stress state occurs typically for the fatigue crack in plates where the stress state at the crack tip is mainly determined by the relative ratio for the crack tip plastic xone sixe and the plate thickness. The change of stress state has a significant influence on fatigue crack growth behaviour. Weight functions were combined in the modified Dugdale model because crack surface displacement solutions can be derived and the stress intensity factors for complex stress fields can be obtained from corresponding weight functions. It has been shown that a great many problems cau be solved when approximate weight function techniques are used. Comparisons were made between predictions from the model and experimental data available in the literature. There was very good agreement for the fatigue crack growth under constant amplitude loading with different stress ratios (R-ratios), for the load interaction effects in both plane stress and plane strain conditions, for the fatigue crack growth in plates under specnum loading, for the fatigue crack growth in residual stress fields, and for the small crack growth behaviour.
A study of the effect of mechanical variables on fatigue crack closure and propagation
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.
Fatigue crack surface area and crack front length: new ways to look at fatigue crack growth
MATEC web of conferences, 2018
This paper discusses the appropriateness of crack length as a reference dimension for fatigue damage. Current discussion on short crack versus long crack data is still divided between various approaches to model small crack growth. A proper physical explanation of the probable cause of the apparent differences between short crack and long crack data is not yet provided. Long crack data often comprises crack growth in constant thickness specimens, with a through crack of near constant crack front geometry. This is not true for corner cracks or elliptical surface crack geometries in the small crack regime where the crack front geometry is not symmetric or through-thickness. This affects similitude parameters that are based on the crack length. The hypothesis in this paper is that a comparison between long crack data and short crack data should be made using similar increments in crack surface area. The work applied to the specimen is dissipated in generation of fracture surface, whereas fracture length is a result. The crack surface area approach includes the two-dimensional effect of crack growth geometry in the small crack regime. A corner crack and a through crack are shown to follow the same power law relationship when using the crack area as base parameter. The crack front length is not constant, and its power law behaviour for a corner crack is shown.
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.).
F2004/26 Quantitative Evaluation of Fatigue Crack Growth Retardation Due
2014
Fatigue crack kinking and bifurcation are phenomena capable of inducing significant growth retardation or even crack arrest. However, bifurcated crack models available in the literature cannot account for the subsequent propagation behavior observed in practice. In this work, specialized Finite Element (FE) and life assessment software are used to predict the reduction in the propagation rates in kinked and bifurcated cracks. The crack path and associated stress intensity factors (SIF) of bifurcated cracks are numerically obtained for several bifurcation angles and branch lengths. From these results, empirical crack retardation equations are proposed to model the retardation factor along the crack path, allowing for a better understanding of the influence of crack deflection in the propagation life.
International Journal of Fatigue, 2011
During overloads in variable amplitude fatigue, local stresses at small cracks growing from notches reach yield stress magnitude. Such high stress levels result in a large decrease in crack opening stress and an increase in the fatigue damage of subsequent smaller stress cycles. This paper presents a methodology for modeling changes in crack opening stress level and fatigue damage using data derived from periodic underload fatigue tests of smooth specimens. Predicted crack closure stress levels agree well with those obtained from crack growth observations made with a high magnification microscope.