Developing fatigue crack growth data using the round compact tension specimen (original) (raw)
Related papers
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.
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.).
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.
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.
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.
Fatigue & Fracture of Engineering Materials & Structures 3.pdf
A B S T R A C T In this paper, results from the linear normalization (LN ) technique of Reese and Schwalbe for deriving J-crack resistance ( J±R) curves have been compared, related to J±Da ( J-integral±ductile crack growth) data points, to those obtained from traditional elastic compliance technique. Research results regarding a nuclear grade steel exhibiting a wide range of elastic±plastic fracture resistance agree quite well for both techniques until a certain level of toughness of the material. Below this critical level, LN produces inconsistent results for the sub-sized compact tension specimens (0.4T C[T]). The evidence suggests that the loss of applicability of the LN technique can be determined on the basis of the Z plastic factor (Z pl ) for the best linear correlation achieved for DP N ±Da (normalised load gradient±ductile crack growth) data.
A double edge notch specimen design for tension–compression fatigue crack growth testing
Engineering Fracture Mechanics, 2012
Compact, C(T), specimens are commonly used for fatigue and creep-fatigue crack growth testing under constant-load-amplitude conditions. The use of the standard C(T) specimens [ASTM] is limited to positive load ratios. They are also limited in the amount of crack growth data that can be developed at high stress intensity values due to accumulation of plastic and/or creep strains leading to ratcheting in the specimen. Testing under displacement control can potentially address these shortcomings of the load-controlled tests for which the C(T) geometry is unsuitable. A double edge notch tension-compression, DEN(T-C), specimen to perform displacement controlled creep-fatigue crack growth testing is developed and optimized with the help of finite element and boundary element analyses. The specimen has threaded cylindrical ends for applying the loads. The test section is rectangular with a half width, W, and half height, H, and a constant thickness, B. The height to width ratio, H/W, of the specimen must be kept low to avoid buckling during compressive loading while ensuring that the stress-intensity parameter values are not strongly dependent on the chosen H/W value. Simple optimization analysis shows that a H/W ratio of 1.2 is likely to best meet these requirements. However, only performing actual experiments will confirm whether this choice of H/W value is optimum. In the finite element analyses, the specimen is modeled using an effective height (h eff) to width (W) ratio, h eff /W, of 1.46 to account for the fillet regions on the two ends of the specimens. Accurate expressions for estimating the fracture mechanics crack tip parameters such as the stress intensity parameter, K, the crack mouth opening displacement (CMOD), and the load-line displacement (LLD) are developed over a wide range of crack sizes for the DEN(T-C) specimen. Expressions are also developed for crack size as a function of specimen compliance.
3671-T-STRESS and Mode I Fatigue Crack Growth
Fatigue crack growth is dependent on material properties and loading parameters. Critical areas of industrial components are often subjected to complex stresses, either because multiaxial loads are applied or because residual stresses are present in the material. In plane stresses are taken into account through mode I, II and III stress intensity factors. Out of plane stresses are usually not included in fatigue crack growth models. However, experimental evidences of the influence of out-of plane stresses on fatigue crack growth are reported in the literature. This paper is centered on the role of the T-stress during mode I fatigue crack growth. The effect of a T-stress on fatigue crack growth is studied through its effect on crack tip blunting. As a matter of fact, fatigue crack growth is characterized by the presence of striations on the fracture surface, which implies that the crack grows by a mechanism of blunting and re-sharpening. From a mechanical point of view, this means that forward and reverse plasticity occurs within the crack tip plastic zone. The presence of a T-stress modifies significantly the evolution of plastic deformation at crack tip and thus plastic blunting. In the present study, crack tip blunting is a global scalar variable, which is calculated using the finite element method as the average value of the permanent displacement of the crack faces over the whole Kdominance area. Experimental measurements have also been conducted using cross-correlation of images taken at various stress intensity factors, in order to measure the displacement field in the K-dominance area. A yield stress intensity factor is defined for the cracked structure, as the stress intensity factor for which crack tip blunting exceeds a given value. The variation of the yield stress intensity factor as a function of the T-stress was studied. It is found that the T-stress can modify very significantly the yield point of the cracked structure. Secondly it is found that the yield limit in a (T,K I) plane is not-dependent on the crack length, A yield criterion is proposed for the cracked structure. This criterion is an extension of the Von-Mises yield criterion to the problem of the cracked structure. As a matter of fact, the present yield criterion is based on the elastic shear energy within the K-dominance area, as calculated using the Westergaard stress functions. The proposed criterion matches almost perfectly the results stemmed from the FEM. Finally the evolution of the yield limit of the cracked structure in a (T,K I) plane was studied for various loading scheme. These results allow developing a plasticity model at the global scale for the cracked structure taking into account the effect of the T-stress.
Fatigue Crack Growth under Mode I, II and III for Plane-strain and Plane-stress Conditions
Procedia Engineering, 2014
Fatigue crack growth (FCG) could be encountered in many mechanical components, which can be made from either thin or thick steel plates (or shells) and, therefore, be subjected to a plane-stress or a plane-strain condition, respectively. The loads applied in a solid body containing a narrow notch or a sharp crack will induce a yield zone near its tip with a dimension that will depend on the mechanical properties of the material, as well as on the thickness of the body, the crack length and the magnitude of the loads applied. Crack propagation can then occur under mode I, II, III or mixed-mode for general loading. The paper presents J I , J II and J III integral functions, which were correlated with the elastic stress intensity factors K I , K II and K III , for thin and thick CT specimens. The evaluation of J-Integral values was carried out for different crack lengths, along the crack front, and using the Finite Element Method (FEM), with collapsed nodes and midside nodes dislocated to ¼ of the edge's length, in order to simulate the crack tip singularity. Interaction between in-plane, in-plane sliding and out-of-plane modes are also discussed in the paper. In addition, FCG rates under mode I, mode III and a mixed-mode (mode I+III) were experimentally determined, at room temperature, for a high-strength Cr-Mn austenitic stainless steel.