Advances on creep-fatigue damage assessment in notched components (original) (raw)
Related papers
Creep behavior of V-notched components.PDF
Geometrical discontinues such as notches play a significant rule in structural integrity of the components, especially when the component is subjected to very severe conditions, such as the high temperature fatigue or creep. In this paper, a generalized form of the existing notch tip creep stressstrain analysis method developed by Nuñez and Glinka, is developed and extended to a wide variety of blunt V-notches. Assuming the generalized Lazzarin-Tovo solution that allows a unified approach to the evaluation of linear elastic stress fields in the vicinity of both cracks and notches is the key in getting the extension to blunt V-notches. Numerous cases have been analysed and the stress fields obtained according to the proposed method were compared with proper finite element data, showing a very good agreement.
Journal of Pressure Vessel Technology, 2019
Structural discontinuities (e.g., nozzle, hole, and groove) widely occur in many high temperature components of nuclear and fossil power plants. In general, the notched component is used for simplified tests and analyses due to the complexity of the introduction of a practical component. In the previous work, the effects of the notch on failure life of the components have been reported experimentally, including the strengthening and weakening effects; however, the internal mechanisms have not been clearly demonstrated. This work reviews the notch effects on the structural strength of the notched components at elevated temperatures under creep, fatigue, and creep-fatigue loading conditions. Experimental phenomena (i.e., strengthening or weakening effects) for typical notched specimens subjected to the above three loading conditions are summarized, and the related factors for notch effects on creep rupture life or cycle to failure of the components are discussed. The mechanisms for th...
Two-parameter Description Of Creep/high-cycle-Fatigue Behaviour Of Notched 9% Cr Steel
WIT transactions on engineering sciences, 1970
A notch effect under creep/high-cycle-fatigue loading conditions was studied both theoretically and experimentally in notched specimens of an advanced 9%Cr steel of the type P91 at 600°C. Experiments were performed under condition of the constant maximum net stress. The stress ratio R, i.e., the cyclic stress component of creep/fatigue loading was varied. The dependence of time to fracture on R exhibits nonmonotonous behaviour with a pronounced maximum coinciding with the fracture mode transition from pure creep to fatigue. The notched specimens show increased rupture strength when compared with smooth bars. The strengthening due to the notch depends on the notch geometry. It is shown that the notch tip stress triaxiality parameter is not sufficient to describe this phenomenon.
Crack initiation life at notch root under the transition of creep condition
Engineering Fracture Mechanics, 1985
Ahshe-Acceleration of creep fracture is posible in a high-strength material such as a superalloy under the transition from the small scale creep (SSC) condition to the large scale creep (LSC) condition. In this study, an analytical method of predicting the creep crack initiation life for a notched body was presented. In order to assess the validity of this method, the crack initiation at a notch root was also experimentally observed on a high-strength Ni-base superalloy through load-controlled creep-fatigue tests at 923 K. As a result, this method was found to be sulliciently applicable to the crack initiation life prediction for a notched body under the transition from SSC to LSC. NOMENCLATURE temperature-dependent factor in power law creep relation Young's modulus elastic stress concentration factor stress concentration factor, K, = CN/am stress exponent in power law creep relation number of cycles to crack initiation (i.e. crack initiation life) normalized crack initiation life, N: = NJ{ZJ(uJE)} predicted crack initiation life stress ratio, R =a&~_ real time loading (or tension-going) period unloading (or compression-going) period stress hold-time equivalent creep strain equivalent creep strain rate equivalent creep strain accumulated at a notch root during each tensile stress hold-time equivalent creep fracture strain stress equivalent stress maximum stress minimum stress equivalent stress at a notch root net section stress during tensile stress hold-time non-dimensional time, r = BEu:; ' t non-dimensional characteristic time nondimensional hold-time
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.
An iterative technique to assess the fatigue strength of notched components
Procedia Structural Integrity, 2020
The present work provides an efficient formulation to assess the growth of short fatigue cracks in metallic components. The proposed technique consists on the iterative combination of a micromechanical short-crack growth model and the Finite Elements Method. The interaction of the crack with the microstructure of the material is evaluated through the dislocations distribution technique. The finite elements analysis of the problem is needed to obtain the stress gradient ahead of the notch. The division of the main problem into simpler scenarios makes the resolution of the method easier since cases with known solutions are required exclusively. The iterative method formulation is properly described and application examples are given in order to show its usefulness.
Fatigue Failure of Notched Specimen—A Strain-Life Approach
Materials Sciences and Applications, 2011
Failure cycles of notched round specimens under strain controlled cyclic loading are predicted using strain-life relations obtained from experiment for plain fatigue round specimens. For notched specimens, maximum strain occurs at notch root and is different from applied controlled strain. The maximum strain is computed by appropriate Finite element analysis using the FE software ABAQUS. FE model and material parameters are validated by comparing the FE results and experimental results of LCF tests of round specimens. This value of maximum strain is used for prediction of failure cycles. Prediction is compared with the experimental results. The results show good matching.
A non-linear model for the fatigue assessment of notched components under fatigue loadings
International Journal of Fatigue, 2016
This paper presents a general theory for the estimations of an entire fatigue curve in ductile materials based on the implicit gradient approach. In order to modify the slope of the Woehler curves, the material was considered non-linear. The average stress of the hysteresis loop was taken into account by means of Walker's model. Subsequently, the implicit gradient method was adopted for the numerical evaluation of the effective stress and strain at low-and medium-cycle fatigue life and was then related to the fatigue strength of the material. The characteristic length, relating to the fatigue behaviour of the material, was considered constant for the fatigue lifetime. In order to confirm the proposed method, new experimental data were obtained, relating to axisymmetric notched specimens loaded with nominal stress ratio R=-1 and R=0. In terms of the effective strain amplitude, evaluated by means of the implicit gradient approach, the different Woehler curves of notched specimens were summarised in a unique fatigue curve as a function of Walker's cycle parameter.
Fatigue & Fracture of Engineering Materials & Structures, 2015
paper presents a damage mechanics method applied successfully to assess fatigue life of notched specimens with plastic deformation at the notch tip. A damage-coupled elasto-plastic constitutive model is employed in which nonlinear kinematic hardening is considered. The accumulated damage is described by a stress-based damage model and a plastic strain-based damage model, which depend on the cyclic stress and accumulated plastic strain, respectively. A three-dimensional finite element implementation of these models is developed to predict the crack initiation life of notched specimens. Two cases, a notched plate under tension-compression loadings and an SAE notched shaft under bending-torsion loadings including non-proportional loadings, are studied and the predicted results are compared with experimental data.
Mode I fatigue crack growth evaluation at notches
An analytical elastic-plastic model describing the fatigue life of components with elliptical notches under constant amplitude loading has been proposed. The calculation occurs by integrating a crack growth law from a starting crack size of micro-structural dimension till up to the total fracture of the component. Plasticity induced crack opening and closure effects are explicitly taken into account. Thereby, calculated opening load levels for cracks growing in notch affected areas have been found out to be in good agreement with corresponding experimental values determined from notched specimens made of two different metallic materials. Furthermore, the comparison of experimentally determined and calculated crack growth curves for specimens with central notches confirm the calculation accuracy of the model.