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The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1. [ABSTRACT FROM AUTHOR]

Austenitic stainless steels are candidate materials for many engineering applications owing to their excellent strength, ductility, toughness even at low temperatures and non-magnetic nature. In heat-transfer pipelines of heavy water reactors of nuclear power plants, underground pipelines etc. the steel is potentially used, where components frequently encounter cyclic loading. However, during prolonged service at high temperature and with aggressive environments the steel suffers from grain boundary corrosion known as sensitization. Further, sensitization in conjugation with asymmetric cyclic loading makes the situation more complex. In such cases usually, premature failure takes place. This investigation is intended to study the effect of sensitization on the asymmetric cyclic loading or ratcheting behaviour of a non-conventional austenitic stainless steel (X12CrMnNiN17-7-5). Relationships between sensitization treatments for different durations with microstructural and tensile property variation were established. Further, ratcheting experiments were carried out at different combinations of mean stress and stress amplitudes. The results indicated that accumulation of ratcheting strain increased with stress amplitude. However, it was inversely proportional to the degree of sensitization (DoS). The increase in brittleness of the material with sensitization led to decreased accumulation of ratcheting strain. The analyses of the obtained results may help the design and safety aspects of structural components made up of similar steels. [ABSTRACT FROM AUTHOR]

As part of a project within the Swedish nuclear industry with the objective to investigate the performance of different constitutive models in ratchet simulation, an extensive experimental program has been conducted on pressurized tube specimens. In total 30 test specimens made of two different materials, 316L and P235, have been manufactured and tested. In order to determine material properties, monotonic tensile load and internal pressure experiments have been performed. The remaining test specimens have been used for ratcheting experiments. The experimental results show ratcheting in the hoop direction when the tube is subjected to certain combinations of internal pressure and cyclic axial strains. The higher the pressure is and the larger the strain ranges are, the higher the ratcheting response becomes. Measured ratcheting strains are compared to numerical simulations using different constitutive models. In this paper the interrelated models of Prager, Armstrong-Frederick and Chaboche are investigated. In addition to these, the Besseling model is investigated. Based on the result from this investigation, recommendations on how to conduct ratcheting simulation of pressurized equipment subjected to cyclic secondary loading are presented. [ABSTRACT FROM AUTHOR]

In a national project, with the aim to develop a robust method for structural verification of pressure equipment subjected to ratcheting, a two-rod test approach is developed. Two testing machines, with one specimen in each, are controlled in such way that a constant primary load and a cyclic secondary load are applied on the specimens. The ferritic steel P265 and the austenitic steel 316L are tested for a number of load combinations. Test results show that ratcheting is produced in all tests. As expected, the amount of ratcheting increases with the load level. At the start of a two-rod test, structural ratcheting dominates over material ratcheting. Depending on the load level and combination of primary load and cyclic secondary load, material ratcheting might eventually become more important. Based on results from tensile testing and fully-reversed strain controlled cycling, five different constitutive models are calibrated. The possibility to simulate the response of the conducted experiments with the different models is investigated. The most advanced model investigated, i.e. the nonlinear kinematic Chaboche model, does not necessarily give the best predictions. Up to a strain level of 5%, i.e. the criterion in ASME NB-3228.4, the bi-linear model best predicts the experiments. Recommendations for how ratcheting in structures subjected to cyclic plastic deformation can be predicted by numerical simulation are given. [ABSTRACT FROM AUTHOR]

The paper shows results of experiments focused on the influence of loading frequency on ratcheting as well as interesting results obtained from uniaxial tests and biaxial tests with rectangular and circular loading paths under variable amplitude loading realized on the ST 52 steel at the VŠB-Technical University of Ostrava. The model based on the AbdelKarim-Ohno kinematic hardening rule and Calloch isotropic hardening rule was used for stress-strain behavior description in finite element analysis using commercial software Ansys. The material model was implemented into the FE code using a fortran subroutine. Results of simulations containing ratcheting and cyclic hardening effects correspond with experiments very well. [ABSTRACT FROM AUTHOR]

CSIR - Structural Engineering Research Centre (CSIR-SERC), Chennai, has state-of-the-art facilities and expertise for carrying out R&D studies in the areas related to fatigue and fracture behaviour of structural components and materials. In the last 25 years, the laboratory has efficiently utilised the facilities and expertise available in serving various institutions and industries related to energy sector, automobile applications, railways, aerospace applications, construction industry etc. Fatigue and fracture studies have been carried out on steel tubular joints of offshore jacket platforms, carbon steel and stainless steel piping components used in power plants, automobile components (connecting rods, air springs, air suspension systems, industrial and automobile chains, stabiliser legs, hub brackets, sub-frames, rubber bushes, parallel link brackets etc.), tyre curing press, alumino-thermit and flash-butt welded rail joints used in Indian Railways, reinforcing bars (thermo-mechanically treated bars, corrosion resistant steel etc.), and many other components and materials. This paper describes some of the important investigations carried out in the Fatigue and Fracture Laboratory (FFL) of CSIR-SERC and presents some salient test results. [ABSTRACT FROM AUTHOR]

Abstract: The present investigation has been carried out to understand the effect of mean stress on room temperature ratcheting behaviour of solution annealed AISI 304 stainless steel at constant stress amplitude. It is found that the ratcheting strain to failure linearly increases with increase of mean stress and there is no significant difference in the accumulated ratcheting strain with change in solution annealing temperature vis-a-vis grain size. While the number of cycles to complete failure increases exponentially with mean stress irrespective of solution annealing temperature, the ratcheting life is found to increase when soltionin annealing is done at lower temperature. This variation is believed to be associated with the tensile yield strength of the specimens solutionised at two different temperatures. [Copyright &y& Elsevier]

Abstract: The effect of monotonic and cyclic deformation on the evolution of back stress vis-à-vis Bauschinger effect due to different amounts of unidirectional plastic deformation in annealed OFHC copper has been investigated. It is observed that for the same amount of prestrain the back stress evolution and the corresponding Bauschinger phenomenon depend upon the mode of deformation. It is found that higher back stress develops in case of monotonic deformation as compared to ratcheting deformation for same amount of unidirectional prestrain. It is also found that the effect of stress rate on the change of Bauschinger parameters is comparatively less than the effect of increasing prestrain in both types of deformation. [Copyright &y& Elsevier]

Abstract: With an objective of understanding the cyclic fracture, fatigue crack growth and ratcheting behaviour of Type 304LN stainless steel piping components used in primary heat transport system of nuclear power plants, subjected to various operational, loading and environmental conditions, experimental studies were carried out on totally 36 straight pipes and eight elbows. The pipe dimensions were in two categories: 170mm OD and 15mm thickness or 324mm OD and 25mm thickness. These studies on actual size piping components of power plants have resulted in very valuable fatigue behaviour related data, which would form important inputs in assuring safety and damage tolerant design of power plants. An overview of the studies is presented in this paper. [Copyright &y& Elsevier]

Abstract: Ratcheting experiments were carried out on 316LN stainless steel as a function of temperature ranging from 523 -923K for different mean stress (σ m )-stress amplitude (σa) combinations. The effect of temperature on ratcheting strain accumulation as well as cyclic life is found to be anomalous due to occurrence of Dynamic Strain Aging (DSA). While the peak DSA temperature is at 823K for all mean stress-stress amplitude combinations, the temperature regime in which DSA operates varied for different σm - σa combinations. [Copyright &y& Elsevier]

Abstract: The integrity assessment of the primary piping components needs to be demonstrated under normal operation cyclic loadings as well as under complex cyclic loadings of extreme magnitude as may come during a severe earthquake event. In order to understand material's cyclic plasticity and fatigue-ratcheting behaviour, systematic experimental and analytical investigations have been carried out on specimens of SA333Gr.6 carbon steel and SS304LN stainless steel. The material specification of SA333Gr.6 is same as used in Primary Heat Transport (PHT) piping of Pressurized Heavy Water Reactors (PHWRs) and material specification of SS304LN steel is same as proposed for Indian Advance Heavy Water Reactor's (AHWR's) Main Heat Transport (MHT) piping. The test program included the tensile, axial fatigue and uniaxial ratcheting tests to establish the material's mechanical properties and cyclic plasticity behavior. The results of these tests have been investigated in details using few popular finite element cyclic plasticity models to understand and quantify the material's cyclic plasticity behavior. The studies revealed the need to modify the Chaboche model to simulate the LCF/cyclic plasticity and ratcheting under different strain/stress amplitudes loading conditions. On accounting for modification, the Chaboche model nicely predicted the LCF and ratcheting response for all the tests. The tests, finite element analyses result and their interpretations have been presented in this paper. [Copyright &y& Elsevier]

Abstract: ‘Fatigue Ratcheting’ is a phenomenon which leads to reduction in fatigue life of a structural component by loss of ductility due to cycle-by-cycle accumulation of plastic strain. One real-life example of a situation with fatigue ratcheting possibility is piping of power plants subjected to internal pressure (primary load) and cyclic bending (secondary load). Fatigue ratcheting studies were carried out on TP304 LN stainless steel straight pipes of 168 mm outer diameter subjected to steady internal pressure and four point cyclic bending. The length and average thickness of the pipes were 2800 mm and 15 mm respectively. The thickness was reduced to 12 mm in the gauge length portion of 200 mm at the centre of the pipe. Post-yield two element rosette strain gauges were mounted at various locations within the gauge length to measure the longitudinal and circumferential strains. The pipes were filled with water and pressurized; the pressure was maintained at 35 MPa till the first through-thickness crack was observed. Cyclic bending load was applied on the pipes by subjecting them to different levels of load-line displacements. Number of cycles corresponding to through-thickness crack/s and final failure of the component were recorded. During the tests, load, load-line displacement and deflections at three locations were continuously monitored. Local bulging, ovalization and consequent thinning of the pipe cross-section were observed due to fatigue ratcheting. These ratcheting studies on pressurized piping components of power plants give valuable inputs necessary for designing the components and assuring the integrity of pressure boundary under design basis loads such as loads arising during an earthquake event. [Copyright &y& Elsevier]

Abstract: Two carbon black filled SBRs were submitted to various uniaxial tension cyclic tests, including stretch-control tests and stresscontrol tests. Stress relaxation and stretch creep were measured according to the number of cycles. Results show a strong dependence of the material softening to the maximum loading applied and the number of cycles. After a few cycles (around 40), the softening evolves linearly according to the logarithm of the number of cycles. The cyclic softening is also shown to be locked by a low number (5) of overload cycles. More interestingly, the material softening appears to be unaffected by the loading cycle amplitude. This study aims at defining the parameters to include in a subsequent cyclic stress-strain model. [Copyright &y& Elsevier]