Xaver Schuler

Fracture mechanics evaluation of cracked components with consideration of multiaxiality of stress state

Abstract The fracture-mechanics evaluation of cracked components is an essential part of safety analyses. This evaluation is usually based on one-parametric evaluation procedures without taking into account the multiaxiality of the stress state. Considering the multiaxiality of the stress state across the flawed component cross-section, it is possible to recognize and extend the limits of application of fracture mechanics, which among others, are given by the limited transferability of fracture-mechanics material laws. Within the scope of the research project “Phenomenological Vessel Burst Tests-Phase IV”, T-branches and elbows with dimensions like the primary coolant lines of PWR plants were investigated. In addition to the experimental investigations, extensive numerical calculations were performed by means of the finite element method (FEM). To determine the stress and the gradient of multiaxiality across the ligament of the component, 3-D finite-element analyses were carried out concerning elastic-plastic material behaviour. The evaluation with regard to crack initiation has been proven by experimental results as well as the qualitative assessment of the fracture behaviour on the basis of the multiaxiality analyses.

Application of Advanced Fatigue Damage Parameters in Comparison With Fatigue Analysis Included in Codes and Standards

Technical components are subjected to cyclic loading conditions that can be arbitrarily complex in the most general case. For analytical fatigue strength verifications in the finite life regime both the uniaxial material characteristics by means of Wohler curves as well as a representative equivalent fatigue damage parameter (FDP) for multiaxial cyclic loadings have to be determined. For simple loading conditions, the fatigue assessment can be performed using well-known and verified strength hypotheses for quasi-static loading conditions. However, for complex non-proportional cyclic loading conditions with rotating principle stress directions the application of these hypotheses is not sufficiently verified. Hence, advanced stress, strain or energy based strength hypotheses in critical plane formulation are used. These hypotheses require considerable numerical efforts.The fatigue concept (MPA AIM-Life) enables an assessment of complex fatigue loading conditions with different advanced strength hypotheses. An interface to the finite element code ABAQUS allows the fatigue assessment of complex component geometries. Based on fatigue tests of specimens made from ferritic and austenitic materials under uniaxial and multiaxial loading conditions (tension/torsion) the accuracy of different strength hypotheses is demonstrated. Therefore the fatigue analysis assessment included in codes and standards is compared to different advanced fatigue damage parameters.Copyright

Thermal and Mechanical Fatigue Loading: Mechanisms of Crack Initiation and Crack Growth

The present contribution is focused on the experimental investigations and numerical simulations of the deformation behaviour and crack development in the austenitic stainless steel X6CrNiNb18-10 (AISI–347) under thermal and mechanical cyclic loading in HCF and LCF regimes. The main objective of this research is the understanding of the basic mechanisms of fatigue damage and development of simulation methods, which can be applied further in safety evaluations of nuclear power plant components. In this context the modelling of crack initiation and crack growth inside the material structure induced by varying thermal or mechanical loads are of particular interest. The mechanisms of crack initiation depend among other things on the art of loading, microstructure, material properties and temperature. The Nb-stabilized austenitic stainless steel in the solution-annealed condition was chosen for the investigations. Experiments with two kinds of cyclic loading — pure thermal and pure mechanical — were carried out and simulated.The fatigue behaviour of the steel X6CrNiNb18-10 under thermal loading was studied within the framework of the joint research project [1]. Interrupted thermal cyclic tests in the temperature range of 150 °C to 300 °C combined with non-destructive residual stress measurements (XRD) and various microscopic investigations, e.g. in SEM, were used to study the effects of thermal cyclic loading on the material. This thermal cyclic loading leads to thermal induced stresses and strains. As a result intrusions and extrusions appear inside the grains (at the surface), at which micro-cracks arise and evolve to a dominant crack. Finally, these micro-cracks cause continuous and significant decrease of residual stresses.The fatigue behaviour of the steel X6CrNiNb18-10 under mechanical loading at room temperature was studied in the framework of the research project [2]. With a combination of interrupted LCF tests and EBSD measurements the deformation induced transformation of a fcc austenite into a bcc α′-martensite was observed in different stages of the specimen lifetime. The plastic zones develop at the crack tips, in which stress and strain amplitudes are much higher than the nominal loading, and enable martensitic transformation in the surrounding of the crack tip. The consequence of this is that cracks grow in the “martensitic tunnels”. The short and long crack growth behaviours of the steel X6CrNiNb18-10 under mechanical loading at room temperature and T = 288 °C were studied for different loading parameters. Moreover, the R-ratio was modified in order to study the effect of crack closure at the crack tip for long cracks.Several FE-models of specimens with different geometries and microstructures were created and cyclically loaded according to the experimental boundary conditions. A plastic constitutive law based on a Chaboche type model was implemented as a user subroutine in the FE software ABAQUS. The corresponding material parameters were identified using uniaxial LCF tests of X6CrNiNb18-10 with different strain amplitudes and at different temperatures. These calculations aimed in the estimation of stress and strain distributions in the critical areas in which the crack initiation was expected.Copyright

Further Development of the Nonlocal Damage Model of Rousselier for the Transition Regime of Fracture Toughness and Different Stress States

In nuclear power engineering failure has to be excluded for components with high safety relevance. Currently, safety assessments mainly use fracture mechanics concepts. Especially in the transition region of fracture toughness where limited stable crack extension may appear before cleavage fracture the currently applied methods are limited.This Paper deals with the development and verification of a closed concept for safety assessment of components over the whole range from the lower shelf to the upper shelf of fracture toughness. The results of classical used local damage mechanics models depend on the element size of the numerical model. This disadvantage can be avoided using an element size depending on microstructure. With high stress gradients and small crack growth rates usually smaller elements are required. This is in conflict with an element size depending on microstructure. By including the damage gradient as an additional degree of freedom in the damage mechanics model the results depend no longer at the element size. In the paper damage mechanics computations with a nonlocal formulation of the Rousselier model are carried out for the evaluation of the upper transition area. For the prediction of fracture toughness from the ductile to brittle transition area the nonlocal Rousselier model is coupled with the Beremin model. Thus ductile crack growth and failure by brittle fracture can be described in parallel. The numerical prediction of the behaviour of fracture toughness specimens (C(T)-specimens and SE(B)-specimens with and without side grooves) and the experimental results are highly concordant. The load displacement behavior of the specimens and the developed crack front from the ductile to brittle transition area can be well calculated with the nonlocal damage model. The instability in relation to temperature calculated with the coupled damage mechanics model predicts the variations of the experimental results very well.For further application of the nonlocal Rousselier model experiments and numerical calculations of specimens with different stress states and multi-axiality are carried out. Modified fracture toughness specimens like CTS-specimens (compact tension shear specimens) are taken to investigate the applicability of the nonlocal damage model of Rousselier to mixed mode fracture.© 2014 ASME

Extension of fracture mechanics evaluation methods by consideration of multiaxiality of stress state for piping components

Abstract Experimental investigations and numerical calculations by means of the finite element method concerning linear elastic as well as elastic-plastic material behaviour were performed to develop a methodology for the fracture mechanics evaluation taking into account the multiaxiality of stress state. A description of this fracture mechanics evaluation methodology and its application on degraded piping components (T-branches and elbows with dimensions like the primary coolant lines of PWR-plants) is provided and discussed.

Environmental Influences on the Fatigue Assessment of Austenitic and Ferritic Steel Components Including Welds

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability. For cyclic stress evaluation, different country specific design codes and standards provide fatigue analysis procedures to be performed considering the various mechanical and thermal loading histories and geometric complexities of the components. For the fatigue design curves used as limiting criteria, the influence of different factors like e.g. environment, surface, temperature and data scatter must be taken into consideration in an appropriate way. In this context there is a need of consolidating and increasing the current knowledge.In the framework of an ongoing three years German cooperative project performed by Materials Testing Institute MPA Stuttgart and AREVA GmbH (Erlangen) it is the aim to both improve the state of the art based on an experimental program on the factors mentioned above including hold-times at transient free static load and on the derivation of a practicable engineering fatigue assessment concept. Emanating from a review of the current state of the art the cooperative project is split up into three major parts:1) Experimental investigations concerning the influence of loading parameters and environmentally assisted fatigue (EAF) effects (light water reactor environment) on the fatigue strength of ferritic steels including weldments.2) Experimental investigations concerning the influence of long hold times and the EAF effects on the fatigue strength of austenitic and ferritic steels.3) The results of the outlined experimental program and published results will constitute the input for the proposal of an engineering fatigue assessment concept. This concept includes the differentiation between numerous factors of influence as an essential feature. In this context the margins between mean data curves and design curves are to be discussed in detail considering the factors of influence in general and EAF in particular.Based on a comprehensive consolidation of the state of the art and previous investigations in air and in light water reactor environment an experimental program is set up with the following key aspects:- Strain controlled fatigue tests on welded (microstructure of the weldment excluding microscopic and macroscopic weld notch effects) and unwelded smooth laboratory specimens subjected to constant and variable strain amplitude loading in air and light water reactor environment.- Strain controlled fatigue tests on notched specimens for the consideration of multi-axiality effects in air and light water reactor environment.- Strain controlled fatigue tests on smooth round laboratory specimens in air and in light water reactor environment focusing on long (power plant relevant) hold time effects.© 2014 ASME

Derivation of Design Fatigue Curves for Austenitic Stainless Steel Grades 1.4541 and 1.4550 Within the German Nuclear Safety Standard KTA 3201.2

Titanium and niobium stabilized austenitic stainless steels X6CrNiTi18-10S (material number 1.4541, correspondent to Alloy 321) respectively X6CrNiNb18-10S (material number 1.4550, correspondent to Alloy 347) are widely applied materials in German nuclear power plant components. Related requirements are defined in Nuclear Safety Standard KTA 3201.1. Fatigue design analysis is based on Nuclear Safety Standard KTA 3201.2. The fatigue design curve for austenitic stainless steels in the current valid edition of KTA 3201.2 is essentially identical with the design curve included in ASME-BPVC III, App I (ed. 2007, add. July 2008 respectively back editions).In the current code revision activities of KTA 3201.2 the compatibility of latest in air fatigue data for austenitic stainless steels with the above mentioned grades were examined in detail. The examinations were based on statistical evaluations of 149 strain controlled test data at room temperature and 129 data at elevated temperatures to derive best-fit mean data curves. Results of two additional load controlled test series (at room temperature and 288°C) in the high cycle regime were used to determine a technical endurance limit at 107 cycles. The related strain amplitudes were determined by consideration of the cyclic stress strain curve. The available fatigue data for the two austenitic materials at room temperature and elevated temperatures showed a clear temperature dependence in the high cycle regime demanding for two different best-fit curves. The correlation of the technical endurance limit(s) at room temperature and elevated temperatures with the ultimate strength of the materials is discussed.Design fatigue curves were derived by application of the well known factors to the best-fit curves. A factor of SN = 12 was applied to load cycles correspondent to the NUREG/CR-6909 approach covering influences of data scatter, surface roughness, size and sequence. In terms of strain respectively stress amplitudes in the high cycle regime, for elevated temperatures (>80°C) a factor of Sσ = 1.79 was applied considering and combining in detail the partial influences of data scatter surface roughness, size and mean stress. For room temperature a factor of Sσ = 1.88 shall be applied.As a result, new design fatigue curves for austenitic stainless steel grades 1.4541 and 1.4550 will be available within the German Nuclear Safety Standard KTA 3201.2. The fatigue design rules for all other austenitic stainless steel grades will be based on the new ASME-BPVC III, App I (ed. 2010) design curve.Copyright

Quantification of Crack-Tip Constraint Effect on Master Curve Reference Temperature Based on Two-Parameter Approach

The master curve has evolved into a mature technology for characterizing the fracture toughness transition of ferritic steels. However, it is well known that the master curve reference temperature (To) values estimated from small laboratory specimen may be biased low due to loss of crack-tip constraint. To quantify such variations of To resulting from differences of crack-tip constraint of testing specimen, two-parameter fracture mechanics approaches are employed in the present study. In this context, fracture toughness test and 3-dimensional finite element (FE) analysis for several standard and nonstandard test specimens are performed to quantify relationship between variations of To and constraint parameters and to find best constraint parameter representing effect of crack-tip constraint on To values evidently. Based on testing and present FE results, To and constraint parameter loci are constructed and engineering To correlation models considering crack-tip constraint are suggested

Fracture mechanics evaluation of small diameter piping considering the latest experimental results

Abstract The results obtained from investigations carried out on austenitic piping of small nominal diameter (DN80 and DN50) are introduced and discussed together with their assessment using fracture mechanics methods. Essential results are summarised as following. The pipes with flaws (fatigue crack) down to a depth to a max / t =0.51 (DN80) as well as a max / t =0.62 (DN50) and a circumferential extension of results 2 α =120° reached bending angles up to 26°. The ASME collapse load (test collapse load) was exceeded considerably and the experimental maximum load could not be reached. Failure due to a leakage or rupture did not occur in any test. The maximum crack extension was 0.69 mm (DN80, a max / t =0.51) resp. 0.3 mm (DN50, a max / t =0.62). The experimental maximum load can approximately be assessed by the limit analysis. The fracture mechanics approximation methods GE/EPRI and LBB/NRC calculated a / t =0.4 and 2 α =120° initiation loads above the experimental maximum load for pipes containing flaws. These results confirmed the procedures for the proof of integrity of small diameter piping by updating information on load, deformation and failure behaviour of austenitic piping damaged with circumferential flaws. Using these results may formulate a final safety concept for the proof of integrity of small diameter piping by completing the current concepts.

Fatigue Behavior of Cladding Material for Nuclear Components

To evaluate the fatigue behavior of the austenitic cladding of pressurized components, the fatigue design curves are usually based on experiments on specimens taken from plates, pipes or bars. Due to rapid heat transport into the ferritic base material, the cladding has a distinctive anisotropic structure that results from the manufacturing process. Therefore, it may not be assumed a priori that the fatigue curves of the various austenitic product forms used in the Safety Standards are also representative for the material of austenitic cladding. It is therefore necessary to determine and assure experimentally a fatigue curve of the cladding material and compare the results with the database of austenitic stainless steels used in German NPPs.Flat specimens specially adapted to the geometric conditions were prepared out of the austenitic cladding of a RPV which had been manufactured according to nuclear specifications. To check a possible influence of the specimen geometry, flat specimens were prepared in advance from an austenitic pipe of known fatigue behavior and tested at different strain amplitudes. A comparison of the results with the fatigue curve determined with cylindrical specimens showed no influence at the higher strain amplitudes. Altogether, the fatigue data with flat specimens made of the austenitic cladding fit well into the scatter of the fatigue database of austenitic base materials tested with cylindrical specimens.Copyright