Karl-Heinz Herter

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

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

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

Crack Growth Behavior of Ferritic Pressure Vessel Steels in Oxygenated High Temperature Water Under Transient Loadings

The assessment of the influence of the LWR coolant environment and postulated chloride transients on the crack growth is of importance for ageing management with regard to safety and reliability. Aim of the investigations was to determine cyclic crack growth rates at LWR conditions and to study possible size effects and the impact of chlorides on environmentally assisted cracking.Crack growth experiments were performed with fracture mechanics specimens of different size in simulated BWR water of high purity and under the effect of chloride transients with RPV steel 22NiMoCr3-7.Subsequent to a phase of cyclic loading, the specimens were exposed to static load, interrupted by partial unloadings.All cyclic crack growth rates da/dN vs. ΔK in high purity water were in good agreement with ASME XI water curves.No significant influence of specimen size on the crack growth behavior and with regard to SCC could be detected in high purity water environment. Cyclic induced crack propagation immediately stopped when turning to static load. Under static load the chloride transients did not cause crack initiation by SCC.Load transients in chloride containing environment initiated significant SCC-induced crack growth. A “chloride memory effect” with regard to a preceding chloride transient at static load, leading to SCC-induced crack propagation during subsequent load transients in high purity water environment did not arise.Copyright

Fatigue Behavior of Nuclear Materials Under Air and Environmental Conditions

The assessment of fatigue and cyclic crack growth behavior of safety relevant components is of importance for ageing management with regard to safety and reliability. For cyclic stress evaluation different 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 finish and temperature must be taken into consideration in an appropriate way.Fatigue tests were performed in the low cycle fatigue and high cycle fatigue regime with low-alloy steels as well as with Nb- and Ti-stabilized German austenitic stainless steels in an air and high temperature BWR environment to extend the state of knowledge of environmentally assisted fatigue as it can occur in BWR plants.Using the RPV steel 22NiMoCr3-7 experimental data was developed to verify the influence of BWR coolant environment (high purity water as well as water containing sulphate with 90 ppb SO4 and water containing chloride with 50 ppb Cl at a test temperature of 240 °C and an oxygen content of 400 ppb) on the fatigue life and to extend the basis for a reliable estimation of the remaining service life of reactor components. Corresponding experiments in air were performed to establish reference data to determine the environmental correction factor Fen.The experimental results are compared with available international mean data curves, the new design curves and on the basis of the environmental factor Fen.Copyright

Fatigue Behavior of Dissimilar Welds Used for Nuclear Piping

Due to specific requirements in NPP piping different materials are used and connected by dissimilar welds (DM). The fatigue behavior of such welds must be known for design and safety evaluations. The overall fatigue behavior of welds depends on the properties of the different weld sections and their interaction. The welding may influence the fatigue behavior of the base materials in the vicinity of the weld.The investigation deals with the fatigue behavior of DM typical for German NPP: ferritic steel 20MnMoNi5-5 welded to austenitic steel X6CrNiNb18-10 using nickel based alloy for buttering. Fatigue specimens were taken from each region of the weld (ferritic steel near weld, buttering, connection weld, austenitic steel near weld). Additionally specimens were taken containing two adjacent material regions and the respective fusion line. For each position specimens were tested in fully reversed strain controlled conditions at room temperature and total strain amplitude of 0.25%. The results were compared with the best-fit curves for austenitic and ferritic steels.Copyright

Strain Limits for the Analysis of the Mechanical Behaviour of Pressurized Components

In technical codes and standards the strength assessment of pressurized components and systems is usually performed by limitation of fictitious elastic stresses as well as by limitation of the fatigue level. It shall be proven that the highest stresses resp. stress ranges caused by external loading show a specific margin against the resistance of the material used (concept of stress limitation). This concept is based on the principle that in a component with inhomogeneous stress distribution plastic deformation are allowed at the locations which sustain the highest loads. If in special cases the stress categorization is unclear, the effect of plastic deformation on the mechanical behavior shall be decisive. The same holds for postulated load levels with low occurrence probability (severe accidents) for which the stress limits do not utilize fully the deformation capacity of ductile materials. For that reason solutions that are based on the “Strain Limitation Concept” (SLC) shall be developed. Strength assessment using the concept of strain limitation relies on limiting strains to a specified allowable strain value, i.e. “Limit Strain Curve” (LSC). Within this presentation the approach of the concept of strain limitation is described with a special attention for the determination of the strain limits that can be allowed. With the help of Finite Element damage calculations (Rousselier model) on notched round bars the deformation behavior of an austenitic steel is described until failure. The dependency of the local failure strain on size, stress triaxiality and stress gradients is analyzed. In order to validate the numerical investigation these results are compared with experiments.© 2012 ASME

Probabilistic Procedure to Evaluate Integrity of Degraded Pipes

The determination of critical crack sizes or permissible/allowable loading levels in pipes with degraded pipe sections (circumferential cracks) for the assurance of component integrity is usually based on deterministic approaches. Therefore along with numerical calculation methods (finite element (FE) analyses) limit load calculations, such as e.g. the “Plastic limit load concept” and the “Flow stress concept” as well as fracture mechanics approximation methods as e.g. the R-curve method or the “Ductile fracture handbook” and the R6-Method are currently used for practical application. Numerous experimental tests on both ferritic and austenitic pipes with different pipe dimensions were investigated at MPA Stuttgart. The geometries of the pipes were comparable to actual piping systems in Power Plants. Different crack geometries and dimensions were considered. A new post-calculation of the above mentioned tests was performed using probabilistic approaches to assure the component integrity of degraded piping systems. As a result the calculated probability of failure was compared to experimental behaviour during the pipe test. The influence of the performance of non-destructive tests on the probability of failure was also taken into account. Different reliability techniques were used for the verification of the probabilistic approaches.

Detailed Assessment of Fatigue Life: A Critical Review

For the construction, design and operation of technical components and systems the appropriate technical codes and standards provide detailed stress analysis procedures, material data and a design philosophy which guarantees a reliable behaviour of the structural components throughout the specified lifetime. Especially for cyclic stress evaluation the different codes and standards provide different fatigue analyses procedures to be performed considering the various (specified or measured) loading histories (of mechanical and thermal origin) and geometric complexities of the components. In order to fully understand the background of the fatigue analysis included in the codes and standards as well as of the fatigue design curves used as a limiting criteria (to determine the fatigue life usage factor), it is important to understand the history and the methodologies which are available for the design engineers. In the paper the different fatigue analysis procedures in the technical codes and standards (e.g. ASME-III, KAT 3201.2, prEN 13445-18) are discussed in detail. The most important parameters influencing the fatigue analysis, like plastification factor Ke , the correction factors with respect to mean stress, to surface finish, to tempeature, to the environment and to unwelded and welded components are verified on the basis of experimental results. Thus safety margins relevant for the assessment of fatigue life are shown and compared with the safety factors implemented in the different technical codes and standards.Copyright