Search Results for fatigue crack growth rates

The fatigue crack propagation thresholds of SAW weld metal of 25Cr2Ni2MoV simulating product of fossil and nuclear power low pressure turbine rotor at different stress ratios are tested. There is a big dispersity of the test results, even at the same stress ratio. The double logarithm curves of the fatigue crack growth rate and stress intensity factor range are researched. The difference of critical points between stable propagation region and near-threshold region in different specimens is found to be an important cause to the dispersity. Their locations in the specimens can be determined by the method of backward inference. After the observation of the microstructures around the critical points, a good correspondence between the size of prior austenite grain and the maximum size of monotonic plastic zone on the crack tip is confirmed. The difference of the critical points at the same stress ratio is caused by the inhomogeneous microstructures. So the inhomogeneous microstructures in the multi-pass and multi-layer weld metal contribute to the dispersity of the experimental threshold values.

This paper reports on a study that investigated how low cycle fatigue (LCF) and fatigue crack propagation (FCG) properties of P92, P122, and P23 steels vary between 600°C and 700°C depending on the location relative to a cross weld. Microstructure analysis was also performed on fractured specimens. Due to its higher yield strength, P122 exhibited the best continuous LCF life. However, creep-fatigue interaction (CFI) in the weld heat-affected zone (HAZ) of P122 and P23 steels significantly reduced their lifespans compared to continuous LCF tests. This reduction is attributed to the effect of weld thermal cycles on fine precipitates. FCG tests revealed that the base metal consistently outperformed the HAZ in all tested steels and temperatures. P92 and P122 showed similar FCG rates except for P92's behavior at 600°C, which resembled P23. In both steels, the HAZ exhibited faster FCG rates at 600°C and 700°C compared to the base metal, particularly at lower stress intensity factor ranges (ΔK). Within the HAZ, the region 1 mm from the fusion line displayed the slowest FCG rates, followed by the base metal, while the fusion line and the region 2 mm from it showed the fastest. Fracture surfaces near the fusion line displayed cleavage-like features, while the region 1 mm away exhibited features associated with higher crack growth resistance.

This paper presents the creep and creep-fatigue crack growth behaviors of 30Cr1Mo1V turbine rotor steel which had been in service for 16 years. Two typical sections of the rotor, i.e. high and low temperature sections, are examined at 538°C, with crack initiation and propagation monitored by D.C. potential drop method in a compact tension (CT) specimen. The material of the high temperature section has the lower resistance to creep and creep-fatigue crack growths than the low temperature section. The creep crack initiation (CCI) time decreases with the increase of initial stress intensity factor. The creep-fatigue crack growth (CFCG) is dominated by the cycle-dependent fatigue process when the hold time at the maximum load is shorter, but it becomes dominated by the time-dependent creep process when the hold time becomes longer. The high temperature section shows a larger influence of time-dependent creep behavior on CFCG than the low temperature section.

High temperature components with notches, defects and flaws may be subject to crack initiation and crack propagation under long-term service conditions. To study these problems and to support an advanced remnant life evaluation, fracture mechanics procedures are required. Since a more flexible service mode of power plants causes more start up and shut down events as well as variable loading conditions, creep-fatigue crack behavior becomes more and more decisive for life assessment and integrity of such components. For steam power plant forged and cast components, the crack initiation time and crack growth rate of heat resistant steels were determined in long-term regime up to 600 °C. Component-like double edge notched tension specimens have been examined. The results are compared to those obtained using the standard compact tension specimen. Crack initiation time and crack growth rate have been correlated using the fracture mechanics parameter C*. The applicability of the stress intensity factor K I to describe the creep crack behavior is also being assessed. A modified Two-Criteria-Diagram was applied and adapted in order to recalculate crack initiation times under creep-fatigue conditions. Recommendations are given to support the use of different fracture mechanics parameters in order to describe the long-term crack behavior under creep and/or creep-fatigue conditions.

For the safe life prediction of components under high cycle fatigue loading at high temperature, such as gas turbine blades and turbocharger components, the behavior of initial defects, which are physically short cracks below the long crack threshold ΔK is of crucial importance. The evolution of different crack closure mechanisms (such as plasticity, roughness and oxide induced crack closure) can lead to crack arrest by a reduction of the effective crack tip loading. To visualize the crack growth behavior of such cracks, cyclic crack resistance curves (cyclic R-curves) are used. The experimental determination of cyclic R-curves is challenging, especially under high temperature conditions due to a lack of optical accessibility. The formation of very short cracks in high strength materials makes it even more complicated to reliably determine these data. Within this study the crack growth behavior of physically short fatigue cracks in three different material states of the nickel alloy IN718 (wrought, cast and PBF-LB/M - processed) is experimentally determined at 650 °C. Based on a load increase procedure applied on Single Edge Notched (SEN) specimens with a compression pre-cracking procedure in advance, crack propagation of physically short cracks is measured with alternating current potential drop systems in air and under vacuum conditions. These examinations are carried out for three different load ratios (R = -1, 0 and 0.5) to investigate the amount of certain crack closure mechanisms active under different loading conditions. Moreover, the formation of a plastic wake along the crack flanks is determined by a finite element simulation. The results determined in air and under vacuum conditions are used to describe the impact of oxide induced crack closure on the behavior of physically short cracks. This allows the evaluation of the behavior of both near-surface and internal defects that are not accessible to the atmosphere.

Creep-fatigue crack formation (endurance) and crack growth rate data are necessary inputs for assessing the structural integrity and for estimating the design life of high temperature components in power generation and aircraft engine industries. Ensuring consistency in the reported test data, as well as an understanding of the inherent scatter and its source in the data, are both necessary for assuring quality and limitations of the analyses that rely on the data. In 2008, the American Society for Testing and Materials (ASTM) under the umbrella of its subcommittees E08.05 on Cyclic Deformation and Crack Formation and E08.06 on Crack Growth, and the sponsorship of Electric Power Research Institute (EPRI) through its international experts’ working group on creep-fatigue embarked on the task of developing separate standard test methods for creep-fatigue crack formation and creep-fatigue crack growth. The first standard entitled, “E-2714-09: Standard Test Method for Creep-fatigue Testing” was developed in 2009 and was followed up with a round-robin consisting of 13 laboratories around the world for testing the newly developed standard. This paper discusses the results of this round-robin concluded in 2012 using the widely used P91 steel that led to the formulation of the Precision and Bias statement contained in the version of the ASTM standard E2714 that was successfully balloted in the year 2013.

In this study, fatigue crack propagation behavior at lower temperature in single crystal nickel-base superalloys was investigated experimentally and analytically. Four types of compact specimens with different combinations of crystal orientations in loading and crack propagation directions were prepared, and fatigue crack propagation tests were conducted at room temperature and 450°C. It was revealed in the experiments that the crack propagated in the shearing mode at room temperature, while the cracking mode transitioned from the opening to shearing mode at 450°C. Both the crack propagation rate and the transition behavior were strongly influenced by the crystallographic orientations. To interpret these experimental results, crystal plasticity finite element analysis was carried out, taking account some critical factors such as elastic anisotropy, crystal orientations, 3-D geometry of the crack plane and the activities of all 12 slip systems in the FCC crystal. A damage parameter based on the slip plane activities derived from the crystal plasticity analysis could successfully rationalize the effect of primary and secondary orientations on the crystallographic cracking, including the crack propagation paths and crack propagation rates under room temperature. The proposed damage parameter could also explain the transition from the opening to crystallographic cracking observed in the experiment under 450°C.

A research program has been initiated to develop the first predictive methodology for corrosion fatigue life in steam turbine blades, addressing a critical gap in current understanding despite extensive research into corrosion pitting and fatigue failure. The study focuses initially on dual-certified 403/410 12% Cr stainless steel, utilizing a newly developed test facility capable of conducting high-cycle fatigue tests in simulated steam environments at 90°C with controlled corrosive conditions. This testing platform enables the investigation of various steady and cyclic stress conditions, establishing a foundation for future testing of other blade steels and the development of comprehensive blade life estimation techniques.

Single crystal Ni-base superalloys are subjected to tension hold at high temperature in addition to cyclic loading during the operation of gas turbines. Various studies have investigated creep-fatigue crack propagation in superalloys under trapezoidal loadings and evaluated the life time based on parameters such as creep J-integral. However, it is still unclear how damage field and stress-strain condition change at the crack tip during hold time, and how it affects on fatigue crack propagation. In this study, the influence of the tension hold and accompanying creep at crack tip on subsequent fatigue crack propagation behavior was evaluated by introducing single tension holds into pure cyclic loadings. The series of the experiments revealed that because of the tension hold, material degradation and stress relaxation occurred simultaneously ahead of crack tip. In the region where material was degraded, the resistance against crack propagation was reduced, while in the region where stress was relaxed, the crack driving force was lowered.

In today’s market place power generation plants throughout the world have been trying to reduce their operating costs by extending the service life of their critical machines such as steam turbines and gas turbines beyond the design life criteria. The key ingredient in plant life extension is remaining life assessment technology. This paper will outline remaining life procedures which will incorporate the defect tolerant design concepts applied to the various damage mechanisms such as creep, fatigue, creep-fatigue and stress corrosion cracking. Also other embrittlement mechanisms will also be discussed and how they will influence the life or operation of the component. Application of weld repairs to critical components such as rotors and steam chest casings will be highlighted and how defect tolerant design concept is applied for the repair procedure and the acceptance standard of the nondestructive testing applied. Also highlighted will be various destructive tests such as stress relaxation tests (SRT) which measures creep strength and constant displacement rate test (CDRT) which evaluates fracture resistance or notch ductility. Also shown will be actual life extension examples applied to steam turbine components and weld repairs. Utilization of computer software to calculate fatigue and creep fatigue crack growth will also be presented

Significant development is being carried out worldwide for establishing advanced ultra supercritical power plant technology which aims enhancement of plant efficiency and reduction of emissions, through increased inlet steam temperature of 750°C and pressure of 350 bar. Nickel base superalloy, 50Ni-24Cr-20Co-0.6Mo-1Al-1.6Ti-2Nb alloy, is being considered as a promising material for superheater tubes and turbine rotors operating at ultra supercritical steam conditions. Thermal fluctuations impose low cycle fatigue loading in creep regime of this material and there is limited published fatigue and creep-fatigue characteristics data available. The scope of the present study includes behavior of the alloy under cyclic loading at operating temperature. Strain controlled low cycle fatigue tests, carried out within the strain range of 0.2%-1%, indicate substantial hardening at all temperatures. It becomes more evident with increasing strain amplitude which is attributed to the cumulative effects of increased dislocation density and immobilization of dislocation by γ′ precipitates. Deformation mechanism which influences fatigue life at 750°C as a function of strain rate is identified. Hold times up to 500 seconds are introduced at 750°C to evaluate the effect of creep fatigue interaction on fatigue crack growth, considered as one of the primary damage mode. The macroscopic performance is correlated with microscopic deformation characteristics.

Solar salts are used as an energy storage media and heat transfer fluid in power plants. The salts can cause significant corrosion to various steels that are in contact with the salt. Static corrosion tests performed with different steels show, that the corrosive attack by industrial grade salt melts is more severe than by defined grade salt melts and the sample corrosion is faster (i.e. the weight gain is larger) for higher temperatures. Slow strain rate (SSR) tests in salt are difficult to conduct due to the corrosive attack of the salt also on the test setup. The SSRT setup in salt could be realized and tests could be conducted successfully. No clear evidence for an accelerated failure of samples tested in salt compared to samples tested in air could be found on Alloy 347 Nb. Comparative low cycle fatigue (LCF) tests at air and in molten salt atmosphere were successfully performed and showed similar results on tubes out of Sanicro 25. No evidence of accelerated crack growth in molten salt could be found.

The use of the bainitic creep strength enhanced ferritic steel T/P23 has increased over the last decade in a wide range of applications including headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in weldments of this material, such as hydrogen induced cracking, reheat cracking and stress corrosion cracking. In order to help characterize high temperature cracking phenomena, including reheat cracking, a limited number of laboratory creep crack growth tests are being conducted as part of an ongoing project. Tests were run on as-welded sections with the test specimen crack-tip located in select zones of the weldment. Test temperatures are intended to bookend the range of applications from a waterwall condition of ~482°C (900°F) to the superheat/reheat condition of 565°C (1050°F). This paper describes the results of some early testing at 482°C (900°F). The tests provided useful insight into the cracking susceptibility of the material at this temperature with respect to not only time-dependent cracking, but also fatigue crack growth and fracture toughness. The paper includes details of the test method and results, as well as findings from post-test metallographic examinations of the tested specimens.

To improve efficiency and flexibility and reduce CO 2 emissions, advanced ultra super critical (AUSC) power plants are under development, worldwide. Material development and its selection are critical to the success of these efforts. In several research and development programs / projects the selection of materials is based on stress rupture, oxidation and corrosion tests. Without doubt, these criteria are important. To improve the operational flexibility of modern power plants the fatigue properties are of increased importance. Furthermore, for a safe operation and integrity issues the knowledge about the crack behavior is essential. Crack initiation and crack growth may be caused by natural flaws or cracks induced by component operation. In order to develop new materials, properties like tensile strength and creep strength are an important part of qualification and subsequent approval by notified bodies. Consequently short term properties as well as time-temperature dependent properties are generated and taken into considerations. In the case of high strength γ'-strengthening nickel-base alloys investigating the creep crack behavior is also strongly recommended. This article shows results of currently investigated nickel-based alloys for newly developed headers, pipes and other high temperature boiler applications and their critical creep crack propagation behavior.

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.

This research compares creep crack growth behavior of two heats of creep strength enhanced ferritic (CSEF) steel, grade 91. These heats represent extremes of creep damage susceptibility, one heat exhibiting low creep ductility and the other high creep ductility. Creep crack growth tests were performed with compact tension specimens and were monitored with direct current potential drop and optical surface measurements. Load line displacement was measured throughout the duration of the tests. Specimens were sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the presence of oxidation, micro-cracking, creep damage, and void density. Tests were performed over a range of initial stress intensities on the low ductility material to investigate the impact of creep ductility. Metallurgical evidence and test data for each crack growth test was assessed to evaluate crack growth behavior linked to creep crack growth parameter (C*) and stress/creep damage distribution in the vicinity of the crack.

High-performance Ferritic (HiperFer) steels represent a promising materials innovation for next-generation thermal energy conversion systems, particularly in cyclically operating applications like concentrating solar thermal plants and heat storage power plants (Carnot batteries), where current market adoption is hindered by the lack of cost-effective, high-performance materials. HiperFer steels demonstrate superior fatigue resistance, creep strength, and corrosion resistance compared to conventional ferritic-martensitic 9-12 Cr steels and some austenitic stainless steels, making them potentially transformative for future energy technologies. This paper examines the microstructural mechanisms underlying HiperFer’s enhanced fatigue resistance in both short and long crack propagation, while also presenting current findings on salt corrosion properties and exploring potential alloying improvements for fusion reactor applications, highlighting the broad technical relevance of these innovative materials.

Monoblock low-pressure (LP) turbine rotor shaft forgings for nuclear power plants have been produced from up to 600 ton ingots. However, ingots greater than 600 tons are necessary to increase the generator capacity. Segregation, non-metallic inclusions, and micro porosities inevitably increase with the increase in ingot size. Manufacturing such massive ingots with high soundness is quite difficult. Thus, the development of 650 ton ingot production was carried out in 2010. The 650 ton ingot was dissected and investigated to verify its internal quality. The internal quality of the 650 ton ingot was found to be equal to that of 600 ton ingots. Subsequently, in 2011, we produced a 670 ton ingot, the world’s largest, to produce a trial LP rotor shaft forging with a diameter of 3,200 mm. Results show that the internal quality, mechanical properties, and heat stability are the same as LP rotor shaft forgings made from 600 ton ingots.

A failure of the upper casing of the circulation pump led to a big damage in the PP Staudinger unit 5 on 12th of May 2014. According to the §18(2) BetrSichV an extensive root cause analysis (RCA) was started. From the beginning on different lines of activities were initiated to handle the situation with the required diligence. Decisions were made, taking into account safety regulations, possibility of repair and best practice engineering. Following the board decision to repair the unit 5, a lot of detailed work was done. All of the performed work packages were linked in different timelines and needed to meet in the key points. Consequently it was a challenge to achieve the agreed date of unit 5 restart on 15th of January 2015. The unit restart on the targeted date was a proof of the excellent collaboration between all involved parties. The presentation gives a summarizing overview about the damage, the main results of the RCA and the repair activities.

The extrapolation of short-term laboratory test results to predict long-term high-temperature component failure remains challenging, particularly for P91 steel due to its phase transformation during extended service and susceptibility to type IV cracking. While the NSW model successfully predicts creep crack growth bounds using short to medium-term test data (<10,000 hours), recent literature suggests materials may exhibit more brittle behavior and reduced failure strain in longer-term tests. This study examines whether the NSW model, using short-term uniaxial data, can effectively predict these long-term behavioral changes for more accurate service life assessment.