CONTRIBUTIONS OF OXIDATION AND CREEP TO HIGH TEMPERATURE FATIGUE CRACK SUSCEPTIBILITY IN WASPALOY

The aggressive mechanical and environmental conditions in the hot sections of jet engines leads to creep and oxidation enhanced fatigue damage on high-performance metal components. Understanding the relative contribution of oxidation, cyclic damage accumulation, and creep is needed. Crack growth kinetics data were gathered on SENT fracture mechanics specimens using the direct current potential drop method. Specimens were tested in lab air and vacuum at elevated temperature at a constant ΔK and loaded according to a trapezoidal waveform with dwells ranging from 1 to 300 seconds. Plastic deformation and dislocation cell structure along the crack wake was assessed using high resolution electron backscatter diffraction and TEM. Quantitative comparisons of crack growth kinetics and deformation character for each test condition are performed and show variations in crack growth rates and size and intensity of plastic damage along the crack wake. These results provide insight into the contribution of creep and oxidation to crack growth and inform modeling high temperature fatigue behavior.
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LCF AND TMF OF SINGLE-CRYSTAL AND DIRECTIONALLY-SOLIDIFIED NI-BASE SUPERALLOYS PREDICTED USING A PROBABILISTIC PHYSICS-GUIDED NEURAL NETWORK

Predicting the life under thermomechanical fatigue (TMF) is challenging because there are several parameters defining the mechanical and thermal cycles including dwell periods within the cycle that can impact life. The relationships between these TMF history parameters and fatigue life are not always clear. A probabilistic physics-guided neural network was developed and trained to learn these relationships and predict the cycles to failure for a wide range of possible creep-fatigue and TMF histories using life data extracted from the literature.
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COMPUTATIONAL PREDICTIONS OF HYDROGEN ASSISTED FRACTURES [Keynote]

Hydrogen significantly reduces the ductility, toughness and fatigue crack growth resistance of metals, which leads to premature failures across many industrial sectors and compromises the role of hydrogen as energy carrier in the transition to a low carbon economy. This paper provides an overview of the efforts by the author and his collaborators in developing a computational framework capable of predicting hydrogen-assisted failures by providing a mechanistic description of hydrogen uptake, transport and embrittlement. Phase field and multi-physics (electro-chemo-mechanical) modelling are combined to resolve the physical processes at play. The ability of the computational models developed to deliver predictions in agreement with laboratory experiments and over scales relevant to engineering practice is showcased.
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REPURPOSING EXISTING NATURAL GAS PIPELINES FOR HYDROGEN SERVICE

In efforts toward decarbonization of the existing energy system, hydrogen has emerged as an attractive option for low carbon fuel. Currently, proposed applications for hydrogen include direct use as a fuel, either in pure form, or blended into natural gas supply, and as an option for long-term energy storage
option.
While there are numerous potential opportunities for blending hydrogen into natural gas systems, it is recognized that the introduction of hydrogen into existing natural gas infrastructure may present multiple challenges related to material, operational, and process
compatibility.

This paper presents an overview of the technical, operational, and integrity challenges associated with transitioning line pipe to blended hydrogen service. Additionally, it provides a framework and high-level guidance for how to transition natural gas systems to enable safe and reliable operation withhydrogen.
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FATIGUE CRACK GROWTH RATE OF VINTAGE PIPELINE STEELS IN GASEOUS HYDROGEN – EFFECT OF FREQUENCY AND K

There is currently significant focus on developing carbon neutral energy solutions. As part of this effort, there is an increasing effort aimed at using hydrogen as a fuel. This in turn requires it’s in the existing natural gas pipeline infrastructure as an efficient delivery method. The existing natural gas pipeline infrastructure is over 50 years old, with large parts of the infrastructure consisting of vintage steel pipes manufactured in the 1950’s and 1960’s. These pipelines are often subject to various fatigue loading events. Over the course of normal operations, the pipelines are subject to frequent low amplitude ripples, as well as occasional large amplitude pressure cycling. The role of hydrogen on accelerating fatigue behavior of steels has been extensively studied at high pressures of H2 and under modestly high frequencies (~1Hz). However, these pressures and loading conditions are not typical of pipeline operations, in addition, there has been very little work performed on vintage grade pipeline steels.
A program was performed to develop relevant fitness for service data on vintage pipeline steel at hydrogen partial pressures in the range of 300 psia. The intent was to develop crack growth rate information to perform fitness for service assessments under conditions associated
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FATIGUE LIFE PREDICTION OF WELDED JOINTS AT SUB-ZERO TEMPERATURES USING MODIFIED PARIS-ERDOGAN PARAMETERS

The fatigue life of welded steel components is usually determined by the weldment; in these cases, fracture mechanical approaches are widely used for their prediction. Ferritic steels are known to have a fatigue strength that is dependent on temperature. Therefore, this study evaluates fatigue tests of cruciform joints and transverse stiffeners at different sub-zero temperature levels, with regard to fatigue life. Simultaneously, the stress intensity factors over the crack length are calculated for the individual experiments using analytical solutions. Then, using the Paris-Erdogan relation with temperature and material adjusted C and m parameters as well as tabular values, the fatigue lives are calculated with analytical solutions and compared with the experimental results. It is shown that, the prediction accuracy is significantly increased for the sub-zero temperature range by using temperature-adjusted Paris-Erdogan parameters, as long as the temperature are above the fatigue transition temperature.
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COMPONENT INTEGRITY IMPLICATIONS FROM CREEP DAMAGE TOLERANCE AND FRACTURE CHARACTERISTICS OF CREEP STRENGTH ENHANCED FERRITIC STEELS

Creep Strength Enhanced Ferritic Steels (CSEF) are an import class of alloys used in high temperature components of power generating plants. These steels, while having excellent high temperature strength, can exhibit variable creep ductility and fracture tolerance due to undesirable metallurgical features. Methods for component assessment must capture this variability in behavior which can result in quite different failure times and modes. Examples are provided from multiaxial laboratory tests and failed components to illustrate variability in creep fracture behavior and application of integrity assessment methods. This illustrates the importance of understanding creep damage evolution, particularly for welded components, and shows how rapid fracture can occur in some circumstances. The implications for component life management and fitness-for-service are outlined with some practical examples.
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A PHENOMENOLOGICAL MODEL FOR CREEP CRACK GROWTH BEHAVIOR IN FERRITIC STEELS

A micro-mechanics model is described that rationalizes the effects of test temperature and microstructural variables such as grain boundary particles on the creep crack growth rate (CCGR) behavior of ferritic steels. The model predicts that as the average spacing between particles that initiate creep cavities on grain boundaries decreases, the CCG rates are expected to increase. CCGR data at several temperatures can be collapsed into a single trend using the newly proposed temperature compensated creep crack growth rate plot derived from the proposed model. The applicability of the model is demonstrated for Grade 22 and Grade 91 steels using extensive amounts of data available in the literature on new and service degraded conditions. The trends from the model are compared to CCGR data trends noted in fitness for service codes. It is shown that differences between the CCGR behavior of Grade 22 steel in new and ex-service are negligible in the base metal region but not near weldments. Significant differences were observed between new and ex-service materials in the CCGR behavior of Grade 91 materials even in the base metal region.
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MEASUREMENT METHOD OF CYCLE SEQUENTIAL CHARACTERISTICS OF STRESS REDUCTION UNDER STRAIN-CONTROLLED CREEP-FATIGUE CONDITIONS USING CIRCULAR SHARP NOTCHED ROUND BAR SPECIMEN

Concerning strain-controlled creep-fatigue conditions of a notched specimen, since cycle sequential characteristics of stress reduction are affected not only by materials and test conditions but also by a dimension of notch, estimation of experimental result by conventional method is not feasible. In order to establish a unified measurement method of the cycle sequential characteristics of the stress reduction for a notched specimen under strain-controlled creep-fatigue condition, the stress range Δσ (=σmax-σmin) and its reduction ratio η has been proposed. In this study, strain-controlled creep-fatigue crack growth tests were conducted for the CNS (Circular sharp Notched round bar Specimen) using two types of the extensometers which has different gauge lengths, and the effect of gauge length on cycle sequential characteristics of the stress reduction and failure life was investigated in relation to the crack length. In addition, the optimum gauge length was discussed using FEM.
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ON THE CYCLIC SOFTENING AND RATCHETING BEHAVIOUR OF A CSEF GAS TURBINE ROTOR STEEL AT 600 C

Cyclic softening under load-controlled low cycle fatigue typically manifests itself by the presence of the ratcheting, which induces an accumulation of plastic and creep strain with the increasing number of cycles, leading to progressive damage and shorter fatigue lives. Therefore, improved understanding of the physical mechanisms related to the high-temperature cyclic plasticity softening behaviour of tempered martensitic CSEF steels is crucial for more accurate fatigue behaviour assessments and to maintain the safety and integrity of critical energy components. In this study, fully reversed uniaxial and multi-axial load-controlled tests on FV566 martensitic gas turbine rotor steel at 600oC were utilized to examine the evolution of cyclic softening, ratcheting and fatigue damage under different degrees of stress states. Microstructural characterizations were conducted on the tested samples to study the evolution of the key microstructural features in the material and their roles in the damage development. A modelling framework is developed by coupling the unified viscoplastic constitutive material model with a physically based damage model, which can be used to simulate the ratcheting response and the associated microstructural degradation.
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