Probabilistic Life Models for Steel Structures Subject to Creep- Fatigue Damage

When metal structures are subjected to long-term cyclic loading at high temperature, simultaneous creep and fatigue damage may occur. In this paper probabilistic life models, described by hold times in tension and total strain range at elevated temperature have been derived based on the creeprupture behavior of 316FR austenitic stainless steel, which is one of the candidate structural materials for fast reactors and future Generation IV nuclear power plants operating at high temperatures. The parameters of the proposed creepfatigue model were estimated using a standard Bayesian regression approach. This approach has been performed using the WinBUGS software tool, which is an open source Bayesian analysis software tool that uses the Markov Chain Monte Carlo sampling method. The results have shown a reasonable fit between the experimental data and the proposed probabilistic creep-fatigue life assessment models. The models are useful for predicting expended life of the critical structures in advanced high temperature reactors when performing structural health management.

Comparison of Evolution Fatigue With Simplified Elastic Plastic Analysis, Plastic Analysis and a Fracture Mechanics Approach

Fatigue analyzes are performed to ensure that no damage leading to failure will occur in a component. In ASME III NB-3222.4(e), a method is provided for analyzing a component’s resistance against cyclic loads. In ASME NB-3228.5 or alternatively NB-3653.6 (c) a simplified elastic-plastic method is given to take plasticity into account. According to NB-3228.4 (c), fatigue evaluation is allowed with plastic analysis where the stress amplitude is determined from the numerically maximum principal total strain range. This report examines the fatigue life, using a continuum mechanics approach, of a pipe penetration that is subjected to a rapid cooling down followed by a slow heating up. Simplified elastic-plastic analysis and plastic analysis are performed. The analysis also takes into account the FEN-factor according to the method given NUREG/CR6909. The fatigue life is also determined by a fracture mechanics approach using a R6 based fracture assessment and crack growth analysis. The results show a large difference in fatigue life depending on the method used. For the plastic analyzes it is of great importance to the result in which direction the evaluation is made. For the present case the maximum principal total strain range is obtained in the radial direction. The stresses in this direction are small. A crack tangentially to this direction lacks physical significance to a pipe geometry and cannot lead to failure. In this case it is recommended that evaluation is made for the strain range in the axial direction for pipe geometries instead of the maximum principal total strain range.

Length Change Pattern of the Medial Structures of the Knee and Related Reconstructions

Aims and Objectives: Chronic medial instability presents a severe problem both for the patient and the surgeon, and may result into anterior cruciate ligament graft failure in a combined anteromedial instability. Thus, reconstructions have been developed to conquer this problem. However, these are not capable of mimicking the flat anatomy of the medial structures of the knee. The goal of the present study was to examine the length change patterns of the native medial structures of the knee and their related reconstructions. It was hypothesised that the different portions of the medial collateral ligament present different length change patterns, which cannot be imitated by current reconstructions. Materials and Methods: Eight cadaveric knees were dissected of skin and subcutaneous fat. The satorius fascia was removed to get a clear vision of the medial structures. The knee was then mounted in a rig and the quadriceps muscle and the iliotibial tract were loaded, using cables and hanging weights, according to its fiber orientations and cross-sections. Threads attached to three tibial pins at the anterior/middle/posterior portion of the medial collateral ligament (MCL) were then guided to three femoral eyelets at the anterior/middle/posterior portion of the femoral MCL insertion and analogous with the tibial/femoral posterior oblique ligament (POL) insertion. A tibial pin was also put at the semitendinosus insertion to imitate the Lind reconstruction. Between 0-120 degree knee flexion, the distances between each possible tibiofemoral combination were measured using a linea variable differential transformer (LVDT). Statistical analysis was performed using two way repeated measurements ANOVA. Results: The anterior MCL showed an initial slackening (2%) until 20° flexion, followed by a tightening (5%) towards deep flexion (120°), meaning that it is tight in flexion. The posterior MCL also showed an initial slackening (4%) until 20° of flexion. However, then followed by an isometric area (20-80°) and a further slackening (8%) towards deep flexion (120°), meaning that it is tight in extension. The three portions of the POL showed a linear slackening between 0-120° (25%). The middle MCL showed a sine wave behaviour, slackening from 0- 60° (3%) and tightening between 60-100° (1%). This behaviour was similar in the Lind and Robinson reconstruction, which were the most isometric tibiofemoral combinations (total strain range: 5,3 ± 2,1). The native POL length changes showed the most non-isometric behaviour resulting into a total strain range of 28,8 ± 6,2, which was significantly different from the native MCL and MCL reconstructions (p< .001) Conclusion: The anterior, middle, and posterior parts of the MCL showed different length change patterns. The anterior part tightened in flexion, whereas the posterior part tightened in extension. This behavior could not be reproduced by the current reconstructions, such as the Lind and Robinson procedure, which only could imitate the middle portion of the native MCL.

Critical Review of Strain Measures for Characterisation of Fatigue Damage in ASME Section III Fatigue Assessments

Rules for fatigue evaluation of nuclear pressure vessels and piping components are provided in Subsection NB of Section III of the ASME code. The code prescribed fatigue procedure requires the comparison of an alternating stress amplitude with fatigue allowables (design fatigue curves), usually derived through uniaxial specimen testing. For elastic assessments of multiaxial loading, typical from thermal shocks, a Tresca stress is used to characterise the stress field into a single effective stress measure for comparison with ASME fatigue allowables. For nonlinear elastic-plastic assessments, Appendix XIII-3440(b) of Section III specifies that “the numerically maximum principal total strain range” (interpreted as Maximum Total Principal (MTP) strain range) should be used for comparison with fatigue allowables. Two alternative methods for the characterisation of multiaxial strain fields are presented in the ASME code. Section VIII Division 2 provides alternative rules for the construction of pressure vessels, with Part 5 specifying the use of a Von Mises based Effective Strain Range (ESR) for elastic-plastic analysis. Section III Division 5 Subsection NBB provides rules for the assessment of components at elevated temperatures, also specifying the use of a Von Mises based Equivalent Total Strain Range (ETSR) measure. The two alternative strain measures are differentiated by their treatment of the elastic strain contribution. In the ESR method an equivalent elastic strain is calculated and summated with the plastic strain component. In the ETSR method the total strain (elastic plus plastic) is used thus evaluating the elastic and plastic contributions simultaneously. More complex critical plane approaches have also been proposed in recent years to better characterise multiaxial loading conditions. This paper presents a comparison of the various ASME specified strain measures and simplified critical plane approaches for fatigue evaluation of complex multiaxial loading. In support of this comparison, predictions of initiation lives to 0.254 mm defect in the stepped pipe specimen reported in PVP2004-2748 are provided to quantify the additional conservatism contained in elastic-plastic fatigue assessments of nuclear components. Predictions use the methodology presented in the companion paper PVP2019-93847 for the generation of short crack fatigue curves and the associated modification to environmental enhancement factors. It is concluded that use of the ASME specified strain measures, in conjunction with lower bound stress-strain data, conservatively underestimate the initiation life to a 0.254 mm defect by a factor of four for the example considered. However, use of more complex critical plane strain measures were observed to provide significant improvement in prediction accuracy of elastic-plastic fatigue evaluations.

Low Cycle Fatigue Life Evaluation According to Temperature and Orientation in Nickel-Base Superalloy

Components of gas turbines must be extremely resistant to high temperatures, high stresses, high-temperature corrosion, and erosive environments. The materials used in these environmental conditions are mainly nickel-based superalloys. In this study, the low-cycle fatigue of the nickel-based superalloy Inconel 792 was examined. The total strain range of a gas turbine between 760 °C and 870 °C was considered as the parameter representing the actual gas turbine operation. In addition, tests were performed using a trapezoidal waveform of the total strain to reflect the operation-stop conditions of a gas turbine with frequent shutdowns. The results of the fatigue test were compared with the Coffin–Manson method and energy method. The fractured surface was analyzed using a scanning electron microscope (SEM).

Low Cycle Fatigue and Relaxation Performance of Ferritic–Martensitic Grade P92 Steel

Due to their excellent creep resistance and good oxidation resistance, 9–12% Cr ferritic–martensitic stainless steels are widely used as high temperature construction materials in power plants. However, the mutual combination of different loadings (e.g., creep and fatigue), due to a “flexible” operation of power plants, may seriously reduce the lifetimes of the respective components. In the present study, low cycle fatigue (LCF) and relaxation fatigue (RF) tests performed on grade P92 helped to understand the behavior of ferritic–martensitic steels under a combined loading. The softening and lifetime behavior strongly depend on the temperature and total strain range. Especially at small strain amplitudes, the lifetime is seriously reduced when adding a hold time which indicates the importance of considering technically relevant small strains.

Low Cycle Fatigue Behavior of Alloy 800H Base Metal and Weldments at 700°C

Alloy 800H is currently being considered as one of the near-term candidate materials for design and construction of some major high temperature components of a very high temperature reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low-cycle fatigue loadings of components. The aim of this work is to study the low cycle fatigue behavior of Alloy 800H base metal and weldments at 700°C. The weldment specimens were machined from gas tungsten arc welding (GTAW) butt-welded plate such that the loading direction was oriented transverse to the welding direction. Fully reversed total-strain controlled low-cycle fatigue tests have been performed at total strain ranges of 0.6, 0.9, 1.2 and 1.5%. For all the low-cycle fatigue tests, triangular test waveforms with a constant strain rate of 10−3/s were applied. Low-cycle fatigue testing was conducted in accordance with ASTM Standard E606 on servo-hydraulic test machines. And also, creep-fatigue experiments were carried out at 700°C employing 0.6% total strain range and 10−3/s strain rate using trapezoidal waveform with tension hold time. The main focus is to characterize the low-cycle fatigue properties for Alloy 800H weldment specimens from the cyclic deformation behavior and fatigue fracture behavior. The cyclic deformation behavior was influenced by total strain range and material property. The fatigue life was decreased with increasing the total strain range for both base metal and weldment. However, the lives of weldment specimens have a longer life than that of base metal at lower total strain ranges. It was also observed that creep effects play a significant role in fatigue life reduction.

AFM Study of Microstructure Role in the Cyclic Plasticity of Martensitic Steel

The paper presents the description of FSMs (fatigue slip markings) and the evaluation of cyclic plasticity markings of a 12%Cr martensitic steel by AFM surface analyses. The microstructure of a 12%Cr martensitic steel, quenched and tempered in air, consists of prior austenite grains, packets, blocks, and laths. The low cycle fatigue (LCF) test at a total strain range Δεt=1.2% was interrupted at different life fractions for the surface relief investigation by AFM. The localization of FSMs relatively to the different microstructural interfaces of the studied steel was proposed. The principal FSMs, appeared in the first fatigue cycle, are likely to be localized at the packet and block boundaries, while the secondary one (appeared later at 44% of lifetime) are at the lath boundaries or in the laths. The height of principal and secondary FSMs increases constantly during LCF cycling.