可重复使用液体火箭发动机高温焊缝接头疲劳寿命评估与验证

GRCop-84 (Cu-8 at.% Cr-4 at.% Nb) is a new high-temperature copper-based alloy. It possesses excellent high-temperature strength, creep resistance and low-cycle fatigue up to 700 C (1292 F) along with low thermal expansion and good conductivity. GRCop-84 can be processed and joined by a variety of methods such as extrusion, rolling, bending, stamping, brazing, friction stir welding, and electron beam welding. Considerable mechanical property data has been generated for as-produced material and following simulated braze cycles. The data shows that the alloy is extremely stable during thermal exposures. This paper reviews the major GRCop-84 mechanical and thermophysical properties and compares them to literature values for a variety of other high-temperature copper-based alloys.

In this review article, research papers related to recent developments in Ni‐superalloy technologies have been reviewed in order to provide an insight into recent achievements and the potential for further study, research, and development in this field. In this paper, studies on various aspects of Ni‐based superalloys are reviewed, such as production methods, which include widely used casting methods, as well as unconventional alternative procedures, novel techniques, or simulation and prediction of certain alloy casting properties. Reviewing was done by categorising the papers into 4 major categories: manufacturing of Ni‐based superalloys, effects of alloying elements, physical and mechanical properties of Ni‐based superalloys, and defects in Ni‐based superalloys. The process used to make Ni‐superalloy parts can have a huge impact on the production process efficiency, the final product’s quality and properties, and the defects formed in it. Investment casting is one of the most common methods for making Ni‐superalloy parts. Manufacturing covers studies on various casting methods used to make Ni‐based superalloy components, novel techniques and methods developed to improve casting procedures to produce better products, and alternative manufacturing methods like AM and HIP processing. Similar to production process, the role of alloying elements is also very important. Even minor changes in their compositions can cause significant changes in the final product. Simultaneously, these alloying elements appear to be more efficient in the development of new methods to control product quality, suppress defect formation, and improve material properties such as the creep and fatigue. As a result, the effects of various alloying elements used in castings of Ni‐based superalloys are thoroughly examined. A material’s properties are its most important components. They assist the industrialist in selecting or developing a material based on the needs of the application/use. With this in mind, many researchers have conducted extensive research on physical and mechanical properties, as well as how to improve them. Fatigue life, stress rupture, creep properties, impact ductility, strain response, stress relaxation behaviour, and so on are some of the most important physical and mechanical properties of Ni‐superalloys. This article thoroughly reviews various studies on these properties, how and by what factors they are affected, and how they can be improved. Another important factor to consider when making Ni‐superalloy castings is defect formation, which can affect the properties of the final product. Freckle defects, hot tears, porosities, and slivers are some of the major defects that occur in Ni‐superalloys during the casting process. This article also reviews in detail about these defects, how they form, and how they affect the final product. These defects were found to have a significant influence on a variety of properties, such as creep, fatigue behaviour, and fracture mechanism. Topics and areas such as reinforcement of Ni‐superalloys with the help of CNCs and 3D printing of Ni‐superalloys that can provide scope for potential future research are highlighted based on the above‐reviewed papers.

A technique for conducting strain-controlled, thermochemical, axial-torsional fatigue tests on thin-walled tubular specimens was developed. Three waveforms of loading, namely, the axial strain waveform, the engineering shear strain waveform, and the temperature waveform were required in these tests. The phasing relationships between the mechanical strain waveforms and the temperature and axial strain waveforms were used to define a set of four axial-torsional, thermomechanical fatigue (AT-TMF) tests. Real-time test control (three channels) and data acquisition (a minimum of seven channels) were performed with a software program written in C language and executed on a personal computer. The AT-TMF testing technique was used to investigate the axial-torsional thermomechanical fatigue behavior of a cobalt-base superalloy, Haynes 188. The maximum and minimum temperatures selected for the AT-TMF tests were 760 and 316°C, respectively. Details of the testing system, calibration of the dynamic temperature profile of the thin-walled tubular specimen, thermal strain compensation technique, and test control and data acquisition schemes, are reported. The isothermal, axial, torsional, and in- and out-of-phase axial-torsional fatigue behaviors of Haynes 188 at 316 and 760°C were characterized in previous investigations. The cyclic deformation and fatigue behaviors of Haynes 188 in AT-TMF tests are compared to the previously reported isothermal axial-torsional behavior of this superalloy at the maximum and minimum temperatures.

Damage Mechanisms and Life Assessment of High-Temperature Components deals with the underlying causes of high-temperature failures and their effect on component life and reliability. The first few chapters develop the theory necessary to understand and analyze high-temperature damage phenomena, including fracture, creep, and fatigue. Various forms of embrittlement and corrosion are also addressed as are creep-fatigue, thermal fatigue, and welding defects. The chapters that follow discuss the practical implications of these phenomena, explaining how to assess damage and estimate the remaining service life of boiler tubes, turbine blades, reactor vessels, nozzles, and other components. Life-assessment procedures draw on a knowledge of design, material behavior, and nondestructive inspection techniques, which are covered as well. The book makes extensive use of data plots, diagrams, and images and includes many worked-out examples and case histories. It also serves as a ready source of material property data. For information on the print version, ISBN 978-0-87170-358-3, follow this link.

This paper presents an overview of the R5 high temperature creep-fatigue procedure for austenitic materials. In particular revision to the weld procedures to be incorporated in the upcoming version of R5 are discussed in detail. The main feature of the new approach is that the traditional fatigue strength reduction factor (FSRF) which is applied to both creep and fatigue strain predictions is separated into a local weld strain enhancement factor (WSEF) and weld fatigue endurance reduction (WER). The new method is applied to the example of a fillet welded attachment on a boiler tube where a significant increase in predicted life is illustrated by adopting the new method new method. The principal advantage stems from a reduction in the predicted stress at the start of creep dwells and hence the level of creep damage in a cycle.

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The fatigue properties of a low strength weld metal in a dissimilar welding joint in high cycle and very high cycle regimes were investigated by fully reversed axial tests in air at room temperature and 370°C. A clear duplex S-N curve existed as a result of the transition of fatigue failure mode from surface-induced failure to internal-induced failure at 370°C, while the S-N curve was continuously decreased at room temperature. A new model was successfully proposed to predict fatigue life, and interpret the crack initiation modes transition from surface inclusion to interior inclusion. It was concluded that cracks were initiated by competition among non-metallic inclusions, welding pores and discontinuous microstructures in high cycle regime. While in the very high cycle regime, non-metallic inclusions were the dominant crack initiation mechanism which depended on stress level, inclusion size as well as inclusion depth.

Abstract In recent years, the role of thermal power plants has shifted from providing a baseload to providing supplemental supply to compensate for fluctuations in the output of renewable energy sources. Thus, the operation of these plants involves frequent startup and shutdown cycles, which lead to extensive damage caused by creep and fatigue interactions. In addition, the piping utilized in thermal plants is subjected to a combined stress state composed of bending and torsional moments. In this study, a high-temperature fatigue testing machine capable of generating such a bending-torsional loading was developed. Creep-fatigue tests were conducted on P91 steel piping with weldment. The results clarified that the creep-fatigue life was reduced by the superposition of the torsional and bending moments and that it was further reduced by a holding load. It was shown that the creep-fatigue life of piping with weldment can be estimated accurately using the equivalent bending moment, which is composed of the torsional and bending moments. It was also confirmed that crack occurred in the heat-affected zone (HAZ) of the welded part, which has been often observed in actual thermal power equipment. From the finite element analysis, it was identified that cracking was initiated in the HAZ due to the accumulation of creep strain and increase in the hydrostatic pressure component during a holding load.

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The Paton Welding Journal, 2014, №02. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

The Paton Welding Journal, 2016, №03. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

This poster discusses Creep-fatigue life assessment of high-temperature weldments using the linear matching method.

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High-cycle fatigue tests were conducted to investigate the effects of temperature, stress ratio (R), specimen orientation, welding and specimen size on the fatigue behavior of type 316L stainless steel. The high-cycle fatigue test results indicated that the fatigue limits significantly decreased when the stress ratio (R) decreased. The corresponding fatigue limits were reduced to lower values when tests were conducted at 300°C, compared to those obtained at room temperature. The fatigue behavior and fatigue limits of standard and subsize specimens were observed to be consistent at both room temperature and 300°C. The constant life diagram was established from the S–N curves acquired. The fatigue limit strongly depended on the materials strength, which was a function of specimen orientation, test temperature, and welding processes. The dimension of the fatigue damaged area on a fracture surface increased as the stress ratio decreased. In the case of R=−1.0, the fatigue damaged region extended over the whole fracture surface. The subgrain boundaries after high-cycle fatigue tests were clearly demonstrated by their diffraction patterns, which were related to the dynamic recovery of multiple dislocations.

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The objective of this study is to examine the feasibility of the X-ray diffraction method for the fatigue life assessment of high-temperature steel pipes used for main steam pipelines, re-heater pipelines and headers etc. in power plants. In this study, X-ray diffraction tests were performed on the specimens simulated for low cycle fatigue damage, in order to estimate fatigue properties at the various stages of fatigue life. As a result of X-ray diffraction tests, it was confirmed that the full width at the half maximum (FWHM) decreased with an increase in the fatigue life ratio, and that the FWHM and the residual stress due to fatigue damage were algebraically linearly related to the fatigue life ratio. From this relationship, a direct assessment of the remaining fatigue life was feasible.

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The fatigue life and tensile strength of JLF-I steel (Fe-9Cr-2W-V-Ta) and its TIG weldment were investigated at the room temperature and 400 ℃. Four kinds of test specimens, which associated with the rolling direction and the TIG welding direction were machined. The base metal of JLF- I steel represented almost anisotropy in the tensile properties for the rolling direction. And the base metal of JLF- I steel showed lower strength than that of TIG weldment. Also, the strength of all materials entirely decreased in accordance with elevating test temperature. Moreover, the fatigue limit of weld metal was largely increase than that of base metal at both temperatures. The fatigue limit of JLF - I steel decreased in accordance with elevating test temperature. The fatigue limit of JLF-l steel decreased in accordance with elevating test temperature. The SEM fractography of tensile test specimen showed conspicuous cleavage fracture of a radial shape. In case of fatigue life test specimen, there were so many striations at crack initiation region, and dimple was observed at final fracture region as a ductile fracture mode.

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.

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This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses.

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Welding alloy 617 with other metals and alloys has been receiving significant attention in the last few years. It is considered to be the benchmark for the development of economical hybrid structures to be used in different engineering applications. The differences in the physical and metallurgical properties of dissimilar materials to be welded usually result in weaker structures. Fatigue failure is one of the most common failure modes of dissimilar material welded structures. In this study, fatigue life prediction of dissimilar material weld was evaluated by the accelerated life method and artificial neural network approach (ANN). The accelerated life testing approach was evaluated for different distributions. Weibull distribution was the most appropriate distribution that fits the fatigue data very well. Acceleration of fatigue life test data was attained with 95% reliability for Weibull distribution. The probability plot verified that accelerating variables at each level were appropriate. Experimental test data and predicted fatigue life were in good agreement with each other. Two training algorithms, Bayesian regularization (BR) and Levenberg–Marquardt (LM), were employed for training ANN. The Bayesian regularization training algorithm exhibited a better performance than the Levenberg–Marquardt algorithm. The results confirmed that the assessment methods are effective for lifetime prediction of dissimilar material welded joints.

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Resistance spot welding is a widely used joining process in the structural components of various engineering products such as white goods, aerospace equipment and especially automobiles. Hence it is crucial that the mechanical performance of such joints be well understood in order to design and manufacture reliable engineering products. In service, Spot-welded joints undergo fatigue failure under cyclic loadings. In order to examine and improve the fatigue life of spot welded structures, various assessment methods have been used on standardized test samples such as tensile shear (TS), modified tensile shear (MTS), coach peel (CP) and modified coach peel (MCP) specimens. The fatigue life studies have shown that among the most important design variables for spot-welded structures influencing joint strengths, sheet thickness, spot weld nugget diameter, number of spot welds and the joint type can be counted. In this study, a numerical examination was carried out to observe the effects of sheet thickness and nugget diameter on the fatigue life of tensile shear (TS) samples. In the analysis both Morrows mean stress and Coffin Manson approach were utilized. The performance of both methods was verified on the basis of experimental data available in literature. The elastic and plastic strains necessary for the calculations were found out by means of finite element method. The results presented herein provide variation of fatigue life predictions according to selected geometry parameters along with valuable interpretations that can be used as guidelines for designers.

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New laser-based welding technology leads to differences in the geometrical and mechanical properties of welded joints as compared to those encountered in conventional arc-welded joints. The present work concentrates on the effects of these properties on the fatigue resistance of steel butt joints welded using laser-based methods. The investigation exploited both experimental and theoretical methods.\n\nIn laser-based joints, the mechanical properties within the joint varied strongly, affecting notch stresses and strains. Therefore, the response analysis based on Neuber's rule requires a separate structural and local approach. The structural analysis, based on the stress-strain curve of parent material, gives the loading for the weld notch. This is applied to the local analysis, where the stress-strain curve of the heat-affected zone (HAZ) is used.\n\nThe initiation life of macro cracks of laser-based joints was observed to form an essential part of the total fatigue life, suggesting that the mechanical properties of the material in the weld notch have a strong influence on fatigue resistance. The strain-based approach was applied with several discrete growth steps to model the propagation of short cracks up to the length of a macro crack in the weld notch. Within each step, the number of load cycles causing the increase of fracture was calculated with the Coffin-Manson formula. The length of a discrete growth step, which describes the damage zone, was based on the multiple of the averaged grain size of the HAZ material. This growth length also defines the averaging distance of effective notch stresses and strains. The approach was validated with the results of fatigue tests, using welded miniature and plate specimens.\n\nThe new theoretical approach with several discrete growth steps was especially relevant to laser-based joints with fine-grained microstructure in the HAZ. The short crack modelling was required to attain reliable predictions for the fatigue resistance of welded butt joints when the life of the macro crack initiation constituted a significant portion of the total fatigue life or the initiation life varied as the function of the applied stress range. The outcome of this important result explained the increase of the slope of S-N curves from the generally accepted value of 3.0 for arc-welded joints up to values above 10 for laser-based joints.\n\nThe main reason for the differences in the initiation process of fatigue crack between the laser-based and arc-welded joints was the dissimilar grain size in the HAZ. The average grain size of the laser-based joints was about seven times smaller than that of the arc-welded joint. The other parameters causing the difference were the hardness of the weld materials and the weld size, which affected the joint stiffness, and thus the notch stresses and strains.

Alloy 617 is the one of the leading candidate materials for intermediate heat exchangers (IHX) of a Very High Temperature Reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. As a series of the work to better understand the LCF properties of Alloy 617 weld joints at high temperature, firstly, in this work, strain-controlled LCF testing of Alloy 617 base metal (BM) and weld joint (WJ) by a gas tungsten arc weld process (GTAW) were carried out. Fully reserved total-strain controlled LCF tests have been conducted at room temperature with four total strain ranges of 1.5, 1.2, 0.9, and 0.6%. For the LCF tests triangular test waveforms with a frequency of 0.25 Hz were applied. The present paper is to characterize the LCF properties for Alloy 617 base metal (BM) and weld joint (WJ) from the cyclic stress response behavior and fatigue fracture behavior, with the comparative method. The cyclic stress response behavior was influenced by the level of total strain ranges and the material properties. Though base metal (BM) had shown higher plastic strain accumulation, the observed fatigue life of the weld joint (WJ) is lower than the base metal (BM). Coffin-Manson relationship and energy-life models can be used to determine the fatigue life.

In this study, we comparatively investigate the low cycle fatigue behavior of Alloy 617 (INCONEL 617) weldments by gas tungsten arc welding process at room temperature and 800 °C in the air to support the qualification in high temperature applications of the Next Generation-IV Nuclear Plant. Axial total-strain controlled tests have been performed with the magnitude of strain ranges with a constant strain ratio (Rε = −1). The results of fatigue tests consistently show lower fatigue life with an increase in total strain range and temperature at all testing conditions. The reduction in fatigue life may result from the higher cyclic plastic strain accumulation and the material ductility at high temperature conditions. A constitutive behavior of high temperature by some cyclic hardening was observed. The occurrence of serrated yielding in the cyclic stress response was also observed, suggesting the influence of dynamic strain aging during high temperature. We evaluated a well-known life prediction model through the Coffin-Manson relationship. The results are well matched with the experimental data. In addition, low cycle fatigue cracking occurred in the weld metal region and initiated transgranularly at the free surface.

This paper is concerned with the effect of welding on the fatigue behaviour of X100 material for steel catenary risers. The methodology includes both modelling and experimental characterisation. The modelling combines (i) a physically-based yield strength model to capture the thermally-induced microstructural heterogeneity and associated spatial variations in relative contributions of the key strengthening mechanisms due to welding, and (ii) a five-material cyclic plasticity model with a Coffin-Manson strain-life fatigue model for prediction of cross-weld heterogeneity in cyclic plasticity and fatigue response. The combined non-linear isotropic-kinematic cyclic plasticity behaviour of the five weld joint constituent materials (PM, weld metal (WM) and heat-affected zone (HAZ) subregions) is implemented via a user material (UMAT) subroutine, including Kocks-Mecking monotonic-cyclic evolution of yield stress. The experimental methodology consists of tensile tests with digital image correlation (DIC) for X100 PM and cross-weld samples. The results indicate that the primary phenomenon driving the detrimental effect of welding on fatigue is the evolution of cyclic strain localisation in the inter-critical heat-affected zone (ICHAZ), leading to predicted ICHAZ failure.

The article presents the results of research on low cycle fatigue strength of welded joints of structural steel S960QL. Two types of butt welds were analysed: I-joints and V-joints. The tests were performed under load controlled using the total strain amplitude ε ac . Fatigue life analysis was conducted based on the Manson-Coffin-Basquin equation, which made it possible to determine fatigue parameters. High concordance was found of the adopted description model with experimental results. Studies have shown differences in the fatigue life of the various joints analysed, wherein I-joints showed about 20-50% higher fatigue life. Fractographic tests of fatigue fractures in joints revealed the details of fatigue cracking and differences in the propagation rate of fatigue cracks.

In the present study, the fatigue behavior and tensile strength of A6061-T4 aluminum alloy, joined by friction stir spot welding (FSSW), are numerically investigated. The 3D finite element model (FEM) is used to analyze the FSSW joint by means of Abaqus software. The tensile strength is determined for FSSW joints with both a probe hole and a refilled probe hole. In order to calculate the fatigue life of FSSW joints, the hysteresis loop is first determined, and then the plastic strain amplitude is calculated. Finally, by using the Coffin-Manson equation, fatigue life is predicted. The results were verified against available experimental data from other literature, and a good agreement was observed between the FEM results and experimental data. The results showed that the joint's tensile strength without a probe hole (refilled hole) is higher than the joint with a probe hole. Therefore, re-filling the probe hole is an effective method for structures jointed by FSSW subjected to a static load. The fatigue strength of the joint with a re-filled probe hole was nearly the same as the structure with a probe hole at low applied loads. Additionally, at a high applied load, the fatigue strength of joints with a refilled probe hole was slightly lower than the joint with a probe hole.

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This work aims to investigate on the low cycle fatigue life assessment, which is adopted on the strain-life relationship, or better known as the Coffin-Manson relationship, and also the strain energy density-based model. The low cycle fatigue test results of Alloy 617 weldments under <TEX>$900^{\circ}C$</TEX> have been statistically estimated through the Coffin-Manson relationship according to the provided strain profile. In addition, the strain energy density-based model is proposed to represent the energy dissipated per cycle as fatigue damage parameter. Based on the results, Alloy 617 weldments followed the Coffin-Manson relationship and strain energy density-based model well, and they were compatible with the experimental data. The predicted lives based on these two proposed models were examined with the experimental data to select a proper life prediction parameter.

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

The material quality, the deformation rate, the temperature and the stress state influence mechanical behaviour and properties of different materials. Due to this great variety of the influencing factors we do not have one model of general validity describing the behaviour of materials, but we have to use a great number of material constants in order to characterize the properties. The exponents of the Manson-Coffin, the Basquin and the Paris-Erdogan laws were applied for the verification of the connection among the fatigue fracture types. Own measured values and test results can be found in the literature were used for the illustration of the connections. “Fracture surface”-s were determined for characterizing of different steel grades and their welded joints. It can be concluded that “fracture surface”-s are suitable for the describing of the fracture behaviour and the conversion of different fracture parameters of steels.

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Nowadays the marine industry plays an important role in our everyday life, offering a cost effective way for international freight transfer. From a financial point of view the more cargo can be transferred, the better. For a vessel with a set dimensions this can be achieved by increasing the deadweight, hence decreasing the lightweight. To that end, there is an increasing trend of application of HSS material, thin plate structures or both. Key challenges with these new structure is the consideration of their fatigue strengths.&#13;\n&#13;\nThe Thesis project deals with the fatigue life of fully penetrated fillet welds and the effects of the weld toe radius in HSS and LSS. A localised strain based approach was used for the fatigue strength assessment. This approach applies a step by step discrete crack growth simulation for the total fatigue damage process (crack initiation and propagation) within the LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) range, respectively. The model considers the effects of the microstructure, i.e. hardness and grain size for the two material, and the toe radius (R= 1 mm and 0.01 mm). Using Abaqus, the FEM analysis was carried out on a simplified parametric model of the fillet welds created with Matlab. The resulting stress and strain values were averaged using a characteristic length of the material, and the summation of the cycles, estimated by the Coffin-Manson relation for each discrete step, gave the overall fatigue life of the welded structure.&#13;\n&#13;\nThe predicted fatigue strength and the shape of the S-N curve for both materials are in line with expectations. However, due to the material model used, where the strain vs life curves of the two materials differ significantly in the LCF range but seem to be similar at HCF one, the resulting S-N curves differ. The smooth-weld, R= 1 mm without any initial cracks seem to have a higher FAT class for both materials, whereas the weld with R= 0.01 mm gave a lower FAT class than the IIW recommended values. Also, the long crack propagation rate is reasonably in line with the recommendations for fillet welds. The differences of the short crack initiation and propagation period are significant for the two materials, whereas the long crack propagation and final fracture showed fairly similar behaviour. &#13;\n&#13;\nThe initiation period in the presence of a larger toe radius, R= 1 mm lasts longer and is more dominant for the HSS material than for the LSS material. Hence, for this geometry, the HSS material could offer a better fatigue resistance. On the contrary, a smaller toe radius, R= 0.01 mm, or the presence of initial cracks of the size of 0.1 or 0.2 mm, seems to significantly lower the fatigue strength more for the HSS, than for the LSS weldments. This result shows the high dependency on the geometry, related to the actual material and implies that, due to the higher notch sensitivity of HSS, the fatigue strength will be lower for the R= 0.01 mm case than for the R = 1 mm. This tendency is not prominent in the LSS material.

Du to high strength and toughness, high oxidation resistance, and high ductility, the Inconel 600 alloy is an ideal choice for the components used in combined heat and power turbines. Therefore, in this paper, the authors conducted experimental tests to better underestand the mechanical behavior of superalloys Inconel 600. The experiments included tensile, fatigue, and creep tests. The material deformation and stress-strain behavior were measured. In addition the yield strength, ultimate tensile strength, elongation, and modulus of elasticity were captured. By illustrating the engineering and true stress-strain curves the Ramberg-Osgood relation were extracted. As a result of fatigue test, the relationship between strain amplitude and the number of cycles to failure for specimens were obtained. The creep tests were conducted at a constant temperature of 650℃. The strain-time data were collected, and the resulting creep strain-time curve were plotted for smooth samples under their respective stress conditions. REFERENCES G. Becerra, M.R.B. Alvarez, V.H.L Morelos, A. Ruiz. Creep behavior and microstructural characterization of Inconel-625/Inconel-600 welded joint. MRS Adv. 8, (2023) 1217–1223, https://doi.org/10.1557/s43580-023-00662-7. Baig, S.H.I. Jaffery, M.A. Khan, M. Alruqi, Statistical analysis of surface roughness, burr formation and tool wear in high speed micro milling of Inconel 600 alloy under cryogenic, wet and dry conditions. Micromachines 14(1) 2023, 13. https://doi.org/10.3390/mi14010013. Nanaware, S. Pawar, M. Ramachandran. Mechanical characterization of nickel alloys on turbine blades. REST J.E.M.M., 1(1) (2015), 15–19. D. Kwon, D.K. Park, S.W. Woo, D.H. Yoon, I. Chung. A study on fretting fatigue life for the Inconel alloy 600 at high temperature. Nucl. Eng. 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Materials 12 (2019) 3165. https://doi.org/10.3390/ma12193165. Kuzmanov, B. Borisov, I. Muhtarov. Tensile testing of Inconel 600 wire at high temperatures. IOP Conf. Ser.: Mater. Sci. Eng. 878 (2020) 012057.https://doi.org/10.1088/1757-899X/878/1/012057. V.C.S. Rao, A. Manoj, B.R. Swathi. Residual stress measurement of Inconel 600 on different welding techniques by using conventional and XRD methods. Mater. Today: Proc. 41 (2021) 1160–1163. https://doi.org/10.1016/j.matpr.2020.09.403. Reza Kashyzadeh, G.H. Farrahi, A. Ahmadi, M. Minaei, M. Ostad Rahimi, S. Barforoushan. Fatigue life analysis in the residual stress field due to resistance spot welding process considering different sheet thicknesses and dissimilar electrode geometries. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 237(1) (2023) 33-51. ‏ https://doi.org/10.1177/14644207221101069. A. Monteiro, S.L.V. Silva, L.V. Silva, A.H.P. Andrade, L.C.E. Silva. Characterization of nickel alloy 600 with ultra-fine structure processed by severe plastic deformation technique (SPD). J. Mater. Sci. Chem. Eng. 5 (2017) 33–44. https://doi.org/10.4236/msce.2017.54004. Yi, G.S. Was. Stress and temperature dependence of creep in alloy 600 in primary water. Metall. Mater. Trans. A 32, (2001) 2553–2560. https://doi.org/10.1007/s11661-001-0045-6ю Karthik, S. Swaroop. Laser shock peening enhanced corrosion properties in a nickel based nconel 600 superalloy. J. Alloys Compd. 694 (2017) 1309–1319. https://doi.org/10.1016/j.jallcom.2016.10.093. Makuch, M. Kulka. Microstructural characterization and some mechanical properties of gas-borided Inconel 600-alloy. Appl. Surf. Sci., 314 (2014) 1007–1018. https://doi.org/10.1016/j.apsusc.2014.06.109. ASTM E8-04: Standard Test Methods for Tension Testing of Metallic Materials, ASTM, https://doi.org/10.1520/E0008-04. 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ABSTRACT The purpose of this paper is to present a unified analysis to both high and low cycle fatigue based on shakedown theories and dissipated energy. The discussion starts with a presentation of the fatigue phenomena at different scales (microscopic, mesoscopic and macroscopic) and of the main shakedown theorems. A review of the Dang Van high cycle fatigue criterion shows that this criterion is essentially based on the hypothesis of elastic shakedown and can therefore be expressed as a bounded cumulated dissipated energy. In the low cycle fatigue regime, recent results by Skelton and Charkaluk et al . show that we can speak of a plastic shakedown at both mesoscopic and macroscopic scale and of a cumulated energy bounded by the failure energy. The ideas are also justified by infrared thermography tests permitting a direct determination of the fatigue limit.

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The main objectives of the present study were the application and validation of the newly proposed Digital Image Correlation equivalent structural strain approach for assessing the low-cycle fatigue life of S235 welded joints. Low-cycle fatigue tests were performed at a displacement ratio of minus one. Experimental tests were performed using two different ways of controlling the displacement amplitude: applying traditional low-cycle fatigue tests at a constant amplitude and stepwise succession tests at increasing amplitudes. A comprehensive, independent experimental procedure, proposed by the authors and not yet validated for steel welded joints, was applied to assess the equivalent structural strain range using the Digital Image Correlation technique for the traditional low-cycle fatigue tests and stepwise succession tests at increasing amplitudes. It is noteworthy that the values of the DIC equivalent structural strain (ΔEs from the DIC), calculated on the external sides of the samples, were utilized to predict fatigue life in correlation with the ASME mean curve and fall within the ±3σ scatter bands (external bands). In particular, most of the tests lie within the ±2σ boundary of the design curves except for some tests at low applied displacements. Moreover, it was shown that this method is applicable to stepwise succession tests with increasing displacement amplitudes, leading to significant time savings compared to conventional experimental tests.

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Abstract Strengths for monotonic and cyclic loadings of similar overmatching Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si (Ti6242) linear friction welds (LFW) were studied and compared with the parent material (PM) behaviour. Non‐destructive synchrotron observations revealed the presence of pores in the weld interface. The weld centre zone (WCZ) showed a higher strength leading to lower macroscopic ductility of the cross‐weld samples. Local strain and normalized strain rate have been assessed by stereo digital image correlation (DIC) and revealed an early plastic activity at yielding in the vicinity of the WCZ attributed to residual stresses. For the target life, the fatigue strength was slightly reduced but compromised by a strong scatter. Indeed, an internal fish‐eye fatigue crack initiation was found on an unexpected dendritic defect that was very different from the PM microstructure and the known martensitic α ′ in the WCZ. The dendritic defect was linked to surface contamination prior to welding and led to melting.

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The aim of this research was to examine the mechanical and fatigue properties of friction stir welded Sc-modified 5 mm thick AA2519-T62 extrusion. The joint was obtained using the following parameters: 800 rpm tool rotation speed, 100 mm/min tool traverse speed, 17 kN axial, and MX Triflute as a tool. The investigation has involved microstructure observations, microhardness distribution analysis, tensile test with digital image correlation technique, observations of the fracture surface, measurements of residual stresses, low cycle fatigue testing, and fractography. It was stated that the obtained weld is defect-free and has joint efficiency of 83%. The failure in the tensile test occurred at the boundary of the thermo-mechanically affected zone and stir zone on the advancing side of the weld. The residual stress measurements have revealed that the highest values of longitudinal stress are localized at the distance of 10 mm from the joint line with their values of 124 MPa (the retreating side) and 159 MPa (the advancing side). The results of low cycle fatigue testing have allowed establishing of the values of the cyclic strength coefficient (k' = 504.37 MPa) and cyclic strain hardening exponent (n' = 0.0068) as well as the factors of the Manson-Coffin-Basquin equation: the fatigue strength coefficient σ'_f = 462.4 MPa, the fatigue strength exponent b = -0.066, the fatigue ductility coefficient ε'_f = 0.4212, and the fatigue ductility exponent c = -0.911.

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Abstract Melted zones, microcracks, shear bands, and elastic incompatibility of explosively welded materials are features that may initialize cracks at the interface and reduce fatigue strength. This study aims to determine the effect of interfacial defect-like structures on the fatigue strength of explosively welded corrosion-resistant plates. Cyclic axial loading was applied to seven distinct layer-by-layer compositions of Ti Gr 1, Zr 700 alloys, and carbon steels. The interfacial wave height as a metric of potential fatigue life influencing factors along with measured strain amplitude was applied as the input quantities for the Machine Learning based model, i.e. the Gaussian process for regression (GPR). This is a novel and successful application of GPR to estimate the effect of interfacial wave height on the fatigue life of explosively welded plates. For the first time, the effect of the interface feature on fatigue life was estimated quantitatively. The Digital Image Correlation technique was applied to measure the field of cyclic strain for the purpose of verifying if a single strain amplitude is representative of a heterostructured plate. It was found that interfacial wave height is an important feature and its increase by 100 µm reduces the fatigue life of analysed plates by 36%. Additionally, to validate the applicability of explosively welded plates to engineering structures under cyclic loading, the experimental fatigue lives were compared with the design curve of the American Society of Mechanical Engineers (ASME) code.

The aim of this scientific work was to evaluate the compression instability effects during static and low-cycle fatigue loadings of AA 5083 welded joints, commonly used in marine structures. Low-cycle fatigue assessment in marine structures is of utmost importance since high levels of plastic deformation can arise in the proximity of high-stress concentration areas. Displacement ratios equal to minus one and zero were used to perform experimental low-cycle fatigue tests. The tests were monitored by means of the Digital Image Correlation technique in order to detect the strain patterns, with particular attention paid to stress concentration areas, indicating that a specimen tends to buckle during high compression loads, for tests with a displacement ratio of minus one. The tests at displacement ratios equal to −1 showed a lowering of the strain–life curve revealing a considerable effect on compression instability. A nonlinear finite element modelling procedure, depending only on hardness measurements, was developed. The hardness measurements were used in order to assess the distinct mechanical properties of the different zones that were included in the finite element model. The finite element model results were compared to the data achieved by means of the digital image correlation technique, demonstrating that hardness measurements can help predict the low-cycle fatigue behaviour of welded joints and consider compression instability phenomena.

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It is intended as a practical and reproducible procedure to identify macroscopic cracks at an early stage and monitor crack propagation during fatigue tests. It consists of strain field measurements at the weld using DIC. Images are taken at fixed load cycle intervals. Cracks become visible in the computed strain field as elevated strains. This way, the whole width of a small-scale specimen can be monitored to detect where and when a crack initiates. Subsequently, it is possible to monitor the development of the crack length. Because the resulting images are saved, the results are verifiable and comparable. The procedure is limited to cracks initiating at the surface and is intended for fatigue tests under laboratory conditions. By visualizing the crack, the presented procedure allows direct observation of macrocracks from their formation until rupture of the specimen.

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