Creep Fatigue Research Articles

Laves Phase Precipitation Behavior in HiperFer (High Performance Ferritic) Steel with and without Boron Alloying

High-chromium ferritic stainless HiperFer steels were developed for high-temperature applications in power conversion equipment. The presented research describes the precipitation behavior of the Laves phase after the thermomechanical treatment of Fe-17Cr-0.6Nb-2.4W HiperFer alloys with and without the addition of 55 ppm boron. The boron-alloyed variant was produced with the aim of enhancing grain boundary strengthening and consequently increasing creep resistance. The focus is set on the effect of boron on the thermomechanically induced precipitation of (Fe,Cr,Si)2(Nb,W) Laves phase at grain boundaries. The addition of boron modifies the diffusion conditions in the area of grain boundaries. Consequently, the formation of Laves phase is promoted and the particle growth and coarsening process are suppressed. The impact of boron addition was validated by performing creep and thermomechanical fatigue testing in the standard processing state of HiperFer steel. In the B-alloyed variant, increased creep ductility through the modification of the particle-free zone widths at high-angle grain boundaries was encountered. Nevertheless, an optimized thermomechanical treatment is necessary to fully utilize the increased ductility effect for the creep strength optimization of the B-alloyed grade.

Creep fatigue analysis of DEMO divertor components following the RCC-MRx design code

In the DEMO fusion reactor, in-vessel components will be subjected to very high thermo mechanical steady and cyclic loads. A design check that is required by the RCC-MRx code used for nuclear installations and fusion reactors is a creep-fatigue check. The fatigue damage is caused by the pulsed operation of the fusion reactor while creep damage occurs during the hold time of loads at elevated temperatures. The temperature of the main divertor components is kept below that which causes creep by using cooling fluid that flows through channels fabricated within the components themselves. Other components such as the shielding liner and reflector plate supports on the divertor cassette cannot be cooled as such and so their temperature can rise high enough so that they sustain creep damage. In the presence of creep, the fatigue life of a component is reduced. In this work, a creep fatigue assessment of a representative simple geometry is carried out. The representative geometry is that of a thick cylinder under the action of steady and fluctuating loads similar to those seen by DEMO in-vessel components while in service. The cylinder example creep fatigue results are used as a benchmark and compared with those obtained using the creep fatigue assessment (CFA) tool developed at KIT (Karlsruhe Institute of Technology). Methodologies used for creep fatigue assessments within RCC-MRx are presented and explained and results discussed. The work should provide a contribution towards any necessary creep fatigue assessments of DEMO divertor components currently being developed.

Effect of Load Changing on Creep fatigue Life of Pipe Joints

Effect of Load Changing on Creep fatigue Life of Pipe Joints

Fatigue life evaluation of gold wire bonding solder joints in MEMS pressure sensors

The failure of the gold wire bonding solder joint of a MEMS pressure sensor caused by the thermal cycling test was investigated with thermal cycling experiments and fatigue simulations. The results show that the maximum stress and strain occurred at the root of the solder joint on the substrate. With the accumulation of strain caused by thermal cycles, the bonding area of solder joint became smaller and the shear force of solder joints reduced to 22.3% after 1600 thermal cycles. The solder joints on the substrate failed after 1940 thermal cycles in plastic fatigue simulations and 3.16 × 108 thermal cycles in creep fatigue simulations. Plastic fatigue is the major factor for thermal failure of gold wire bonding.

Experimental Study on the Influence of Different Loading Rates on Fatigue Mechanical Properties of Sandstone

Underground rock engineering often encounters long-term cyclic loading and unloading. Under the influence of this effect, the mechanical characteristics of rocks will inevitably change, which will affect the stability and safety of underground engineering. Therefore, it is necessary to study the fatigue characteristics of rocks under a certain period of action. With an RDL series electronic creep relaxation testing machine, fatigue loading and unloading tests of sandstone at different loading rates were carried out, followed by uniaxial compression on the samples. The study shows that the stress–strain curves of the uniaxial compression specimens have three stages: a compaction pore fracture stage, an elastic deformation stage, and an unstable fracture developing to failure stage. The stress–strain curves of the samples with a certain number of cycles of loading and unloading give the thinning and dense phenomenon, and the axial upper limit strain and axial cumulative residual strain gradually decrease as the loading rate increases. With the increase, the uniaxial compressive strength of the reloaded samples increases gradually, which is higher than the ordinary uniaxial compressive strength. In the process of cyclic loading and unloading, the internal particles of the sample present fracture and reorganization of the fragile structure and, at the same time, compaction stability.

Research on the Creep and Fatigue Evaluation Method of the Double-Layered Annulus Metal Hydride Bed Combined with Numerical Modeling and ASME Code

Applied to a hydrogen absorption-desorption cycle, the hydrogen storage bed will experience higher exchange temperatures and variety of mechanical load. Due to the complex structure of the double-layered annulus metal hydride bed and the importance of thermal stress on the failure of a metal hydride bed, the numerical modeling of its hot spot stress was carefully carried out. Moreover, in this paper, an analysis method considering the limit failure mode condition is proposed to deal with the stress analysis in the process of hydrogen absorption and desorption. According to the proposed analysis method, the maximum stress of thermal-structural coupling in the process of hydrogen absorption occurs at about 1/3 along the diameter direction and at the geometric mutation of the connection between the cooling pipe and the main body of the hydrogen storage bed in the process of hydrogen desorption. Apart from that, the hydrogen storage bed is also subjected to thermal-mechanical fatigue by the iterative process of absorption and desorption, and its operating temperature range is in the thermal creep temperature region. Based on the distribution of the stress and temperature, evaluation hot sites were selected and the ASME-NB and ASME-NH codes were used to evaluate fatigue and fatigue creep, respectively. The evaluation showed that the fatigue damage generated during its service life is small, while the creep damage is relatively large and the total damage generated during the service life of the hydrogen storage bed is within the safe range. Aiming at the structure and complex process of a double-layer hydrogen storage bed, creep and fatigue evaluation methods are proposed, various failure modes of hydrogen storage bed are highlighted, and the relationship between process parameters and life is established.

Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell: A dislocation density-based crystal plasticity study

To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800∘C, a physics-based crystal plasticity model that describes the complex rate- and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.

Effect of cure temperature on the mechanical properties of epoxy-aluminium single lap joints

ABSTRACT Mixed adhesive joints combining different stiffnesses and strengths in the same overlap is a good strategy to increase the mechanical properties. For this purpose, different adhesives with distinct curing temperatures are used, whose optimization is crucial to maximize the mechanical performance. In this context, this study considers Araldite® AV 4076–1/HY 4076 and Araldite® AW 106/HV 953 U adhesives to evaluate the effect of curing temperature on tensile strength, creep and stress relaxation response and fatigue strength of single lap joints. It was possible to conclude that both adhesives exhibited higher static strength, lower stress relaxation and lower creep displacements with increasing curing temperature, but while for the brittle adhesive this trend occurred up to 150°C for the ductile one it was only up to 70°C. After this value, all properties regressed due to the degradation of the ductile adhesive with temperature. Considering values corresponding to 25% of the tensile strength, for example, it was noted that the ductile adhesive relaxes 54.7% more than the fragile one and presents a creep displacement around 95.1% higher. On the other hand, while the fatigue life was shown to be insensitive to the curing temperature for the brittle adhesive, in the case of the ductile one it decreases after 70°C. Therefore, when these adhesives are combined into a mixed adhesive joint, the curing temperature must never exceed 70°C.

Data‐driven prediction of the probability of creep–fatigue crack initiation in 316H stainless steel

AbstractStainless steel components in advanced gas‐cooled reactors (AGRs) are susceptible to creep–fatigue cracking at high temperatures. Quantifying the probability of creep–fatigue crack initiation requires probabilistic numerical simulations; these are complex and computationally intensive. Here, we present a data‐driven approach to develop fast probabilistic surrogate models of creep–fatigue crack initiation in 316H stainless steel. We perform a set of Monte Carlo simulations based on the R5V2/3 high temperature assessment procedure and determine the sensitivity of the probability of crack initiation to loads and operating conditions. The data are used to train different supervised machine learning models considering Bayesian hyperparameter optimization. We discuss the relative performance of such models and show that a gradient tree boosting algorithm results in surrogate models with the highest accuracy.

On the compatibility of liquid sodium as heat transfer fluid for advanced concentrated solar thermal energy systems

The use of liquid sodium as a heat transfer fluid has shown great promise and application in nuclear power generation and it is now being utilized in concentrated solar thermal power (CSP) applications, owing to its favorable thermodynamic properties. Its implementation, however, comes with a unique array of technical issues in CSP applications, primarily the incompatibility of structural materials with liquid sodium in these operational environments. In this review, major damage mechanisms will be discussed, with a focus on their relevance to advanced CSP plants. Such mechanisms include corrosion, liquid metal embrittlement, carburization/de-carburization, erosion, creep, and thermal fatigue. The degradation factors such as impurities in the sodium (e.g. oxygen) and the dissolution of the structural material's alloying elements (Cr, Mn, Ni and Si, etc.) are also discussed. This review presents a holistic overview of these inter-connected mechanisms, and most importantly explores potential solutions to mitigate these issues, including better structural material candidates, robust plant operational parameters/design, better service life predictions, and improved purification and monitoring methods for stringent control of impurities. The future directions of research are also discussed to ensure the successful use of liquid sodium in the next generation of CSP technology.

Realistic modelling of microstructural features in numerical simulations of wave propagation in metals

Research in numerical modelling in recent years has enabled realistic simulations of wave propagation in three dimensions in large volumes of material, while including features at small scale. A critical enabler has been the growth of computer power, especially Finite Element computations on GPUs. The presentation will start with the development of modelling capability for polycrystalline materials, for which extensive recent work has created realistic simulations for wave speed and grain-scattering attenuation, representing the microstructure at grain scale. This topic has received much attention in theoretical work over several decades, so the simulations have been helpful to evaluate and understand the theoretical models and the physics of the behaviour. Subsequently, the modelling has addressed several other microstructural interests, including simulations for creep damage and fatigue damage, each of which cause reductions of wave speed; models of equivalent macro material properties for these are developed based on studies of the microstructural information. Finally, recent work will be presented on wave propagation and scattering in Titanium alloys containing Macro Textured Regions (MTRs/Macrozones); these comprise microstructures of mixed scale, for which there is interest to use ultrasound to characterise the MTRs (large scale) against a background of a smaller scale regular microstructure.

Microstructure and mechanical properties of in-situ oxide-dispersion-strengthened NiCrFeY alloy produced by laser powder bed fusion

Oxide dispersion strengthened (ODS) superalloys are widely used in aerospace and nuclear reactors due to their exceptional creep strength and fatigue crack resistance at elevated temperature. In this work, an ODS NiCrFeY alloy for high temperature applications was fabricated by laser powder bed fusion (L-PBF). The microstructure and mechanical properties at room and elevated temperature of the L-PBF processed ODS NiCrFeY alloy were investigated. It was demonstrated that equiaxed grains with large fraction of low angle grain boundaries and cellular structures with high density of dislocations were obtained in the as-built alloy. Dispersed Y2O3 nanoparticles were in-situ formed in the ODS NiCrFeY alloy. Most of the Y2O3 nanoparticles were located at the cellular walls and pinned with a large number of dislocations. Due to the strengthening effects by dispersed Y2O3 nanoparticles and fine cellular structures, the ODS NiCrFeY alloy showed high ultimate tensile strength of 810, 560 and 250 ​MPa at room temperature, 600 ​°C and 800 ​°C, respectively.

Study on thermodynamic behaviour of natural gas and thermo-mechanical response of salt caverns for underground gas storage

Using salt caverns for underground gas storage (UGS) is an important energy storage solution. A coupled aero-thermo-mechanical (ATM) model with an integrated wellbore–3D realistic cavern geometry was established to investigate the thermodynamic behaviour of natural gas and the thermo-mechanical response of rock salt for UGS caverns. Unlike classical thermo-gas-mechanical (TGM) models that assume the cavern gas is an ideal gas and in an isotropic thermodynamic state, the ATM model solved thermodynamic properties for the real gas, and revealed the tempo-spatial varying gas state and its mechanism. Predictions of gas temperature, pressure and flux by ATM modelling well captured the in-situ measurements obtained by the distributed optical cable monitoring system during a one-month field testing at Jintan UGS site. Effects of the tempo-spatial varying gas state on the mechanical response were unveiled by ATM modelling. Predictions of mechanical response by TGM modelling shared similar patterns as that by ATM modelling, however, the TGM model tended to underestimate the risk of creep failure, tensile failure, fatigue failure and plastic damage due to the isotropic gas state. The proposed model could be used to improve the thermodynamic and mechanical predictions that are pertinent to the optimal design and the stability of caverns.

Evaluation of fatigue and creep-fatigue damage levels on the basis of engineering damage mechanics approach

Progressive degradation of material mechanical properties in the low cycle fatigue (LCF) and creep-fatigue (CF) interaction at high temperature affects the safe operation of in-service materials. By considering material degradation, the present work aims to establish a method for evaluating LCF and CF damage levels with wide applicability. Material-level data accumulations as well as theoretical foundations of LCF and CF are presented, including interrupted LCF and CF tests, subsequent tensile tests and energy-based damage models. A damage variable representing the degradation of material mechanical properties is then defined based on the tensile plastic strain energy density (TPSED), the physical mechanism of which is reflected in the microstructure evolution and fracture appearance. By taking into consideration the material degradation threshold in the traditional damage summation rule, a new three-dimensional (3D) damage interaction diagram is established, where the additional third axis indicates the material degradation level. Finally, taking GH4169 alloy and P92 steel as examples, this work demonstrates the implemented procedures of damage level evaluation, which has been validated via the experimental data.

The Reliability of SAC305 Individual Solder Joints during Creep–Fatigue Conditions at Room Temperature

The failure of one solder joint out of the hundreds of joints in a system compromises the reliability of the electronics assembly. Thermal cycling is a result of both creep–fatigue mechanisms working together. To better understand the failure process in thermal cycling, it is crucial to analyze both the effects of creep and fatigue mechanisms in a methodical manner. In this work, individual solder junctions are subjected to accelerated shear fatigue testing to investigate the effects of creep and fatigue on joint dependability at room temperature. A modified fixture is used to conduct fatigue tests on an Instron 5948 micromechanical tester. SAC305 joints with an OSP surface finish were cycled under stress control at first, and then the strain was maintained for a set amount of time. In this investigation, three stress amplitudes of 16, 20, and 24 MPa are used, together with varying residence periods of 0, 10, 60, and 180 s. The fatigue life of solder junctions is described for each testing condition using the two-parameter Weibull distribution. Additionally, as a function of stress amplitude and residence time, a dependability model is created. For each testing scenario, the progression of the stress–strain loops was studied. By quantifying relevant damage metrics, such as plastic work per cycle and plastic strain at various testing circumstances, the damage due to fatigue is distinguished from creep. To investigate the relationships between plastic work and plastic strain with fatigue life, the Coffin–Manson and Morrow Energy model is used. The results indicate that using greater stress magnitudes or longer dwell periods significantly shortens fatigue life and dramatically increases plastic work and plastic strain. The housing impact is significant; in some circumstances, testing with a longer dwelling period and lower stress amplitude resulted in more damage than testing with a shorter dwelling period and higher stress levels. When illustrating the fatigue behavior of solder junctions under various stress amplitudes or dwellings, the Coffin–Manson and Morrow Energy model were both useful. In the end, general reliability models are developed as functions of plastic work and plastic strain.

An integrated unified elasto-viscoplastic fatigue and creep damage model with characterization method for structural analysis of nickel-based high-temperature structure

This paper proposes an integrated unified elasto-viscoplastic fatigue and creep damage model with modified hardening equations to simulate the elasto-viscoplasticity, primary-secondary and tertiary creep, relaxation, cyclic softening, temperature-dependency, and creep-fatigue interaction (CFI) damage. For this purpose, we integrate the creep damage and fatigue damage model into the viscoplasticity-damage model to predict tertiary creep and cyclic softening behavior caused by creep and fatigue damage modes. Interaction effects between each damage mode are considered by the creep-fatigue interaction (CFI) damage scheme. This paper also introduces a genetic algorithm-based integrated characterization method that can effectively optimize material input parameters of the unified elasto-viscoplastic-damage material model with limited experimental data. Implicit Euler's backward iteration scheme is adopted to improve the convergence and accuracy of solutions. The proposed material model is implemented in UMAT for finite element (FE) analysis of high-temperature structures under cyclic-dwell loading. Experimental data of tensile, dwell, creep, and cyclic loading at various temperatures are used to verify the model. Finally, full-scale turbine blade FE analysis is performed under anisothermal conditions. The proposed material modeling framework can also be utilized for other high-temperature structures, such as power plants, leading-edge nose cones, etc.

Effects of strain rate on the tensile and creep-fatigue properties of 316H stainless steel

Since the unstable start-ups and shutdown processes of nuclear power plants, the effect of strain rate on tensile and creep-fatigue (CF) properties has attracted attention. This paper aims to study the strain rate dependent tensile and CF behavior of 316H stainless steel. The tensile and CF tests were independently performed with different strain rates of 0.0001, 0.0002, 0.0005, 0.001 and 0.005 s−1 at 600 °C. Due to the dynamic strain aging (DSA), the tensile stress-strain curve appears serrated flow phenomenon and shows negative strain rate sensitivity (SRS), which means the tensile strength of the material does not increase significantly with an increase of the strain rate, and even decreases. Meanwhile, it was found that peak tensile stress and inelastic strain increase in CF tests under low strain rates. The CF life of 316H steel was predicted using the hysteresis energy method. At the same time, based on microstructure and fracture observation, different dominant damage mechanisms were discussed.

Creep-fatigue characteristics of rock salt under different loading paths

The salt behavior subjected to creep-fatigue load is meaningful to ensure the stability of the salt cavities used for underground energy storage. The salt specimens are tested under three paths of creep-fatigue load, including creep followed by fatigue (C + F), fatigue followed by creep (F + C) and creep-fatigue (CF). The stress-strain curves, deformation, loading modulus and acoustic emission (AE) count were systematically analyzed. The axial, lateral and volumetric deformations present a four-phase and two-phase change trend at C + F (F + C) and CF loading, respectively. The deformation curve for an individual creep or fatigue stage at C + F and F + C loading resembles that in pure creep or fatigue tests. The prior creep or fatigue load decreases the strain rate in the subsequent fatigue or creep phase. The prior creep load decreases the loading modulus of rock salt in the subsequent fatigue stage, and the loading modulus presents a downward trend with increasing the cycle numbers for the salt samples at C + F loading. The hold time during the loading-unloading stage at CF loading exerts little effect on the variation trend of loading modulus. The hold time increases the irreversible deformation for each cycle and induces the salt samples to be prone to dilate at CF loading. Continuous AE activities were monitored in the creep stage at C + F loading, while minor AE responses develop in the creep phase at F + C loading and the hold time stage at CF loading. These results suggest that the combined creep and fatigue load in sequence and simultaneously result in a negative and positive interaction between them, respectively. The deformation behavior is explained from the viewpoint of micro-mechanism.

Remnant tensile and creep properties of aluminized AISI 321 austenite stainless steel under prior creep–fatigue interaction

AbstractAluminized AISI 321 steels applied as heat exchange tube are generally subjected to creep–fatigue (C‐F) exposure during service. Remaining tensile and creep performance of this steel at 620°C were therefore studied under prior C‐F deformation. Results revealed that residual properties exhibit an initial increase and followed degradation with rising lifetime fraction of prior C‐F. The tensile strength and creep lifespan reach highest at 30% lifetime fraction, since dislocation cell networks are well developed and secondary nanotwins are activated at substrate. Additionally, dynamic recovery and wavy slips occurred in coatings partially accommodate local plastic deformations and inhibit defect initiations, avoiding premature material fracture. On the other hand, these networks could restrain the increase in coating thicknesses. When the lifetime fraction of prior C‐F increases to 80%, declined remnant properties are observed, which is attributed to the recovery of dislocation cells and carbide coarsening. Meanwhile, coating microcracks also accelerate steel failure.

Study on the shear creep characteristics of anchored jointed rock masses under creep fatigue loading

Study on the shear creep characteristics of anchored jointed rock masses under creep fatigue loading