Creep Fatigue Research Articles

Establishment of unified creep–fatigue life prediction under various temperatures and investigation of failure physical mechanism for Type 304 stainless steel

AbstractInvestigations into creep–fatigue life and the corresponding failure physical mechanism are crucial for guaranteeing the structural integrity of components. In this work, a series of strain‐controlled fatigue and creep–fatigue tests were performed at different temperatures. Then, the EBSD‐TEM combinative analysis was performed to reveal the microstructure evolution. The creep failure parameter dependence derived from standard creep experimental data and their importance in further creep–fatigue employment were discussed. Results show that strain energy density has better relevance than ductility in connecting with creep failure. The temperature‐dependent critical strain energy density and an equivalent failure strain energy density, considering geometric effect, were incorporated with the current energy‐based model, which enables creep–fatigue life scatter within a factor of 1.5. Moreover, multi‐slip activations and severe slip interactions under creep–fatigue conditions were responsible for the ultimately lifetime reductions based on microstructure observations.

Life prediction of P91 piping accounting for creep fatigue conditions

ABSTRACT To address the lack of available standard codes that deal with interactions between creep and fatigue, which is generally approached with a simple superposition law, the parameter associated with CFCG is introduced, under the hypothesis of partial creep strain reversal based on the creep reversal parameter . The TDFAD methodology used to predict initial crack growth during creep and creep-fatigue regime with P91 grade steel operating at 600°C is reviewed and integrated. This state-of-the-art crack tip parameter is introduced and accounts for small-scale creep, in the creep-fatigue crack propagation assessment.

High temperature rheological properties of surface pretreated PVA fiber modified asphalt

In order to improve the compatibility of polyvinyl alcohol (PVA) fiber and asphalt in application of asphalt pavement, the surface of PVA fiber is pretreated with KH-570 silane coupling agent. The tests such as contact angle, rotational viscosity, temperature scanning, multiple stress creep and recovery (MSCR) and the linear amplitude sweep (LAS) are conducted to study the compatibility and high temperature rheological properties of original PVA fiber (T) and surface treated PVA fiber (ST) modified asphalts with different contents, so as to evaluate the effect of surface modification process on the rheological properties of PVA fiber modified asphalt. The results show that the surface treatment process improves the compatibility of asphalt and PVA fiber. Compared with T, ST has better viscosity increasing effect, rutting resistance, and permanent deformation resistance at the same content, and can slightly improve the fatigue resistance of asphalt at the lower strain level. However, T and ST have no obvious improvement on creep deformation ability and fatigue life of asphalt at high stress level under repeated load. The surface pretreatment process can effectively improve the storage stability, viscosity and high temperature deformation resistance of the binder and the results provide reference for the preparation and application of PVA fiber modified asphalt.

Using machine learning to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading

In this article, machine learning is used to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading, both of which represent the most complex loadings that couple creep, fatigue and oxidation damage. A uniaxial fatigue and fatigue–creep dataset, which was obtained for temperatures of between 300°C and 600°C for a low-alloy martensitic steel, is utilized in this study. Two different machine learning based approaches to lifetime prediction are demonstrated. The first approach is based only on a shallow neural network, whereas the second approach is proposed as a combination of a sequence learning based model – either long short-term memory network or gated recurrent unit – with the shallow neural network. A good correlation between the experiment and the prediction suggests that lifetime under complex thermo-mechanical loading can be reasonably predicted via the proposed machine learning based damage models.

Stress-Controlled Creep–Fatigue of an Advanced Austenitic Stainless Steel at Elevated Temperatures

Creep–fatigue interaction occurs in many structural components of high-temperature systems operating under cyclic and steady-state service conditions, such as in nuclear power plants, aerospace, naval, and other industrial applications. Thus, understanding micromechanisms governing high-temperature creep–fatigue behavior is essential for safety and design considerations. In this work, stress-controlled creep–fatigue tests of advanced austenitic stainless steel (Alloy 709) were performed at a 400 MPa stress range and 750 °C with tensile hold times of 0, 60, 600, 1800, and 3600 s, followed by microstructural examinations. The creep–fatigue lifetime of the Alloy 709 was found to decrease with increasing hold time until reaching a saturation level where the number of cycles to failure did not exhibit a significant decrease. Softening behavior was observed at the beginning of the test, possibly due to the recovery of entangled dislocations and de-twining. In addition, hysteresis loops showed ratcheting behavior, although the mean stress was zero during creep–fatigue cycling, which was attributed to activity of partial dislocations. Microstructural examination of the fracture surfaces showed that fatigue failure dominated at small hold times where the cracks initiated at the surface of the sample. Larger creep cracks were found for longer hold times with a lower probability of dimpled cavities, indicating the dominance of creep deformation. The results were compared with other commonly used stainless steels, and plausible reasons for the observed responses were described.

A New Creep-Fatigue Interaction Model for Predicting Deformation of Coarse-Grained Soil.

Studying the creep–fatigue interaction of the coarse-grained soil (CGS) is very important for safety assessment and disaster prevention in subgrade engineering. Current research work is mainly focused on single creep or fatigue deformation. In this paper, a new creep–fatigue interaction model is established to predict the creep–fatigue interaction deformation of different gradation CGS based on the rheological mechanics and the interactive relationship between creep and fatigue complex compliance method. Triaxial creep–fatigue interaction tests of different gradations CGS under different average stresses and frequencies were conducted to verify the new creep–fatigue interaction model. Research results show that for the creep–fatigue and fatigue–creep interaction, the fatigue deformation is always larger than the creep deformation under the same stress level. For the creep–fatigue multi-interaction, the second creep and fatigue deformation are always smaller than the first creep and fatigue deformation. The results of the triaxial creep–fatigue interaction tests verify the validity of this new model.

On multiaxial creep–fatigue considering the non-proportional loading effect: Constitutive modeling, deformation mechanism, and life prediction

In this paper, a series of strain-controlled fatigue and creep–fatigue tests under proportional/non-proportional loadings were performed for type 304 stainless steel at 873 K. Then, post-test metallographic observations were performed through the electron back scattered diffraction (EBSD) and transmission electron microscope (TEM) combinative characterizations. In this aspect, the wavy slip dominated deformation mechanism under non-proportional loadings was considered as the essence for additional hardening, while the introduction of creep resulted in further microstructure evolutions by facilitating recrystallization. Afterward, a unified viscoplasticity constitutive model was proposed to simulate the cyclic stress–strain responses, in which an additional hardening parameter combined with a loading-path parameter was used to describe the cyclic hardening curves. Concurrently, stress triaxiality was introduced to provide accurate descriptions for the stress relaxation behavior. Semi-physical continuum damage models involving multiaxial damage factor and non-proportional strain energy parameter was proposed to predict the multiaxial creep–fatigue damage evaluations. Good agreements between experimental data and simulated results were achieved with the help of the proposed numerical procedures.

Studies of High-Temperature Fatigue Behavior and Mechanism for Nickel-Based Superalloy Inconel 625

Strain-controlled continuous fatigue and creep–fatigue tests were carried out at 700 °C and 800 °C on Inconel 625 alloy. The effects of strain rate and tensile-hold time on cyclic stress response and fatigue life were investigated. Then, the microstructural analysis and the fractographic analysis of fatigue-fractured specimens were performed by scanning electron microscopy and transmission electron microscopy. The cyclic stress responses during high-temperature fatigue and the creep–fatigue–oxidation damage mechanism were discussed. The results showed that the strain rate and the tensile-hold time had little effect on the fatigue life at 700 °C, but there was a significant impact at 800 °C due to the creep–fatigue–oxidation interaction. The cyclic plastic deformation accelerated the precipitation of the γ″ phase, resulting in a continuous cyclic hardening and negative strain rate sensitivity. The fatigue failures at 700 °C under continuous fatigue conditions occurred with a transgranular fracture mode, while a transgranular-intergranular hybrid fracture manner was found at 800 °C. Furthermore, a frequency-modified total strain energy density model was proposed to consider the effects of creep and oxidation on fatigue life, and the predicted fatigue lives were located within the 1.5 times error band.

Effect of TaC/Ti/Si3N4 Hard Ceramics on Mechanical and Microstructural Behaviour of AA7075 Processed through Stir Casting Process

A liquid metallurgical stir casting route was utilized for developing aluminium alloy (7075) matrix composites blended with fluctuating wt% of silicon nitride (Si3N4), tantalum carbide (TaC), and titanium (Ti) for sake of generating a profitable composite with enhanced properties. The physical, microstructural, and mechanical characteristics such as density, high temperature tensile, fatigue, and time-dependent creep experiments were conducted and evaluated. SEM microstructural examinations were performed on the composite samples and proved that there was a good interfacial bonding and uniform dissemination among the reinforcement particulates in the matrix. Physical characteristics were analysed and the outcomes showed that AA7075 blended with 1 wt% TaC, 8 wt% Si3N4, and 0.5 wt% Ti proved higher experimental and theoretical densities of 3.2464 g/cm3 and 3.3038 g/cm3, respectively, with maximum porosity of 1.7758%. At 30°C, the tensile properties of the developed MMCs showed improved ultimate tensile strength (UTS) of 137.64 N/mm2. Beyond 100°C, AA7075 reinforced with 0.75 wt% TaC, 6 wt% Si3N4, and 1 wt% Ti proved to have a higher strength of 232.17 N/mm2. Fatigue properties of the developed MMCs discovered were found to be reformed when proportionate to the base alloy by operating at 14 × 103 cycles at the stress of 42.72 MPa. Time-dependent creep analysis of the developed MMCs found to be advanced due to the addition of tough ceramic particulates.

Time-dependent creep fatigue damage evolution in C/SiC composite: Theory and analytical prediction

In this paper, the time-dependent damage evolution in carbon fiber-reinforced silicon carbide ceramic-matrix composites (C/SiC CMCs) under creep fatigue loading at elevated temperature was analyzed using a micromechanical approach. Synergistic effects of creep fatigue peak stress level and damage in the matrix and the interface on time-dependent damage evolution in C/SiC composites were analyzed. Next, relationships of time-dependent creep fatigue damage mechanisms, damage parameters, environmental temperature and duration were determined. Then, the effects of composite constituent properties, creep fatigue damage state, environmental temperature, and duration on time-dependent damage evolution in C/SiC composites were analyzed. Furthermore, time-dependent composite strain response and interface debonding and oxidation state in C/SiC composite under different creep fatigue peak stress levels were predicted. Under creep fatigue loading, the composite internal damage evolution contributed to the increased time/temperature-dependent composite peak strain. Hysteresis-based damage parameters were developed and utilized to monitor C/SiC composite internal damage evolution. Under creep fatigue loading, the hysteresis area and width increased with oxidation duration to a peak value and remained constant, and the hysteresis modulus decreased with oxidation duration.

Thermal–Mechanical FEM Analyses of a Liquid Rocket Engines Thrust Chamber

The Italian Ministry of University and Research (MIUR) funded the HYPROB Program to develop regeneratively cooled liquid rocket engines. In this type of engine, liquid propellant oxygen–methane is used, allowing us to reach very good performances in terms of high vacuum specific impulse and high thrust-to-weight ratio. The present study focused on the HYPROB final ground demonstrator, which will be able to produce a 30 kN thrust in flight conditions. In order to achieve such a thrust level, very high chamber pressures (up to 50 bar) and consequently high thermal fluxes and gradients are expected inside the thrust chamber. Very complex and high-fidelity numerical FEM models were adopted here to accurately simulate the thermal–mechanical behavior of the thrust chamber cooling channels, accounting for plasticity, creep, and low-cycle fatigue (LCF) phenomena. The aim of the current work was to investigate the main failure phenomena that could occur during the thrust chamber’s service life. Results demonstrated that LCF is the main cause of failure. The corresponding number of loading cycles to failure were calculated accordingly.

Improved Kriging-based hierarchical collaborative approach for multi-failure dependent reliability assessment

Reliability assessment considering multi-failure dependency brings in highly complex computing tasks, which is impracticable for some complex structures like turbine cooling blades. To efficiently investigate the failure dependency between multiple frail sites and multiple failure modes, an improved Kriging (IK) model is first established by optimizing model parameters with dynamic hybrid ant colony optimal algorithm, and the IK-based hierarchical collaborative (IK-HC) method is further developed by absorbing the advantages of IK model into HC strategy. Based on the multidimensional Copula function with dependency capture power and the IK-HC method with high-fidelity computing power, the multi-failure dependent reliability framework is first built. The computing efficiency and accuracy of the proposed IK-HC method are pre-verified by several numerical cases, and the method is applied to the multi-site (i.e., blade root, front edge) multi-mode (i.e., low-cycle fatigue, creep fatigue) failure dependent reliability assessment of a high-pressure turbine cooling blades. Method comparisons reveal that the proposed method can perform the multi-failure dependent reliability assessment with high accuracy and efficiency.

Influence of 650 °C Thermal Aging on Microstructure and Creep–Fatigue Behaviors of P92 Steel

Abstract In this paper, P92 steel was subjected to thermal aging treatment at 650 °C for 800 h, and then basic mechanical and creep–fatigue test was performed. The creep–fatigue cycle response trend is consistent before and after aging. Subsequently, microscopic observation shows that P92 steel after aging still has typical lamellar martensite and prior austenite grains. The thermal aging of 650 °C resulted in more precipitates of martensite lath, obvious lath boundary, coarsening of martensite lath, and decreased dislocation density. Furthermore, thermal aging results in the increase of precipitates (Laves phase) and martensite width of P92 steel. The fine Laves phase located on the grain boundary can effectively nail the grain boundary and play the role of precipitation strengthening. Besides, the Laves phase located on the dislocation has the effect of diffusion strengthening, which prevents dislocation slip and improves the creep–fatigue resistance of P92 steel. Finally, four creep–fatigue life model parameters of aging P92 steel were obtained according to the test, including strain range partitioning (SRP), strain energy partitioning (SEP), frequency separation life model (FSL), and strain energy density exhaustion model (SEDE). The prediction results of the four models fall within the double tolerance zone. The SRP and SEP are found to be conservative, while the FSL and SEDE are recommended herein due to their suitability of predicting creep–fatigue life of aging P92 steel.

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

Unraveling the Complexities of Deformation/Damage Incurred in P91 Steel Weld Joint During Creep–Fatigue Interaction Loading at 873 K

Numerical simulation of thermal stress and life assessment of a thin double-layer metal hydride bed under combined thermal and mechanical loads

In this work, the finite element model of the hydrogen absorption process in the thin double-layered metal hydride bed (MHB) has been established. The Johnson–Cook constitutive model, considering the temperature softening and strain rate hardening effect, was used to simulate the mechanical behavior of 316L stainless steel during the hydrogen absorption and desorption processes. The simulation results agree well with the experimental results, which verifies the effectiveness of the numerical model. The equivalent stress distribution of the stainless steel bed in the process of hydrogen absorption and desorption has been calculated by the aforementioned model. The influence of the defect depth and opening angle on tank safety has been calculated and analyzed considering the possible defects in the welding process and service life. Finally, according to the American Society of Mechanical Engineers boiler and pressure vessel code criterion, the fatigue–creep interaction condition of the MHB was predicted and analyzed. This work can provide an effective reference for the design of a hydrogen storage bed.

Dependence of Creep Strain and Fatigue Behavior on Surface Characteristics of Resistive Strain Gauges.

Creep behavior and fatigue life are important performance indexes that affect the long-term stability of resistive strain gauges. The resistive strain gauges, fabricated with wet etching and resistance trimming, present micro-morphology such as textures and uneven edges on the surface and side-wall profile of sensitive grids. This paper observed the micro-morphology of the sensitive grids by microscope and analyzed its range of geometric dimensions. A sine function was used to establish equivalent geometric models for the surface textures and side-wall profile. Based on time hardening theory and the S–N curve, the dependence of micro-morphology of metal resistive strain gauges on creep behavior and fatigue life was studied. The results indicate that the roughness of micro-morphology has an influence on creep behavior and fatigue life. The surface textures and side-wall profile lead to the increase of creep strain and the decrease of fatigue life in varying degrees. When 60% of the ultimate stress of the strain gauges is loaded, the average creep strain in steady-state calculated by the maximum roughness of the side-wall profile reaches up to 6.95 times that of the perfect flat surface. Under the condition of loading 70% of the ultimate stress and the same roughness, the fatigue life led by side-wall profile could be reduced to 1/25 of the textured surface. The obtained achievements promote an understanding for optimizing the fabrication process of resistive strain gauges as well as developing high-precision and long-life force sensors.

Life evaluation of sandwich panels based on coupled creep and fatigue damage evolution

Creep fatigue is the main failure style in sandwich panels due to the viscous behavior of adhesive. Focusing on the failure of interfacial adhesive layer, a life evaluation method for sandwich panels has been developed by coupling damage evolution of creep and fatigue. Creep failure and creep fatigue tests have been carried out by sandwich panel specimens. The life evaluation method has then been examined by sandwich panel specimen subjected to cyclic T-shaped loadings. It is found that the scattering of sandwich panel's creep fatigue life can be well covered and evaluated by considering the scatter of initial damage.

Creep-Fatigue Response, failure mode and deformation mechanism of HAYNES 282 Ni based superalloy: Effect of dwell position and time

Creep-fatigue (C-F) response of HAYNES-282 as a function of dwell position and time, along with corresponding deformation micro-mechanisms and failure modes are presented here. An inertia effect in inelastic strain beyond the total-strain reversal point is reported for the first time. Damage (DC-F) in the material is found to decrease in the order of both-dwell, compressive-dwell and tensile-dwell. DC-F, an effective stress relaxation per cycle and strain energy density are found to correlate well with life. A change in deformation mechanism from shearing to Orowan-looping explains the difference in damage as a function of dwell position. HAYNES 282 shows an inherent anisotropy in strain hardening behavior, being higher under compressive loading. We postulate that compressive dwell would be more damaging in such material, with dwell-time-softening nullifying the beneficial effect of strain hardening of the same segment. Failure occurs through a mixed-mode, with dominance of either transgranular or intergranular fracture, depending on dwell position and time.

Effects of the solution and first aging treatment applied to as-built and post-HIP CM247 produced via laser powder bed fusion (LPBF)

In this work CM247LC, a low weldable Ni-Based alloy, was produced using selective laser melting (SLM). Despite the initial process parameter optimization, the low defect volume fraction was still uncompliant with manufacturing standards. This condition is principally caused by the high γ’ volume fraction which strongly affects the alloy weldability. Nonetheless, a crack free condition was eventually achieved applying a γ’-sub-solvus Hot Isostatic Pressing Cycle (HIP) which lowered the defects fraction down to 0.04%. The HIP cycle also demonstrated to play an important role in the stabilization of the microstructure, considerably limiting the carbides coarsening during the following heat treatment. Apart from the effectiveness of the healing process brought by HIP, the material microstructure still needs an optimization process which will be described along this paper. In fact, the Initial microstructure obtained after the printing process (the as-built condition) as well as the one obtained after HIP (post-HIP) won’t meet the desired requirements. Namely, the dendritic and γ’ free microstructure of the as-built material or the one with coarse and disordered particles obtained right after HIP, still need a tailored homogenization process. This paper will show how the combined effect of the solution and first aging treatment will profoundly alter the γ’ precipitation. More specifically, here, a new heat treatment recipe was developed to promote the precipitation of ordered cuboidal primary γ’ so as to improve creep and high temperature fatigue resistance. Moreover, the use of a γ’ super-solvus temperature allowed to achieve a γ’ volume fraction as high as 73% reducing its average size to 520 nm. At the same time, such heat treatment caused a profound alteration of the crystalline structures of the material promoting a general grain coarsening and the formation of equiaxial grains.

Understanding the relation between creep-fatigue fracture mechanisms and intergranular dislocation accommodation of a high chromium steel using nanoindentation characterization

Relying on nanoindentation technology, the effect of intergranular dislocation accommodation on fracture behavior for the P92 steel that had experienced creep-fatigue (CF) loading was studied. The results indicate that the fracture mechanism of P92 steel gradually changes from fatigue-dominating to creep-fatigue interactive damage with increasing dwell time. According to the variations of hardness (H), elastic modulus (E), creep resistance and strain rate sensitivity (m), a normalized damage indicator ln(H/E)/m is proposed to characterize the fracture mechanism. Thermally activated process of intergranular dislocation accommodation results in the formation of creep voids at GBs and a remarkable stress relaxation, leading to a creep-fatigue fracture mechanism for the CF specimens with long dwells. However, mechanically driven of dislocation accommodation contributes to enhanced grain size and lower hardness, resulting in a fatigue-dominated fracture mechanism.