Load Control Mode Research Articles

Effect of microstructure rafting on deformation behaviour and crack mechanism during high-temperature low-cycle fatigue of a Ni-based single crystal superalloy

The low cycle fatigue behaviours of a microstructure rafting Ni-based single crystal superalloy have been experimentally investigated at 980 ℃. Deformation of γ/γ’ phases and the corresponding dislocation configurations were investigated, highlighting rafting γ/γ’ morphology that contributes to crack initiation and propagation, as well as macro-scale accumulated plastic strain. Unlike the discrete slip lines of a virgin superalloy, intense slips developed along the parallel {111} slip plane result in crossed slip bands in the rafting superalloy. The decreased resistance of widened γ channels to dislocation movement, along with the prevention of dislocation cutting through γ’ precipitates in pre-existing dense dislocation networks, facilitates crack propagation in the γ channel in the slightly rafting superalloy. As the rafting state increases, the dislocation network loses its protective effect by reducing coherency stress and acting as a superdislocation source, which facilitates crack propagation along the γ/γ’ interface. Finally, a microstructure-based fatigue model is developed considering the reduction of deformation resistance induced by rafting. The fatigue loading control mode effect is introduced by a combination of resolved shear stress and tensile stress effects on crack initiation. The LCF life of rafting Ni-based superalloys significantly decreases under stress-controlled conditions compared to strain-controlled conditions due to the increase in cumulative plastic strain. However, the insignificant impact of the initial surface oxide layer on LCF life is revealed.

Role of Al2O3 reinforcements in polymer-based nanocomposites for enhanced nanomechanical properties: Time-dependent modeling of creep and stress relaxation

Alumina (Al2O3) based nanocomposites mark a paradigm shift in materials science, offering unprecedented opportunities for multifunctional applications. This study meticulously examines the mechanical properties of polymer nanocomposites (PNCs), with a specific focus on the impact of varying sizes and concentrations of Al2O3 nanoparticles. By investigating these parameters, the research enables the tailoring of mechanical properties to meet specific requirements. The study reveals remarkable enhancements in the reduced modulus (Er), reaching up to 350%, 175%, and 100% under load and displacement control modes for 25 nm, 80 nm, and 200 nm average particle sizes, respectively. Beyond mechanical strength, the research delves into the nanocomposites resistance to long-term deformation (creep) and their ability to maintain consistent performance under load (stress relaxation). Additionally, the tribological performance lays the groundwork for the development of high-performance materials. The insights gained from this study represent more than an incremental improvement; they signify a transformative leap forward in addressing the multifaceted challenges of advanced material technology, with the potential to revolutionize various industries.

Notch effect in tension-tension fatigue of short glass fibre reinforced polyphenylene sulfide composites

An experimental investigation of the fatigue behaviour of plain and notched specimens made of 40 % wt. short glass fibre-reinforced PolyPhenylene sulfide is presented. To this end, plain and V-notched specimens (with notch root radius ranging from 0.2 mm to 10 mm) were produced by injection moulding, and the final geometries were obtained by means of two different technological processes (i.e., directly after the injection moulding process and by machining them from injected plates). To investigate the effect of notch root radius and fibre orientation, tension–tension fatigue tests were carried out in load control mode, and the experimental results were reanalysed in terms of net stress amplitude defining the traditional stress-life curves. During the fatigue tests, the damage evolution was monitored using a travelling microscope to define the number of cycles spent for fatigue crack nucleation. Furthermore, the fracture surfaces were observed at the microscopic level to investigate the damage mechanisms and the actual fibre orientation was evaluated by computed tomography analysis. Then, by employing coupled finite element analysis of the injection moulding process and the structural behaviour, four different approaches available in the open literature were used to correlate the geometrical and/or manufacturing process effects in the fatigue strength of the considered material.

Cyclic deformation behavior of non-oriented electrical steel sheets

The cyclic deformation behavior of a fully processed, non-oriented electrical steel sheet is investigated in dependence on loading condition and control mode. Therefore, strain- and stress-controlled fatigue tests are performed to determine cyclic deformation curves in the low- and high-cycle fatigue regimes. With symmetric strain control, continuous cyclic hardening occurs, while in asymmetric stress control with non-zero mean stress, pronounced ratcheting takes place. Depending on the loading condition, the material can exhibit a transitional period with cyclic softening. This effect is present at intermediate amplitudes in the yield point region. It emerges as the dislocation movement is initially inhibited, while at higher amplitudes, cyclic hardening occurs from the very beginning. The evolving dislocation structures are characterized by the predominantly planar slip behavior of the alloy.

Nano indentation studies on ceramic thinfilms coatings deposited using sputtering process for energy applications

Nanoindentation technique is generally used for measuring thinfilm mechanical properties such as hardness, modulus and stiffness. Nanoindentation of ceramic thinfilms of SiO2, Si3N4 and Al2O3 was deposited by radio-frequency (RF) magnetron sputtering on the stainless steel (SS304) substrates using a nanoindenter. Under varied sputtering conditions, the “as-deposited” film was amorphous. The as-deposited thin film had a thickness of 200 nm. The amorphous film was loaded/unloaded only once while operating in load control mode. Hardness and Young's modulus, two mechanical properties of the ceramic thinfilms, were also measured. When SiO2, Si3N4, and Al2O3 thinfilms are deposited onto stainless steel substrates using an RF magnetron sputtering, the roughness of the ceramic thinfilms is in the range of 8 to 12 nm. The nanoindentation results were compared, the hardness of the coatings is in the range of 6 to 9 GPa, and these ceramic coatings can be used as an adhesive layer for multilayer thin film coating.

Fatigue behavior evaluation in Fe-20Mn-3Al-0.9C alloys by dynamic nanoindetation

The fatigue response by applying cyclic loads on individual grains using monotonic and cyclic nanoindetation to analyze the effect on load or displacement control in an austenitic Fe-20Mn-3Al-09C alloy were analyzed. The material response to the cycles considering the load control and the displacement control to properties such as hardness, elastic's modulus, and contact stiffness, was studied by analyzing the load-unload curves (P-h). The material softening Analysis showed that these properties decrease with increasing cycles due to the activation of deformation mechanisms. The elasticity's Modulus and hardness values for monotonic loads in load control presented values close to those reported in the literature. On the other hand, in displacement control, the values were lower. In load and displacement control mode, the first two cycles presented no overlap between unloading and loading between cycles, due to large irreversible plastic deformation and low elastic recovery after unloading. In the subsequent cycles, overlaps between the hysteresis curves were predominant, due to the activation of the deformation mechanisms.

An insight from energy index characterization to determine the proneness of rockburst for hard rock

The evaluation index and classification criterion of coal bursting liability are often used to evaluate hard rock rockburst proneness. In view of the great difference between the engineering conditions of embedding rocks (hard) and coal seam (softer), the evaluation results of rockburst proneness of hard rock are not accord with the actual situation. Aiming at this problem, rockburst tendency indices were studied by laboratory test in this paper. Based on the analysis of the data, it is concluded that the peak-strength strain energy storage index WETP and residual elastic energy index AEF can reflect the energy and rockburst proneness of hard rock from the elastic energy storage and residual elastic energy. By designing the cyclic loading and unloading test and the uniaxial compression test of rock, the loading rates of the above two kinds of energy indices were determined, and the determination and calculation process of the above two kinds of energy indices were clarified. The experimental results show that the linear elastic energy storage law obtained by cyclic loading and unloading of one sample is not very different from that obtained by multiple samples, both methods can be used for measurement. The peak-strength elastic energy density, pre-peak total input energy density and post-peak failure energy density of rock increase slightly with the strain rate, and the degree of rock fragmentation also has the same trend. The energy index of rock cannot fully reflect the rockburst proneness of rock. Based on the test process and results, in the case of hard rocks, 0.3–1 mm/min could be used for displacement control mode, and 50–140 kN/min should be used for load control mode. Extensometer or instrument sensor can be the strain acquisition methods.

Thermal-Mechanical Fatigue Behavior and Life Assessment of Single Crystal Nickel-Based Superalloy

Thermal-mechanical fatigue (TMF) tests and isothermal fatigue (IF) tests were conducted using thin-walled tubular specimens under strain-controlled conditions. The results of TMF tests showed a strong correlation between mechanical behavior and temperature cycling. Under different phases of temperature and mechanical loading, the hysteresis loop and mean stress of the single crystal superalloy showed noticeable variations between the stress-controlled and strain-controlled conditions. In the strain-controlled TMF test, temperature cycling led to stress asymmetry and additional damage, resulting in a significantly lower TMF life compared to IF life at the maximum temperature. Moreover, the OP TMF life is generally lower than that of the IP TMF at the same strain amplitude. The Walker viscoplastic constitutive model based on slip systems was used to analyze the TMF mechanical behavior of the single crystal superalloy, and the change trends of the maximum Schmid stress, the maximum slip shear strain rate, and the slip shear strain range were analyzed, and their relationship with the TMF life was investigated. Finally, a TMF life prediction model independent of the loading mode and phase was constructed based on meso-mechanical damage parameters. The predicted TMF lives for different load control modes and phases fell within the twofold dispersion band.

Ways to increase the maneuverability characteristics of the CCGT when operating in load control modes (for example, CCGT-450)

THE PURPOSE The article is devoted to the analysis of the problems that arise during the operation of heating CCGTs of the type CCGT-450 at reduced loads with their participation in the regulation of electrical and thermal loads. The description of the main limitations of the adjustment range of gas turbines, and the maneuverability characteristics of the CCGT, an analysis of some existing technical proposals for expanding the adjustment range is given. It is shown that the application of these proposals, expanding the boundaries of the adjustment range in small quantities, does not solve the problem as a whole. The description and research results of the proposed new technical solutions to expand the adjustment range and maneuverability characteristics of the CCGT - 450 are given.METHODS. When solving the tasks, the analysis, comparison and generalization of energy characteristics, modeling of modes were carried out.RESULTS. It is shown that: the use of methods for converting part of the central heating system and the central heating system as a whole of the T-125/150 steam turbine into a low-steam mode together with bypass steam distribution will expand the adjustment range to a level comparable to the adjustment range of condensing power units on gas-fuel oil;. when shutting down the CCGT-450 operating in condensation mode and when operating it in the GTU-CHP mode during periods of night failure of the electrical load, the conversion of the steam turbine to motor mode significantly improves the maneuverability and reliability characteristics of the steam turbine and the CCGT as a whole. conclusion. The use of methods for transferring part of the central heating system and central heating system as a whole and bypass steam distribution expand the adjustment range of the CCGT-450, respectively, by 7.8, 10.1 and 16.0% of the rated power; When the CCGT-450 is operating in condensation mode when the CCGT is stopped and when it is operating in the GTU-CHP mode during periods of night failure of the electric load, the steam turbine is transferred the MR significantly improves the maneuverability and reliability characteristics of the steam turbine by reducing the duration of start-up and loading of the steam turbine by 40 and 14.5 minutes, respectively, reducing starting fuel losses, eliminating temperature fluctuations of steam and metal of the steam intake organs of the central pump and the control stage during start-up;

The effect of tightening again on bolt loosening under transverse load: Experimental and finite element analysis

Bolt loosening is one of the most common failure modes in bolted joints. Tightening again is often used to compensate for the loss of clamping force in many critical applications. In this study, the effect of load control modes and lubrication conditions on bolt loosening were studied by experiments. Then, a series of experiments were carried out on the tightening again after bolt loosening under transverse load. Finally, in order to study the cause of fatigue fracture, FEA was carried out by using an accurate three-dimensional finite element model. The results show that the bolt loosening is mainly caused by relative motion between the two clamped plates. With oil lubrication, the bolt clamping force was able to reach the magnitude comparable to the initial preload when the nut is released to a specific angle (30° ∼ 60°) before tightening again. Nevertheless, the times of tightening again should be limited to avoid fatigue fracture of the bolt.

Simplified Elastoplastic Fatigue Correction Factor Analysis Approach Based on Minimum Conservative Margin

ASME and RCC-M codes specify an elastoplastic fatigue analysis technique: a simplified elastoplastic fatigue analysis method based on linear elastic analysis. In this method, the elastic strain range is multiplied by the elastoplastic correction factor (Ke) to envelope the actual plastic strain range for fatigue evaluation. The ASME or RCC-M provide the Ke parameters of typical materials, such as austenitic stainless steel and low alloy steel. However, how can the parameters of the material not included in the codes be determined? Based on the existing material Z2CND18.12 (nitrogen control) in the codes and taking into account various sensitive factors, the minimum conservative margin of Ke for this material is calculated, and then the parameters of nonstandard materials are determined iteratively based on the conservative margin. The sensitive factors include the different structure model, load types, the loading control mode, temperature value and the material constitutive model. Based this approach, the Ke parameters of TA16 are determined and verified by the transient with drastic change in temperature and pressure. The results of the case show that the simplified elastoplastic fatigue analysis can envelope the results of cyclic plastic fatigue analysis. The minimum margin approach established in this paper can reasonably determine the Ke value of materials beyond the ASME and RCC-M codes.

Effect of load control mode on the post-peak behaviour of siltstone

Effect of load control mode on the post-peak behaviour of siltstone

Post-peak Stress–Strain curves of brittle hard rocks under axial-strain-controlled loading

A super stiff rock mechanics testing machine called Stiffman is introduced in this paper. This test facility uses composite loading frames and relay loading by two loading hydraulic actuators and overcomes the technical difficulties of conventional rock mechanics testing machines such as insufficient loading system stiffness, inability to use axial-strain-controlled loading in the post-peak deformation stage, limited straining amount to reach the residual deformation stage. To validate the reliability of the newly developed test machine, a benchmark study was carried out first. Then, systematic uniaxial and triaxial compression tests under axial-strain-controlled loading for three brittle hard rocks were conducted and the results are compared with those obtained using a conventional rock mechanics testing system called Rockman. The test results of Rockman show that under uniaxial compression and triaxial compression under axial-strain-controlled loading at low confining pressures, the rock specimens failed violently in the post-peak deformation stage and it was not possible to obtain complete stress–strain curves of the rocks. On the other hand, it was possible to load the rocks under axial-strain-controlled loading in a stable fashion using Stiffman and complete stress–strain curves of the rocks under uniaxial compression and triaxial compression at low confining pressures could be obtained. Stiffman-tested rock specimens showed signs of spalling but stayed mostly in one piece. Finally, peak and residual strength envelopes of the three brittle rocks are given based on the test results of Stiffman. The experiment results show that Stiffman can be used to capture complete post-peak stress–strain curves of brittle rocks reliably under axial-strain-controlled loading. It provides a new platform for studying post-peak deformation behaviors of brittle hard rocks reliably in a loading control mode that other conventional test machines cannot provide.

Bi-level decentralized control of electric heating loads considering wind power accommodation in real-time electricity market

Flexible resources on demand-side, such as electric heating loads (EHLs), are perceived as an effective way of supporting the power balance of power grids with large scale renewable energy integration. However, it is difficult to quickly, effectively and evenly aggregate and schedule a large population of these loads because of their heterogeneity and geographic dispersion. The efficiency and effectiveness of optimal control are important for achieving the power balance between power source and load, especially in real-time scheduling. Moreover, the even load allocation between these heterogeneous loads also needs to motivate users participation in real-time power balance. To follow wind power in the real-time electricity market, this study presents a bi-level control method for EHLs based on distributed optimization computing, which considers the bi-level queuing method, different load control modes and multi-time scales between different control levels. First, each load group, which includes a large population of EHLs is controlled by a distributed server (DS); all the load groups are scheduled by the central management server (CMS), which deals with the wind farm or electricity retailer. Next, a distributed optimization model is built based on the bi-level control structure, in which the different power balance statuses and multi-time scales between the electricity market and EHLs are considered. Furthermore, considering the power balance on the multi-time scales, layer-by-layer load allocation optimization through the bi-level time-varying temperature queuing method, and the load-leveling optimization, serve as the sequential optimization problems. Finally, a simulation including 12,500 EHLs based on measured data in northern China is carried out. The results show that the proposed approach can effectively realize real-time power balance between wind power and EHLs, while maintaining uniform load allocation between the EHLs.

Effects of Notches and Defects on Dwell Fatigue Mechanism and Fatigue Life of Ti-6Al-4V ELI Alloy Used in Deep-Sea Submersibles

The notch (i.e., stress concentration) and defect are important factors influencing the conventional fatigue behavior of metallic materials. What is the influence of notches and defects on the dwell fatigue mechanism and fatigue life? In this paper, the effects of notches and defects on the dwell fatigue behavior of the Ti-6Al-4V ELI alloy used in deep-sea submersibles are investigated under the load control mode. It is shown that the dwell fatigue is insensitive to the defect size (190–438 μm) compared to the conventional fatigue. For notched specimens, they all present fatigue failure mode under dwell fatigue testing, and the dwell fatigue life is higher than that of the smooth specimen at the same local maximum stress. The dwell of the maximum stress has no influence on the fatigue life and failure mechanism for notched specimens. Moreover, the facet feature is observed in the crack initiation region for both the conventional and dwell fatigue of notched specimens. Electron backscatter diffraction observation indicates that the feature of the fine line markings on the facet in the image by scanning electron microscope is due to the steps on the fracture surface of the α grain.

Creep and fracture of warm columnar freshwater ice

Abstract. This work addresses the time-dependent response of 3 m×6 m floating edge-cracked rectangular plates of columnar freshwater S2 ice by conducting load control (LC) mode I fracture tests in the Aalto Ice Tank of Aalto University. The thickness of the ice plates was about 0.4 m and the temperature at the top surface about −0.3 ∘C. The loading was applied in the direction normal to the columnar grains and consisted of creep/cyclic-recovery sequences followed by a monotonic ramp to fracture. The LC test results were compared with previous monotonically loaded displacement control (DC) experiments of the same ice, and the effect of creep and cyclic sequences on the fracture properties were discussed. To characterize the nonlinear displacement–load relation, Schapery's constitutive model of nonlinear thermodynamics was applied to analyze the experimental data. A numerical optimization procedure using Nelder–Mead's (N-M) method was implemented to evaluate the model functions by matching the displacement record generated by the model and measured by the experiment. The accuracy of the constitutive model is checked and validated against the experimental response at the crack mouth. Under the testing conditions, the creep phases were dominated by a steady phase, and the ice response was overall elastic–viscoplastic; no significant viscoelasticity or major recovery was detected. In addition, there was no clear effect of the creep loading on the fracture properties at crack growth initiation: the failure load and crack opening displacements.

Experimental investigation on the rotating bending fatigue behaviour of glass and carbon fibre-reinforced epoxy composites

ABSTRACT Composite materials have seen a substantial increase in use in aerospace, aircraft and automobile industries as a result of the high expectations on material performance raised by emerging technologies. Fatigue loads are mostly inevitable in these applications. As a consequence, in addition to static strength, current designs require fatigue analysis. Hence, in this investigation, hollow cylindrical composites, constituted by glass and carbon fibres with an epoxy matrix, were considered for evaluating their static and fatigue behaviour. These specimens were tested in load control mode using an in-house built rotating bending fatigue machine at a test frequency of 10 Hz with zero mean stress. To determine the scatter in fatigue life of the developed composite specimens and to create life distribution graphs at any probability of survival percentage, the two-parameter Weibull distribution function was used. Results indicate higher shape parameters for both the composites, at lower stress levels, which represent less dispersions in the fatigue life under high cycle fatigue. Matrix cracking, interfacial delamination, fibre pull-out, and longitudinal splitting of the matrix along fibre failures are observed using a scanning electron microscope.

Mean Stress Effect on the Fatigue Life of 304L Austenitic Steel in Air and PWR Environments Determined with Strain- and Load-Controlled Experiments

The mean stress effect on the fatigue life of 304L austenitic steel was evaluated at 300 °C in air and pressurized water reactor (PWR) environments. Uniaxial tests were performed in strain-control and load-control modes, with zero mean stress and a positive mean stress of 50 MPa. A specific procedure was used for the strain-controlled experiments to maintain the strain amplitude and mean stress constant. The strain-controlled data indicate that the application of positive mean stress decreases the fatigue life for a given strain amplitude in air and PWR environments. The data also show that the life reduction is independent of the environments, suggesting that no synergistic effects between the mean stress and the LWR environment occur. The load-controlled experiments confirm that the application of positive mean stress increases fatigue due to cyclic hardening processes. This observation is much less pronounced in the PWR environment. All data were analyzed using the Smith–Watson–Topper (SWT) stress–strain function, which was shown to correlate well with all strain- and load-controlled data with and without mean stress in each environment. In the SWT–life curve representation, the life reduction in the PWR environment was found fully consistent with the NUREG-CR6909 predictions.

Development and application of large-scale model test system for underwater shield tunnels

In order to simulate the characteristics of underwater shield tunnels, a new large-scale model test system with high water pressure and high stress loading capacity was developed. The loading control mode of the hydraulic system can perform uniform loading and gradient loading. Moreover, the vertical and horizontal hydraulic loading can reach 0·8 MPa and 1·6 MPa. The water pressure loading system has a maximum capacity of 0·5 MPa. Based on an actual project, a model test of an underwater shield tunnel lining under real soil and water pressure conditions was carried out. New segment joints of the model were developed during the model testing. The lateral pressure coefficient at rest of the model strata was slightly larger than 0·5, and close to that of the field strata. It was also found that the water pressure had a reducing effect on the soil pressure under large buried depth and high water pressure conditions. This system can mostly meet the requirements of underwater or underground engineering model tests.

Multiaxial notch fatigue life prediction based on the dominated loading control mode under variable amplitude loading

AbstractA new calculation approach is suggested to the fatigue life evaluation of notched specimens under multiaxial variable amplitude loading. Within this suggested approach, if the computed uniaxial fatigue damage by the pure torsional loading path is larger than that by the axial tension–compression loading path, a shear strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage; otherwise, an axial strain‐based multiaxial fatigue damage parameter is assigned to calculate multiaxial fatigue damage. Furthermore, the presented method employs shear strain‐based and axial strain‐based multiaxial fatigue damage parameters in substitution of equivalent strain amplitude to consider the influence of nonproportional additional hardening. The experimental data of GH4169 superalloy and 7050‐T7451 aluminium alloy notched components are used to illustrate the presented multiaxial fatigue lifetime estimation approach for notched components, and the results reveal that estimations are accurate.