Number Of Stress Cycles Research Articles

Correlation between polymer relaxation time and loading frequency based on LWLRA

ABSTRACT In classical viscoelastic theory, phase difference between loading and viscoelastic solid response does not change if loading frequency remains constant. In our experiments, however, it was found that phase difference was gradually decreased as the number of stress cycles was increased. Using local weighted linear regression algorithm (LWLRA), experimental data were successfully analysed and phase difference values between loading and response were obtained. The relation between initial phase difference and loading frequency obeys power-law. Based on Maxwell viscoelastic model, the relationship between relaxation time and loading frequency was proposed, which could be described by a power function.

Failure cause analysis of an expansion bellow of integrated turbine unit

The bellow is a flexible component which compensates an expansion allowance imposed by axial strain during high temperature service. The bellow failure during turbine operation of boiler unit has been systematically investigated. The metallurgy of bellow was AISI 316L stainless steel. The comprehensive laboratory investigation includes visual inspection, chemical analysis, mechanical tests, metallographic and fractographic studies. Visual inspection in the field revealed multiple cracks in the bellow and failed debris were collected for laboratory investigation. The mechanical tests of the expansion bellow confirmed to material property specification. The microstructure revealed austenitic structure and fracture surface revealed striation marks. The striation marks attributed to fatigue failure. Finite element analysis revealed that maximum stress induced in the trough region and stress could be higher as compared to ASME VIII Div 1 design equations. It can be concluded that actual operating pressure range could resulted in sufficient stress cycles to initiate fatigue cracks from surface of the bellow. The large number of stress cycles owing to actual varied pressure range attributed to fatigue failure. It is recommended to reduce many intermittent operating conditions by good operational practice and installation of strain gauges at critical locations of the bellow. It is further proposed to increase the existing ply thickness and install reinforced expansion bellow & sandwich/layered plies for prevention of fatigue failure.

Fatigue Models Based on Real Load Spectra and Corrected S-N Curve for Estimating the Residual Service Life of the Remanufactured Excavator Beam

To more accurately predict the residual fatigue cycles and estimate the service life of the remanufactured excavator, the fatigue models integrating the corrected S-N curve, the RFC algorithm, the FEA model, and the mechanism dynamic model are established depending on the real load spectra under experimental working conditions and the corrected S-N curve of the beam metal remanufactured with the welding process. Depending on the test data of the unidirectional stress history and the servo displacements of the major cylinders, the mechanism dynamic model was first established to illustrate the real load spectra applied on the pivots of the working beam. The load spectra are further used in the finite element analysis (FEA) model to obtain the stress contours of the beam relevant to the sampling time, which is the stress spectra at any nodes on the beam in theory. Subsequently, the rain flow counting (RFC) algorithm based on the dual parameters of the cyclic stress amplitude and mean is established to provide the frequency spectra in the longevity region on the beam. Furthermore, due to the fatigue property changes of the beam metal remanufactured with the welding process, its S-N curve is corrected based on the detail fatigue rating (DFR) method to compute the stress cycles at each stress level on the crisis nodes. Finally, the total stress cycles that can be burdened by the remanufactured beam is computed under the Miner’s linear fatigue cumulative criterion. The total number of stress cycles is eventually converted to the fatigue and service life depending on the proportion of the sampling time under relevant working conditions. The results show that integrated fatigue models provide a practical approach to enhancing the accuracy of the estimation on the residual service life of the remanufactured excavator beam. It is significant for improving the reliability and safety of the remanufactured excavator.

Hemolysis and von Willebrand factor degradation in mechanical shuttle shear flow tester

Chronic blood trauma caused by the shear stresses generated by mechanical circulatory support (MCS) systems is one of the major concerns to be considered during the development of ventricular assist devices. Large multimers with high-molecular-weight von Willebrand factor (VWF) are extended by the fluid forces in a shear flow and are cleaved by ADAMTS13. Since the mechanical revolving motions in artificial MCSs induce cleavage in large VWF multimers, nonsurgical bleeding associated with the MCS is likely to occur after mechanical hemodynamic support. In this study, the shear stress (~ 600 Pa) and exposure time related to hemolysis and VWF degradation were investigated using a newly designed mechanical shuttle shear flow tester. The device consisted of a pair of cylinders facing the test section of a small-sized pipe; both the cylinders were connected to composite mechanical heads with a sliding-sleeve structure for axial separation during the withdrawing motion. The influence of exposure time, in terms of the number of stress cycles, on hemolysis and VWF degradation was confirmed using fresh goat blood, and the differences in the rates of dissipation of the multimers were established. The plasma-free hemoglobin levels showed a logarithmic increase corresponding to the number of cycles, and the dissipation of large VWF multimers occurred within a few seconds under high shear stress flow conditions.

High-Cycle-Fatigue Characterization of an Additive Manufacturing 17-4 PH Stainless Steel

On the one hand, many mechanical components manufactured through additive technologies are optimized in terms of stiffness/weight or strength/weight thanks to lattice structures. On the other hand, the high complexity of these components often impedes further finishing operations and, therefore, the fatigue strength can be compromised. The high surface to volume ratio together with the high roughness, typical of additive manufactured components, promote the crack nucleation. In this paper, the High-Cycle-Fatigue (HCF) behavior of the 17-4 PH stainless steel (SS) was characterized. Cylindrical samples, manufactured via Selective Laser Melting (SLM) with an EOS M280, were tested in the as-build condition through a STEPLab UD04 fatigue-testing machine. In particular, a preliminary quasi-static traction test was performed on a sample to obtain the yield strength (σY = 570 MPa) and the ultimate tensile strength (UTS = 1027 MPa). Fatigue tests were performed on samples at different stress levels in order to characterize the whole Stress-Number of cycles (S-N) curve (Wöhler diagram). More specifically, the stair-case method combined with the Dixon approach were exploited to calculate the fatigue limit (σF = 271 MPa). The obtained results were compared with those present in literature for the same material and they are coherent with previous researches

Essential structure of S-N curve: Prediction of fatigue life and fatigue limit of defective materials and nature of scatter

Historically, S-N curve has been expressed in the form of an exponential relationship between applied stress σ and number of cycles to failure Nf. This paper elucidates the essential structure of S-N curve based on the prediction method for fatigue life and limit of materials containing defects from the viewpoint of mechanics of small crack. The extended application of the proposed method to step loading and variable amplitude loading is discussed. The question about the applicability of Miner’s rule is also discussed from the viewpoint of continuous decrease in fatigue limit with number of stress cycles and crack growth.

Fatigue and Tensile Properties of Aramid Fibers Measured by Single Fiber Mechanical Tests

Aramid fibers, with superior mechanical properties, have been widely used in a lot of industrial fields. From a safety point of view, long-term durability is required for these aramid fibers. In this study, the fatigue and tensile properties of two types of single aramid fibers, poly(para-phenylene terephthalamide) (PPTA) and poly(para-phenylene-co-3,4′-oxydiphenylene terephthalamide) (PPODTA), were investigated. Single fiber tensile and fatigue tests, cyclic tensile tests, tensile tests for post-fatigue test specimens, and optical microscopic observations were conducted. The stress–strain curves of the cyclic tensile tests indicated that PPTA and PPODTA fibers could be deformed above the elastic limit, and the strain at 2.0 GPa stress and the initial tensile modulus in subsequent tensile tests were changed by the cyclic stress. In fatigue tests, the cumulative probabilities of fatigue failure increased with the number of stress cycles for both the PPTA and PPODTA fibers. The fatigue behavior was influenced by the stress conditions and gauge length of the specimens. The initial tensile modulus in subsequent tensile tests was increased by the fatigue tests for both the PPTA and PPODTA fibers. However, regarding the tensile strength, significant decreases due to the fatigue tests were not observed. Fibrils detached from the fiber surface were observed in the post-fatigue test PPTA and PPODTA fiber specimens by optical microscope observations, with the loss of these fibrils contributing to the changes in modulus.

Experimental and analytical study of random fatigue, in time and frequencies domain, on an industrial wheel

Industrial wheels are components subject to fatigue under a high number of stress cycles, depending on the type of vehicle they are installed to.The reliability of these components is strictly dependent on the accuracy of the fatigue validation method adopted.A common practice in the industry is to test loads under constant amplitude in laboratory through fatigue and test parameters according to the standards used in the industry.The object of this research is to compare the results in terms of damage and the number of failure cycles, adopting both the time domain and frequency domain approach on a real industrial component.Different theories were developed and applied to the real case of study of an industrial wheel, under specific load cases. Eventually, we applied different criteria to the numerical analysis.The added value of this study is the application of the fatigue criteria on a real industrial component, in order to check the reliability of the criteria.Eventually, we determined that the results show that, for these specific experimental load conditions, frequency-domain methods are a bit more conservative than time-domain methods.

ASPECTS REFERRING TO FATIGUE TESTING OF EPOXY POLYMERIC MATERIALS

Concomitant with the development of new technologies in usage of renewable energies and with the necessity of replacing metal, epoxy polymeric materials are becoming more frequently used. Their main advantages are the reduced weight, high mechanical and corrosion resistance, the possibility to recover, recondition and reutilise the components. Thus, they become suitable for various industries, such as: automobile manufacturing, wind turbine blades, naval industry etc. Increased fragility and low resistance in initiating and developing cracks lead to varied fatigue performance. One way of reducing these disadvantages is to test fatigue in order to observe and analyse its manifestation, before projecting components. This paper presents tests carried on cylindrical epoxy polymeric samples. The samples were manufactured through casting in tubular moulds because of great advantages of this method. The manufacturing time is short, the material usage is maximised and after casting, the samples are smooth and don’t have sharp edges. Hence, the risk of unwanted concentrated stress is eliminated. The testing means assessing the samples to variable loading in rotating bending. For a better precision of test results, it is very important that his method could be accurately repeated with samples, preparing them in similar conditions. The results are used to draw Wohler`s curve using stress-number of cycles (S-N) as coordinates, specific to the tested material. Finally, the level of maximum stress of a material that resists fatigue, without having any ulterior damage can be determined.

ИССЛЕДОВАНИЕ ИЗНОСОСТОЙКОСТИ ЧУГУННЫХ ЗУБЧАТЫХ КОЛЕС С ДИФФУЗИОННЫМИ КАРБИДНЫМИ ПОКРЫТИЯМИ

The article is concerned with wear resistance increase of saw-tooth enclosed drives applying instead of iron gear wheels, made from cement steel quality, gear wheels from high-duty cast iron of HF 60 with thermodiffusion carbide layer. Elements- diffusants were chosen according to level of carbide forming and hardness of formed layersvanadium- for forming very hard, chrome-medium according to coating and manganese- for low layers according to coating hardness. Layers are formed on spur gears, which are reverse gear of light vehicle. Setting for research included draw gear- asynchronous electric drive, placed on lathe-bed with the help of which rotation of gear shift transmission with powered reverse gear was done. Transmission output shaft was pressed into chunk. Kinematick of disconnected machine provided necessary moment resistance. Through given number of cycles the box was taken down, gear set was removed and metal graphic research was carried out. Residual layer thickness and linear wear were diagnosed, on the basis of obtained experimental data we built characteristic curves of linear wear from number of stress cycles. For comparative evaluation in this type of research we carried out experiments on measurement of linear wear of case – hardening steel 20 ChGn after cementation and further quenching and research of hardened cast iron HF mark 60. It was found out that the highest quality wearing in this type of research vanadium layers have, slightly lower quality wear chromium plate have and on the 3rd place manganese layers. The worst quality wear we observed in steel 20 ChGN. On the basis of obtained –experimental data empirical regularities of wear rate of layers from their Brinnel hardness and diameter of obtained copy were determined.

Development and validation of a cumulative exposure shoulder risk assessment tool based on fatigue failure theory

Objective To present a new risk assessment tool for shoulder intensive occupational tasks based on fatigue failure theory. Methods The tool estimates cumulative damage (CD) based on shoulder moments and loading cycles using an S–N curve derived from in vitro tendon fatigue failure tests. If multiple shoulder tasks are performed, the CD for each is summed. In the validation, 293 workers were evaluated for five separate shoulder outcomes. Logistic regression was used to assess the log CD against five shoulder outcomes adjusted for covariates including age, sex, body mass index (BMI), and plant site. Results Both crude and adjusted logistic regression results demonstrated strong dose-response associations between the log CD measure and all five shoulder outcomes (continuous ORs ranged from 2.12 to 5.20). Conclusions The CD measure of The Shoulder Tool demonstrated dose-response relationships with multiple health outcomes. This provides further support that MSDs may be the result of a fatigue failure process. Practitioner summary This study presents a new, easy-to-use risk assessment tool for occupational tasks involving stressful shoulder exertions. The tool is based on fatigue failure theory. The tool was tested against an existing epidemiology study and demonstrated strong relationships to multiple shoulder outcomes. Abbreviations MSD: musculoskeletal disorder; NORA: national occupational research agenda; RULA: rapid upper limb assessment; REBA: rapid entire body assessment; S-N: stress-number of cycles; EDL: extensor digitorum longus; DPC: damage per cycle; CD: cumulative damage; UTS: ultimate tensile strength; FTOV: first time office visit; 3DSSPP: 3-dimensional static strength prediction program; AS: visual analogue scale; BMI: body mass index; CI: confidence interval; Nm: newton-metre; LiFFT: lifting fatigue failure tool; DUET: distal upper extremity tool; OMNI-RES: OMNI resistance exercise scale.

A lubricated wear model for determining wear surface geometry on journal-bearing surfaces

In this study, the friction fatigue mechanism is employed to describe the quantitative information concerning characterization of the steady-state wear process in journal bearings, which combined with the load sharing notion in mixed lubrication. For this approach, the asperity stress cycle number is estimated by combining the statistical dynamic contact model with the mixed elasto-hydrodynamic lubrication (EHL) simulation model. The maximum shear stress theory is applied together with the subsurface stress distribution of contact asperities to predict the average volume of wear fragment for asperity. Quantitative analysis of steady-state wear profiles for the bearing reveals that the maximum wear depth appears at both ends of the bearing. With the increase of running time, the wear gradually extends to the bearing centre, and the wear rate rapidly decreases to a certain value and then tends to be stable. With this lubricated wear model, the influences of surface roughness, external load, and shaft speed are also discussed. The proposed wear simulation method is based on the interdisciplinary theory, which includes the description of surface micro-morphology (Gaussian rough surfaces), the friction fatigue theory for explaining the generation of wear debris, and the coupling effect of wear and lubrication that is expressed by considering the wear surface geometry in oil film thickness equation. Additionally, the simulation results and available experiment data are compared to demonstrate the fatigue wear modeling scheme.

Conformational Transitions of Receptor Binding Domain of SARS-COV-2 under Mechanical Stress at 0K and Physiological Temperatures

Understanding the workings of the novel coronavirus (SARS-CoV-2) is crucial to develop counter therapeutic measures. SARS-CoV-2 gains entry into human cell by binding its receptor Binding Domain (RBD) of Spike protein (S1) to ACE2 receptors. In order to study the effect of mechanical stress on the RBD of SARS-CoV-2, it is modelled as viscoelastic material using Burgers Model. Strain response of RBD under constant stress is analyzed, which gives useful insights into the conformational transitions of RBD at 0K and physiological temperatures. The theoretical underpinning has shown that with increase in the number of stress cycles, the binding affinities of RBD conformational states to ACE2 receptor decrease, decreasing the binding reaction rate between ACE2 receptor and SARS-CoV-2. This analysis gives theoretical evidence that ultrasonic therapy and photo therapy (UV) can be potential candidates to reduce binding reaction rates between ACE2 and SARS-CoV-2.

Dynamic Visible Monitoring of Heterogeneous Local Strain Response through an Organic Mechanoresponsive AIE Luminogen.

Local strain concentration is critically important for damage formation of structural components. Therefore, it is of particular interest in developing the structural health monitoring (SHM) method for large-scale, full-field, and on-site monitoring of local strain response in complicated structural components in service. The present work investigated a SHM method based on a pure organic mechanoresponsive luminogen (MRL), 1,1,2,2-tetrakis(4-nitrophenyl)ethane, for heterogeneous local strain concentration. Invisible heterogeneous local strain response in complicated weld joints is transformed into visible fluorescence under monotonic tension and cyclic stress loading. The local strain (<15%) calculated by fluorescence intensity has a good agreement with the results obtained by the conventional digital image correlation method, indicating good measurement accuracy of the calibrated organic MRL method. The heterogeneity of local strain in complicated weld joints increases along with elongation and number of stress cycles. Moreover, the higher mean stress and stress amplitude can induce significantly higher accumulated local strain in the relatively soft fusion zone region. Compared with conventional strain measurement methods, the present organic MRL method opens up new possibilities for large-scale, full-field, and on-site monitoring of local strain concentration and damage in complicated structural components.

Assessment of Dynamic Response of Cohesionless Soil Using Strain-Controlled and Stress-Controlled Cyclic Triaxial Tests

Soils subjected to earthquake motions undergo random variations in stress, strain and frequency during the entire period of shaking. The spatial and temporal complexity of the strong motions is commonly addressed through simplistic strain-controlled or stress-controlled cyclic loading tests as a part of laboratory investigations. Such a methodology is utilized and reported in the present study for assessing the dynamic response of cohesionless sand obtained from River Brahmaputra, India. In order to assess the dynamic response and liquefaction potential of the cohesionless Brahmaputra sand, both stress-controlled and strain-controlled cyclic triaxial tests were performed on reconstituted cylindrical specimens prepared at different relative densities (Dr) ranging from 30 to 90%. The specimens were subjected to varying effective confining pressures (σ′c: 50–200 kPa), shear strain amplitudes (γ: 0.015–4.5%) and cyclic stress ratios (CSR: 0.05–0.3). For all the tests, the frequency of the applied harmonic loading was maintained at 1 Hz. The magnitude of excess pore-water pressure (PWP) generated during successive loading cycles of the strain-controlled tests was found to be considerably lower than that generated in a stress-controlled test. The strain-controlled test reveals a reduction in the development of excess PWP with the increase in confining pressure and relative density, thereby indicating a decrease in liquefaction potential with increasing confining pressure. The stress-controlled test highlighted that based on a particular CSR, an increase in confining pressure results in the requirement of higher deviatoric stress and smaller numbers of cycles are required to initiate liquefaction. Although apparently misleading, it is imperative that during earthquake, same deviatoric stress is applied on the entire soil deposit. Thus, for specimens at larger depths having higher confining pressure, the CSR value is smaller, and thereby successive deeper layers require more number of stress cycles to liquefy. Hence, the study revealed that both stress-controlled and strain-controlled tests can be successfully used to assess the dynamic properties and liquefaction potential of cohesionless specimens. Based on the findings, the developed cyclic shear stress ratio (CSR) = 0.5 and cyclic shear strain amplitude (γ) = 0.5% are considered as the limiting conditions to achieve the onset of liquefaction through strain-controlled and stress-controlled approaches, respectively.

Characteristics of Permeability Changes in Bituminous Coal Under Conditions of Stress Variation Due to Repeated Mining Activities

The factors affecting permeability change under repeated mining of coal seams are important study aspects that need to be explored. This study combined various stress variation characteristics of protective seam mining and simplified the stress path of repeated mining in protective seam mines. Based on the results from the bespoke gas flow and displacement testing apparatus, seepage tests for simulated repetitive mining were carried out. The results simulated the actual behavior very well. With any drastic increase in the mining influence, the axial deviation stress in the stress path increased, and the greater the difference in coal permeability during the unloading and stress recovery stage, the more substantial the increase in permeability. The change in coal permeability was significantly influenced by the severity of simulated repeated mining cycles. When the mining stress exceeded a critical value, the permeability of the coal sample increased with the increase in the number of loading and unloading cycles, but the reverse was true when the mining stress was lower than the critical value. The effective sensitivity of seepage to the applied stress decreased with an increase in the number of stress cycles. With a decrease in the deviation stress, that is, with lower severity of mining influence, the effective sensitivity of coal seepage to stress gradually decreased.

Fatigue crack initiation and propagation in re-entrant auxetic cellular structures

The investigation of the fatigue behaviour of an auxetic cellular structure made of Al-alloy 7075-T651 is presented in this study. The complete fatigue process of the analysed cellular structure is divided into the crack initiation (Ni) and crack propagation (Np) period, where the total fatigue life (Nt) is defined as the sum of Ni and Np. The crack initiation period, Ni, is determined using the strain life approach in the framework of the Fe-Safe computational code, where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical cross-section of the analysed cellular structure. The number of stress cycles, Np, required for the fatigue crack propagation from the initial to the critical crack length is also determined numerically using a finite element model in the framework of the Abaqus computation FEM code. The Maximum Tensile Stress (MTS) criterion is considered when analysing the crack path inside the cellular structure. Finally, experimental fatigue testing has also been performed to validate the computational results.

Micropitting Fatigue Wear Simulation in Conformal-Contact Under Mixed Elastohydrodynamic Lubrication

In this study, a physics-based fatigue wear model is proposed to evaluate the reliability and to predict the life of cumulative micropitting wear for lubricated conformal contacts on rough surfaces. The surface normal load, mean film thickness, and frictional shear traction are simulated by a mixed elastohydrodynamic lubrication (EHL) model for a stress prediction model to calculate the average maximum Hertzian pressure of contact asperities and unit with the statistical contact model and dynamic contact model to obtain the asperity stress cycle number. The wear formula is established through combining a micropitting life prediction model of surface asperities and a mean micropitting damage constant of asperities. The four dominant aspects affecting wear behaviors of the surface contact pairs, working conditions, structure and surface topographies, material properties and lubrication conditions are all taken into account in the model. It is a high-fidelity and comprehensive model that can be used to analyze and optimize the tribological design of rolling–sliding pairs in machinery. The micropitting fatigue wear modeling scheme is validated by comparison of theoretical calculations and available experimental wear data.

Tether analyses of offshore triceratops under ice force due to continuous crushing

Oil exploration and production in ultra-deep ice-infested waters demands an innovative structural form to withstand the ice loads. Offshore triceratops is one such new-generation innovative, compliant platforms, which show good adaptability in extreme sea state conditions even under the combined action of wind, waves, and current. As tethers are the crucial components in compliant platforms like a triceratops, it is of paramount importance to study the effect of dynamic ice loads on tether tension variation. The main aim of this study is to carry out the tether tension analysis under the ice force action in different ice sea states. The ice force time history is developed from the continuous crushing ice spectral model, and numerical studies are carried out using Ansys Aqwa by applying the ice force on buoyant legs. The study reveals that the tether tension variation is less than 10% even under extreme ice sea state. The parametric studies are also carried out to assess the effect of ice velocity, ice crushing strength, and ice thickness on tether tension variation. It is found that the maximum stress developed in the wires of tethers is very less compared to the yield stress of the material, ensuring no tether failure. However, a large number of stress cycles may lead to fatigue failure of tethers. Hence, fatigue analysis is also carried out to assess the fatigue damage and service life of tethers. The results show that fatigue damage increases from normal to extreme ice sea state. The results are also compared with open-water load case. Studies carried out under lock-in ice condition shows the increase in the fatigue damage and reduction in the service life of the tethers.

Evolution of the degradation mechanisms with the number of stress cycles during an accelerated stress test of carbon supported platinum nanoparticles

Development of high performance electrodes for polymer electrolyte membrane fuel cells (PEMFCs) requires fast screening of the catalysts for their catalytic activity and durability by subjecting them to a suitable accelerated stress test (AST). Electrochemical potential cycling in acidic media is one of such frequently used ASTs of platinum nanoparticle-based catalysts. The activity degradation of such catalysts during AST takes place through different mechanisms including particle size growth, dissolution and detachment of catalyst particles, electronic connectivity loss, etc. Here, we present a quantification of these degradation mechanisms with the number of potential cycles (N) during the AST of carbon supported platinum catalyst. Among various mechanisms, the activity degradation due to particle size growth attributed to the Ostwald ripening is found to be the most prominent during the initial part of the AST (N<500). On the other hand, the loss due to dissolution/detachment varies linearly during the AST cycling. Such understanding of the evolution of degradation mechanisms with N may be utilized effectively towards development of high performance PEMFC electrocatalysts.