Life Curves Research Articles

Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Varied approaches to loading assessment in fatigue studies

In this paper, the authors want to draw readers’ attention to one long-standing question: which approach is preferable for estimation longevities in fatigue problem, the time domain approach (Rainflow) or the frequency domain one (Dirlik and others)? This question is important in engineering problems, particularly in problems of prolongation of the guaranteed service life. The discussion here is restricted by the longevity evaluation only at the post-processing stage of unidirectional loaded machine parts. It means the realizations might be recorded.Some experimental and speculative evidence of preferable use the Rainflow method is shown. Taking into account the huge computer's power nowadays, the question of the irrelevance of appellation to the calculation accelerating using the spectral methods is specially discussed.There are areas, where the spectral methods are really necessary. There is only a need to recommend the restriction of their application scope to these special situations. It seems there is no need in inventing new spectral complicated algorithms only to stress out at the end, that their result coincides with the Rainflow outcome. That might be confusing for the practicing engineers.Currently, the main attention among supporters of spectral methods is focused on non-stationary and non-Gaussian random processes. (to estimate the spectral density is impossible for non-stationary processes, according to definition). These researchers seem to have forgotten, that even for these complicated situations the decision has already existed: that is the Rainflow and its analogues.The paper shows extensive laboratory experiment results of random fatigue testing of aluminum flat specimens under regular (to build the fatigue curve) and irregular (random) loading. The fatigue life curve (Gassner curve) has been built. These results allowed to compare the existing computation methods of longevity estimation. In the particular situation of narrow-band process, the methods seem to provide comparative results.Considered are methodological issues related to the assessment of the necessary and sufficient realization length, the influence of RMS, cycle counting methods and some possibilities of computing resources saving when using the Rainflow method. The stability of the Rainflow estimates is confirmed. Some problems with the choice of parameters during the longevity assessment by the Dirlik method were noted.

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

AbstractSuperaustenitic steel Sanicro 25 has been subjected to out‐of‐phase thermomechanical fatigue cycles with 10‐min dwell at peak temperature in the temperature range from 250°C to 700°C. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated. The internal structure and nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X‐ray spectroscopy in scanning transmission electron microscope. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for fatigue crack nucleation and propagation. Effect of dwells in in‐phase and in out‐of‐phase cycling on the fatigue behaviour, on the modification of internal structure and damage mechanisms, were analysed and discussed.

Multiaxial fatigue life prediction based on a simplified energy-based model

Thanks to the reliable correlation with experimental data, conventional energy-based models are widely employed in multiaxial fatigue life prediction of structural components, by combining energy-based variables with reference life curves. However, in practical applications, additional calibration factors and incremental plasticity theories are usually indispensable to characterize fatigue damage along multiaxial loading paths, whose procedures are usually rather complicated and time-consuming. In this paper, a simplified energy-based model, as an alternative to the incremental plasticity-based model, is proposed to predict the fatigue life under multiaxial loading. Being different from conventional models, the proposed model is based on a series of multiaxial energy-life curves, which avoids the introduction of additional calibration factors. Particularly, to eliminate the model’s dependence on incremental plasticity theory, the effect of loading paths is captured by a loading path-dependent factor which is derived from the non-proportionality factor and the Moment of Inertia method. The availability of the proposed model is validated by reasonable correlations with experimental data of four kinds of materials under diverse loading paths.

Rubber–fibre composite modelling and its influence on fatigue damage assessment

AbstractA novel multiaxial energy‐based approach is presented and used to demonstrate the influence of different finite element (FE) modelling techniques on the prediction of the fatigue life of a rubber composite with long oriented fibres. It is shown that the simplest modelling methods using 2‐D elements with rebar layers, layered 2‐D elements or layered 3‐D elements do not allow for a precise determination of the critical location and damage value. In contrast, modelling methods with 3‐D matrix and discrete reinforcement provide much better results. The predicted critical location corresponds to the measured one, although the predicted fatigue life still differs from the measured results. The most complex microscopic modelling method shows the best agreement between the predicted and measured fatigue life. Because microscopic modelling is not suitable for modelling larger products made of rubber–fibre composite, it is also noted that modelling techniques with 3‐D matrix and discrete reinforcing elements can be used with the same accuracy if the fatigue life curve is obtained from measurements on the specimens made of composite material rather than the specimens made of the critical base material (rubber).

Character of Fatigue Damage under Axial, Torsional and Biaxial Loading of 316L Stainless Steel

Characteristic features of fatigue damage of 316L austenitic stainless steel cyclically strained axially in tension-compression, reversed torsion and combined axial-torsional mode were studied. All loading modes resulted in the formation of persistent slip markings (PSMs). Predominantly one slip system was activated in the case of axial and torsional loading while at biaxial loading, activation of several slip systems was involved. PSMs acted as sites of multiple fatigue crack initiation. The path of subsequent crack growth at a macroscopic scale differed considerably in dependence on loading mode and applied amplitude. The hardening-softening curves and fatigue life curves were evaluated and results were compared and discussed in terms of the type of applied loading.

Study on the shape change of crack growth rate curve under different forms of additional stress field

In this paper, FRANC3D is used to simulate the fatigue crack growth process under different additional stress fields to explore the key mechanical parameters influencing the fatigue crack growth curve. For finite large plate model with penetrating crack, the crack growth rate is calculated under different forms and different values of applied load and additional stress coupling, the fatigue life curves, as well as crack growth rate curves and Pairs formulas under different applied loads are obtained. Through the analysis, the relationship between the additional stress field and the parameters in Pairs formula of crack growth rate is obtained. Because of the existence of the additional stress field, the stress intensity factor amplitude of crack tip changes, which is the root cause of the change of crack growth rate curve shape.

Development of an enhanced single point milling procedure to screen metalworking cutting fluid performance in terms of tool wear when machining aerospace alloys

Metalworking fluids (MWFs) can greatly improve the machinability of materials and increase cutting tool life. There are a range of MWF products available on the market, however there are very few reliable low cost machining based fluid screening tests which can help select the most suitable candidate. This study developed a novel and rigorous single point milling (SPM) procedure carried out under controlled conditions, which would provide fluid performance differentiation for a range of typical aerospace alloys. The use of a single insert with a controlled geometry reduced machining variance and ensured performance repeatability. Tool life curves were used to determine optimum machining surface speeds for Inconel 718 (In718) of 80 m/min and Ti-6Al-4V (Ti64) of 160 m/min. Carrying out trials using five different cutting fluid products within a controlled tool life window clearly demonstrated that the SPM machining test was able to differentiate performance on both In718 and Ti64 material. Overall a 65% and 53% performance difference in tool life behaviour was observed between the best and worst performing fluids for In718 and Ti64, respectively.

Experimental Study on Medium and Low Cycle Fatigue Properties of Cast Steel GS20Mn5V

Owing to the remarkable advantages in mechanical behavior, cast steel nodes have been widely used in static structures. Nowadays, cast steel nodes also gain increasing popularity due to the superior fatigue performance in dynamic structures, but they are not yet widely used because the fatigue properties of cast steel are not well understood. In this paper, the fatigue test of cast steel GS20Mn5V commonly used in steel castings is carried out. The strength of medium and low cycle fatigue and the fatigue limit are obtained. The feasibility of the estimated S-N (fatigue stress versus life) curve is tested. The double logarithmic linear model (DLLM) and the reversed generalized Pareto model (RGPM) are used to fit the experimental data, and the comparison is made. The P-S-N (the relationship between fatigue stress and life at different survival rate) curve obtained by the RGPM is proposed. The results show that the estimated S-N curve is not suitable for low cycle fatigue life, fitting the experimental data with the RGPM is the best, and obtaining the P-S-N curve from the RGPM is feasible.

Effect of Voltage Waveforms of HVDC Converter Transformer on Lifetime Characteristics

The converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage waveforms superimposed with dc voltages would occur. The effect of these waveforms on the failure and endurance of the valve side winding insulation of the transformers is not yet understood, which is investigated here. In this paper, the voltage waveforms occurring in a converter transformer are obtained through PSCAD/EMTDC for a typical double pole ±500 kV, 1000 MW DC-link HVDC transmission system based on the CIGRE benchmark system. The waveforms are then amplified using a High Voltage amplifier for experimental breakdown investigations. Stepped-stress damage equalization method has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under pure sinusoidal and actual alternating voltages with superimposed dc and harmonic voltage has been made to understand why the converter transformers generally failed on the valve side. The results first time give fundamental reasons for the possible failure of the converter transformers and put forth important design considerations.

High-cycle fatigue and fracture behaviours of SLM AlSi10Mg alloy

Abstract The high-cycle fatigue and fracture behaviours of the selective laser melting (SLM) AlSi10Mg alloy were investigated. Flat specimens were designed directly in the shape required for the fatigue tests under pulsating loading in tension (R=0, R is the dynamic factor). The fatigue–life (S–N) curves were modelled with a conditional Weibull's probability density function, where the real-valued genetic algorithm (GA) and the differential ant-stigmergy algorithm (DASA) were applied to estimating the needed Weibull's parameters. The fractography of the fatigue specimens showed that the fatigue cracks initiated around the surface defects produced by SLM and then propagated in an unstable manner. However, the presence of large SLM defects mainly influenced the crack initiation period and did not have a strong influence on the crack propagation. The obtained experimental results present a basis for further investigation of the fatigue behaviour of advanced materials and structures (e.g. cellular metamaterials) fabricated by additive manufacturing (AM). Especially, in the case of two-dimensional cellular structures, the cross-section of cellular struts is usually rectangular which corresponds to the specimen shape considered in this work.

Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy

To investigate the influence of shot peening (SP) on very high cycle fatigue (VHCF) performance of 2024-T351, the specimens with three surface conditions were performed under ultrasonic fatigue tests: mechanicallypolished without peening (NP), ceramic shot peening (SP1), steel and glass mixed shot peening (SP2). The roughness, microhardness, residual stress, fractography measurement and scanning electron microscopy (SEM) were applied before fatigue test to characterize the effective layer induced by the peening treatment. For the failed specimens, the fracture surface were analysed using SEM to study the mechanisms of fatigue crack propagation. In addition, the fatigue life curve in ultra-high cycle region continuously decreased in the three series of specimens. However, the experimental results revealed that fatigue strength improvement resulting from shot peening treatment was negligible in very high cycle regime. Furthermore, the stress intensity factor for the surface crack initiation (SCI) and interior crack initiation (ICI) was discussed based on quantitative analysis on the fracture surface. The average values of ΔKfish-eye for NP, SP1 and SP2 specimens are about 2.22, 1.48 and 1.61 MPa · m1/2, respectively.

Development and experimental research of a test bench for sliding friction and bending fatigue

In order to research the fatigue damage characteristics of components under the coupling of sliding friction and bending fatigue. A test bench of the coupling of sliding friction and bending fatigue was developed based on the analysis of the principle of bending fatigue and friction experiment. The test bench mainly includes drive module, load application module, data collection and processing module. The single sliding friction test, single bending fatigue test, coupling test of sliding friction and bending fatigue can be carried out by this test bench. The three functions of test bench were carried out by 45steel experiment. The rationality and validity of the test bench were well verified by experimental measurement. The results show that the variation of life curve of coupling of sliding friction and bending fatigue is consistent with that of standard rotary bending fatigue life curve. The fatigue life of specimens is reduced on account of the effect of sliding friction. Under the low bending load, the influence of sliding friction effect on fatigue life is more significant. The fracture has typical characteristics of fatigue striations and dimples morphology.

Low cycle fatigue and dwell-fatigue of diffusion coated superalloy Inconel 713LC at 800 °C

The charm of diffusion coatings is mainly in their protective function of the surface of gas turbine components, which are often subjected to fatigue-creep loading in extremely hostile environments. Their impact gains in importance with the increasing time that the components are exposed to high temperatures and aggressive environments. This motivates the authors to study the low cycle fatigue behaviour of Al-Cr coated nickel-based superalloy Inconel 713LC. Polycrystalline diffusion Al-Cr coating was produced by out-of-pack Cr-modified aluminising chemical vapour deposition technique on the surface of cylindrical fatigue specimens manufactured by investment casting. Continuous low cycle fatigue (CLCF) tests and dwell-fatigue (DF) tests where a 10-minute tensile dwell was included in each cycle were conducted in strain control mode with constant total strain amplitude at 800 °C. The fatigue behaviour was characterised by cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves. The Al-Cr coating provided excellent protection against surface oxidation of Inconel 713LC, especially during dwell-fatigue tests. The substrate as well as the Al-Cr coating were examined in as-coated conditions and after fatigue loading through scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Dislocation arrangement in the substrate material was investigated in a transmission electron microscope (TEM). The microstructural observations and the discussion on the degradation mechanisms of surface-treated superalloy have been provided for both regimes of cycling.

Influence of polyester microfiber reinforcement on flexural fatigue characteristics of concrete

The concrete with polyester microfiber is being investigated for the behaviour of flexural strength under static loading. But their behaviour under dynamic loading is unclear and hence there is a need for a comprehensive study. The fatigue performance of concrete with polyester microfiber (0.0, 0.9, 2.0, and 3.0 kg/m3 by volume of concrete) under flexural loading is studied. The cyclic loads are applied at constant maximum stress levels of 80, 85, and 90%, whereas the minimum stress level is fixed at 2% of the static flexural strength of the concrete. The stress level vs. fatigue life (S-N) curves are formulated to observe the effectiveness of microfiber on fatigue performance, follow the logarithmic relationship. Under all the stress levels, concrete with fibre dosage of 0.9 kg/m3 is shown pronouncing effect on fatigue lives compared to concrete with other fibre dosages. The fatigue results are in better agreement with two-parameter Weibull and lognormal distributions. The analysis of covariance (ANCOVA) indicated that the initial stiffness and stress levels are significant in affecting fatigue lives. The unified model is proposed for damage evolution, which can be satisfactorily used to describe the damage of concrete.

An Experimental Evaluation of High-Cycle Gear Tooth Bending Fatigue Lives Under Fully Reversed and Fully Released Loading Conditions With Application to Planetary Gear Sets

Abstract High-cycle gear tooth bending fatigue lives of spur gears under fully reversed and fully released loading conditions are compared in this experimental study. The experimental methodology described in an earlier publication, (Hong et al. 2020, “A Rotating Gear Test Methodology for Evaluation of High-Cycle Tooth Bending Fatigue Lives Under Fully Reversed and Fully Released Loading Conditions,” Int. J. Fatigue, 133, p. 105432. 10.1016/j.ijfatigue.2019.105432), is employed to perform two sets of rotating, gear tooth bending fatigue tests. Statistical analyses are performed to regress stress versus life (S–N) curves under both loading conditions. These curves indicate that a gear under fully reversed loads has a shorter bending life at the same maximum tooth root stress as a gear under fully released loads. Various planetary gear set kinematic conditions with different stationary members are considered to determine the equivalent number of tooth loading cycles per revolution of the sun gear. They are combined with established S–N curves under both loading conditions to determine the ratios of allowable maximum tooth root stresses amongst the gear components of a P-planet gear set such that each gear in the set has the same bending fatigue life. A “stress-balanced” gear set designed to these stress ratios is expected to have the same bending fatigue life for its sun, ring, and planet gears, ensuring that the planetary gear set life is the longest.

Advantages of “Life Curve” Mobile Application: An Easier Alternate of Paper Partograph

Background: The WHO approved paper Partograph is a time tested tool for monitoring the progression of labour to reduce life-threatening complications during labour as well as maternal mortality and morbidity. However, inept manpower for recording and interpreting data, chance of retrograde plotting and limited scope of distance monitoring have become a significant barriers to use this tool. The ‘Life Curve’, an Android Apps, is put forward as an easier alternate of the paper partograph.
 Objective: To see the advantages of the ‘life curve mobile application’: an Android apps which support any Android operating system. It is a new tool based on WHO partograph to monitor the progression of labor with automatically generated interpretation. Early warning and alarm system is included here for creating an opportunity of early referral and intervention.
 Method: A cross sectional study was conducted in Department of Gynaecology and Obstetrics, Sir Salimullah Medical College & Mirford Hospital from December 2016 to May 2017 on 30 mothers admitted with labour pain. Labor and all other parameters were recorded and monitored by using “life curve” mobile application along with paper partograph. The research protocol including the ethical clearance was approved by Bangladesh Medical Research Council. It is part of the feasibility study of the “life Curve” mobile application.
 Results: Mean age of the subjects was 25.9±5.7 yrs, mean gestational age 39.6±.9 wks, and 83% had vaginal delivery. The apps is easy to fill up, automatically generates graph, interesting to work with, provides timely automated reminder to evaluate the parturient, generates digital color coded warning figures, sends automated text message to the supervisor in abnormal situation. The performance parameters were scored in a 1-5 scale. For comparing the advantages of using these two partographs, nine features were compared with a total score of 45. Total score for live curve has 42.7 and that for paper postograph was 19.52 the difference is highly significant (p=.001). The mean±SD score of life curve (4.74±.52) was statistically significantly higher than that of paper partograph (2.17±1.18): p<.0002.
 Conclusions: The life curve mobile application is found to be a easier tool for monitoring of labour with many advantages; and can be used as an alternative of paper partograph.
 Bangladesh J Obstet Gynaecol, 2017; Vol. 32(2) : 84-89

A residual stiffness-based model for the fatigue damage of biological soft tissues

Biologically-derived and chemically-treated collagenous tissues such as glutaraldehyde-treated bovine pericardium (GLBP) are widely used in many medical applications. The long-term cyclic loading-induced tissue fatigue damage has been identified as one of the primary factors limiting the durability of such medical devices and an in-depth understanding of the fatigue behaviors of biological tissues is critical to increase device durability. However, a limited number of fatigue damage experiments were performed on biological tissues due to complexity and time-consuming nature of such fatigue experiments. Consequently, accurate constitutive models for fatigue damage are also lacking. In this study, we performed a rigorous fatigue experiment on GLBP tissues. The stress, strain and permanent set at a maximum of 8 different fatigue cycles, up to 15 million cycles, were obtained, which demonstrated a nonlinear stress softening and a nonlinear permanent set accumulation. Based on the experimental data, we developed a novel residual stiffness-based fatigue model. The fatigue model considers the fatigue-induced reduction of initial stiffness and stiffening effect, in contrast to our previous damage-based model that only considered the fatigue-induced reduction of the initial stiffness. Moreover, a new constitutive relation was proposed to describe how the fatigue life (the cycle number at failure) depends on the equivalent strain, analogous to the stress versus fatigue life (S-N) curve for traditional engineering material. The new fatigue model can characterize the stress softening and nonlinear permanent set effects when referring to the pre-fatigued configuration. It can also describe the nonlinear stress stiffening effect when referring to the post-fatigued configuration. The model predictions are in good agreement with the experiment. The experimental results and the novel model could be applied to fatigue analyses of medical devices to improve the durability.

A Deformation History-Based Approach for Ultra-Low Cycle Fatigue Damage Evaluation of Steel Structures

Ultra-low cycle fatigue (ULCF) damage is one of the main failure modes of steel structures when subjected to intense earthquake action, such as near-field action. However, existing ULCF evaluation methods are based on the plastic strain history of structures, which requires fine numerical simulation and causes high calculation cost. In order to improve and simplify the ULCF evaluation process for steel structures, a new damage index based on the structure deformation history was proposed in this paper, with the application of structure life curve and Miner’s rule. Two types of steel components, notched round steel bar and steel pier, were employed as the research objectives to verify the accuracy of proposed damage index. The predicted ULCF life was compared with the results of tests and finite element simulations, which showed that the application of damage index was of acceptable accuracy. Compared with the traditional plastic strain history-based ULCF evaluation methods, the advantage of proposed damage index is that ULCF life of a given steel structure can be determined quickly according to the loading condition once its life curve is realized, thus eliminating the cumbersome numerical simulation process.

Experimental study on wear life of journal bearings in the rotor system subjected to torque

In the rotor system subjected to torque, the lubrication state of the journal bearings will change, which can lead to wear life change of such bearings. Therefore, the experimental study on the wear life prediction of the journal bearings in the rotor system subjected to torque was carried out. An improved Archard model for wear rate prediction was proposed, which can be applied to determine the relationship among speed, torque, and wear of different bearings. The 20 h wear of a single-span rotor system was tested at three speeds and torque. Test results show that the impact of torque on wear is greater than that of speed. The wear life of a bearing can be predicted with determined wear threshold. The 20 h wear was calculated by using the wear data of the rotor test bench in this model and compared with the actual wear. The comparison results indicate that the accuracy is higher than 92%. The torque – wear life curve shows that wear life is logarithmic to torque at a constant speed and significantly affected by changes in the low torque. Given a constant torque, the wear life will decrease logarithmically with the speed increasing.