Higher Number Of Cycles Research Articles

Role of applied stress level on the actuation fatigue behavior of NiTiHf high temperature shape memory alloys

This study presents the actuation fatigue response of nano-precipitation hardened Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) undergoing thermal cycling between martensite and austenite under various tensile stress levels up to 500 MPa. Changes in fatigue life, and actuation and irrecoverable strains were monitored as a function of the number of cycles to failure. The experimental results revealed a consistent increase in actuation strain concomitant with the applied load at the expense of fatigue life. Significantly high number of cycles to failure were observed for this class of materials: specimens tested under 200 MPa achieved ∼21,000 cycles with the average actuation strain of ∼2.15% while those tested under 500 MPa experienced ∼2,100 cycles to failure with the average actuation strain of 3.22%. Fracture surface and crack density analyses revealed notable crack formation prior to failure at all stress levels. However, the rate of crack formation during repeated transformation increased with the applied stress. It was also demonstrated that the actuation fatigue lives of the present HTSMAs exhibit an almost perfect power law correlation with average actuation work output. The same work-based power law was shown to successfully capture actuation fatigue lives of several low temperature SMAs. Remarkably, the power law exponents for many SMAs were shown here to be either ∼ −0.5 or ∼ −0.8, which points out the likelihood of the existence of a universal empirical rule for actuation fatigue response of SMAs. The current findings constitute the first report on the applied stress - actuation fatigue interrelationship in NiTiHf HTSMAs.

Design of Shaft Respecting the Fatigue Limit for Ultra-High Number of Cycles

Advances in engineering solutions in recent few decades caused that conventional fatigue limit (for steels and cast irons given for number of 107 cycles) is no more sufficient. Construction parts of newly introduced transport vehicles, operating at high velocities and at long distances, reach during their lifetime very high numbers of loading cycles, in order of 109. For this reason, values of fatigue limits for 109 cycles must be considered in design and construction of transport vehicles. In this study, authors present how dramatically will change the design of shaft, when fatigue limit for 109 cycles is considered.

Improved lifetime of new fibrous carbon/ceramic composites

New carbon/ceramic composites have been synthesized from low-cost phenol–formaldehyde resin and polysiloxane preceram. A reference carbon composite reinforced with carbon fibre (CC composite) is obtained in first place from a carbon fibre roving impregnated with a solution of phenol–formaldehyde resin in isopropyl alcohol. To obtain fibrous carbon/ceramic composites the CC perform is impregnated with polymethylphenylsiloxane polymer and then a thermal treatment in an inert atmosphere is applied. Depending on the temperature of this process, the resulting ceramics can be silicon carbide (SiC) or silicon oxycarbide (SiCO). Three representative samples, named CC/SiCO(a) (obtained at 1000 °C), CC/SiCO(b) (1500 °C) and CC/SiC (1700 °C), have been tested for fatigue behaviour and oxidation resistance. The value of the Young’s modulus remains constant in fatigue tests done in flexion mode for the three new composites during a high number of cycles until sudden degradation begins. This is an unusual and advantageous characteristic for this type of materials and results in the absence of delamination during the measurements. In contrast, the CC reference composite shows a progressive degradation of the Young’s modulus accompanied by delamination. SEM micrographs revealed that the formation of filaments of submicrometer diameter during the heat treatment can be responsible for the improved behaviour of these composites. The CC/SiC composite shows the best oxidation resistance among the three types of composites, with a 44% mass loss after 100 h of oxidation.

X-ray computed tomography comparison of individual and parallel assembled commercial lithium iron phosphate batteries at end of life after high rate cycling

X-ray computed tomography (X-ray CT) across multiple length scales is utilized for the first time to investigate the physical abuse of high C-rate pulsed discharge on cells wired individually and in parallel.. Manufactured lithium iron phosphate cells boasting high rate capability were pulse power tested in both wiring conditions with high discharge currents of 10C for a high number of cycles (up to 1200) until end of life (<80% of initial discharge capacity retained). The parallel assembly reached end of life more rapidly for reasons unknown prior to CT investigations. The investigation revealed evidence of overdischarge in the most degraded cell from the parallel assembly, compared to more traditional failure in the individual cell. The parallel-wired cell exhibited dissolution of copper from the anode current collector and subsequent deposition throughout the separator near the cathode of the cell. This overdischarge-induced copper deposition, notably impossible to confirm with other state of health (SOH) monitoring methods, is diagnosed using CT by rendering the interior current collector without harm or alteration to the active materials. Correlation of CT observations to the electrochemical pulse data from the parallel-wired cells reveals the risk of parallel wiring during high C-rate pulse discharge.

Long-term settlements of a ship lock: Measurements vs. FE-prediction using a high cycle accumulation model

During their lifetime many geotechnical structures are subject to a high number of repetitive loading cycles with small strain amplitudes, a so-called high-cyclic loading. Well-known examples are foundations for offshore wind turbines exposed to a cyclic loading caused by wind and water waves. In the case of ship locks, being the subject of the present paper, the high-cyclic loading results from the repeated filling and emptying of the lock chamber which leads to a cyclic change of the water pressures acting on the walls and the bottom of the chamber. Generally, a high-cyclic loading may lead to progressive settlements and a tilting of foundations. Up to now, the availability of validated and standardized analysis procedures for the proof of the serviceability limit state for foundations subject to high-cyclic loading is still limited. One procedure which proved good results for that purpose in recent years is a numerical approach based on finite element (FE) calculations using a high-cycle accumulation (HCA) model to predict the long-term behavior of the subsoil. Subject of this contribution is the application of the HCA model in FE simulations of the ship lock Uelzen I, aiming on a further validation of the proposed procedure. This ship is a well documented structure with long-term monitoring, significantly differing from foundations for offshore wind turbines, which were studied for the validation of the HCA-model so far. Simplifications of the subsoil and the determination of the input parameters for the FE simulations are presented. The long-term settlements predicted by the HCA model are compared to field measurements over two decades. Furthermore, a sensitivity study regarding the influence of various parameters on the predicted settlements is shown.

Pressurized hot water extraction (PHWE) for the green recovery of bioactive compounds and steviol glycosides from Stevia rebaudiana Bertoni leaves

Stevia rebaudiana Bertoni leaves are a natural source of diterpenic glycosides, and various bioactive compounds. The objectives were to characterize antioxidants and steviol glycosides in the extracts obtained from Stevia after “green” pressurized hot water extraction (PHWE). PHWE extracts were obtained at different temperatures (100, 130, 160 °C); static extraction times (5 and 10 min), and cycle numbers (1, 2, 3) using a constant pressure of 10.34 MPa. Temperature was the most important parameter for extraction, where the highest recoveries of all bioactive compounds (except for carotenoids) were at 160 °C. Extracts obtained at longer static times had more steviol glycosides, condensed tannins, and chlorophyll A. Higher amounts of total phenols, condensed tannins, and steviol glycosides were obtained under higher cycle numbers. This study indicated that PHWE is useful for recovering polar and nonpolar antioxidants and steviol glycosides. PHWE may be a suitable technique for scale-up to industrial applications.

Towards a durability test for washing-machines

Durability plays a key role in enhancing resource conservation and contributing to waste minimization. The washing-machine product group represents a relevant case study for the development of a durability test and as a potential trigger to systematically address durability in the design of products. We developed a procedure to test the durability performance of washing-machines as a main objective of this research. The research method consisted of an analysis of available durability standards and procedures to test products and components, followed by an analysis of relevant references related to frequent failures. Finally, we defined the criteria and the conditions for a repeatable, relatively fast and relevant endurance test. The durability test considered the whole product tested under conditions of stress. A series of spinning cycles with fixed imbalanced loads was run on two washing-machines to observe failures and performance changes during the test. Even though no hard failures occurred, results clearly showed that not all washing-machines can sustain such a test without abrasion or performance deterioration. However, the attempt to reproduce the stress induced on a washing-machine by carrying out a high number of pure spinning cycles with fixed loads did not allow equal testing conditions: the actions of the control procedure regarding imbalanced loads differ from machine to machine. The outcomes of this research can be used as grounds to develop standardised durability tests and to, hence, contribute to the development of future product policy measures.

A model for predicting plastic strain and surface cracks at steady-state wear and ratcheting regime

Unidirectional plastic strain accumulation (ratcheting) is one of the main causes of surface crack nucleation in rails and wheels in dry condition. It is related to frictional forces which develop at the contact interface due to sliding, especially in curve and in braking. Surface cracks generated by ratcheting can subsequently lead to severe damage when environmental fluid contaminants (such as rain or snow) are added at the contact interface, due to the complex solid-fluid interaction, which can enhance crack growth.In dry condition, wear and ratcheting can reach an equilibrium, such that the strain field and the crack depth are stationary. Understanding such steady-state deformation regime can be a key factor for predicting the expected crack depth and scheduling a correct maintenance programme.In this paper a model for predicting the strain field at steady-state regime is proposed. Such model, based on an integral equation, allows predicting the strain field and crack morphology at high cycle number with no need of iterative numerical simulation. The potentiality of the model was proven both in characterizing the cyclic plasticity behaviour of materials and in predicting the maximum expected crack depth in full scale railway wheels.

Fatigue Lives of Power Plant Structures Due to Load Sequence Effects Originating from Fluctuating Production of Renewable Energy

The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudo-random sequence derived from measurements in power plants. The developed non-linear fracture mechanics based model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.

Influence of structure sensitising of the AlSi 316Ti austenitic stainless steel on the ultra-high cycle fatigue properties

Austenitic stainless steels are the wide-spread materials, used mainly in the power industry. In that kind of engineering application, structural parts of rotating elements reach during their lifetime very high numbers of loading cycles, exceeding 107 numbers of cycles. With regard to this fact, the data of ultra-high cycle fatigue properties are needed to be used in the qualified design. Increasing demands on the efficiency cause the increase of the operating temperature, and exposition of these materials to the elevated temperatures can cause some important structural changes, which result in the sensitising of the structure. In this study authors present their own experimental results about fatigue properties of AISI 316Ti austenitic stainless steel after sensitising, in the ultra-high cycle region (Nf = 106 ~ Nf = 3×109 cycles). Fatigue tests were carried out using ultrasonic fatigue testing device with frequency f = 20 kHz at the coefficient of cycle asymmetry R = -1, and temperature T = 20±5°C. In the ultra-high cycle region was observed the continuous decrease of the fatigue properties of the AISI 316Ti, and there was recorded the negative effect of the sensitising on the ultra-high cycle fatigue properties of the AISI 316Ti.

Influence of surface morphology on the very high cycle fatigue behavior of metastable and stable austenitic Cr-Ni steels

The present study investigates conventional and cryogenically turned specimens of metastable austenitic steel AISI 347 and stable austenitic steel AISI 904L in the VHCF regime. The cryogenic turning process includes cooling by CO2 snow and generates a surface layer on the specimens of metastable austenitic steel, which is characterized by a phase transformation from paramagnetic fcc - austenite to ferromagnetic bcc - martensite and grain refinement. The stable austenitic steel retains its purely austenitic structure after cryogenic turning, but also shows grain refinement in the surface layer. The specimens with different surface morphology were cyclically loaded at ambient temperature using an ultrasonic fatigue testing system developed and built at the authors’ institute. The testing machine operates at frequencies of approx. 20 kHz to achieve high numbers of load cycles within a reasonable time. To avoid self heating of the specimen, the tests were performed in pulse-pause mode. All specimens were tested with a load ratio of R = -1. During cyclic loading, the metastable austenitic steel partially transformed from paramagnetic fcc - austenite to ferromagnetic bcc - martensite, while no phase transformation could be detected in the stable austenitic steel.

Cost effective strategy using Kriging surrogates to compute fatigue at multiple locations of a structure: Application to offshore wind turbine certification

Offshore wind energy development has experienced a rapid development over the last few years encouraged by the of carbon reduction policy and the energy mix objectives of several countries worldwide. Nowadays, the offshore projects under investigation are composed of tens to hundreds of units reaching impressive dimensions with total rotor diameters from 150 to 220 meters and production capacity of 5 to 12 MW per turbine. Mechanical analyses have to be performed to validate the design regarding the solicitations it may face during its lifetime (20 years). Because of the high number of solicitation cycles the structure is confronted to, an estimation of the cumulated damage is mandatory and has to be carefully assessed. As presented in standards, this verification requires massive computation investments and is usually a challenging task for project engineers. This paper presents the “MultiSite” extension of the AK-DA numerical strategy (“Adaptive Kriging for Damage Assessment”). After being formalized, an illustration of its behaviour and performances is proposed for the validation of a design regarding its cumulated damages at different locations.

Civil Engineering Applications: Specific Properties of NiTi Thick Wires and Their Damping Capabilities, A Review

This study describes two investigations: first, the applicability of NiTi wires in the damping of oscillations induced by wind, rain, or traffic in cable-stayed bridges; and second, the characteristic properties of NiTi, i.e., the effects of wire diameter and particularly the effects of summer and winter temperatures and strain-aging actions on the hysteretic behavior. NiTi wires are mainly of interest because of their high number of available working cycles, reliable results, long service lifetime, and ease in obtaining sets of similar wires from the manufacturer.

Synthesis and characterization of Na2Ti6O13 and Na2Ti6O13/Na2Ti3O7 sodium titanates with nanorod-like structure as negative electrode materials for sodium-ion batteries

In this work, we report microwave assisted hydrothermal synthesis of monoclinic sodium titanates with either tunnel or with mixed tunnel/layered structure. Single phase Na2Ti6O13 and mixed phase Na2Ti6O13/Na2Ti3O7 titanates were studied by electrochemical measurements with cyclic voltammetry, impedance spectroscopy and galvanostatic cycling. SEM images revealed that all the synthesized materials consist of rods with widths in the range of tenths to hundreds nanometers. XRD analysis was used for determination of the phase composition and crystallite size.It was shown that mixed phase electrode material reveals higher capacity and wider active voltage range but has also lower cycling stability. The insertion potential of layered titanate Na2Ti3O7 is much lower than in the case of Na2Ti6O9 and the specific capacity of mixed titanate in the first cycle is double the capacity of pure Na2Ti6O9. Although presence of layered titanate contributed to high specific capacity in early cycling, it allowed a considerable decrease in specific capacity at higher numbers of cycles.

Thermally-Induced Crack Evaluation in H13 Tool Steel

This study reported the effect of thermal wear on cylindrical tool steel (AISI H13) under aluminum die-casting conditions. The AISIH13 steels were immersed in the molten aluminum alloy at 700 °C before water-quenching at room temperature. The process involved an alternating heating and cooling of each sample for a period of 24 s. The design of the immersion test apparatus stylistically simulated aluminum alloy dies casting conditions. The testing phase was performed at 1850, 3000, and 5000 cycles. The samples were subjected to visual inspection after each phase of testing, before being examined for metallographic studies, surface crack measurement, and hardness characteristics. Furthermore, the samples were segmented and examined under optical and Scanning Electron Microscopy (SEM). The areas around the crack zones were additionally examined under Energy Dispersive X-ray Spectroscopy (EDXS). The crack’s maximum length and Vickers hardness profiles were obtained; and from the metallographic study, an increase in the number of cycles during the testing phase resulted in an increase in the surface crack formation; suggesting an increase in the thermal stress at higher cycle numbers. The crack length of Region I (spherically shaped) was about 47 to 127 µm, with a high oxygen content that was analyzed within 140 µm from the surface of the sample. At 700 °C, there is a formation of aluminum oxides, which was in contact with the surface of the H13 sample. These stresses propagate the thermal wear crack length into the tool material of spherically shaped Region I and cylindrically shape Region II, while hardness parameters presented a different observation. The crack length of Region I was about 32% higher than the crack length of Region II.

An experimental investigation on the shear mechanism of fatigue damage in rock joints under pre-peak cyclic loading condition

In this study, the shear mechanism of fatigue damage in rock joints with first-order and second-order triangular asperities under pre-peak cyclic loading conditions is investigated in laboratory. A monotonic shear test is firstly conducted to determine the shear strength in rock samples containing joints with different orders of asperities. Secondly, influence of the pre-peak cyclic loading conditions with various numbers of cycles on the shear mechanism of fatigue damage in rock joints is studied at constant normal stress. In the cyclic loading conditions, two consecutive steps, such as load-controlled and displacement-controlled, are applied in tests. The load-controlled step is used to achieve the cycles of pre-peak loading in shear tests, and the displacement-controlled step is then applied to realize the final failure of rock samples with constant rate of 0.5mm/min. Moreover, the effects of shear loading rates, numbers of cycle and cyclic shear loading magnitude on fatigue damage, peak shear strength and residual shear strength of rock joints are researched. We found that fatigue damage occurs at the second-order asperities in the upper and lower blocks within low number of cycles, but the fatigue cracks initiated with initiation angle of 90° with respect to the first-order asperities in the upper and lower blocks coalesce with each other (or rock joints) within the high number of cycles. Meanwhile, the variations of peak shear strength and residual shear strength of rock joints within low number of cycles are also different from ones within the high number of cycles.

Modeling the Behavior of Geotechnical Constructions Under Cyclic Loading with a Numerical Approach Based on J. Lemaitre Model

A concise prediction of the cyclic accumulation of deformations in non-cohesive soils becomes important for a high number of cycles, but such prediction is troublesome because even small errors of the general purpose constitutive models are quickly accumulated. The solution could be an explicit model that treats accumulation as a sort of creep process. This article presents a numerical fashion the phenomenon of accumulation resulting from cyclic loading in sand in drained state. The first cycle is performed using Hypoplastic model of Wolffersdorff 1996 with the improvement of intergranular strain (IGS). From the second cycle, the behavior of the soil is simulated as a pseudo creep (J. Lemaitre model) where we seek an equivalence between the parameters of J. Lemaitre model and cyclic parameters, Replacing the cyclic effect by a cumulative volumetric strain and the number of cycles (N) is considered equivalent time (t). The prediction of model is compared with experimental values. Good correlation exists between predicted and experimental response.

Hydrogen effects on Ni-Ti fatigue performance by self -heating method

Ni-Ti superelastic alloys are extensively used in manufacturing biomedical devices because of their high mechanical performance, good fatigue durability and biocompatibility compared to traditional metallic materials. During clinical use, most of these devices are intended to work under cyclic or repetitive loadings and may be in contact with corrosive environments leading to unexpected failures. It is however recognized that the fatigue–environment interaction, especially fatigue–hydrogen absorption, can be the main cause of these failures. The aim of this work is to investigate the fatigue behavior of superelastic Ni-Ti intended for manufacturing medical devices at high number of cycles (HCF) with a particular emphasis to the effect of hydrogen on fatigue properties. Fatigue tests were analyzed using self-heating measurements based on observing thermal effects during cyclic loadings. The results obtained with self-heating approach showed a trend of a decrease in the fatigue life of Ni-Ti alloys after hydrogen absorption and the fatigue limit extrapolated will be compared with the results obtained with the classical S-N curves method.

High pressure processing and retrogradation of potato starch: Influence on functional properties and gastro-small intestinal digestion in vitro

A combined approach of high pressure processing (HHP) along with retrogradation was used to treat potato starch samples in order to influence their functional properties and kinetics of glucose release during gastro-small intestinal digestion in vitro. Native potato starch was treated at two levels of high hydrostatic pressure (400 MPa and 600 Mpa) and number of cycles (3 and 6) for 10 min and then subjected to retrogradation at 4 °C for 7 days. The granular starch samples treated at 400 MPa for 6 cycles showed higher peak and breakdown viscosities whereas peak time and final viscosity for the same samples treated for 3 cycles followed by retrogradation were higher than the native control sample. HHP in combination with retrogradation promoted an increase in the relative crystallinity whereas reduction in To and Tp was observed with an increase in the number of cycles at 400 MPa. The use of 3 cycles of 600 MPa led to significant changes in morphology of the granules, causing light eruptions on the surface of most granules, abrasions and major disruptions in some granules. After applying 6 cycles at 400 Mpa, a significant reduction was observed in starch hydrolysis (during gastro-small intestinal digestion in vitro) of the HHP-retrograded sample throughout the small intestinal phase, with a final reduction of 10–15% compared to the native and pressure-treated only sample.

Physicochemical Properties of Dialkyl Ethers

Ethylene glycol diethyl ether (1,2-diethoxyethane: DEE) is dialkyl ethers of ethylene glycol. The solubility of lithium salts in DEE is much less than that in ethylene glycol dimethyl ether (1,2-dimethoxyethane: DME) and ethylene glycol ethyl methyl ether (1-ethoxy-2-methoxyethane: EME). Partial fluorination of DEE increases the polarity. We have investigated the effect of the partial fluorination on the physical and electrochemical properties of DEE in some detail. The mass densities decreased in the order of decreasing the molar mass: tetrafluorinated DEE (FETFEE) > trifluorinated DEE (ETFEE) > monofluorinated DEE (EFEE) (≈ diethyl carbonate (DEC)) > DEE. The molar concentration of FETFEE was higher than that of ETFEE in spite of the large molar size: DEC > EFEE > DEE > FETFEE > ETFEE. The relative permittivity and viscosity of FETFEE were higher than those of ETFEE and EFEE: FETFEE > ETFEE ≈ EFEE > DEE > DEC for the relative permittivity and FETFEE > EFEE > ETFEE > DEC > DEE for the viscosity. The kinematic viscosity of ETFEE was as low as that of DEE above 50 °C. The partial fluorination of DEE improved the solubility of LiPF6. The electrolytic conductivity of 1 mol dm−3 LiPF6/FETFEE was lower than that of 1 mol dm−3 LiPF6/EFEE, but higher than that of 1 mol dm−3 LiPF6/ETFEE. The partial fluorination can increase the ability of the solvent molecules to form hydrogen bonds. The fluoromethyl group may serve as an anion-attracting group. The use of an EC−FETFEE equimolar binary mixture suppressed the discharge capacity fading at high cycle numbers.