Low Cyclic Frequency Research Articles

Fatigue responses of metallic glass‐based stochastic network nanomaterial: Superior strain‐hardening ability

AbstractEngineering materials commonly suffer from cumulative and irreversible damage during cyclic deformation, leading to fatigue failure. Developing materials with better fatigue resistance is imperative. This work reports a metallic glass network nanomaterial that possesses superior strain‐hardening ability. After a certain number of fatigue tests, the peak stress of the nanomaterial can even be up to four times its original value. This superior strain‐hardening ability originates from the unique plastic mechanism. During cyclic deformation, localized shear transformation zones form and develop, especially in the stress concentration regions. Ligaments crosslinked to these regions cannot sustain their original configurations and contract toward these plastic hinges, resulting in the collapse and coalescence of voids. This densification enhances the engineering stress during each cycle. The fatigue features can be altered by adjusting the cyclic frequency. Stronger strain‐hardening ability can be achieved at lower cyclic frequencies.

Effects of Pulsatility on Arterial Endothelial and Smooth Muscle Cells

Continuous flow ventricular assist device (CFVAD) support in advanced heart failure patients causes diminished pulsatility, which has been associated with adverse events including gastrointestinal bleeding, end organ failure, and arteriovenous malformation. Recently, pulsatility augmentation by pump speed modulation has been proposed as a means to minimize adverse events. Pulsatility primarily affects endothelial and smooth muscle cells in the vasculature. To study the effects of pulsatility and pulse modulation using CFVADs, we have developed a microfluidic co-culture model with human aortic endothelial (ECs) and smooth muscle cells (SMCs) that can replicate physiologic pressures, flows, shear stresses, and cyclical stretch. The effects of pulsatility and pulse frequency on ECs and SMCs were evaluated during (1) normal pulsatile flow (120/80 mmHg, 60 bpm), (2) diminished pulsatility (98/92 mmHg, 60 bpm), and (3) low cyclical frequency (115/80 mmHg, 30 bpm). Shear stresses were estimated using computational fluid dynamics (CFD) simulations. While average shear stresses (4.2 dynes/cm²) and flows (10.1 mL/min) were similar, the peak shear stresses for normal pulsatile flow (16.9 dynes/cm²) and low cyclic frequency (19.5 dynes/cm²) were higher compared to diminished pulsatility (6.45 dynes/cm²). ECs and SMCs demonstrated significantly lower cell size with diminished pulsatility compared to normal pulsatile flow. Low cyclical frequency resulted in normalization of EC cell size but not SMCs. SMCs size was higher with low frequency condition compared to diminished pulsatility but did not normalize to normal pulsatility condition. These results may suggest that pressure amplitude augmentation may have a greater effect in normalizing ECs, while both pressure amplitude and frequency may be required to normalize SMCs morphology. The co-culture model may be an ideal platform to study flow modulation strategies.

Towards High-Frequency Shape Memory Alloy Actuators Incorporating Liquid Metal Energy Circuits

Large shape memory alloy (SMA) actuators are currently limited to applications with low cyclic actuation frequency requirements due to their generally poor heat transfer rates. This limitation can be overcome through the use of distributed body heating methods such as induction heating or by accelerated cooling methods such as forced convection in internal cooling channels. In this work, a monolithic SMA beam actuator containing liquid gallium–indium alloy-filled channels is fabricated through additive manufacturing. These liquid metal channels enable a novel multi-physical thermal control system, allowing for increased heating and cooling rates to facilitate an increased cyclic actuation frequency. Liquid metal flowing in the channels performs the dual tasks of inductively heating the surrounding SMA material and then actively cooling the SMA via forced internal fluid convection. A coupled thermoelectric model, implemented in COMSOL, predicts a possible fivefold increase in the cyclic actuation frequency due to these increased thermal transfer rates when compared to conventional SMA forms having external heating coils and being externally cooled via forced convection. The first ever experimental prototype SMA actuator of this type is described and, even at much lower flow rates, is shown to exhibit a decrease in cooling time of 40.9%.

Effect of low cyclic frequency on fatigue crack growth behavior of a Mn–Ni–Cr steel in air and 3.5% NaCl solution

Corrosion fatigue phenomenon involves the time dependent corrosion and cycle dependent fatigue loading. The intention of this study is to develop a test method to evaluate the crack growth rate of a material in a corrosive environment at low frequencies. The effect of cyclic frequency on crack growth rate of a Mn–Ni–Cr steel in lab air and 3.5% NaCl solution is evaluated using frequency shedding method where the cyclic frequency is decreased exponentially as a function of crack length at a constant stress intensity factor range. A Zn sacrificial anode coupled to the specimen slows down the rate of crack growth.

Effect of Oxygen on Ethanol Stress Corrosion Cracking Susceptibility: Part 2—Dissolution-Based Cracking Mechanism

The mechanism of stress corrosion cracking (SCC) of carbon steel in fuel-grade ethanol was investigated. Repeated potentiodynamic polarization measurements were made during slow strain rate (SSR) tests. Current difference curves were determined from measurements made in the plastic and elastic regions. Peaks in the current difference curves were observed in the same potential region where SCC was found previously with potentiostatic SSR tests, which reveals the relation of cracking susceptibility and slip-enhanced anodic dissolution. Notched bar SSR tests exhibited consistent evolution of measured current vs. strain at constant applied potentials. A high R-ratio and low cyclic frequency crack growth rate (CGR) test was intended to simulate the real service condition of pipelines. The CGR at different applied anodic potentials was higher in the same SCC-susceptible potential range determined by the other approaches. The results support a slip-enhanced dissolution-based mechanism of ethanol SCC.

Fatigue of steel in air at low cyclic loading frequency

This investigation was aimed at studying crack growth behaviour at extremely low loading frequencies of steels exhibiting low temperature creep. It was found that crack dormancy at low loading frequency is an intrinsic characteristic of the steels. Frequency can be an independent driving force, like the stress intensity factor range (ΔK) and the maximum stress intensity factor (Kmax), in low cyclic loading frequency regimes. A dormant state or threshold condition can be achieved by reducing loading frequency.

FATIGUE CRACK GROWTH IN NICKEL‐BASED SUPERALLOYS AT 500‐700°C. I: WASPALOY

Abstract— The effects of cyclic frequency, hold time, and stress‐intensity‐factor range (δK) on rates of fatigue crack growth in air at 500‐700°C have been studied for Waspaloy—a nickel‐based superalloy used for gas‐turbine engine discs. The main effects observed were: (i) higher rates of crack growth for lower cyclic frequencies at high δK at 600 and 700°C. and (ii) lower rates of crack growth at low δK (and higher δK thresholds) for longer hold times at 700°C, compared with those at a baseline frequency of 2 Hz. Metallographic and fractographic observations suggested that the effects of cyclic frequency and hold time could be rationalised in terms of the competing effects of enhancement of cracking due to creep and inhibition of cracking caused by oxide‐induced crack closure, fracture‐surface‐roughness induced crack closure, and crack‐branching/deflection. Possible mechanisms for promoting intergranular and transgranular cracking at low cyclic frequencies or long hold times are discussed.

FATIGUE CRACK GROWTH IN NICKEL‐BASED SUPERALLOYS AT 500‐700°C. II: DIRECT‐AGED ALLOY 718

Abstract— The effects of cyclic frequency, hold time, and stress‐intensity factor range (ΔK) on rates of fatigue crack growth in air at 500°C and 700°C have been studied for the direct‐aged version of the nickel‐based superalloy 718. The main effects were similar to those observed for Waspaloy (Part I), namely: (i) small effects of cyclic frequency and hold time at 500°C, (ii) higher rates of crack growth at lower cyclic frequencies at high ΔK at 700°C, and (iii) lower rates of crack growth at low ΔK (and higher ΔK thresholds) for longer hold times at 700°C. For DA 718, there was no increase in crack growth rates with increasing hold times (0–60 s) at high ΔK at 700°C (unlike the large increases reported for standard processed Alloy 718). Metallographic and fractographic observations showed that crack growth was predominantly transgranular at 500°C, and predominantly intergranular at 700°C except at 2 Hz at high ΔK. The precise intergranular crack path, extent of branching, and fracture‐surface appearance depended on ΔK, wave‐form and cycle‐period. The mechanisms responsible for the observed effects, and possible explanations for the different behaviour of DA 718 and standard processed Alloy 718, are discussed.

高張力鋼の腐食疲労とフラクトグラフィ

Considering that the corrosion fatigue is especially time-dependent, it is necessary to confirm the long-life fatigue properties of structural steels to be adoptable to the practical design curve. Fatigue tests have been performed on three kinds of high tensile steels for offshore structures in air or by dropping fresh water and sea water to the midsection of each specimen under rotating bending load at various cyclic speeds including lower frequencies. In addition, fractographic observation has been carried out on the crack initiation and fracture surface of each specimen.The main results obtained on the basis of the long-time corrosion fatigue tests (maximum period; 1.45 years/specimen) are as follows:(1) The fatigue strength decreases with higher cyclic numbers and lower frequencies. There exists a good linear relation between stress amplitude and cyclic frequency. The relation can be expressed by a linear logarithmic equation.(2) The effect of corrosion liquid on fatigue strength is remarkable in long period tests and the fatigue strength in sea water becomes lower than that in fresh water in the frequency range between 1500 and 4rpm.(3) The fatigue strength is a little dependent on tensile strength in the low cycle region and the difference in fatigue strength among high tensile steels becomes reversed in the high cycle region. This phenomenon appears not only in fresh water but also in sea water.(4) The crack initiates from pits and propagates mainly by transgranular fracture. Both quasi-cleavage fracture and intergranular fracture are observed in the heat-treated steels under low cyclic frequencies in the high cycle region. These fracture patterns would be caused by corrosive effect and accelerate the crack propagation rate of the heat-treated steels.

Predictions of the initial non- steady- state crack growth behavior in the creepjfatigue of the nickel- base superalloy AP1

Using the fracture mechanics parametersK and C* to analyze cyclic crack growth test results carried out on the nickel-base superalloy API, the effects of test frequency on the initial incubation time followed by transient cracking rates and the steady-state secondary crack growth rates were considered. The crack-ing behavior at 700 °C for this material exhibits a frequency dependence over a range of 0.001 to 10 Hz. It has been shown that, at high temperatures under steady-state cracking conditions, fatigue processes are most dominant at high frequencies, and conversely, time-dependent creep dominates at low frequen-cies. The creep cracking rate is described by a model linked to the exhaustion of available ductility in a creep process zone at the crack tip, and the fatigue rate is linked to the Paris Law equation. For the sec-ondary regime of crack growth, the effects of frequency are described in a cumulative damage model de-veloped for creepJfatigue interaction. For the crack incubation and the transient process under initial loading, the model is extended to predict the cracking behavior in the creep regime at low cyclic frequen-cies. For the higher frequencies, fatigue dominates and creep transient effects are not observed experi-mentally.

Fatigue fracture of nylon polymers

We determine the effect of frequency on fatigue crack propagation rates in unfilled nylon polymers. Specifically it was of interest to investigate the frequency dependence under conditions where hysteretic heat generation did not occur. For dry injection-moulded nylon the results demonstrate that a strong frequency dependence exists with higher crack propagation rates at lower frequencies. This indicates that the mechanism of fatigue crack growth at room temperature is primarily one of creep crack growth, especially at frequencies below 1.0 Hz. It is also noted that hysteretic heating causes fracture mode transitions to occur during stable; fatigue crack propagation in injection-moulded nylons, even at relatively low cyclic frequencies (5.0 Hz)

Pore pressure generation and dissipation in dense sands under cyclic loading

A triaxial test system capable of subjecting samples of saturated soil to load controlled cyclic loading under various conditions of drainage is described. The system was used to apply cyclic loading, from compression to extension, at a frequency of 0.06 Hz (4 cycles/min) on a relatively dense saturated sand. This relatively low loading frequency is considered applicable to problems associated with wave or wind loadings.The results of a series of undrained cyclic loading tests are presented. This data generally supports concepts developed by previous research on cyclic loadings of sands.Partial drainage was found to be effective in counteracting pore water pressure buildup at this low cyclic frequency. Failure of one test sample was prevented by drainage even though cyclic mobility had been initiated. Data obtained from this type of controlled drainage testing can be used directly as model study data or can be combined with pore pressure dissipation theory to evaluate the risk of failure under cyclic loading and to design relief measures, such as drainage wells, to prevent the initiation of cyclic mobility in dense sands. Key words: cyclic loading, triaxial, sand, undrained, partial drainage, cyclic mobility, offshore.

The effect of grain size upon the fatigue-crack propagation behavior of alloy 718 under hold-time cycling at elevated temperature

Fatigue-crack propagation tests were conducted in air at 538°C on several specimens of Alloy 718 representing several different producers, melt practices and product forms. This variety resulted in a range of grain sizes from ASTM Size 5 to 11.5. Tests at low cyclic frequency employing a tensile hold-time revealed a relationship between crack growth rates and grain size: higher growth rates were associated with fine-grain material and lower rates with larger-grain material. The lowest crack growth rates were associated with a necklace microstructure, whereby large grains are associated with necklaces of very small grains.

Cyclic grain boundary migration during high temperature fatigue—I. Microstructural observations

Experiments were conducted on high purity lead at room temperature using reverse bending and torsion fatigue at low cyclic frequencies (⩽ 1.50 Hz). Metallographie observations after testing show that there is a one-to-one correspondence between the markings from grain boundary migration and the number and pattern of cyclic loading, and this correspondence is maintained up to s>100 cycles. Grain boundary sliding occurs in each cycle in addition to the migration, and this leads to the development of broad triple point folds. If the strain amplitude is maintained constant, it is shown that the average distance migrated in each cycle increases as the imposed frequency is decreased. The distance migrated is often exceptionally large in the first cycle of testing, and there is often a similar large initial displacement if the test is interrupted for periods of time from 1 to 24 h and then continued. For large grain sizes ( ≳ 2000 μm), the migration markings may lead to a zig-zag pattern where the individual segments lie fairly close to 45° to the stress axis. A model is described which accounts for the one-to-one correspondence and which is consistent with a fine structure observed within the migration markings.

Influence of frequency and microstructure on fatigue crack growth in Ti-6Al-6V-2Sn

Fatigue crack growth behaviour for a mill-annealed Ti-6Al-6V-2Sn alloy tested in air was found to be very sensitive to frequency and microstructure near threshold and again at higher stress intensities. Fracture near threshold was characterized by a combined failure mode involving striation formation, cleavage, and grain boundary separation. A modified Dugdale–Barenblatt model, which quantifies microstructural effects, predicted reasonably good threshold values as a function of grain size. At high stress intensities and a low cyclic frequency, crack growth rates exhibited enhanced values over those observed at a higher frequency. Fracture associated with the enhanced rates was characterized by both cleavage and striation formation. The appearance of the cleaved grains was identical to that observed in sustained load testing and associated with hydrogen embrittlement.

On the S-N Curve of Carbon Steel under Rotary Bending Conditions in City Water

Specimens of annealed 0.35 % C steel were submitted to rotary bending tests in city water at speeds of 1500, 750 and 50 rpm. It was found that the S-N curve of each cyclic frequency, tends to approach the horizontal, however, under continued testing conditions it begins to descend again. In the range below the horizontal, the time until fracture at low cyclic frequency, was shorter than that at higher frequencies. In this range the intermittent pause test, during which the spechnen are in contact with water, shortened the life of specimen in comparison with the sontinuous test. The author endeavours to explain this phenomenon by considering that at high stress, growth of cracks is mainly attributable to stress, and that at low stress it is related to stress and the corrosive effect of water, however, that it is more difficult for the water to enter into the end of deep cracks.