Fatigue Failure Research Articles

Bending Fatigue Properties of Ultra-High Toughness Cementitious Composite (UHTCC).

Ultra-High Toughness Cementitious Composite (UHTCC) represents a composite material meticulously engineered on the foundation of micromechanical principles. The multi-crack cracking and strain-hardening characteristics of UHTCC enable it to be applied to orthotropic steel decks to control the crack width. Different from most studies which only focus on hybrid fiber or fatigue characteristics, this paper studies the influence of hybrid fiber content on static mechanical properties, flexural toughness, and flexural fatigue characteristics of UHTCC under different stress levels. The compressive and flexural strength, bending toughness, and fatigue damage of UHTCC under different fiber ratios were compared, and the fatigue properties of hybrid fiber UHTCC were verified. The results reveal that hybrid fiber exerts a more pronounced effect on toughness, augmenting the maximum folding ratio by 23.7%. Single-doped steel fiber UHTCC exhibits a characteristic strain-softening phenomenon attributable to inadequate fiber content, whereas the bending toughness index of hybrid fiber UHTCC surpasses that of SF1.5P0 by 18.6%. Under low-stress conditions, UHTCC demonstrates a nearly threefold increase in bending fatigue life with a mere 1% steel fiber content, while the influence of polyvinyl alcohol (PVA) fiber on fatigue life is more significant: with an increase of only 1/5 volume content, the fatigue life increased by 29.8%, reaching a maximum increase of 43.2% at 1/4 volume content. Furthermore, the fatigue damage accumulation curve of UHTCC follows a three-stage inverted S-shaped trajectory. The inclusion of PVA fiber facilitates early initiation of stable cracking during the fatigue failure process, thereby advancing the entire strain stability development stage and mitigating external load forces through the proliferation of micro-cracks. Consequently, compared to SF1P0, the ε0 of SF1P5 experiences a significant increase, reaching 143.43%.

Fatigue properties of SiC graded ceramic lattice structures with a triply periodic minimal surface manufactured by laser powder bed fusion

SiC graded ceramic lattice structure (GCLS) offers superior mechanical strength and its impact on fatigue warrants investigation. In this work, SiC triply periodic minimal surface (TPMS) GCLSs were prepared via laser powder bed fusion technology and liquid silicon infiltration process, alongside SiC TPMS uniform ceramic lattice structure (UCLS) as a reference. Fatigue properties, fatigue failure, and strengthening mechanisms were systematically investigated through compression fatigue tests, finite element (FE) simulations, and theoretical analysis. Fatigue failures of SiC UCLSs and GCLSs are influenced by cyclic ratcheting and fatigue damage, with cyclic ratcheting dominance. The fatigue strength ratios of UCLS and GCLS are 0.7 and 0.74, indicating that GCLSs exhibit stronger deformation resistance and superior fatigue performance. FE results show that surface tensile stress level in GCLS is lower than in UCLS, resulting in slower fatigue crack initiation. Stress redistribution and higher crack thresholds contribute to enhanced fatigue resistance in SiC TPMS GCLSs.

Effect of electromagnetic force installation method on critically reinforced interference sizes and fatigue behavior of CFRP bolted joints

AbstractThe electromagnetic force dynamic installation (DI) method of interference fit fasteners has significant advantages in improving the quality of the assembly interface of composite structure. This study aims to investigate how improving assembly interface quality may help in achieving high‐precision interference‐fit joints in composites. Single‐lap joint specimens were prepared using electromagnetic installation technique, the fatigue life and failure behaviors of the joints under different bolt‐hole clearances were measured and analyzed experimentally. In addition, a three‐dimensional finite element model was developed to predict the radial pre‐compression stresses around the hole and the progressive failure behavior of various typical damage modes in carbon fiber‐reinforced polymer induced by the interference. The results show that the critical interference range for the DI‐composite bolted structure is 1.0%–1.2%. The fatigue life of the best interference (1.0%) and the worst interference (0%, neat fit) have a difference of 36 times in amplitude. The residual stress with uniform distribution and high amplitude reduces the stress concentration of the hole wall and improves the failure evolution of the loaded hole. The contribution of this study provides vital guidance for improving the fatigue performance of CFRP bolted structures using a novel DI method.Highlights I = 1.0% is the critical damage‐enhancing interference size for dynamically installation interference fit bolts with electromagnetic forces. The residual stress amplitude introduced by the interference does not increase uniformly as the interference size increases. Excessive interference significantly reduces the uniformity and magnitude of residual stresses. Under subsequent cyclic loading, the fifth layer near the inlet and outlet of the extruded lower laminate appears to be the critical ply of extrusion deformation, and severe damage is concentrated in the critical ply of deformation. The decrease of stress amplitude and the variation of mean stress are the mechanisms of fatigue life enhancement in interference fit joints under external cyclic loading.

High‐Cycle‐Fatigue Anisotropy of an Aluminum Alloy Superthick Plate

The effect of anisotropy on high‐cycle fatigue (HCF) property along the rolling direction (RD) and the transverse direction (TD) of a 7××× aluminum alloy super thick plate is mainly investigated in this study. The results show a similar microstructure at different thicknesses, and the HCF properties are close as well. The fatigue fracture morphologies show that the fatigue crack sources are all located on the surfaces of the RD and TD specimens. By considering the differences in the HCF properties, there is only a slight anisotropy effect at the center thickness of the plate, which can be attributed to the tensile anisotropy caused by textures. Besides, the fatigue damage degree along the TD is slightly higher than that along the RD at the center thickness. The reason can be explained by two aspects: one is the higher probability of defect occurrence along the TD compared with that along the RD which is analyzed by the previously proposed Yield strength‐Tensile strength‐Fatigue strength model, and the other is the influence of the textures. The combination of hard and soft orientations may cause fatigue damage localization easily in the soft‐oriented grains which accelerates the fatigue failure.

АНАЛІЗ КОНСТРУКТИВНО-ТЕХНОЛОГІЧНИХ ОСОБЛИВОСТЕЙ ХВОСТОВИХ БАЛОК ВЕРТОЛЬОТІВ ТРАНСПОРТНОЇ КАТЕГОРІЇ

An analysis of the structural and technological features of the design of the tail beam of transport category helicopters was carried out. The main zones of probable fatigue failure of the tail girders were determined. Based on the analysis of the structural and technological features of the design of tail beams of transport category helicopters, the tail beam design of the physical model was developed. In the practice of world helicopter industry, single-screw helicopters, which are characterized by relative simplicity of design, control system and relatively low cost, have become the most widespread. The tail beam is the most complex part in the production of a helicopter. For a single-screw helicopter, the tail beam is a very critical part of the structure, the failure of which, in most cases, leads to an aviation incident. Ensuring strength requirements is an extremely important task; therefore it is regulated by mandatory airworthiness standards for designers, which are an integral part of the Aviation Rules. In these standards, the initial requirements for the calculation and experimental works to ensure strength are set, load conditions are established, and instructions are given for determining the magnitude of the loads. In the practice of helicopter construction, the experience accumulated in aircraft construction is used in the formation of structural and power schemes of frame units. The tail beam of helicopters is made according to beam, truss or mixed schemes. Optimization methods of compressed and stretched panels of frame units have a common criterion for aviation - the minimum weight of the structure while meeting the requirements of strength, rigidity, survivability, resource, manufacturability, operational efficiency, etc. The task of selecting a rational structural and force scheme of the tail beam is solved by indirect methods involving weight statistical data, parametric dependencies and information about the force schemes of previous structures. In most cases, the type of tail beam design is selected based on the requirements for the helicopter, operating conditions and production capabilities. The task is to find the best option within a given structural type. The helicopter tail beam is part of the structure of the helicopter, has the shape of a truncated cone and consists of a set of frames, stringers and skin. For ease of manufacturing, the tail beam has a longitudinal technological connector, which allows separate assembly of each half of the tail beam. The tail beam is connected to the central part of the fuselage by bolts that fasten the end power frames with the help of fittings at the ends of the stringers. The end part of the tail beam is connected to the keel, similar to the connection of the tail beam to the central part of the fuselage. The following are installed on the tail beam: stabilizers, rudder control rods, rudder drive system, intermediate and tail gear, navigation lights, assemblies and components of hydraulics and tail shaft. Modern transport helicopters are designed and manufactured with a specified resource and service life in mind, which range from 15,000 to 30,000 hours with a service life of 25-30 years. Increasing the fatigue life of the zones of structural irregularities of the tail beams of transport category helicopters is an urgent task, which is of great practical importance for ensuring the safety of flights during the operation of helicopters during the established resource, which in turn is an important indicator of the competitiveness of helicopters.

Fracture mechanism of Ti60 alloy at high temperature in very high cycle fatigue regime and fatigue life prediction

AbstractThis study aimed to investigate the failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at 500°C. Ti60 specimens were characterized before and after the 500°C VHCF test, and the fracture surfaces were observed. The results show that there are three different fatigue failure mechanisms at 500°C: (i) equiaxed primary α phase (αp) cleavage, forming small unsmooth facets and rough fracture area (RA), and fusion of non‐adjacent facets to grow into the main crack. (ii) αp grains gather into large grain with triple junction, and large grain slip to form large fusion facet. (iii) Oxide shedding and then cracks form. A dynamic recurrent neural network model is used to predict the fatigue life of Ti60 alloy, 91% of the overall predictions were within the scatter band of 3.0, and 100% were within the scatter band of 5.0.

Accelerated fatigue bench test method for rubber vibration isolators based on load spectrum compilation

The present study is focused on the development of a novel method for compiling load blocks to accelerate the fatigue bench test of rubber vibration isolators. This approach involves three main steps: Firstly, a uniaxial tensile fatigue test is conducted on rubber specimens under a constant amplitude load. According to the fatigue test data, a rubber fatigue life prediction model is established. Secondly, the mount load spectrum is statistically analyzed, and the fatigue damage distribution of the mount is calculated using the rubber fatigue life prediction model. Lastly, following the principle of damage equivalence, the load spectrum is compiled into load blocks. The fatigue bench test of the mount is then conducted using both the load spectrum and the load block as loading data, respectively. The test results demonstrate that the fatigue failure locations of the mounts are essentially identical under both loading data types, verifying the effectiveness of the proposed load block compilation method.

Fatigue behavior and damage mechanism of steel plate-UHPC composite bridge deck subjected to negative bending moment

In order to investigate the fatigue behavior and failure mechanism of steel plate-UHPC composite bridge deck under hogging moment, a total of four full-scale specimens with two class of reinforcement ratio (Three specimens with reinforcement ratio of 1.58 %, 1.69 %, 1.86 %, respectively, which called longitudinal specimen. One specimen with reinforcement ratio of 0.89 %, which called transverse specimen.) were designed and tested under fatigue loading. Test results reveal that when the fatigue loading ratio was 0.25 with maximum load of 0.8 times of the static cracking load, taking the crack width of 0.10 mm as the fatigue failure criterion, the fatigue life of longitudinal specimen over 16 million cycles. However, the fatigue life of transverse specimen was only 9.08 million cycles. The longitudinal reinforcement restricts the propagation of fatigue cracks along the depth of the UHPC slab, and tensile fracture of the steel fibers occurred in the region between the top layer of longitudinal reinforcement and the UHPC surface. The fatigue damage mechanism resulting from fatigue degradation of the UHPC layer in the tension zone is identified. In addition, The fatigue design of the steel plate-UHPC composite bridge deck should be controlled by the durability rather than the strength. The established tensile S-N curve with 0.05 mm critical UHPC crack width could be used evaluate the fatigue cracking resistance of composite bridge deck.

Investigation of fatigue durability and influencing factors of coil springs: A case study for metro vehicles

This study investigates the fatigue failure behavior of coil compression springs and various influencing factors. The intrinsic and induced causes of fatigue failure in metro steel springs were revealed by systematic tests. Macroscopic observation and metallographic analysis of the fracture indicate that decarburization, which leads to a reduction in the fatigue limit of the material, was the essential cause of the failure. The resonance of the steel spring caused by rail corrugation leads to an increase in dynamic stress, which is the induced cause of the failure. The combined effect of the two factors promoted the failure of the springs. In addition, decarburization led to a decrease in the microhardness of the spring surface from 462 HV to 151 HV and a decrease in fatigue life from over 3.6 million kilometers to 521,000 km (with the decarburization depth of 0.3 mm). The effects of different stress ratios and decarburization depths on the equivalent shear stress (ESS) were comprehensively investigated. The ESS of steel springs with decarburization and different stress ratios were calculated according to the FKM Guideline and the EN 17149 standard. The ESS decreases from 188 MPa to 77 MPa at a stress ratio of R = 0, and from 168 MPa to 76 MPa at a stress ratio of R = 0.5. If the effects of rail corrugation are not taken into account, the ESS meets the design life requirements. Furthermore, the effects of different rail fastener types and track structures on the dynamic stress were compared. It was found that the double-layer nonlinear vibration damping fasteners can significantly improve steel spring vibration. Finally, improvement measures and recommendations are proposed based on the root causes of failure and the influencing factors.

Laser shock peening for enhanced fatigue resistance in steel structures: Insights from Q960 steel study

Fatigue damage of steel structures is often likened to the “cancer” of structural engineering. Fatigue failure, characterized by the absence of apparent signs before occurrence, usually leads to significant losses. To investigate the potential of laser shock peening (LSP) as an effective method for improving the fatigue performance of steel structures, square plates and compact tension (CT) specimens made of Q960 structural steel were designed. The surface integrity, fatigue crack growth (FCG) performance, microstructural grain size, and fracture morphology of specimens were compared at different laser shock parameters. The results show that LSP implanted a high level of beneficial residual compressive stress in a mild way, and no obvious surface defects were introduced. A noticeable retardation effect was found in the FCG of LSPed specimens, and this effect becomes more pronounced with the increase of laser pulse energy. Compressive residual stress is a more dominant factor in enhancing fatigue performance compared to grain refinement for the small-grained metals studied. LSP can effectively reduce the FCG rate of Q960 high-strength steel, showing promising application prospects in strengthening the fatigue hazard zone of steel structures.

Fatigue damage identification based on Kullback-Leibler relative entropy for raw acoustic emission waveform

Acoustic emission (AE) as a non-destructive testing technique is used to identify structural fatigue damage through the evolution of AE characteristic parameters, aiming to prevent significant economic losses and catastrophic consequences caused by fatigue failure. Traditional AE characteristic parameters (counts, hit rate, duration, rise time) are susceptible to the influence of threshold and user-defined acquisition parameters (PDT, HDT, HLT). Inaccurate parameter settings can miss the AE signals of the structural damage, resulting in inaccurate assessment. To reduce the dependency on threshold and other user-defined parameters, the Kullback-Leibler relative entropy (KLRE) is proposed as a feature parameter for identifying structural fatigue damage and performing fatigue life analysis. The effectiveness and feasibility of KLRE were verified through the fatigue tension–compression test on medium carbon steel (MCS) 1045 V-notch specimens. Based on different waveform recording methods, KLRE is divided into the hit KLRE (HKLRE) and waveform stream KLRE (SKLRE). Experimental results demonstrate that:(1) SKLRE outperforms the traditional AE time–frequency domain features in assessing fatigue damage, exhibiting an inflection point in its evolution trend before reaching fatigue critical failure. (2) Node-independent wavelet packet denoising method improves the initial instability of SKLRE caused by work hardening and crack face friction. (3) Amplitude attenuation does not affect the evolution trends of Shannon entropy (SE) and SKLRE. (4) The evolution trend of HKLRE is similar to traditional AE time–frequency domain features, necessitating the use of multiple features together to reduce uncertainty.

EVALUATION OF SURFACE DETERIORATION RESULTING FROM THERMAL FATIGUE IN THE OPERATIONAL STATE OF AN H13 PRESSURE DIE CASTING DIE

The failure rate of pressure die casting dies is increasing due to cracks on the die surfaces, leading to thermal fatigue and fracture. This paper aims to investigate the thermal fatigue failure of H13 die-casting die in running conditions by measuring the surface crack dimensions at core and cavity zones of the die during regular intervals of the die-casting process. The effect of length and width of the cracks on the die surface concerning coolant channel positions was analyzed. Transient thermal analysis was performed to investigate the temperature distribution, principal stress and total deformation of the die concerning to the coolant channel position using ANSYS. The maximum deformation of core insert was 0.0094 mm. Zone C of the core insert, which was 223.52 mm from the coolant channel location and corresponding to 139,237 cycles, had the longest crack, measuring 18.98 mm. Owing to greater temperature gradients, Zone Z of the cavity insert, which is situated 76.25 mm from the coolant channel, had the largest crack width, measuring 0.29 mm. The durability of the die casting die obtained from the numerical model (2.8×105 cycles) was in agreement with experimental studies (2.89×105 cycles).

High-temperature thermal fatigue of Ni3Al-based single crystal alloy affected by wall thickness and peak temperature

Complex thin-walled structures are commonly employed in turbine components to improve cooling efficiency. However, thin-walled blades are prone to thermal fatigue failure due to the high thermal stress induced by frequent and abrupt temperature fluctuations during duty cycles. The thermal fatigue behavior of Ni3Al-based single crystal alloy with thin-walled structure during 25 °C ↔ 1000 °C/1100 °C was investigated by combining thermal fatigue tests and finite element simulation. The thermal fatigue demonstrated a significant thickness debit effect, with thermal fatigue property dropping as peak temperature and wall thickness increased. Wall thickness had a more pronounced effect than peak temperature. Visible slip traces were found on the wall thickness surface, and their density was positively correlated with wall thickness. Further investigation revealed that thermal fatigue is primarily driven by the octahedral slip system ({111}<110>), with significant contributions from (111)[101‾] and (1‾1‾1)[101]. Simulations of thermal stress distribution and J-integral at the crack tip during crack initiation and propagation stages indicated that wall thickness influences thermal fatigue properties by affecting thermal stress concentration at the notch and crack tip. Higher peak temperatures increased thermal stress and reduced yield strength, thus diminishing thermal fatigue performance. A thermal fatigue crack propagation prediction model was constructed incorporating Paris' law and the J-integral. The model revealed an unfavorable correlation between material Cj/ nj and both peak temperature/wall thickness, indicating that both factors adversely affect thermal fatigue resistance.

A framework to identify fatigue failure of asphalt binders under multiple aging levels using linear amplitude and time sweep testing

ABSTRACT The identification of fatigue failure can significantly affect the determination and prediction of fatigue life for a given asphalt binder. In this work, different approaches for identifying the fatigue failure on linear amplitude sweep (LAS) and time sweep (TS) are compared, especially for highly-aged asphalt binders. The candidate methods include traditional 50% or 80% reduction in pseudo-stiffness parameter (denoted as C0.5 and C0.2), peak stored pseudo-strain energy (PSE), peak C × N and peak phase angle. For asphalt binder at multiple aging levels, peak C × N is found to be effective in defining fatigue failure of TS testing. Meanwhile, peak stored PSE is suitable as the failure definition of LAS because it can capture the benefit of polymer modification and has a high accuracy for fatigue life prediction. Nf in strain-control TS test determined by different failure definitions shows that aging can increase the fatigue resistance at early-aged levels and then decrease the fatigue resistance at highly-aged levels. In general, the selected failure definitions have little influence on the trend of Nf with age levels. The purpose of this work is to select the logical failure definitions of asphalt binder under multiple aging levels.

Dynamic calculation of stress influence law of rubber core sealing surface of the rotating blowout preventer with defects

The bubble and crack defects in the production and operation of the rubber core of the rotating blowout preventer (RBOP) easily cause fatigue failure, leading to well-controlled severe accidents. Based on Yeoh’s principal structure equation, the dynamic sealing model of rubber core was established in this paper. The sealing process of the rubber core in lifting and drilling was calculated and analyzed. The stress influence law of typical cracks and bubble defects that affect the fatigue failure of rubber core sealing surface was analyzed. The results are as follows: (1) When the drill pipe joint passes through the RBOP, the structure of the rubber core is significantly deformed. This stage may cause damage and failure of the rubber core. (2) The cracks and bubble defects lead to a decrease in the stability of the sealing surface. With the increase of stress at the defect, the crack of the rubber core is easy to expand. (3) In the same location of the rubber core defects, cracks compared to bubble defects have a more significant impact. For the same type of crack, the bottom left side (A1) and the outer surface underside (D1) crack reduce the stress at the position of the rubber core sealing surface to the maximum. The research results provide a reference basis for defect detection, sealing performance, and service life of rubber cores, in order to improve the reliability evaluation standards of blowout preventer rubber cores, improve well control safety, reduce the occurrence of safety accidents, and improve operational efficiency.

Experimental Structural Safety Analysis of Front-End Loader of Agricultural Tractor

The agricultural front-end loader is an implement attached to the front of tractors to transport various agricultural materials, including soil. Since they are subjected to various loads due to the working environment, their safety analysis in consideration of actual working conditions is required. However, there are no official standardized test codes to consider various actual working environments currently. In this study, the structural safety of a front-end loader for static and fatigue failures was evaluated using new test code reflecting actual working environments. Thirty-four measurement locations were determined as the stress concentration spots of each component of the front-end loader derived through multibody dynamic simulation. The total testing time was set to 1 h, and the test time for each task was determined considering the duty percentage of the actual loader work. The measurement results showed that the maximum stress that exceeds the material’s yield strength occurred at two locations of the mount, which is the connection to the tractor body, resulting in static yielding. For tasks, the pulling and dumping exhibited the highest stress. The task that had the largest impact on fatigue damage was the dumping. The static safety factor was found to be over 1.93 and the fatigue life met the required lifespan at all measurement locations except for those exhibiting static yielding. Therefore, the most vulnerable part of the front-end loader is the mount, and it is necessary to secure the overall structural integrity by robust design for the mount.

The fatigue behavior of TC17 titanium alloy under different test environments: The effect of saltwater on fatigue failure characteristics and life estimation

The study of fatigue behavior in saltwater for TC17 titanium alloy has become a potential research issue in practical engineering and theoretical scientific research. In this study, it is found from the designed fatigue test that the corrosiveness of the test environment has a significant negative effect on the fatigue life of titanium alloy, and the distributions of fatigue life in 3.5%NaCl solution and 5%NaCl solution are similar. Typical surface failure characteristics and obvious corrosion traces are detected due to the coupling effect of corrosion and fatigue load. Then, electron back scatter diffraction (EBSD) is performed to figure out the change of micro grains from fine grains to coarse grains with the test environment changes from water to NaCl solution, and the strain concentration regions are detected at grain boundary and internal of coarse grains. Finally, the comparison of S-N curve with different models are carried out, Basquin model is proved to be suitable for the S-N curve estimation of TC17, and S-N models for different test environments are established. The micro test results provide a clear understanding of the corrosion fatigue failure behavior, and the newly established model considering the corrosiveness of test environment is novel and essential to the further fatigue study in theoretical research and engineering practice.

Non-stationary vibration fatigue life prediction of automotive components based on long short-term memory network

Automotive components are prone to fatigue failure as a result of the long-term effects of vibration loads. Due to the significant non-stationarity of irregular excitations from various road surfaces, the classical frequency-domain method struggles to accurately estimate the fatigue life of automotive components. Based on long short-term memory (LSTM) networks, an efficient time-domain method for non-stationary vibration fatigue life prediction is proposed. Firstly, the data augmentation method for simulating long-time non-stationary loads is studied. Short-time histories are transformed into time–frequency spectrograms, and then the time–frequency spectrums are warped and masked to reconstruct the long-time non-stationary loads. Furthermore, employing only short-time loads and responses as training samples, the LSTM network is trained to construct a surrogate model for calculating structural stress time histories. Finally, the responses of varying-length long-time loads are calculated, and fatigue life is predicted by the combination of rainflow counting and Miner rule. Additionally, the representative response durations required for the fatigue analysis are estimated. Numerical simulation of control arms shows that the fatigue life prediction results using the LSTM surrogate model are within 1.9% difference compared to transient dynamics analysis results based on finite element method, and the calculation efficiency is improved by orders of magnitude.

Pollution concentration monitoring using a new Birnbaum–Saunders control chart

AbstractAir pollution monitoring is an important issue in environmental science. The Birnbaum–Saunders (BS) distribution, originally applied to describe product failure time distribution to fatigue failures and general random wear failures, is also well to describe the pollutant concentration data due to accumulations of various pollutants in the air over time. Sulfur dioxide (SO2) is a critical factor in air pollution. Hence, it is important to monitor its concentration variation for air pollution prevention. Due to the complexity of its distribution form, there is no reliable and easy‐to‐use control chart for monitoring pollutant concentrations based on the BS distribution. We found that the SO2 concentration data follows the BS distribution. In this study, we propose a new median control chart based on the exact sampling distribution of the monitoring statistic to detect shifts in the median of BS distribution. Thus, given the false alarm rate, the control limits for such control charts can be obtained precisely satisfying a preset in‐control average run length using Monte Carlo simulations. The out‐of‐control average run lengths are calculated by simulation to evaluate the detection performance of the proposed chart when the median shifts occur. We further compare the detection performance of the proposed chart and those of the existing control charts based on asymptotic sampling distributions. In order to improve the detection ability of the proposed chart for small median shifts, an exponentially weighted moving average (EWMA) control chart is constructed. The results of numerical analyses demonstrated that the proposed EWMA chart performs much better than all existing control charts for monitoring the median of BS distribution. Finally, the proposed control charts are applied to monitor the median of SO2 concentrations for air pollution control, showing that both charts can effectively detect a shift in the median of SO2 concentrations. The proposed EWMA control chart even detects out a small shift in the median of SO2 concentrations. The results provide a continuous monitoring solution for air pollution prevention.

Investigation on fatigue crack propagation failure mechanism of hydraulic lifting pipe in deep‐ocean natural gas hydrate exploitation

AbstractIn deep‐ocean natural gas hydrate exploitation operation, the fatigue failure mechanism has attracted more and more attention from scholars, but it has not been effectively disclosed. Therefore, in this work, a multifield coupling and multiple‐nonlinear vibration model of lifting pipe is established, which can accurately determine the alternating stress of deep‐ocean lifting pipe. Second, according to Forman theory, the calculation method of crack propagation length and depth on the surface of a deep‐water riser is established, which is verified by the comparison between the experimental and theoretical model calculation results. The results demonstrate that, first, the effect of residual stress in the welded joint of deep‐ocean lifting pipe should be considered in the later parameter influence analysis. Second, the fatigue growth life of deep‐water pipe with small outflow velocity is mainly determined by tensile stress, and that is determined by both tensile stress and bending stress with large outflow velocity. Third, more attention should be paid to the vibration of the lower pipe on‐site to reduce its vibration frequency and vibration stress amplitude, which can effectively reduce the surface crack propagation state of the deep‐ocean pipe and improve the service life of the pipe. Fourth, properly adjusting the tension coefficient of the tensioner during field operation can effectively improve the safety of the pipe, and the optimal tension coefficient is related to the configuration of the deep‐ocean pipe system, which can be analyzed and determined by the model.