Frequency Domain Fatigue Evaluation on SCR Girth-Weld Based on Structural Stress

The Steel Catenary Riser(SCR)is a vital component for transporting oil and gas from the seabed to the floating platform.The harsh environmental conditions and complex platform motion make the SCR’s girth-weld prone to fatigue failure.The structural stress fatigue theory and Master S-N curve method provide accurate predictions for the fatigue damage on the welded joints,which demonstrate significant potential and compatibility in multi-axial and random fatigue evaluation.Here,we propose a new frequency fatigue model subjected to welded joints of SCR under multiaxial stress,which fully integrates the mesh-insensitive structural stress and frequency domain random process and transforms the conventional welding fatigue technique of SCR into a spectrum analysis technique utilizing structural stress.Besides,a full-scale FE model of SCR with welds is established to obtain the modal structural stress of the girth weld and the frequency response function(FRF)of modal coordinate,and a biaxial fatigue evaluation about the girth weld of the SCR can be achieved by taking the effects of multi-load correlation and pipe-soil interaction into account.The research results indicate that the frequency-domain fatigue results are aligned with the time-domain results,meeting the fatigue evaluation requirements of the SCR.

Effects of Intermittent Hypoxia Exposure on Symptoms of Chronic Fatigue Syndrome in Repeated Restraint Stress and Forced Swimming Induced-Mouse Model

Background: Chronic fatigue syndrome (CFS) shows as its main symptoms debilitating fatigue that is not relieved by physiological rest, depression, inflammation, learning disability and memory impairment. But, intermittent hypoxia, consisting of alternating exposure to hypoxia and normoxia, plays a very important role in improving CFS. However, the essential components for improving learning and memory in CFS patients as well as their mechanism are largely unknown. Objectives: This study aims to analyze the effects of 12% and 15% hypoxia on the expression of alpha tumor necrosis factor (TNF-α) and nuclear factor kappa B (NF-κB) in CFS induced-mouse model for clarifying the effects on the learning and memory function. Methods: A total of 48 type IC mice were used. The CFS mouse model was established using restrained stress and repeated forced swimming. Treatment of CFS was done by exposing CFS mice to intermittent hypoxia at 12% and 15%. The effects of intermittent hypoxia on learning and memory as well as its mechanism of action on inflammation were tested respectively with the Morris test, the SDS page, the immunohistochemistry technique and the Nissl staining. Results: We found that 12% and 15% intermittent hypoxia exposure improved learning capacity and memory of CFS induced-mice. SDS page showed that CFS caused higher TNF-α expression. By exposing CFS mice to 12% and 15% intermittent hypoxia, TNF-α expression decreased significantly, with a much better effect at 15%. Both TNF-α and NF-κB increased in CFS state and decreased after treatment with intermittent hypoxia. Conclusion: Intermittent hypoxia improves learning capacity and memory. It acted by decreasing NF-κB come to down-regulating TNF-α and ameliorates learning capacity and memory impairment in CFS mice.

Fatigue reliability quantitative analysis of cement concrete for highway pavement under high stress ratio

In order to obtain the change law of the fatigue reliability of cement concrete for highway pavement under high stress ratios, first, the probability densities of monotonic random variables including concrete fatigue life are deduced. And then, the fatigue damage probability densities of the Miner and Chaboche-Zhao models are deduced. By virtue of laboratory fatigue test results, the fatigue damage probability density functions of the two models can be obtained, considering different stress ratios. Finally, substituting load cycles into them, the change law of cement concrete fatigue reliability about load cycles can be acquired. The results show that under the same stress ratio, with the increase in the load cycle, the fatigue reliability declines from almost 100% to 0% gradually. No matter under what stress ratio, during the initial stage of the load action, there is always a relatively stable phase for fatigue reliability. With the increase in the stress ratio, the stable phase gradually shortens and the load cycle corresponding to the reliability of 0% also decreases. In the descent phase of reliability, the higher the stress ratio is, the lower the concrete reliability is for the same load cycle. Besides, compared with the Chaboche-Zhao fatigue damage model, the Miner fatigue damage model is safer.

Fatigue Life Prediction for SiC/Al Materials Based on Path Planning Algorithm Considering Residual Stress

To explore the influence of path deflection on crack propagation,a path planning algorithm is presented to calculate the crack growth length.The fatigue crack growth life of metal matrix composites(MMCs)is estimated based on an improved Paris formula.Considering the different expansion coefficient of different materials,the unequal shrinkage will lead to residual stress when the composite is molded and cooled.The crack growth model is improved by the modified stress ratio based on residual stress.The Dijkstra algorithm is introduced to avoid the cracks passing through the strengthening base and the characteristics of crack steps.This model can be extended to predict crack growth length for other similarly-structured composite materials.The shortest path of crack growth is simulated by using path planning algorithm,and the fatigue life of composites is calculated based on the shortest path and improved model.And the residual stress caused by temperature change is considered to improve the fatigue crack growth model in the material.The improved model can well predict the fatigue life curve of composites.By analyzing the fatigue life of composites,it is found that there is a certain regularity based on metal materials,and the new fatigue prediction model can also reflect this regularity.

The Stress Combination Method for the Fatigue Assessment of the Hatch Corner of a Bulk Carrier Based on Equivalent Waves

The stress combination method for the fatigue assessment of the hatch comer of a bulk carrier was investigated based on equivalent waves. The principles of the equivalent waves of ship structures were given, including the determination of the dominant load parameter, heading, frequency, and amplitude of the equivalent regular waves. The dominant load parameters of the hatch comer of a bulk carrier were identified by the structural stress response analysis, and then a series of equivalent regular waves were defined based on these parameters. A combination method of the structural stress ranges under the different equivalent waves was developed for the fatigue analysis. The combination factors were obtained by least square regression analysis with the stress ranges derived from spectral fatigue analysis as the target value. The proposed method was applied to the hatch comer of another bulk carrier as an example. This shows that the results from the equivalent wave approach agree well with those from the spectral fatigue analysis. The workload is reduced substantially. This method can be referenced in the fatigue assessment of the hatch comer of a bulk carrier.

Study on Fatigue Life Prediction of TC4-DT Structural Simulants by Using Critical Distance Theory

In the actual engineering of the structure, mechanical notch is inevitable, which will significantly reduce the fatigue life of the structure. In order to ensure the application of notch structures in engineering, the accurate evaluation of the impact of notch on fatigue life has become the basis of fatigue reliability design of structures. To investigate the influence of the concave structure on the overall fatigue life in practical engineering, three different sizes of V-notch parts and concave structure simulation parts were designed, and the life prediction was carried out by using the critical distance method. The results show that the stress gradient of the concave structure with the same stress concentration coefficient is much greater than that of the V-notch structure. Considering the notch effect, the S-T model in the critical distance method is modified by the stress concentration coefficient and stress gradient, and it is found that the life prediction accuracy reaches the ideal.

Effect of stress level on fatigue behavior of 2D C/C composites

Laminated carbon fiber clothes were infiltrated to prepare carbon fiber reinforced pyrolytic carbon （C/C） using isothermal chemical vapor infiltration （CVI）. The bending fatigue behavior of the infiltrated C/C composites was tested under two different stress levels. The residual strength and modulus of all fatigued samples were tested to investigate the effect of maximum stress level on fatigue behavior of C/C composites. The microstructure and damage mechanism were also investigated. The results showed that the residual strength and modulus of fatigued samples were improved. High stress level is more effective to increase the modulus. And for the increase of flexural strength, high stress level is more effective only in low cycles. The fatigue loading weakens the bonding between the matrix and fiber, and then affects the damage propagation pathway, and increases the energy consumption. So the properties of C/C composites are improved.

EFFECT OF POTENTIAL ON INTERACTION BETWEEN STRESS CORROSION CRACKING AND CORROSION FATIGUE OF AUSTENITIC STAINLESS STEEL IN BOILING 42% MgCl_2 SOLUTION

The environment-sensitive fracture behaviour of 0Cr18Ni9Ti austenitic stainless steel in boiling 42% MgCl_2 under the specific load of low frequency and high mean stress was inves- tigated from the relations and differences of crack growth rates and fractographs between stress corrosion fatigue and stress corrosion cracking.The interaction between stress corro- sion cracking and corrosion fatigue was also studied from fracture characteristics with empha- sis on the effects of applied potential on the interaction.

Correlation of crack growth rate and stress ratio for fatigue damage containing very high cycle fatigue regime

A model is proposed to correlate the crack growth rate and stress ratio containing very high cycle fatigue regime.The model is verified by the experimental data in literature.Then a formula is derived for the effect of mean stress on fatigue strength,and it is used to estimate the fatigue strength of a bearing steel in very high cycle fatigue regime at different stress ratios.The estimated results are also compared with those by Goodman formula.

Effects of stress ratio on the temperature-dependent high-cycle fatigue properties of alloy steels

This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue （HCF） properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they fimcfion as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.

Methodology to Evaluate Fatigue Damage of High-Speed Train Welded Bogie Frames Based on On-Track Dynamic Stress Test Data

The current method of estimating the fatigue life of railway structures is to calculating the equivalent stress amplitude based on the measured stress data. However, the random of the measured data is not considered. In this paper, a new method was established to compute the equivalent stress amplitude to evaluate the fatigue damage based on the measurable randomness, since the equivalent stress is the key parameter for assessment of structure fatigue life and load derivation. The equivalent stress amplitude of a high-speed train welded bogie frame was found to obey normal distribution under uniform operation route that verified by on-track dynamic stress data, and the proposed model is, in effect, an improved version of the mathematical model used to calculate the equivalent stress amplitude. The data of a long-term, on-track dynamic stress test program was analyzed to find that the normal distribution parameters of equivalent stress amplitude values differ across different operation route. Thus, the fatigue damage of the high-speed train welded bogie frame can be evaluated by the proposed method if the running schedule of the train is known a priori. The results also showed that the equivalent stress amplitude of the region connected to the power system is more random than in other regions of the bogie frame.

Review on Stress Corrosion and Corrosion Fatigue Failure of Centrifugal Compressor Impeller

Corrosion failure,especially stress corrosion cracking and corrosion fatigue,is the main cause of centrifugal compressor impeller failure.And it is concealed and destructive.This paper summarizes the main theories of stress corrosion cracking and corrosion fatigue and its latest developments,and it also points out that existing stress corrosion cracking theories can be reduced to the anodic dissolution（AD）,the hydrogen-induced cracking（HIC）,and the combined AD and HIC mechanisms.The corrosion behavior and the mechanism of corrosion fatigue in the crack propagation stage are similar to stress corrosion cracking.The effects of stress ratio,loading frequency,and corrosive medium on the corrosion fatigue crack propagation rate are analyzed and summarized.The corrosion behavior and the mechanism of stress corrosion cracking and corrosion fatigue in corrosive environments,which contain sulfide,chlorides,and carbonate,are analyzed.The working environments of the centrifugal compressor impeller show the behavior and the mechanism of stress corrosion cracking and corrosion fatigue in different corrosive environments.The current research methods for centrifugal compressor impeller corrosion failure are analyzed.Physical analysis,numerical simulation,and the fluid-structure interaction method play an increasingly important role in the research on impeller deformation and stress distribution caused by the joint action of aerodynamic load and centrifugal load.

Study on Fatigue Lifetimes and Their Variation of Mg Alloy AZ61 at Various Stress Ratios

In this study, fatigue tests under different R ratios were conducted on the AZ61 Mg alloy to investigate its fatigue lifetimes and fatigue crack growth (FCG) behavior. The fracture surface of the failed specimens was investigated using a scanning electron microscope to study the size of the intermetallic compounds from which the pioneer fatigue crack initiated and led to the final failure of the specimen. To determine the maximum size of the intermetallic compounds existing within the cross section of the specimen at higher risk, Gumbel’s extreme-value statistics were utilized. In the present study, the intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests. A modified linear elastic fracture-mechanics parameter, M, proposed by McEvily et al., was used to analyze the short FCG behavior under different stress ratios, R. The relation between the rate of FCG and M parameter was found to be useful and appropriate for predicting the fatigue lifetimes under different R ratios. Moreover, the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the FCG law and a statistical distribution of sizes of the most dangerous intermetallic compounds. The evaluated results were in good agreement with the experimental ones. This correspondence indicates that the estimation method proposed in the present study is effective for evaluation of the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios.

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

EFFECT OF MEAN STRESS ON FATIGUE PERFORMANCE OF WELDED JOINTS TREATED BY UPT

The fatigue contrast tests of unload longitudinal direction comer joints asoriginal welded and treated by ultrasonic peening of Q235B in various stress ratio are directed. Theimprovements of fatigue performance of unload longitudinal direction corner joints resulted byultrasonic peening are studied. The effect pattern of stress ratio on fatigue performance of weldedjoints that are treated by ultrasonic peening is studied. As tests results indicate that: ① In thecondition of stress ratio .R=-l, the fatigue strength 'of specimen treated by ultrasonic peening isincreased by 165 percent of that of the original welded specimen. And the fatigue life of specimentreated by ultrasonic peening is as much as 75 approx 210 times of that of the latter. When R=0.1,the fatigue strength is increased by 87 percent and the fatigue life is extended by 21 approx 29times. When R=-0.5, the fatigue strength is increased by 123 percent and the fatigue life isextended by 42 approx 59 times. When R=0.45, the fatigue strength is increased by 51 percent and thefatigue life is extended by 3 approx 14 times. ② If the welded joints are treated by ultrasonicpeening, the fatigue strength is no longer independent on the applied mean stress. The more thestress ratio R, the less the fatigue stress range which can be sustained by the joints is. ③Whether the high value residual stress is in the joints or not, the dead load portion of the appliedload must be considered in the design of the joints which should be treated by ultrasonic peening.

Multiaxial Fatigue Analyses of Stress Joints for Deepwater Steel Catenary Risers

In the present study, the dynamic and fatigue characteristics of two types of stress joints are investigated under ocean environmental condition. Connected with the riser and the platform, stress joint at the vessel hang-off position should be one of the main critical design challenges for a steel catenary riser （SCR） in deepwater. When the riser is under a high pressure and deepwater working condition, the stress state for the joint is more complex, and the fatigue damage is easy to occur at this position. Stress joint discussed in this paper includes two types： Tapered Stress Joint （TSJ） and Sleeved Stress Joint （SSJ）, and multiaxial fatigue analysis results are given for comparison. Global dynamic analysis for an SCR is performed first, and then the local boundary conditions obtained from the previous analysis are applied to the stress joint FE model for the later dynamic and multiaxial fatigue analysis. Results indicate that the stress level is far lower than the yield limit of material and the damage induced by fatigue needs more attention. Besides, the damage character of the two types of stress joints differs： for TSJ, the place where the stress joint connects with the riser is easy to occur fatigue damage; for SSJ, the most probable position is at the place where the end of the inner sleeve pipe contacts with the riser body. Compared with SSJ, TSJ shows a higher stress level but better fatigue performance, and it will have a higher material cost. In consideration of various factors, designers should choose the most suitable type and also geometric parameters.

EFFECTS OF STRESS RATIO AND FREQUENCY ON CORROSION FATIGUE CRACK GROWTH MECHANISM IN LOW ALLOY STEELS

Based on theoretical analysis about local strain,strain rate and dissolving rate at crack tip, the corrosion fatigue crack growth rate of steels ZG20SiMn and SM50B-Zc in fresh water and 3.5% NaCl solution were measured experimentally,and the PH and electrode potential within crack were also measured continuously along with crack propagating.It showed that the increase of crack growth rate,caused by both decreasing frequency and raising stress ratio,was mainly accelerated by hydrogen embrittlement.

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.

EFFECT OF SUBSTRUCTURE AND RESIDUAL STRESS IN STRENGTHENED LAYER ON FATIGUE STRENGTH OF STAINLESS OR LOW CARBON STEEL

In the strengthened layer of stainless steel after shot peening,there are a great amount of deformation microtwins which may act as structural strengthening factor and prevent the gradual relaxation of surface residual stress during fatigue,so as to keep its rather high level of bending fatigue strength.However,in the strengthened surface layer of low carbon steel, dislocation cell structure is so unstalbe during fatigue that its surface residual stress relaxation cannot be retarded.Therefore,the bending fatigue strength of the low carbon steel can not be improred by shot peening.

Effect of fatigue loading-confining stress unloading rate on marble mechanical behaviors: An insight into fracture evolution analyses

Rocks in underground works usually experience rather complex stress disturbance.For this,their fracture mechanism is significantly different from rocks subjected to conventional triaxial compression conditions.The effects of stress disturbances on rock geomechanical behaviors under fatigue loading conditions and triaxial unloading conditions have been reported in previous studies.However,little is known about the dependence of the unloading rate on fatigue loading and confining stress unloading(FL-CSU)conditions that influence rock failure.In this paper,we aimed at investigating the fracture behaviors of marble under FL-CSU conditions using the post-test X-ray computed tomography(CT)scanning technique and the GCTS RTR 2000 rock mechanics system.Results show that damage accumulation at the fatigue stage can influence the final fracture behaviors of marble.The stored elastic energy for rock samples under FL-CSU tests is relatively larger compared to those under conventional triaxial tests,and the dissipated energy used to drive damage evolution and crack propagation is larger for FL-CSU tests.In FL-CSU tests,as the unloading rate increases,the dissipated energy grows and elastic energy reduces.CT scanning after the test reveals the impacts of the unloading rate on the crack pattern and a fracture degree index is therein defined in this context to represent the crack dimension.It shows that the crack pattern after FL-CSU tests depends on the unloading rate,and the fracture degree is in agreement with the analysis of both the energy dissipation and the amount of energy released.The effect of unloading rate on fracture evolution characteristics of marble is revealed by a series of FL-CSU tests.