Both solute-segregated long-period stacking ordered(LPSO)structure and stacking faults(SFs)are essential in strengthening rare-earth(RE)Mg alloys.Herein,LPSO-enriched Mg and SFs-enriched Mg are fabricated and comparab...Both solute-segregated long-period stacking ordered(LPSO)structure and stacking faults(SFs)are essential in strengthening rare-earth(RE)Mg alloys.Herein,LPSO-enriched Mg and SFs-enriched Mg are fabricated and comparably investigated for fatigue performances.During fatigue,the Mg nanolayers between LPSO lamellae or SFs act as the gliding channels of dislocations.However,SFs-enriched Mg exhibits outstanding fatigue strength due to solute strengthening within Mg nanolayers.Solute strengthening is assumed to contribute to the local accumulation of basal dislocations and the activation of non-basal dislocations.Dislocations are restricted locally and cannot glide long distances to specimen surfaces,which mitigates fatigue-induced extrusions and slip markings,ultimately leading to an increase in fatigue strength.These findings guide the development of RE-Mg alloys towards a synergy between high tensile and high fatigue performances.展开更多
AZ31B magnesium alloy is widely used in transportation and aerospace fields due to its light weight and high strength,but it often causes structural failure due to fatigue fracture during service.Fatigue fracture is u...AZ31B magnesium alloy is widely used in transportation and aerospace fields due to its light weight and high strength,but it often causes structural failure due to fatigue fracture during service.Fatigue fracture is usually caused by the initiation of cracks on the surface of structural parts and the propagation of cracks to the interior of the specimen in the form of intergranular fracture.In order to improve the fatigue performance,this study proposes a method of pre-tension deformation and surface mechanical rolling treatment of AZ31B magnesium alloy,thereby changing the crack initiation area and increasing the crack propagation resistance.The experimental results show that:As the pre-tension deformation increases,the fatigue limit shows a trend of first rising and then decreasing.The 5PT specimen exhibits the optimal strengthening effect,with a fatigue limit of 115 MPa,which is a 27.78% improvement.Under surface mechanical rolling treatment,the fatigue limit reaches 140 MPa,which is a 55.56% improvement.When pre-tension deformation and surface mechanical rolling treatment are combined,the fatigue limit is further improved compared to individual strengthening methods.Among these,the 2PT+SMRT specimen shows the most significant strengthening effect,with a fatigue limit of 150 MPa,which is a 66.67% improvement.This study proposes a new strategy for improving the fatigue performance of AZ31B magnesium alloy,and reveals the synergistic strengthening mechanism of pre-tension deformation and surface mechanical rolling treatment of AZ31B magnesium alloy,which is of great significance for improving the fatigue performance of metal materials.展开更多
To expand the application of wire-arc additive manufacturing(WAAM)in aluminum alloy forming com-ponents,it is vitally important to reduce the porosity,refine microstructure,and thereby improve the mechanical propertie...To expand the application of wire-arc additive manufacturing(WAAM)in aluminum alloy forming com-ponents,it is vitally important to reduce the porosity,refine microstructure,and thereby improve the mechanical properties of the components.In this study,the interlayer friction stir processing(FSP)tech-nique was employed to assist the WAAM of 4043 Al-Si alloy,and the related effects on the microstruc-ture evolutions and mechanical properties of the fabricated builds were systematacially investigated.As compared to the conventional WAAM processing of Al-Si alloy,it was found that the introduction of in-terlayer FSP can effectively eliminate the pores,and both theα-Al dendrites and Si-rich eutectic network were severely broken up,leading to a remarkable enhancement in ductility and fatigue performance.The average yield strength(YS)and ultimate tensile strength(UTS)of the Al-based components produced by the combination of WAAM and interlayer FSP methods were 88 and 148 MPa,respectively.Meanwhile,the elongation(EL)of 37.5%and 28.8%can be achieved in the horizontal and vertical directions,respec-tively.Such anisotropy of EL was attributed to the inhomogeneous microstructure in the stir zone(SZ).Notably,the stress concentration can be effectively reduced by the elimination of porosity and Si-rich eu-tectic network fragmentation by the interlayer FSP,and thus the fatigue behavior was improved with the fatigue strength and elongation increased by∼28%and∼108.7%,respectively.It is anticipated that this study will provide a powerful strategy and theoretical guidance for the WAAM fabrication of Al-based alloy components with high ductility and fatigue performance.展开更多
Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its...Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its fatigue strength for 107 cycles is 451 MPa, which is higher than that of common carbon structural steel. This should be at- tributed to the fine-grain strengthening resulting from the high content of alloy element V and Thermo-Mechanical Control Process （TMCP）. The S-N curve function is also obtained by nonlinear regression with three parameters power function. The fatigue fractures of the specimen were further analyzed with Scanning Electron Microscopy （SEM） and Energy Disperse Spectroscopy （EDS） to study the fracture mechanism. Taking into account microstruc- ture, hardness and cleanliness of the material, it implies that the fatigue fractures of HRB500 rebar all arise from surface substrates in which many brittle inclusions are contained, and that the fatigue crack propagation is principally based on the mechanism of quasi-cleavage fracture, because of the intracrystalline hard spots leading to stress con- centration and thus to the cracks. Moreover, the transient breaking area exhibits microvoid coalescence of ductile fracture due to the existing abundant inclusions.展开更多
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 unl...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.展开更多
Fatigue performance is a serious concern for mechanical components subject to cyclical stresses,particularly where safety is paramount.The fatigue performance of components relies closely on their surface integrity be...Fatigue performance is a serious concern for mechanical components subject to cyclical stresses,particularly where safety is paramount.The fatigue performance of components relies closely on their surface integrity because the fatigue cracks generally initiate from free surfaces.This paper reviewed the published data,which addressed the effects of machined surface integrity on the fatigue performance of metal workpieces.Limitations in existing studies and the future directions in anti-fatigue manufacturing field were proposed.The remarkable surface topography(e.g.,low roughness and few local defects and inclusions)and large compressive residual stress are beneficial to fatigue performance.However,the indicators that describe the effects of surface topography and residual stress accurately need further study and exploration.The effect of residual stress relaxation under cycle loadings needs to be precisely modeled precisely.The effect of work hardening on fatigue performance had two aspects.Work hardening could increase the material yield strength,thereby delaying crack nucleation.However,increased brittleness could accel-erate crack propagation.Thus,finding the effective control mechanism and method of work hardening is urgently needed to enhance the fatigue performance of machined components.The machining-induced metallurgical structure changes,such as white layer,grain refinement,dislocation,and martensitic transformation affect the fatigue performance of a workpiece significantly.However,the unified and exact conclusion needs to be investigated deeply.Finally,different surface integrity factors had complicated reciprocal effects on fatigue performance.As such,studying the comprehensive influence of surface integrity further and establishing the reliable prediction model of workpiece fatigue performance are meaningful for improving reliability of components and reducing test cost.展开更多
The fatigue performance and fracture mechanism of laser welded twinning induced plasticity（TWIP）steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.Th...The fatigue performance and fracture mechanism of laser welded twinning induced plasticity（TWIP）steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.展开更多
This paper studies the effect of different environmental factors, including the axle load weight, environmental temperature, vehicle speed, and the aging level of asphalt, on the fatigue performance of asphalt mixes b...This paper studies the effect of different environmental factors, including the axle load weight, environmental temperature, vehicle speed, and the aging level of asphalt, on the fatigue performance of asphalt mixes based on four-point bending beam fatigue tests. A fractional factorial design method named ＂uniform design＂ was applied in experimental design. The relations of the environmental factors to initial stiffness, fatigue life, phase angle and cumulative dissipated energy were established with the general linear modeling method. It is found that there exists very good correlativity between the environmental factors and the fatigue performance indices of asphalt mixes. The coefficients of total correlation are mainly beyond 0. 95. The results indicate that the consideration of the effect of environmental factors is necessary in the fatigue performance evaluation on real asphalt pavement.展开更多
For a longitudinal welded joint, the tensile residual stresses are as high as the yield stress of the metal, so that the weld toes are sensitive to fatigue load. In this case a low transformation temperature electrode...For a longitudinal welded joint, the tensile residual stresses are as high as the yield stress of the metal, so that the weld toes are sensitive to fatigue load. In this case a low transformation temperature electrode (LTTE) is one of the most useful methods used to improve the fatigue strength of the longitudinal welded joint, because the tensile residual stress is reduced or changed into compressive stress. Three kinds of longitudinal welded joints were selected to conduct fatigue tests. The tests results show that the fatigue strengths at 2×10 6 cycles of the joints welded with LTTE were improved by 41%, 47% and 59% respectively compared with those of the joints welded with E5015, and the fatigue lives at 162 MPa were improved by 9.9 times, 9.6 times and 46.8 times respectively. Furthermore, the LTTE method is not necessary to add process after welding and so that it can be valuable method to improve the fatigue performance of longitudinal welded joints.展开更多
Four-point flexural fatigue test for Gussasphalt mixture specimen was carried out at a straincontrolled mode system. The results showed that the development of the tested stiffness modulus and phase angle of the mixtu...Four-point flexural fatigue test for Gussasphalt mixture specimen was carried out at a straincontrolled mode system. The results showed that the development of the tested stiffness modulus and phase angle of the mixtures with increasing load cycles exhibited three periods, initial generation, slow development and failure period. The fatigue crack generation zone formed in the third period, in which the macro mechanical properties were signifi cantly decreased. Moreover, we also analyzed the effects of asphalt content and mixing temperature on the fatigue life of the mixture. The results showed that the fi rst period when the specimen＇s initial stiffness modulus was reduced to 80% accounted for 5%-10% of the total fatigue life; the second period in which the reduction became slow and demonstrated a liner relationship with load cycles occupied 70%-85% of the fatigue life; and the third period was about 5%-10%. The results indicated that the lower the mixing temperature, the longer the fatigue life of Gussasphalt mixture. Besides, the increasing of asphalt content has a minor effect on the fatigue life of Gussasphalt mixture展开更多
To address the design challenges of helicopter hub central components under high-performance requirements,this paper conducts safe-life topology optimization design research considering fatigue performance for rotor h...To address the design challenges of helicopter hub central components under high-performance requirements,this paper conducts safe-life topology optimization design research considering fatigue performance for rotor hub central components under multi-load conditions,combined with helicopter fatigue strength engineering design theory.For dealing with the issues of derivative calculation difficulties when directly considering fatigue constraints in existing topology optimization methods,this study establishes a mathematical formulation suitable for structural topology optimization of hub central components by combining modified structural safety fatigue limits based on isolife curves.Then the sensitivity analysis of design variables is derived,and an optimization designmodel for typical main rotor hub central components is constructed.By controlling the safe-life equivalent stress of the hub central structure,the goal of managing structural fatigue life is achieved,providing new insights for long-life,high-reliability hub central component design.The paper presents a topology optimization case study of a typical five-armhub central component,completes optimized structure reconstruction and fatigue strength analysis,which validates the effectiveness of the proposed methodology.展开更多
Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc...Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc-Zr,with columnar,equiaxed,and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process.The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475±5–516±6 MPa with favorable elongation of approximately 11%,higher than that of most of the other Al alloys,including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys.Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa(at 107 loading cycles),which is the highest among those reported for the LPBF Al alloys.The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue(LCF)regime.The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading,which,in turn,minimizes the strain localization.At the later stages of strain accumulation,microcracks form at the grain boundaries,limiting the further improvement of the alloy's fatigue strength.This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.展开更多
The fatigue life of components can be significantly enhanced by the formation of the surface hardness layer through surface strengthening technology.To avoid the geometric distortion of thin-walled com-ponents caused ...The fatigue life of components can be significantly enhanced by the formation of the surface hardness layer through surface strengthening technology.To avoid the geometric distortion of thin-walled com-ponents caused by strengthening,the strengthening energy is limited and the ideal strengthening effect cannot be obtained.This work aims to propose a novel approach to address this issue effectively.The surface layer with high-density dislocations was obtained by a low-energy surface strengthening method(shot peening)at first.Then the surface strengthening mechanism changes from dislocation strengthen-ing to grain boundary strengthening after electropulsing treatment(EPT).The evolution of residual stress and microstructure was analyzed using multi-scale characterization techniques.The results demonstrate that EPT followed by surface strengthening makes a remarkable 304%increase in fatigue life of TC11 titanium alloy.The enhancement of fatigue life can be attributed to the grain refinement accompanied by the formation of nanotwins and sub-grains in the surface-strengthened layer,as well as the reduction in dislocation density within the substrate after EPT.This study demonstrates the significant potential of EPT in further enhancing the fatigue life of surface pre-strengthened thin-walled components.展开更多
The tidal power has the potential to play a vital role in a sustainable energy future.The main objective of this paper is to investigate the performance and fatigue life of tidal current turbine(TCT)using fluid struct...The tidal power has the potential to play a vital role in a sustainable energy future.The main objective of this paper is to investigate the performance and fatigue life of tidal current turbine(TCT)using fluid structure interaction(FSI)modeling.The performance of TCT was predicted using Ansys CFX.The performance curve,pressure distribution on the blade,and velocity streamline were visualized for eight repetitive analyses at different tip speed ratio.The hydrodynamic load calculated from CFD analysis was transferred to FEA model for investigation of the structural response of TCT.Modal analysis was performed to examine the mode shapes and natural frequencies of TCT.The fatigue analysis were performed and number of cycles and safety factor at different equivalent alternating stresses were investigated.The results of the simulation confirm that the turbine has a maximum value of the coefficient of performance atλ=5,the turbine operating frequency is not close to its natural frequency,and it is safe under the applied fatigue loads with a high factor of safety.展开更多
To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy,the electric pulse has been introduced into the ultrasonic surface rolling process(USRP),which is called electric pulse-assisted ul...To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy,the electric pulse has been introduced into the ultrasonic surface rolling process(USRP),which is called electric pulse-assisted ultrasonic surface rolling process(EUSRP).With the help of“electroplasticity”of the electric pulse,the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level.However,the surface hardness decreases due to the continuous effect of the pulse current and the“skin effect”during treatment.It is worth noting that the higher the applied pulse current,the more severe the softening.This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens.To break this paradox,the EUSRP treatment is followed by a USRP treatment.The EUSRP-2(with a pulse current of 200 A)+USRP specimens exhibit excellent surface hardness,a gradient deformation layer thickness of about 400μm,low surface roughness and high compressive residual compressive stress.Besides,the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis.The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment,the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site.The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.展开更多
The fatigue of concrete structures will gradually appear after being subjected to alternating loads for a long time,and the accidents caused by fatigue failure of bridge structures also appear from time to time.Aiming...The fatigue of concrete structures will gradually appear after being subjected to alternating loads for a long time,and the accidents caused by fatigue failure of bridge structures also appear from time to time.Aiming at the problem of degradation of long-span continuous rigid frame bridges due to fatigue and environmental effects,this paper suggests a method to analyze the fatigue degradation mechanism of this type of bridge,which combines long-term in-site monitoring data collected by the health monitoring system(HMS)and fatigue theory.In the paper,the authors mainly carry out the research work in the following aspects:First of all,a long-span continuous rigid frame bridge installed with HMS is used as an example,and a large amount of health monitoring data have been acquired,which can provide efficient information for fatigue in terms of equivalent stress range and cumulative number of stress cycles;next,for calculating the cumulative fatigue damage of the bridge structure,fatigue stress spectrum got by rain flow counting method,S-N curves and damage criteria are used for fatigue damage analysis.Moreover,it was considered a linear accumulation damage through the Palmgren-Miner rule for the counting of stress cycles.The health monitoring data are adopted to obtain fatigue stress data and the rain flow counting method is used to count the amplitude varying fatigue stress.The proposed fatigue reliability approach in the paper can estimate the fatigue damage degree and its evolution law of bridge structures well,and also can help bridge engineers do the assessment of future service duration.展开更多
Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration（VIV） for a riser wi...Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration（VIV） for a riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a flexible riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line（IL） response is as important as the cross-flow（CF） response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare riser can be quite distinct from that of a riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction.展开更多
The deepwater test string is an important but vulnerable component in offshore petroleum exploration,and its durability significantly affects the success of deepwater test operations.Considering the influence of rando...The deepwater test string is an important but vulnerable component in offshore petroleum exploration,and its durability significantly affects the success of deepwater test operations.Considering the influence of random waves and the interaction between the test string and the riser,a time-domain nonlinear dynamic model of a deepwater test string is developed.The stress-time history of the test string is obtained to study vibration mechanisms and fatigue development in the test string.Several recommendations for reducing damage are proposed.The results indicate that the amplitude of dynamic response when the string is subjected to random loads gradually decreases along the test string,and that the von Mises stress is higher in the string sections near the top of the test string and the flex joints.In addition,the fatigue damage fluctuates with the water depth,and the maximum damage occurs in string sections adjacent to the lower flex joint and in the splash zone.Several measures are proposed to improve the operational safety of deepwater test strings：applying greater top tension,operating in a favorable marine environment,managing the order of the test string joints,and performing nondestructive testing of components at vulnerable positions.展开更多
An inverted asphalt pavement is created by reversing the sequence of the lower and middle layers in a conventional asphalt pavement. The lower layer is composed of material with larger particle size and lower asphalt ...An inverted asphalt pavement is created by reversing the sequence of the lower and middle layers in a conventional asphalt pavement. The lower layer is composed of material with larger particle size and lower asphalt content, which improves its ability to withstand deformation caused by rutting. On the other hand, the middle surface has a higher asphalt content, specifically designed to resist fatigue cracking. This paper examines the mechanical response of two pavement structures and investigates the potential of two measures, inverted asphalt pavement and asphalt mixture design by vertical vibration compaction method(VVCM), in reducing stresses and stress levels in asphalt pavements. Additionally, a large thickness rutting and fatigue test method was developed to study the rutting resistance and fatigue life of the pavement structures, and to construct rutting deformation and fatigue life prediction models. Finally, test sections were paved to verify the feasibility of the inverted pavement and VVCM materials. The findings show that inverted pavement and VVCM materials have a minimal impact on pavement stress, but can reduce pavement shear and tensile stress levels by up to 18%–25%.Furthermore, inverted pavement and VVCM materials have positive effects on improving the rutting resistance and fatigue life of asphalt pavements.展开更多
Although there are many kinds of fracture tests to choose from in evaluating the crack resistance of asphalt mixture,the semi-circular bending(SCB)test has attracted a lot of attention in the academic road engineering...Although there are many kinds of fracture tests to choose from in evaluating the crack resistance of asphalt mixture,the semi-circular bending(SCB)test has attracted a lot of attention in the academic road engineering community because of its simplicity,stability,and flexibility in testing and evaluation.The SCB test has become a common method to study the cracking resistance of asphalt mixture in recent years.This paper mainly summarizes the overview of the SCB test,summarizes some research results and common characterization parameters of the SCB test method in monotone test and fatigue test in recent years,and predicts and suggests the research direction of the SCB test in the future.It is found that the research on the monotonic SCB test is more comprehensive,and the research on the SCB fatigue test needs to be further improved in the aspects of loading mode,characterization parameter selection,and so on.Researchers can flexibly adjust the geometric dimensions and the test parameters of semi-cylindrical specimens,and conduct comprehensive analysis combined with the results of numerical simulation.The crack resistance of asphalt mixture can be comprehensively evaluated by fracture energy,fracture toughness,stiffness,flexibility index and other fracture indicators,combined with the crack propagation of the specimen.The analysis of numerical simulation can confirm the test results.In order to standardize the setting of fatigue parameters for future application,it is necessary to standardize the setting of bending performance.展开更多
基金supported by National Natural Science Foundation of China(Nos.12102280,12172238,12332012)Postdoctoral Fellowship Program of CPSF(No.GZB20230473)+1 种基金Support of Ultramicroscopy Research Center(URC,Kyushu University)are highly acknowledged.Yao Chen acknowledges the support of JSPS Fellowship(No.JP22F22720)JSPS KAKENHI(No JP22K03828).
文摘Both solute-segregated long-period stacking ordered(LPSO)structure and stacking faults(SFs)are essential in strengthening rare-earth(RE)Mg alloys.Herein,LPSO-enriched Mg and SFs-enriched Mg are fabricated and comparably investigated for fatigue performances.During fatigue,the Mg nanolayers between LPSO lamellae or SFs act as the gliding channels of dislocations.However,SFs-enriched Mg exhibits outstanding fatigue strength due to solute strengthening within Mg nanolayers.Solute strengthening is assumed to contribute to the local accumulation of basal dislocations and the activation of non-basal dislocations.Dislocations are restricted locally and cannot glide long distances to specimen surfaces,which mitigates fatigue-induced extrusions and slip markings,ultimately leading to an increase in fatigue strength.These findings guide the development of RE-Mg alloys towards a synergy between high tensile and high fatigue performances.
基金the generous support from natural Science Foundation of Shanxi Province(No.202203021221040 and No.201901D111106).
文摘AZ31B magnesium alloy is widely used in transportation and aerospace fields due to its light weight and high strength,but it often causes structural failure due to fatigue fracture during service.Fatigue fracture is usually caused by the initiation of cracks on the surface of structural parts and the propagation of cracks to the interior of the specimen in the form of intergranular fracture.In order to improve the fatigue performance,this study proposes a method of pre-tension deformation and surface mechanical rolling treatment of AZ31B magnesium alloy,thereby changing the crack initiation area and increasing the crack propagation resistance.The experimental results show that:As the pre-tension deformation increases,the fatigue limit shows a trend of first rising and then decreasing.The 5PT specimen exhibits the optimal strengthening effect,with a fatigue limit of 115 MPa,which is a 27.78% improvement.Under surface mechanical rolling treatment,the fatigue limit reaches 140 MPa,which is a 55.56% improvement.When pre-tension deformation and surface mechanical rolling treatment are combined,the fatigue limit is further improved compared to individual strengthening methods.Among these,the 2PT+SMRT specimen shows the most significant strengthening effect,with a fatigue limit of 150 MPa,which is a 66.67% improvement.This study proposes a new strategy for improving the fatigue performance of AZ31B magnesium alloy,and reveals the synergistic strengthening mechanism of pre-tension deformation and surface mechanical rolling treatment of AZ31B magnesium alloy,which is of great significance for improving the fatigue performance of metal materials.
文摘To expand the application of wire-arc additive manufacturing(WAAM)in aluminum alloy forming com-ponents,it is vitally important to reduce the porosity,refine microstructure,and thereby improve the mechanical properties of the components.In this study,the interlayer friction stir processing(FSP)tech-nique was employed to assist the WAAM of 4043 Al-Si alloy,and the related effects on the microstruc-ture evolutions and mechanical properties of the fabricated builds were systematacially investigated.As compared to the conventional WAAM processing of Al-Si alloy,it was found that the introduction of in-terlayer FSP can effectively eliminate the pores,and both theα-Al dendrites and Si-rich eutectic network were severely broken up,leading to a remarkable enhancement in ductility and fatigue performance.The average yield strength(YS)and ultimate tensile strength(UTS)of the Al-based components produced by the combination of WAAM and interlayer FSP methods were 88 and 148 MPa,respectively.Meanwhile,the elongation(EL)of 37.5%and 28.8%can be achieved in the horizontal and vertical directions,respec-tively.Such anisotropy of EL was attributed to the inhomogeneous microstructure in the stir zone(SZ).Notably,the stress concentration can be effectively reduced by the elimination of porosity and Si-rich eu-tectic network fragmentation by the interlayer FSP,and thus the fatigue behavior was improved with the fatigue strength and elongation increased by∼28%and∼108.7%,respectively.It is anticipated that this study will provide a powerful strategy and theoretical guidance for the WAAM fabrication of Al-based alloy components with high ductility and fatigue performance.
基金Item Sponsored by Fundamental Research Funds for the Central Universities of China(FRF-TP-15-062A3)
文摘Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its fatigue strength for 107 cycles is 451 MPa, which is higher than that of common carbon structural steel. This should be at- tributed to the fine-grain strengthening resulting from the high content of alloy element V and Thermo-Mechanical Control Process （TMCP）. The S-N curve function is also obtained by nonlinear regression with three parameters power function. The fatigue fractures of the specimen were further analyzed with Scanning Electron Microscopy （SEM） and Energy Disperse Spectroscopy （EDS） to study the fracture mechanism. Taking into account microstruc- ture, hardness and cleanliness of the material, it implies that the fatigue fractures of HRB500 rebar all arise from surface substrates in which many brittle inclusions are contained, and that the fatigue crack propagation is principally based on the mechanism of quasi-cleavage fracture, because of the intracrystalline hard spots leading to stress con- centration and thus to the cracks. Moreover, the transient breaking area exhibits microvoid coalescence of ductile fracture due to the existing abundant inclusions.
文摘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.
基金Supported by National Natural Science Foundation of China(Grant No.52005281)Major Program of Shandong Province Natural Science Foundation of China(Grant No.ZR2018ZA0401)Applied Basic Research Projects for Qingdao Innovation Plan(Grant No.18-2-2-67-jch).
文摘Fatigue performance is a serious concern for mechanical components subject to cyclical stresses,particularly where safety is paramount.The fatigue performance of components relies closely on their surface integrity because the fatigue cracks generally initiate from free surfaces.This paper reviewed the published data,which addressed the effects of machined surface integrity on the fatigue performance of metal workpieces.Limitations in existing studies and the future directions in anti-fatigue manufacturing field were proposed.The remarkable surface topography(e.g.,low roughness and few local defects and inclusions)and large compressive residual stress are beneficial to fatigue performance.However,the indicators that describe the effects of surface topography and residual stress accurately need further study and exploration.The effect of residual stress relaxation under cycle loadings needs to be precisely modeled precisely.The effect of work hardening on fatigue performance had two aspects.Work hardening could increase the material yield strength,thereby delaying crack nucleation.However,increased brittleness could accel-erate crack propagation.Thus,finding the effective control mechanism and method of work hardening is urgently needed to enhance the fatigue performance of machined components.The machining-induced metallurgical structure changes,such as white layer,grain refinement,dislocation,and martensitic transformation affect the fatigue performance of a workpiece significantly.However,the unified and exact conclusion needs to be investigated deeply.Finally,different surface integrity factors had complicated reciprocal effects on fatigue performance.As such,studying the comprehensive influence of surface integrity further and establishing the reliable prediction model of workpiece fatigue performance are meaningful for improving reliability of components and reducing test cost.
基金Item Sponsored by National Natural Science Foundation of China(51374151,21201129)Science and Technology Major Project of Shanxi Province of China(20111101053)Natural Science Foundation of Shanxi Province of China(2011011020-2)
文摘The fatigue performance and fracture mechanism of laser welded twinning induced plasticity（TWIP）steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.
文摘This paper studies the effect of different environmental factors, including the axle load weight, environmental temperature, vehicle speed, and the aging level of asphalt, on the fatigue performance of asphalt mixes based on four-point bending beam fatigue tests. A fractional factorial design method named ＂uniform design＂ was applied in experimental design. The relations of the environmental factors to initial stiffness, fatigue life, phase angle and cumulative dissipated energy were established with the general linear modeling method. It is found that there exists very good correlativity between the environmental factors and the fatigue performance indices of asphalt mixes. The coefficients of total correlation are mainly beyond 0. 95. The results indicate that the consideration of the effect of environmental factors is necessary in the fatigue performance evaluation on real asphalt pavement.
文摘For a longitudinal welded joint, the tensile residual stresses are as high as the yield stress of the metal, so that the weld toes are sensitive to fatigue load. In this case a low transformation temperature electrode (LTTE) is one of the most useful methods used to improve the fatigue strength of the longitudinal welded joint, because the tensile residual stress is reduced or changed into compressive stress. Three kinds of longitudinal welded joints were selected to conduct fatigue tests. The tests results show that the fatigue strengths at 2×10 6 cycles of the joints welded with LTTE were improved by 41%, 47% and 59% respectively compared with those of the joints welded with E5015, and the fatigue lives at 162 MPa were improved by 9.9 times, 9.6 times and 46.8 times respectively. Furthermore, the LTTE method is not necessary to add process after welding and so that it can be valuable method to improve the fatigue performance of longitudinal welded joints.
基金Funded by the National Natural Science Foundation of China(No.51202214)
文摘Four-point flexural fatigue test for Gussasphalt mixture specimen was carried out at a straincontrolled mode system. The results showed that the development of the tested stiffness modulus and phase angle of the mixtures with increasing load cycles exhibited three periods, initial generation, slow development and failure period. The fatigue crack generation zone formed in the third period, in which the macro mechanical properties were signifi cantly decreased. Moreover, we also analyzed the effects of asphalt content and mixing temperature on the fatigue life of the mixture. The results showed that the fi rst period when the specimen＇s initial stiffness modulus was reduced to 80% accounted for 5%-10% of the total fatigue life; the second period in which the reduction became slow and demonstrated a liner relationship with load cycles occupied 70%-85% of the fatigue life; and the third period was about 5%-10%. The results indicated that the lower the mixing temperature, the longer the fatigue life of Gussasphalt mixture. Besides, the increasing of asphalt content has a minor effect on the fatigue life of Gussasphalt mixture
基金supported by the National Natural Science Foundation of China(Grant No.52375253)the Outstanding Youth Foundation of Shandong Provincial Natural Science Foundation(Grant No.ZR2024YQ036)+2 种基金the Shandong Provincial Key Research and Development Program(Grant No.2025****0306)the Aeronautical Science Foundation of China(Grant No.202400180Q3002)the Special Fund for the Taishan Scholars Program.
文摘To address the design challenges of helicopter hub central components under high-performance requirements,this paper conducts safe-life topology optimization design research considering fatigue performance for rotor hub central components under multi-load conditions,combined with helicopter fatigue strength engineering design theory.For dealing with the issues of derivative calculation difficulties when directly considering fatigue constraints in existing topology optimization methods,this study establishes a mathematical formulation suitable for structural topology optimization of hub central components by combining modified structural safety fatigue limits based on isolife curves.Then the sensitivity analysis of design variables is derived,and an optimization designmodel for typical main rotor hub central components is constructed.By controlling the safe-life equivalent stress of the hub central structure,the goal of managing structural fatigue life is achieved,providing new insights for long-life,high-reliability hub central component design.The paper presents a topology optimization case study of a typical five-armhub central component,completes optimized structure reconstruction and fatigue strength analysis,which validates the effectiveness of the proposed methodology.
基金the National Natural Science Foundation of China(No.52171026)the Equipment Pre-Research Field Foundation(No.80923010304).
文摘Aluminum alloys that are additively manufactured using the laser powder bed fusion(LPBF)suffer from relatively poor high cycle fatigue(HCF)resistance.In an effort to alleviate this,a high-strength Al alloy,Al-Mn-Mg-Sc-Zr,with columnar,equiaxed,and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process.The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475±5–516±6 MPa with favorable elongation of approximately 11%,higher than that of most of the other Al alloys,including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys.Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa(at 107 loading cycles),which is the highest among those reported for the LPBF Al alloys.The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue(LCF)regime.The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading,which,in turn,minimizes the strain localization.At the later stages of strain accumulation,microcracks form at the grain boundaries,limiting the further improvement of the alloy's fatigue strength.This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.
基金supported by the National Nature Science Foun-dation of China(Grant No.50875061).
文摘The fatigue life of components can be significantly enhanced by the formation of the surface hardness layer through surface strengthening technology.To avoid the geometric distortion of thin-walled com-ponents caused by strengthening,the strengthening energy is limited and the ideal strengthening effect cannot be obtained.This work aims to propose a novel approach to address this issue effectively.The surface layer with high-density dislocations was obtained by a low-energy surface strengthening method(shot peening)at first.Then the surface strengthening mechanism changes from dislocation strengthen-ing to grain boundary strengthening after electropulsing treatment(EPT).The evolution of residual stress and microstructure was analyzed using multi-scale characterization techniques.The results demonstrate that EPT followed by surface strengthening makes a remarkable 304%increase in fatigue life of TC11 titanium alloy.The enhancement of fatigue life can be attributed to the grain refinement accompanied by the formation of nanotwins and sub-grains in the surface-strengthened layer,as well as the reduction in dislocation density within the substrate after EPT.This study demonstrates the significant potential of EPT in further enhancing the fatigue life of surface pre-strengthened thin-walled components.
文摘The tidal power has the potential to play a vital role in a sustainable energy future.The main objective of this paper is to investigate the performance and fatigue life of tidal current turbine(TCT)using fluid structure interaction(FSI)modeling.The performance of TCT was predicted using Ansys CFX.The performance curve,pressure distribution on the blade,and velocity streamline were visualized for eight repetitive analyses at different tip speed ratio.The hydrodynamic load calculated from CFD analysis was transferred to FEA model for investigation of the structural response of TCT.Modal analysis was performed to examine the mode shapes and natural frequencies of TCT.The fatigue analysis were performed and number of cycles and safety factor at different equivalent alternating stresses were investigated.The results of the simulation confirm that the turbine has a maximum value of the coefficient of performance atλ=5,the turbine operating frequency is not close to its natural frequency,and it is safe under the applied fatigue loads with a high factor of safety.
基金supported by the Natural Science Foundation of Guangdong,China(No.2022A1515010023)Zhuhai Industrial Core Research Project(No.2220004002348).
文摘To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy,the electric pulse has been introduced into the ultrasonic surface rolling process(USRP),which is called electric pulse-assisted ultrasonic surface rolling process(EUSRP).With the help of“electroplasticity”of the electric pulse,the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level.However,the surface hardness decreases due to the continuous effect of the pulse current and the“skin effect”during treatment.It is worth noting that the higher the applied pulse current,the more severe the softening.This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens.To break this paradox,the EUSRP treatment is followed by a USRP treatment.The EUSRP-2(with a pulse current of 200 A)+USRP specimens exhibit excellent surface hardness,a gradient deformation layer thickness of about 400μm,low surface roughness and high compressive residual compressive stress.Besides,the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis.The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment,the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site.The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.
文摘The fatigue of concrete structures will gradually appear after being subjected to alternating loads for a long time,and the accidents caused by fatigue failure of bridge structures also appear from time to time.Aiming at the problem of degradation of long-span continuous rigid frame bridges due to fatigue and environmental effects,this paper suggests a method to analyze the fatigue degradation mechanism of this type of bridge,which combines long-term in-site monitoring data collected by the health monitoring system(HMS)and fatigue theory.In the paper,the authors mainly carry out the research work in the following aspects:First of all,a long-span continuous rigid frame bridge installed with HMS is used as an example,and a large amount of health monitoring data have been acquired,which can provide efficient information for fatigue in terms of equivalent stress range and cumulative number of stress cycles;next,for calculating the cumulative fatigue damage of the bridge structure,fatigue stress spectrum got by rain flow counting method,S-N curves and damage criteria are used for fatigue damage analysis.Moreover,it was considered a linear accumulation damage through the Palmgren-Miner rule for the counting of stress cycles.The health monitoring data are adopted to obtain fatigue stress data and the rain flow counting method is used to count the amplitude varying fatigue stress.The proposed fatigue reliability approach in the paper can estimate the fatigue damage degree and its evolution law of bridge structures well,and also can help bridge engineers do the assessment of future service duration.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51279101,51239007 and 51490674)a Research Project on High-Technology Ships supported by the Ministry of Industry and Information Technology of Chinathe Central Financial Support of Local Key Discipline Youth Fund Project(Grant No.YC319)
文摘Laboratory tests were conducted on a flexible riser with and without helical strakes. The aim of the present work is to further understand the response performance of the vortex induced vibration（VIV） for a riser with helical strakes. The experiment was accomplished in the towing tank and the relative current was simulated by towing a flexible riser in one direction. Based on the modal analysis method, the displacement responses can be obtained by the measured strain. The strakes with different heights are analyzed here, and the response parameters like strain response and displacement response are studied. The experimental results show that the in-line（IL） response is as important as the cross-flow（CF） response, however, many industrial analysis methods usually ignore the IL response due to VIV. The results also indicate that the response characteristics of a bare riser can be quite distinct from that of a riser with helical strakes, and the response performance depends on the geometry on the helical strakes closely. The fatigue damage is further discussed and the results show that the fatigue damage in the CF direction is of the same order as that in the IL direction for the bare riser. However, for the riser with helical strakes, the fatigue damage in the CF direction is much smaller than that in the IL direction.
基金supported by the National Key Basic Research Program of China (973 Program,Grant No.2015CB251203)the Fundamental Research Funds for the Central Universities (14CX06119A)
文摘The deepwater test string is an important but vulnerable component in offshore petroleum exploration,and its durability significantly affects the success of deepwater test operations.Considering the influence of random waves and the interaction between the test string and the riser,a time-domain nonlinear dynamic model of a deepwater test string is developed.The stress-time history of the test string is obtained to study vibration mechanisms and fatigue development in the test string.Several recommendations for reducing damage are proposed.The results indicate that the amplitude of dynamic response when the string is subjected to random loads gradually decreases along the test string,and that the von Mises stress is higher in the string sections near the top of the test string and the flex joints.In addition,the fatigue damage fluctuates with the water depth,and the maximum damage occurs in string sections adjacent to the lower flex joint and in the splash zone.Several measures are proposed to improve the operational safety of deepwater test strings：applying greater top tension,operating in a favorable marine environment,managing the order of the test string joints,and performing nondestructive testing of components at vulnerable positions.
基金supported by Shaanxi Province Innovation Capacity Support Program (2022TD-06)Transportation Industry Key Science and Technology Projects (2021-MS1-011)+1 种基金the Science and Technology Project of the Shaanxi Provincial Department of Transportation (20-02K)the Scientific Project from Henan Provincial Communication (2021-2-8)。
文摘An inverted asphalt pavement is created by reversing the sequence of the lower and middle layers in a conventional asphalt pavement. The lower layer is composed of material with larger particle size and lower asphalt content, which improves its ability to withstand deformation caused by rutting. On the other hand, the middle surface has a higher asphalt content, specifically designed to resist fatigue cracking. This paper examines the mechanical response of two pavement structures and investigates the potential of two measures, inverted asphalt pavement and asphalt mixture design by vertical vibration compaction method(VVCM), in reducing stresses and stress levels in asphalt pavements. Additionally, a large thickness rutting and fatigue test method was developed to study the rutting resistance and fatigue life of the pavement structures, and to construct rutting deformation and fatigue life prediction models. Finally, test sections were paved to verify the feasibility of the inverted pavement and VVCM materials. The findings show that inverted pavement and VVCM materials have a minimal impact on pavement stress, but can reduce pavement shear and tensile stress levels by up to 18%–25%.Furthermore, inverted pavement and VVCM materials have positive effects on improving the rutting resistance and fatigue life of asphalt pavements.
基金The authors acknowledge the financial support from the National Natural Science Foundation of China(No.51968006).
文摘Although there are many kinds of fracture tests to choose from in evaluating the crack resistance of asphalt mixture,the semi-circular bending(SCB)test has attracted a lot of attention in the academic road engineering community because of its simplicity,stability,and flexibility in testing and evaluation.The SCB test has become a common method to study the cracking resistance of asphalt mixture in recent years.This paper mainly summarizes the overview of the SCB test,summarizes some research results and common characterization parameters of the SCB test method in monotone test and fatigue test in recent years,and predicts and suggests the research direction of the SCB test in the future.It is found that the research on the monotonic SCB test is more comprehensive,and the research on the SCB fatigue test needs to be further improved in the aspects of loading mode,characterization parameter selection,and so on.Researchers can flexibly adjust the geometric dimensions and the test parameters of semi-cylindrical specimens,and conduct comprehensive analysis combined with the results of numerical simulation.The crack resistance of asphalt mixture can be comprehensively evaluated by fracture energy,fracture toughness,stiffness,flexibility index and other fracture indicators,combined with the crack propagation of the specimen.The analysis of numerical simulation can confirm the test results.In order to standardize the setting of fatigue parameters for future application,it is necessary to standardize the setting of bending performance.
