The fatigue crack growth rate of a novel Ti-6Al-4V-1Mo titanium alloy,which is developed for laser directed energy deposition technique,was investigated before and after cyclic heat treatment(CHT).Changes in microstru...The fatigue crack growth rate of a novel Ti-6Al-4V-1Mo titanium alloy,which is developed for laser directed energy deposition technique,was investigated before and after cyclic heat treatment(CHT).Changes in microstructure,fracture surfaces,and crack growth paths were analyzed before and after CHT.Results indicate that in the stable crack growth region,the growth rates for the as-deposited and cyclic heat-treated specimens follow the relationships da/dN=1.8651×10^(−8)(ΔK)^(3.2271)and da/dN=1.4112×10^(−8)(ΔK)^(3.1125),respectively.Compared with that at the as-deposited state,the microstructure after CHT is transformed from a uniform basket-weave microstructure to a dual-phase microstructure consisting of near-sphericalαandβ-transformed matrix phases.The cyclic process also disrupts the continuity of the grain boundaryα(αGB)at the primaryβ-phase grain boundary.The coarsening of primaryαand the disruption ofαGB continuity are the primary factors to release stress concentration and promote crack deflection,thereby decreasing the fatigue crack growth rate.Additionally,the increased occurrence of crack branching,secondary cracking,and crack bridging in cyclic heat-treated specimens further reduces the crack driving force and slows the fatigue crack growth rate.展开更多
By using fatigue crack propagation testing and microstructural characterization,the crack fracture and propagation mechanisms of K4169 superalloy under various loads were investigated.The results demonstrate that the ...By using fatigue crack propagation testing and microstructural characterization,the crack fracture and propagation mechanisms of K4169 superalloy under various loads were investigated.The results demonstrate that the grain sizes of K4169 superalloy significantly increase,and the precipitation of the needle-likeδphase and the Laves phase is observed.Voids and microcracks form at location of Laves phase enrichment,creating conditions for crack propagation.By the a−N(a is the crack length,and N is the number of cycles)relationship curve,the change in the fatigue crack growth rate with the increasing number of cycles progresses through three separate stages.The fracture process of K4169 superalloy under low-stress cyclic loading(3 kN)exhibits the ductile fracture.Subsequently,the fracture process starts to change from the ductile fracture to the brittle fracture as the stress increases to 4.5 kN.In the microstructures of fractures in both stress states,intergranular propagation is the mechanism responsible for crack propagation.Moreover,the Laves phase exists near the fracture crack,which is in line with the post-service structural phenomenon.展开更多
In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated i...In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated in a bimodal Ti-5Al-4 V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature.The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking.Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure.The fatigue mechanisms were then analyzed with a focus on dislocation activity.α_(p) grains adjacent to cracked BTGBs contained a high dislocation density.It was primarily composed of planar slip bands of dislocations.In addition,<c+a>dislocations were noticed in the vicinity of cracked BTGBs.They supposedly pertain to crack tip plasticity during growth,and no evidence of a role of an incoming slip event in crack nucleation was obtained.Also,basal slip bands extending across adjacent grains were found to emerge from BTGBs.This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane.Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration.This suggests a possible contribution of local solute segregation to the high cracking resistance of general α_(p)/α_(p) grain boundaries.This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.展开更多
The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)...The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)during the early stage,the fourpoint bending fatigue test was carried out on specimens of three different directions,as well as the forged specimens.The results indicate the anisotropic crack initiation and early propagation of LMD Ti64.The direction perpendicular to the deposition direction exhibits a better fatigue resistance than the other two.The crack initiation position and propagation path are dominated by the microstructure in the vicinity of U-notch.LMD Ti64 has a typical small crack effect,and the early crack propagation velocities in three directions are similar.Affected by the slip system of LMD Ti64,secondary cracks frequently occur,which are often found to have an angle of 60°to the main crack.The electron backscatter diffraction analysis indicates that LMD Ti64 has preferred orientations,i.e.,strong 0001//Z texture and 001//Z texture.Their crystallographic orientation will change as the direction of columnarβgrains turns over,resulting in the fatigue anisotropy of LMD Ti64 in crack initiation and early crack propagation process.展开更多
High-performance magnesium alloys are in great demand to meet the lightweight design requirements of aircraft.Grain size has long been recognized as a key factor influencing the mechanical properties of alloys.This st...High-performance magnesium alloys are in great demand to meet the lightweight design requirements of aircraft.Grain size has long been recognized as a key factor influencing the mechanical properties of alloys.This study investigates the effect of grain size,controlled by Zr addition,on the fatigue behavior of a recently developed low-cost Mg-2.6Nd-0.35Zn alloy,through systematic characterization and analysis of stress-life(S-N)curves,fatigue crack propagation,fracture surface morphology,stress intensity factor,and crack propagation threshold.The results show that after heat treatment(solution at 525±5℃ for 8 h and water quenching at 60-80℃,followed by aging at 250±5℃for 14 h and then air cooling),coarse-grained specimens(average grain size approximately 596μm)containing 0.12wt.%Zr exhibit greater resistance to fatigue crack propagation than fine-grained specimens(average grain size approximately 94μm)containing 0.46wt.%Zr.Coarse grains promote intergranular fracture,while fine grains favor transgranular fracture.In addition,coarse grains reduce the sensitivity of the crack tip to stress concentration.Furthermore,fine-grained samples demonstrate a longer total fatigue life,owing to their superior resistance to crack initiation,which significantly prolongs the crack initiation stage.These findings highlight the importance of optimizing grain size to achieve the best possible fatigue resistance in Mg-Nd-Zn-Zr alloys for practical engineering applications.展开更多
Statistical distribution of residual fatigue life(RFL)of railway axles under given loading was computed using the Monte Carlo method by considering random variation of the selected input parameters.Experimental data f...Statistical distribution of residual fatigue life(RFL)of railway axles under given loading was computed using the Monte Carlo method by considering random variation of the selected input parameters.Experimental data for the EA4T railway axle steel,the loading spectrum,the press fit loading and the residual stress induced by surface hardening were considered in the crack propagation simulations.Usually,the material properties measured by tensile tests are considered to be the most informative source of material data.Under fatigue loading,however,the crack growth rates near the threshold are the most critical data.Two important influencing factors on these crack growth rates are presented:first,the air humidity and,second,the near-surface residual stress.The typical variation of these parameters in operation may change the RFL by one or two orders of magnitude.Experimentally obtained crack growth thresholds and residual stress profiles are highly affected by the used methodology.Therefore,the obtained input data may be located anywhere within a large scatter,while the experimenters are completely unaware of it.This can lead to dangerously non-conservative situations,e.g.when the thresholds are measured in a laboratory under humid air conditions and then applied to predictions of RFLs of axles operated in winter in low air humidity.This is significant for the topic of inspection interval optimisation.The results of experiments done on real 1:1 railway axles were close to the most frequent value found in the histogram of the numerically computed RFLs.展开更多
The interrupted fatigue test method was utilized to investigate the damage evolution mechanism of the notch high-cycle fatigue(NHCF)in Ti-55531 alloy with a multilevel lamellar microstructure.The results reveal that s...The interrupted fatigue test method was utilized to investigate the damage evolution mechanism of the notch high-cycle fatigue(NHCF)in Ti-55531 alloy with a multilevel lamellar microstructure.The results reveal that significant microvoids and microcracks predominantly initiate at α/β interfaces under various notch root radii(R).Notably,even under larger R(0.75 mm),mutual interactions of stacking faults(SFs)−deformation twins,twins−twins,and SFs−SFs are observed.Furthermore,with decreasing R(0.34 and 0.14 mm),the volume fraction of SFs escalates significantly and twins are almost absent.Moreover,activated prismatic slip system decreases with a decrease in Schmidt factor and with the further decrease in R.Finally,strain localization near α/β interfaces contributes to the initiation of fatigue microcracks.展开更多
In order to compare the impact of thickness of different layers on fatigue lives of different semi-rigid asphalt pavement structures, the mechanical results from finite element models in ABAQUS are incorporated with t...In order to compare the impact of thickness of different layers on fatigue lives of different semi-rigid asphalt pavement structures, the mechanical results from finite element models in ABAQUS are incorporated with the fatigue results from fatigue models in FE-SAFE to calculate the mechanical response and fatigue lives of semi-rigid pavement structures under heavy traffic loads. Then the influences on fatigue lives caused by the changes in the thickness of layers in pavement structures are also evaluated. The numerical simulation results show that the aggregated base and the large stone porous mixture （LSPM） base have better anti-cracking performance than the conventional semi-rigid base. The appropriate thickness range for the aggregated layer in the aggregated base is 15 to 18 cm. The thickness of the LSPM layer in the LSPM base is recommended to be less than 15 cm.展开更多
Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect,and it is not completely understood yet.This study conducted stress-controlled low-cycle fatigue and dwell fa...Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect,and it is not completely understood yet.This study conducted stress-controlled low-cycle fatigue and dwell fatigue tests on Ti-6Al-3Nb-2Zr-1Mo alloy with bimodal microstructure to reveal its microstructural characteristics and crack initiation mechanisms.The study demonstrated that the faceted primaryα nodules located near the specimen surface acted as crack initiation sites during both fatigue and dwell fatigue tests.Slip trace analysis revealed that faceted cracking occurred at(0001)basal plane with the maximum Schmid factor value through a special cracking mode referred to as(0001)twist boundary cracking.Innovative criteria of parameters C1 and C2 were proposed based on experimental observation and molecular dynamics simulations,which well identify candidates for(0001)twist boundary crack nucleation.It demonstrated that grain pairs combining a moderately high Schmid factor for basal slip and a well-orientated Burgers vector in the out-of-surface plane was the preferable location for surface(0001)twist-boundary crack initiation,and grain pairs combining a high Schmid factor for basal slip and a high normal stress on basal plane are perfect candidates for subsurface cracking.Based on this,phenomeno-logical models are proposed to explain the surface(0001)twist-boundary cracking mechanism from the perspective of surface extrusion-intrusion-induced micro-notches.展开更多
a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respe...a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respectively),the Paris formula of fatigue crack growth was fitted linearly,and the material parameters C and n were measured.The results show that the sample with cooling rate of 0.5℃/s has the fastest crack growth rate,and the sample with salt bath isothermal has the slowest crack growth rate.The coarse M/A islands with irregular shape in bainite structure with cooling rate of 0.5℃/s has poor resistance to fatigue crack propagation,which is not conducive to improving the fatigue performance.However,the sample with salt bath isothermal has longer fatigue life.Due to the combination of bainitie lamellar and retained austenite distributed between them,the salt bath isothermal sample can effectively improve the strength and toughness of bainite steel.The sample with cooling rate of 0.5℃/s is mainly composed of granular bainitie structure,and the fatigue crack growth trajectory is generally gentle without large angle deflection,the sample with salt bath isothermal is mainly composed of bainite lamellar structure,and the fatigue crack growth trajectory is not straight,with a large number of Z-shaped deflection.The fatigue cracks are prone to produce branch cracks at the stress concentration of propagation deflection,and the branch crack consumes the energy of the main fatigue cracks,thus reducing the fatigue crack growth rate and improving fatigue life.展开更多
This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bi...This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bimodal structure)through fatigue crack propagation rate tests and fatigue threshold value tests.The resistance of the alloy to fatigue crack propagation was found to be closely correlated with the morphology and distribution ofαparticles,as evidenced by microscopic examination of fracture surfaces and analysis of crack propagation paths.The primaryαparticles demonstrated superior resistance to crack propagation compared to the secondaryαparticles.The basketweave structure showed exceptional resistance to fatigue crack propagation at all stages.The lamellar structure mainly resists long crack propagation during rapid propagation,and its threshold value is the lowest,which makes it easy to produce microcrack propagation.On the contrary,the bimodal structure has the highest threshold value among the three,so its resistance to short crack growth is more excellent,but it has the highest crack growth rate in the higher stress intensity factor range.Theαparticles in the three microstructures also undergo rotational motion relative to the force axis during fatigue crack propagation,thereby adjusting the uneven stress distribution betweenα/βphases through slip behavior and further coordinating deformation.展开更多
Fatigue cracking behavior from a notch was investigated at room temperature for Ti-6.SAI-3.5Mo-1.5Zr- 0.3Si （TClI） alloys with four different microstructures obtained at different cooling rates from the β transus t...Fatigue cracking behavior from a notch was investigated at room temperature for Ti-6.SAI-3.5Mo-1.5Zr- 0.3Si （TClI） alloys with four different microstructures obtained at different cooling rates from the β transus temperature. It was found that the alloy with lamellar structures consisting of α/β lamellae or acicular α＇ martensite laths had a higher fatigue crack initiation threshold from the notch, while the bimodal structure with coarse a grain had a lower fatigue cracking resistance. The alloy with α/β lamellar structure showed a higher fatigue crack growth resistance. The length scales of the microstructures were characterized to correlate with fatigue cracking behavior. Fatigue cracking mechanism related to microstructures was discussed.展开更多
Crack length measurement algorithms based on computer vision have shown promising engineering application prospects in the field of aircraft fatigue crack monitoring.However,due to the complexity of the monitoring env...Crack length measurement algorithms based on computer vision have shown promising engineering application prospects in the field of aircraft fatigue crack monitoring.However,due to the complexity of the monitoring environment,the subtle visual features of small fatigue cracks,and the impact of structural elastic deformation,directly applying object segmentation algorithms often results in significant measurement errors.Therefore,this paper proposes a high-precision crack length measurement method based on Bidirectional Target Tracking Model(Bi2TM),which integrates crack tip localization,interference identification,and length compensation.First,a general object segmentation model is used to perform rough crack segmentation.Then,the Bi2TM network,combined with the visual features of the structure in different stress states,is employed to track the bidirectional position of the crack tip in the“open”and“closed”states.This ultimately enables interference identification within the rough segmented crack region,achieving highprecision length measurement.In a high-interference environment of aircraft fatigue testing,the proposed method is used to measure 1000 crack images ranging from 1 mm to 11 mm.For more than 90%of the samples,the measurement error is less than 5 pixels,demonstrating significant advantages over the existing methods.展开更多
The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy （SEM）. The size and shape ...The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy （SEM）. The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.展开更多
With the aim of improving the fatigue properties of Mg alloy welded joints under cyclic loading,the effects of laser bionic treatment and ultrasonic impact bionic treatment on the fatigue crack growth(FCG)behavior of ...With the aim of improving the fatigue properties of Mg alloy welded joints under cyclic loading,the effects of laser bionic treatment and ultrasonic impact bionic treatment on the fatigue crack growth(FCG)behavior of AZ31B Mg alloy TIG-welded joints were studied and compared.The results show that bionic treatment refines the grains on the joint surface and improves the microhardness.In the crack stable growth stage,both bionic samples exhibit a lower FCG rate and a higher FCG resistance.The two bionic treatment methods reduce the probability of crack initiation and partially promote crack deflection,providing a new approach for improving the FCG behavior of welded joints.展开更多
Fatigue crack growth is a critical phenomenon in engineering structures,accounting for a significant percentage of structural failures across various industries.Accurate prediction of crack initiation,propagation path...Fatigue crack growth is a critical phenomenon in engineering structures,accounting for a significant percentage of structural failures across various industries.Accurate prediction of crack initiation,propagation paths,and fatigue life is essential for ensuring structural integrity and optimizing maintenance schedules.This paper presents a comprehensive finite element approach for simulating two-dimensional fatigue crack growth under linear elastic conditionswith adaptivemesh generation.The source code for the programwas developed in Fortran 95 and compiled with Visual Fortran.To achieve high-fidelity simulations,the methodology integrates several key features:it employs an automatic,adaptive meshing technique that selectively refines the element density near the crack front and areas of significant stress concentration.Specialized singular elements are used at the crack tip to ensure precise stress field representation.The direction of crack advancement is predicted using the maximum tangential stress criterion,while stress intensity factors are determined through either the displacement extrapolation technique or the J-integral method.The simulation models crack growth as a series of linear increments,with solution stability maintained by a consistent transfer algorithm and a crack relaxation method.The framework’s effectiveness is demonstrated across various geometries and loading scenarios.Through rigorous validation against both experimental data and established numerical benchmarks,the approach is proven to accurately forecast crack trajectories and fatigue life.Furthermore,the detailed description of the program’s architecture offers a foundational blueprint,serving as a valuable guide for researchers aiming to develop their specialized software for fracture mechanics analysis.展开更多
The thermal fatigue cracking behavior of high Si-Mo nodular cast iron （NCI） is investigated by means of optical microscope （OM）, scanning electron microscope （SEM） and energy dispersive spectroscope （EDS）, in ...The thermal fatigue cracking behavior of high Si-Mo nodular cast iron （NCI） is investigated by means of optical microscope （OM）, scanning electron microscope （SEM） and energy dispersive spectroscope （EDS）, in order to find a new material used in exhaust manifolds in First Automotive Works （FAW） .Nodular cast irons with silicon content about 4.7% , in combination with up to 1.1% molybdenum , were produced by Jilin University and FAW. The repeated heating / cooling test was performed under cyclic heating at various maximum heating temperatures （Tmax） ranging from 800to 900℃.Experimental results indicate that the thermal fatigue cracking resistance of high Si-Mo NCI decreases with increasing the maximum heating temperature.The periods for crack initiation are 24-36 , 40-50and 70-90times associated with heating temperature of 900 , 850and 800℃ , respectively , when the holding time is about 10min at Tmax.When thermal fatigue cracking occurs , the cracking always initiates at the bigger surface of specimen.The major positions of cracks propagation are generally at the eutectic oxide boundary region and the region of the graphite disappearance.At the same time , the oxidation may accelerate crack initiation and propagation.On the other hand , micro-crack number varied from large to little because of shielding effect.As exhaust manifolds , the reasonable working temperature of high Si-Mo NCI is no more than 840℃ by test and analysis.展开更多
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 diffe...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.展开更多
This study investigates the corrosion-assisted fatigue crack growth rate(FCGR)of 16 mm thick AA 7075-T651 friction stir welded(FSW)joints.Compact tension(CT)specimens were extracted from both the base material and FSW...This study investigates the corrosion-assisted fatigue crack growth rate(FCGR)of 16 mm thick AA 7075-T651 friction stir welded(FSW)joints.Compact tension(CT)specimens were extracted from both the base material and FSW joints to evaluate FCGR under varying corrosion exposure durations(0,7,30,60,and 90 days)at a constant stress ratio of 0.5.Microstructural analysis of the welds was conducted using optical and transmission electron microscopy(TEM).Results indicate that the critical stress intensity factor range(ΔK_(cr))of FSW joints is lower than that of the base material,primarily due to precipitate dissolution in the weld zone during the FSW process,as confirmed by TEM analysis.The fatigue life of FSW joints was significantly lower than that of the base material,but with prolonged exposure to seawater corrosion,the gap in fatigue life narrowed.Specimens exposed to seawater for more than 60days exhibited minimal differences in fatigue life between the base material and the FSW joints.This was attributed to the higher corrosion rate of the base material compared to the weld nugget,resulting in the formation of deeper pits that facilitated crack initiation and accelerated fatigue failure.The findings conclude that extended corrosion exposure leads to similar fatigue life and crack growth behaviour in both the base material and FSW joints.SEM and EDX analysis of AA7075-T651 revealed corrosion pits and rust products in initiation zones,ductile striations in growth regions,and secondary cracks with micro voids in fracture zones.FSW joints exhibited ultra-fine grains,smooth ductile fracture in initiation and growth regions,and brittle fracture in the fracture zones under both corroded and uncorroded conditions.展开更多
In order to optimize the current grinding procedure of the backup roll of 2050 continuously variable crown (CVC) mills, the behavior of rolling contact fatigue (RCF) cracking was investigated. Two RCF short cracks, in...In order to optimize the current grinding procedure of the backup roll of 2050 continuously variable crown (CVC) mills, the behavior of rolling contact fatigue (RCF) cracking was investigated. Two RCF short cracks, including vertical short crack and ratcheting short crack initiated from ratcheting, were observed. The behavior of both RCF cracks was analyzed in detail. Then a modified grinding procedure was proposed according to the behavior of RCF cracks and the preventive grinding strategy.展开更多
基金National Key Research and Development Program of China(2024YFB4610803)。
文摘The fatigue crack growth rate of a novel Ti-6Al-4V-1Mo titanium alloy,which is developed for laser directed energy deposition technique,was investigated before and after cyclic heat treatment(CHT).Changes in microstructure,fracture surfaces,and crack growth paths were analyzed before and after CHT.Results indicate that in the stable crack growth region,the growth rates for the as-deposited and cyclic heat-treated specimens follow the relationships da/dN=1.8651×10^(−8)(ΔK)^(3.2271)and da/dN=1.4112×10^(−8)(ΔK)^(3.1125),respectively.Compared with that at the as-deposited state,the microstructure after CHT is transformed from a uniform basket-weave microstructure to a dual-phase microstructure consisting of near-sphericalαandβ-transformed matrix phases.The cyclic process also disrupts the continuity of the grain boundaryα(αGB)at the primaryβ-phase grain boundary.The coarsening of primaryαand the disruption ofαGB continuity are the primary factors to release stress concentration and promote crack deflection,thereby decreasing the fatigue crack growth rate.Additionally,the increased occurrence of crack branching,secondary cracking,and crack bridging in cyclic heat-treated specimens further reduces the crack driving force and slows the fatigue crack growth rate.
基金National Natural Science Foundation of China (No. 51975200)Hunan Provincial Innovation Foundation for Postgraduate,China (No. QL20220201)。
文摘By using fatigue crack propagation testing and microstructural characterization,the crack fracture and propagation mechanisms of K4169 superalloy under various loads were investigated.The results demonstrate that the grain sizes of K4169 superalloy significantly increase,and the precipitation of the needle-likeδphase and the Laves phase is observed.Voids and microcracks form at location of Laves phase enrichment,creating conditions for crack propagation.By the a−N(a is the crack length,and N is the number of cycles)relationship curve,the change in the fatigue crack growth rate with the increasing number of cycles progresses through three separate stages.The fracture process of K4169 superalloy under low-stress cyclic loading(3 kN)exhibits the ductile fracture.Subsequently,the fracture process starts to change from the ductile fracture to the brittle fracture as the stress increases to 4.5 kN.In the microstructures of fractures in both stress states,intergranular propagation is the mechanism responsible for crack propagation.Moreover,the Laves phase exists near the fracture crack,which is in line with the post-service structural phenomenon.
基金financially supported by the National Natural Science Foundation of China(No.U21A2050)support of the Chinese Scholarship Council(No.202006290165).
文摘In recent years,(0001)twist grain boundaries(BTGBs)located in primary α grain clusters were identified as fatigue crack nucleation sites in different Ti alloys.In the present study,crack initiation was investigated in a bimodal Ti-5Al-4 V alloy subjected to low-cycle fatigue and dwell-fatigue loadings at room temperature.The low fraction of primary α grains was not associated with a lack of sensitivity to BTGB cracking.Transmission electron microscopy and electron back-scattered diffraction were used to characterize BTGBs in the initial microstructure.The fatigue mechanisms were then analyzed with a focus on dislocation activity.α_(p) grains adjacent to cracked BTGBs contained a high dislocation density.It was primarily composed of planar slip bands of dislocations.In addition,<c+a>dislocations were noticed in the vicinity of cracked BTGBs.They supposedly pertain to crack tip plasticity during growth,and no evidence of a role of an incoming slip event in crack nucleation was obtained.Also,basal slip bands extending across adjacent grains were found to emerge from BTGBs.This feature provides an easier path for crack extension when growth along the grain boundary becomes difficult owing to a deviation from the basal plane.Atom probe tomography analyses evidenced V and Fe segregation at a grain boundary with a significant deviation from the BTGB configuration.This suggests a possible contribution of local solute segregation to the high cracking resistance of general α_(p)/α_(p) grain boundaries.This work provides new insights into the mechanisms involved in cracking of BTGB in Ti alloys subjected to cyclic loadings.
基金National Natural Science Foundation of China(12172292,12072287)。
文摘The crack initiation and early propagation are of great significance to the overall fatigue life of material.In order to investigate the anisotropic fracture behavior of laser metal deposited Ti-6Al-4V alloy(LMD Ti64)during the early stage,the fourpoint bending fatigue test was carried out on specimens of three different directions,as well as the forged specimens.The results indicate the anisotropic crack initiation and early propagation of LMD Ti64.The direction perpendicular to the deposition direction exhibits a better fatigue resistance than the other two.The crack initiation position and propagation path are dominated by the microstructure in the vicinity of U-notch.LMD Ti64 has a typical small crack effect,and the early crack propagation velocities in three directions are similar.Affected by the slip system of LMD Ti64,secondary cracks frequently occur,which are often found to have an angle of 60°to the main crack.The electron backscatter diffraction analysis indicates that LMD Ti64 has preferred orientations,i.e.,strong 0001//Z texture and 001//Z texture.Their crystallographic orientation will change as the direction of columnarβgrains turns over,resulting in the fatigue anisotropy of LMD Ti64 in crack initiation and early crack propagation process.
文摘High-performance magnesium alloys are in great demand to meet the lightweight design requirements of aircraft.Grain size has long been recognized as a key factor influencing the mechanical properties of alloys.This study investigates the effect of grain size,controlled by Zr addition,on the fatigue behavior of a recently developed low-cost Mg-2.6Nd-0.35Zn alloy,through systematic characterization and analysis of stress-life(S-N)curves,fatigue crack propagation,fracture surface morphology,stress intensity factor,and crack propagation threshold.The results show that after heat treatment(solution at 525±5℃ for 8 h and water quenching at 60-80℃,followed by aging at 250±5℃for 14 h and then air cooling),coarse-grained specimens(average grain size approximately 596μm)containing 0.12wt.%Zr exhibit greater resistance to fatigue crack propagation than fine-grained specimens(average grain size approximately 94μm)containing 0.46wt.%Zr.Coarse grains promote intergranular fracture,while fine grains favor transgranular fracture.In addition,coarse grains reduce the sensitivity of the crack tip to stress concentration.Furthermore,fine-grained samples demonstrate a longer total fatigue life,owing to their superior resistance to crack initiation,which significantly prolongs the crack initiation stage.These findings highlight the importance of optimizing grain size to achieve the best possible fatigue resistance in Mg-Nd-Zn-Zr alloys for practical engineering applications.
基金financially supported by the Czech Science Foundation in the frame of the project No.22-28283Sby the Technology Agency of the Czech Republic through the project No.CK03000060.
文摘Statistical distribution of residual fatigue life(RFL)of railway axles under given loading was computed using the Monte Carlo method by considering random variation of the selected input parameters.Experimental data for the EA4T railway axle steel,the loading spectrum,the press fit loading and the residual stress induced by surface hardening were considered in the crack propagation simulations.Usually,the material properties measured by tensile tests are considered to be the most informative source of material data.Under fatigue loading,however,the crack growth rates near the threshold are the most critical data.Two important influencing factors on these crack growth rates are presented:first,the air humidity and,second,the near-surface residual stress.The typical variation of these parameters in operation may change the RFL by one or two orders of magnitude.Experimentally obtained crack growth thresholds and residual stress profiles are highly affected by the used methodology.Therefore,the obtained input data may be located anywhere within a large scatter,while the experimenters are completely unaware of it.This can lead to dangerously non-conservative situations,e.g.when the thresholds are measured in a laboratory under humid air conditions and then applied to predictions of RFLs of axles operated in winter in low air humidity.This is significant for the topic of inspection interval optimisation.The results of experiments done on real 1:1 railway axles were close to the most frequent value found in the histogram of the numerically computed RFLs.
基金supported by the National Natural Science Foundation of China(Nos.52061005,52261025)the Science and Technology Programs of Guizhou Province,China(Nos.YQK[2023]009,CXTD[2023]009)the Technology Innovation Leading Program of Shaanxi Province,China(No.2024ZCYYDP92)。
文摘The interrupted fatigue test method was utilized to investigate the damage evolution mechanism of the notch high-cycle fatigue(NHCF)in Ti-55531 alloy with a multilevel lamellar microstructure.The results reveal that significant microvoids and microcracks predominantly initiate at α/β interfaces under various notch root radii(R).Notably,even under larger R(0.75 mm),mutual interactions of stacking faults(SFs)−deformation twins,twins−twins,and SFs−SFs are observed.Furthermore,with decreasing R(0.34 and 0.14 mm),the volume fraction of SFs escalates significantly and twins are almost absent.Moreover,activated prismatic slip system decreases with a decrease in Schmidt factor and with the further decrease in R.Finally,strain localization near α/β interfaces contributes to the initiation of fatigue microcracks.
基金The National Natural Science Foundation of China(No.51378121)
文摘In order to compare the impact of thickness of different layers on fatigue lives of different semi-rigid asphalt pavement structures, the mechanical results from finite element models in ABAQUS are incorporated with the fatigue results from fatigue models in FE-SAFE to calculate the mechanical response and fatigue lives of semi-rigid pavement structures under heavy traffic loads. Then the influences on fatigue lives caused by the changes in the thickness of layers in pavement structures are also evaluated. The numerical simulation results show that the aggregated base and the large stone porous mixture （LSPM） base have better anti-cracking performance than the conventional semi-rigid base. The appropriate thickness range for the aggregated layer in the aggregated base is 15 to 18 cm. The thickness of the LSPM layer in the LSPM base is recommended to be less than 15 cm.
基金supported by the National Natural Science Foundation of China(Nos.52074231,52274396 and 52001258)the Chongqing Natural Science Foundation(No.cstc2020jcyj-msxmX1056).
文摘Crack initiation mechanism of dwell fatigue has always been a key problem in rationalizing the dwell effect,and it is not completely understood yet.This study conducted stress-controlled low-cycle fatigue and dwell fatigue tests on Ti-6Al-3Nb-2Zr-1Mo alloy with bimodal microstructure to reveal its microstructural characteristics and crack initiation mechanisms.The study demonstrated that the faceted primaryα nodules located near the specimen surface acted as crack initiation sites during both fatigue and dwell fatigue tests.Slip trace analysis revealed that faceted cracking occurred at(0001)basal plane with the maximum Schmid factor value through a special cracking mode referred to as(0001)twist boundary cracking.Innovative criteria of parameters C1 and C2 were proposed based on experimental observation and molecular dynamics simulations,which well identify candidates for(0001)twist boundary crack nucleation.It demonstrated that grain pairs combining a moderately high Schmid factor for basal slip and a well-orientated Burgers vector in the out-of-surface plane was the preferable location for surface(0001)twist-boundary crack initiation,and grain pairs combining a high Schmid factor for basal slip and a high normal stress on basal plane are perfect candidates for subsurface cracking.Based on this,phenomeno-logical models are proposed to explain the surface(0001)twist-boundary cracking mechanism from the perspective of surface extrusion-intrusion-induced micro-notches.
基金Funded by Inner Mongolia Autonomous Region Science and Technology Program(No.2023YFHH0036)Natural Science Foundation of Inner Mongolia(No.2024LHMS05033)the Basic Scientific Research Fees for Colleges and Universities Directly under the Inner Mongolia(Nos.2023QNJS002,2023YXXS007,2024YXXS039)。
文摘a-N curves of fatigue crack growth of U20Mn bainite rail after different heat treatment process were studied(The temperature is cooled from 900 to 20℃,the cooling rate was 0.5℃/s,1℃/s and salt bath isothermal respectively),the Paris formula of fatigue crack growth was fitted linearly,and the material parameters C and n were measured.The results show that the sample with cooling rate of 0.5℃/s has the fastest crack growth rate,and the sample with salt bath isothermal has the slowest crack growth rate.The coarse M/A islands with irregular shape in bainite structure with cooling rate of 0.5℃/s has poor resistance to fatigue crack propagation,which is not conducive to improving the fatigue performance.However,the sample with salt bath isothermal has longer fatigue life.Due to the combination of bainitie lamellar and retained austenite distributed between them,the salt bath isothermal sample can effectively improve the strength and toughness of bainite steel.The sample with cooling rate of 0.5℃/s is mainly composed of granular bainitie structure,and the fatigue crack growth trajectory is generally gentle without large angle deflection,the sample with salt bath isothermal is mainly composed of bainite lamellar structure,and the fatigue crack growth trajectory is not straight,with a large number of Z-shaped deflection.The fatigue cracks are prone to produce branch cracks at the stress concentration of propagation deflection,and the branch crack consumes the energy of the main fatigue cracks,thus reducing the fatigue crack growth rate and improving fatigue life.
基金financial support from the Equipment Pre-research Project(51312030507).
文摘This study investigates the fatigue crack propagation mechanism of a new high-strength and high-tough Ti-Mo-Cr-V-Nb-Al titanium alloy with three types of microstructures(basketweave structure,lamellar structure,and bimodal structure)through fatigue crack propagation rate tests and fatigue threshold value tests.The resistance of the alloy to fatigue crack propagation was found to be closely correlated with the morphology and distribution ofαparticles,as evidenced by microscopic examination of fracture surfaces and analysis of crack propagation paths.The primaryαparticles demonstrated superior resistance to crack propagation compared to the secondaryαparticles.The basketweave structure showed exceptional resistance to fatigue crack propagation at all stages.The lamellar structure mainly resists long crack propagation during rapid propagation,and its threshold value is the lowest,which makes it easy to produce microcrack propagation.On the contrary,the bimodal structure has the highest threshold value among the three,so its resistance to short crack growth is more excellent,but it has the highest crack growth rate in the higher stress intensity factor range.Theαparticles in the three microstructures also undergo rotational motion relative to the force axis during fatigue crack propagation,thereby adjusting the uneven stress distribution betweenα/βphases through slip behavior and further coordinating deformation.
基金supported by the National Basic Research Program of China (No. 2007CB613803)the National Natural Science Foundation of China (No.51071158)the Fundamental Research Funds for the Central Universities (No. N100702001)
文摘Fatigue cracking behavior from a notch was investigated at room temperature for Ti-6.SAI-3.5Mo-1.5Zr- 0.3Si （TClI） alloys with four different microstructures obtained at different cooling rates from the β transus temperature. It was found that the alloy with lamellar structures consisting of α/β lamellae or acicular α＇ martensite laths had a higher fatigue crack initiation threshold from the notch, while the bimodal structure with coarse a grain had a lower fatigue cracking resistance. The alloy with α/β lamellar structure showed a higher fatigue crack growth resistance. The length scales of the microstructures were characterized to correlate with fatigue cracking behavior. Fatigue cracking mechanism related to microstructures was discussed.
基金supported by the New Cornerstone Science Foundation through the XPLORER PRIZE,China(No.XPLORER-2024-1036)the independent research project of the National Key Laboratory of Strength and Structural Integrity,China(No.BYST-QZSYS-24-072-5)。
文摘Crack length measurement algorithms based on computer vision have shown promising engineering application prospects in the field of aircraft fatigue crack monitoring.However,due to the complexity of the monitoring environment,the subtle visual features of small fatigue cracks,and the impact of structural elastic deformation,directly applying object segmentation algorithms often results in significant measurement errors.Therefore,this paper proposes a high-precision crack length measurement method based on Bidirectional Target Tracking Model(Bi2TM),which integrates crack tip localization,interference identification,and length compensation.First,a general object segmentation model is used to perform rough crack segmentation.Then,the Bi2TM network,combined with the visual features of the structure in different stress states,is employed to track the bidirectional position of the crack tip in the“open”and“closed”states.This ultimately enables interference identification within the rough segmented crack region,achieving highprecision length measurement.In a high-interference environment of aircraft fatigue testing,the proposed method is used to measure 1000 crack images ranging from 1 mm to 11 mm.For more than 90%of the samples,the measurement error is less than 5 pixels,demonstrating significant advantages over the existing methods.
文摘The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy （SEM）. The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.
基金financially supported by the National Natural Science Foundation of China(No.51805235).
文摘With the aim of improving the fatigue properties of Mg alloy welded joints under cyclic loading,the effects of laser bionic treatment and ultrasonic impact bionic treatment on the fatigue crack growth(FCG)behavior of AZ31B Mg alloy TIG-welded joints were studied and compared.The results show that bionic treatment refines the grains on the joint surface and improves the microhardness.In the crack stable growth stage,both bionic samples exhibit a lower FCG rate and a higher FCG resistance.The two bionic treatment methods reduce the probability of crack initiation and partially promote crack deflection,providing a new approach for improving the FCG behavior of welded joints.
基金funding of the Deanship of Graduate Studies and Scientific Research,Jazan University,Saudi Arabia,through Project number:JU-20250230-DGSSR-RP-2025.
文摘Fatigue crack growth is a critical phenomenon in engineering structures,accounting for a significant percentage of structural failures across various industries.Accurate prediction of crack initiation,propagation paths,and fatigue life is essential for ensuring structural integrity and optimizing maintenance schedules.This paper presents a comprehensive finite element approach for simulating two-dimensional fatigue crack growth under linear elastic conditionswith adaptivemesh generation.The source code for the programwas developed in Fortran 95 and compiled with Visual Fortran.To achieve high-fidelity simulations,the methodology integrates several key features:it employs an automatic,adaptive meshing technique that selectively refines the element density near the crack front and areas of significant stress concentration.Specialized singular elements are used at the crack tip to ensure precise stress field representation.The direction of crack advancement is predicted using the maximum tangential stress criterion,while stress intensity factors are determined through either the displacement extrapolation technique or the J-integral method.The simulation models crack growth as a series of linear increments,with solution stability maintained by a consistent transfer algorithm and a crack relaxation method.The framework’s effectiveness is demonstrated across various geometries and loading scenarios.Through rigorous validation against both experimental data and established numerical benchmarks,the approach is proven to accurately forecast crack trajectories and fatigue life.Furthermore,the detailed description of the program’s architecture offers a foundational blueprint,serving as a valuable guide for researchers aiming to develop their specialized software for fracture mechanics analysis.
基金Item Sponsored by Science and Technology Supporting Project of Jilin Province of China (2007301)
文摘The thermal fatigue cracking behavior of high Si-Mo nodular cast iron （NCI） is investigated by means of optical microscope （OM）, scanning electron microscope （SEM） and energy dispersive spectroscope （EDS）, in order to find a new material used in exhaust manifolds in First Automotive Works （FAW） .Nodular cast irons with silicon content about 4.7% , in combination with up to 1.1% molybdenum , were produced by Jilin University and FAW. The repeated heating / cooling test was performed under cyclic heating at various maximum heating temperatures （Tmax） ranging from 800to 900℃.Experimental results indicate that the thermal fatigue cracking resistance of high Si-Mo NCI decreases with increasing the maximum heating temperature.The periods for crack initiation are 24-36 , 40-50and 70-90times associated with heating temperature of 900 , 850and 800℃ , respectively , when the holding time is about 10min at Tmax.When thermal fatigue cracking occurs , the cracking always initiates at the bigger surface of specimen.The major positions of cracks propagation are generally at the eutectic oxide boundary region and the region of the graphite disappearance.At the same time , the oxidation may accelerate crack initiation and propagation.On the other hand , micro-crack number varied from large to little because of shielding effect.As exhaust manifolds , the reasonable working temperature of high Si-Mo NCI is no more than 840℃ by test and analysis.
文摘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.
文摘This study investigates the corrosion-assisted fatigue crack growth rate(FCGR)of 16 mm thick AA 7075-T651 friction stir welded(FSW)joints.Compact tension(CT)specimens were extracted from both the base material and FSW joints to evaluate FCGR under varying corrosion exposure durations(0,7,30,60,and 90 days)at a constant stress ratio of 0.5.Microstructural analysis of the welds was conducted using optical and transmission electron microscopy(TEM).Results indicate that the critical stress intensity factor range(ΔK_(cr))of FSW joints is lower than that of the base material,primarily due to precipitate dissolution in the weld zone during the FSW process,as confirmed by TEM analysis.The fatigue life of FSW joints was significantly lower than that of the base material,but with prolonged exposure to seawater corrosion,the gap in fatigue life narrowed.Specimens exposed to seawater for more than 60days exhibited minimal differences in fatigue life between the base material and the FSW joints.This was attributed to the higher corrosion rate of the base material compared to the weld nugget,resulting in the formation of deeper pits that facilitated crack initiation and accelerated fatigue failure.The findings conclude that extended corrosion exposure leads to similar fatigue life and crack growth behaviour in both the base material and FSW joints.SEM and EDX analysis of AA7075-T651 revealed corrosion pits and rust products in initiation zones,ductile striations in growth regions,and secondary cracks with micro voids in fracture zones.FSW joints exhibited ultra-fine grains,smooth ductile fracture in initiation and growth regions,and brittle fracture in the fracture zones under both corroded and uncorroded conditions.
文摘In order to optimize the current grinding procedure of the backup roll of 2050 continuously variable crown (CVC) mills, the behavior of rolling contact fatigue (RCF) cracking was investigated. Two RCF short cracks, including vertical short crack and ratcheting short crack initiated from ratcheting, were observed. The behavior of both RCF cracks was analyzed in detail. Then a modified grinding procedure was proposed according to the behavior of RCF cracks and the preventive grinding strategy.
