This paper deals with the mechanism of the formation of initial cracks in the yttria partially stabilized zirconia thermal barrier coatings prepared by EB-PVD method.The microcracks were only recognized inside the cer...This paper deals with the mechanism of the formation of initial cracks in the yttria partially stabilized zirconia thermal barrier coatings prepared by EB-PVD method.The microcracks were only recognized inside the ceramic top coat of the thermalcycled TBCs. SEM/EDS observations indicated that some special oxides exist in the area just below the cracks.It seems that the formation of the initial cracks can result from the oxidation stress as well as the thermal stress.展开更多
The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A...The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.展开更多
Based on the theory of hydrogen enhanced localized plasticity of the hydrogen induced cracking and the consideration of the effect of the residual stress produced by eliminated stress heat-treatment, a fractal model o...Based on the theory of hydrogen enhanced localized plasticity of the hydrogen induced cracking and the consideration of the effect of the residual stress produced by eliminated stress heat-treatment, a fractal model of hydrogen induced cracking was presented, and the relationships among the effective surface energy (H), fractal dimension D and stress intensity factor of hydrogen induced cracking, KIH, for welding pipeline under hydrogen environment was set up, from which the relationship of D and KISCC was obtained. The model has been verified experimentally to be correct.展开更多
A new ideological and theoretical model—a technology to control weld hot cracks by transverse compressive pre-stress in the welding of aluminum alloy was put forward,which was further proved by the subsequent self-de...A new ideological and theoretical model—a technology to control weld hot cracks by transverse compressive pre-stress in the welding of aluminum alloy was put forward,which was further proved by the subsequent self-designed test setup.Experiments are conducted on the fishbone shaped specimen under conventional welding and welding with various pre-stress values.The experimental results turn out that,the initiation rate of the weld hot cracks decreases with increasing values of the compressive pre-stress.When the pre-stress reaches 0.3-0.4 of the yield stress,the cracks even disappear.In mechanical viewpoint,the researches here develop a new way to control weld cracks.展开更多
Transverse crack often occurs in the trailing edge region of the bladewhen subjected to the excessive edgewise fatigue load.In this paper a refined model was established through local mesh refinement methods in order ...Transverse crack often occurs in the trailing edge region of the bladewhen subjected to the excessive edgewise fatigue load.In this paper a refined model was established through local mesh refinement methods in order to investigate the initiation mechanism of crack and its extension in blade trailing edge.The material stress around the crack in trailing edge region under different thicknesses is calculated based on the fracture mechanics theory.The factors affecting the fatigue robustness of blade trailing edge are concluded by investigating the results of finite element analysis and coupons test.Compared with the laminate,the lower fatigue strength of the adhesive is the cause of the transverse crack of the adhesive joint at the trailing edge.The increase of the adhesive thickness at the adhesive joint will significantly increase the stress concentration factor at the crack region and accelerate the crack extension of the laminate.In final,a practical design scheme to prevent crack initiation is given for the manufacture of the wind turbine blade.展开更多
The behavior of part-through and through short cracks in single edge blunt notched specimens of a medium carbon steel was investigated by a replication method. It is found that the fatigue failure of these notched spe...The behavior of part-through and through short cracks in single edge blunt notched specimens of a medium carbon steel was investigated by a replication method. It is found that the fatigue failure of these notched specimens is caused mainly by the growth of short surface cracks originating from the surface of notch root. More than 70% of the fatigue life is spent in the regimes in which short surface cracks initiate and propagate as part-through cracks before joining up to form a single through-thickness crack. The effect of original crack profile which is formed through the coalescence of multiple part-through cracks is the main reason causing the 'anomalous' propagation behavior of the through-thickness crack in its early stage.展开更多
Rotary bending fatigue tests were carried out using smooth specimens of a medium-carbon steel with two different grain sizes.The process of early crack development was observed by the replica method,and the effects of...Rotary bending fatigue tests were carried out using smooth specimens of a medium-carbon steel with two different grain sizes.The process of early crack development was observed by the replica method,and the effects of grain size and microstructure on short crack development were studied.It was shown that the initiation process of fatigue cracks is that the damaged region is intensified gradually,and the growth of short cracks relates to the coalescence of cracks.The grain size and microstructure have a great influence upon the initiation and growth of short cracks.A reasonable definition was given in relation to short fatigue crack initiation and propagation.展开更多
In order to simultaneously measure the initiation toughness of pure mode Ⅰ and mode Ⅱ cracks in one specimen,a large-size double-cracked concave-convex plate(DCCP)specimen configuration was proposed.Impacting tests ...In order to simultaneously measure the initiation toughness of pure mode Ⅰ and mode Ⅱ cracks in one specimen,a large-size double-cracked concave-convex plate(DCCP)specimen configuration was proposed.Impacting tests were implemented in the drop plate impact device.Strain gauges were employed to measure impact loads and crack initiation time.The corresponding numerical model was established by using the dynamic finite difference program AUTODYN,and the experimental-numerical method and ABAQUS code were utilized to obtain the initial fracture toughness of the crack.Using experiments and numerical research,we concluded that the DCCP specimen is suitable for measuring the initial fracture toughness of pure mode Ⅰ and mode Ⅱ cracks at the same time;the dynamic initiation toughness increases with the increase of loading rate and the crack initiation time decreases with increasing loading rate;the initiation toughness of mode Ⅱ crack is 0.5 times that of mode Ⅰ crack when subjected to the same loading rate.For the pre-crack in the vicinity of the bottom of a sample,when its length increases from 20 to 100 mm,the dynamic initiation toughness of the pure mode Ⅰ crack gradually decreases,and the longer the lower crack length is,the easier the crack would initiate,but the dynamic initiation toughness of pure mode Ⅱ crack varies little.展开更多
In order to clarify the physical background of the scatter in fatigue behaviour, rotary bending fatigue tests are carried out using smooth speciments of a medium carbon steel with two kinds of grain sizes. The statis...In order to clarify the physical background of the scatter in fatigue behaviour, rotary bending fatigue tests are carried out using smooth speciments of a medium carbon steel with two kinds of grain sizes. The statistical characteristics of short carck initiation and growth lives are investigated by a new definition of short fatigue crack initiation. Detailed analysis reveals that the distribution of short crack initiation life can be expressed by two or threeparameter Weibull distribution, and the threeparameter Weibull distribution is well fitted to the distribution of short crack growth life, and the grain size and stress level have a great influence on the statistical characteristics of crack initiation and growth life.展开更多
The loaded rock experiences multiple stages of deformation.It starts with the formation of microcracks at low stresses(crack initiation,CI)and then transitions into unstable crack propagation(crack damage,CD)near the ...The loaded rock experiences multiple stages of deformation.It starts with the formation of microcracks at low stresses(crack initiation,CI)and then transitions into unstable crack propagation(crack damage,CD)near the ultimate strength.In this study,both the acoustic emission method(AEM)and the ultrasonic testing method(UTM)were used to examine the characteristics of AE parameters(b-value,peak frequency,frequency-band energy ratio,and fractal dimension)and ultrasonic(ULT)properties(velocity,amplitude,energy attenuation,and scattering attenuation)of bedded shale at CI,CD,and ultimate strength.The comparison involved analyzing the strain-based method(SBM),AEM,and UTM to determine the thresholds for damage stress.A fuzzy comprehensive evaluation model(FCEM)was created to describe the damage thresholds and hazard assessment.The results indicate that the optimal AE and ULT parameters for identifying CI and CD stress are ringing count,ultrasonic amplitude,energy attenuation,and scattering attenuation of the S-wave.Besides,damage thresholds were detected earlier by AE monitoring,ranging from 3 MPa to 10 MPa.CI and CD identified by UTM occurred later than SBM and AEM,and were in the range of 12 MPa.The b-value,peak frequency,energy ratio in the low-frequency band(0e62.5 kHz),correlation dimension,and sandbox dimension showed low values at the peak stress,while the energy ratio in a moderate-frequency band(187.5e281.25 kHz)and amplitude showed high values.The successful application of FCEM to laboratory testing of shales has demonstrated its ability to quantitatively identify AE/ULT precursors of seismic hazards associated with rock failure.展开更多
Aiming at mitigating the high risks associated with conventional explosive blasting,this study developed a safe directional fracturing technique,i.e.instantaneous expansion with a single fracture(IESF),using a coal-ba...Aiming at mitigating the high risks associated with conventional explosive blasting,this study developed a safe directional fracturing technique,i.e.instantaneous expansion with a single fracture(IESF),using a coal-based solid waste expanding agent.First,the mechanism of directional fracturing blasting by the IESF was analyzed,and the criterion of directional crack initiation was established.On this basis,laboratory experiments and numerical simulations were conducted to systematically evaluate the directional fracturing blasting performance of the IESF.The results indicate that the IESF presents an excellent directional fracturing effect,with average surface undulation differences ranging from 8.1 mm to 22.7 mm on the fracture surfaces.Moreover,during concrete fracturing tests,the stresses and strains in the fracturing direction are measured to be 2.16-3.71 times and 8 times larger than those in the nonfracturing direction,respectively.Finally,the IESF technique was implemented for no-pillar mining with gob-side entry retaining through roof cutting and pressure relief in an underground coal mine.The IESF technique effectively created directional cracks in the roof without causing severe roadway deformation,achieving an average cutting rate and maximum roadway deformation of 94%and 197 mm,respectively.These on-site test results verified its excellent directional rock fracturing performance.The IESF technique,which is safe,efficient,and green,has considerable application prospects in the field of rock mechanics and engineering.展开更多
Microcrack growth during progressive compressive failure in brittle rocks strongly influences the safety of deep underground engineering.The external shear stressτxy on brittle rocks greatly affects microcrack growth...Microcrack growth during progressive compressive failure in brittle rocks strongly influences the safety of deep underground engineering.The external shear stressτxy on brittle rocks greatly affects microcrack growth and progressive failure.However,the theoretical mechanism of the growth direction evolution of the newly generated wing crack during progressive failure has rarely been studied.A novel analytical method is proposed to evaluate the shear stress effect on the progressive compressive failure and microcrack growth direction in brittle rocks.This model consists of the wing crack growth model under the principal compressive stresses,the direction correlation of the general stress,the principal stress and the initial microcrack inclination,and the relationship between the wing crack length and strain.The shear stress effect on the relationship between y-direction stress and wing crack growth and the relationship between y-direction stress and y-direction strain are analyzed.The shear stress effect on the wing crack growth direction during the progressive compressive failure is determined.The initial crack angle effect on the y-direction peak stress and the wing crack growth direction during the progressive compressive failure considering shear stress is also discussed.A crucial conclusion is that the direction of wing crack growth has a U-shaped variation with the growth of the wing crack.The rationality of the analytical results is verified by an experiment and from numerical results.The study results provide theoretical support for the evaluation of the safety and stability of surrounding rocks in deep underground engineering.展开更多
This study leverages machine learning to perform high-throughput computational screening of n-hexane cracking initiators.Artificial neural networks are applied to predict the chemical performance of initiators,using s...This study leverages machine learning to perform high-throughput computational screening of n-hexane cracking initiators.Artificial neural networks are applied to predict the chemical performance of initiators,using simulated pyrolysis data as the training dataset.Various feature extraction methods are utilized,and five neural network architectures are developed to predict the co-cracking product distribution based on molecular structures.High-throughput screening of 12946 molecules outside the training dataset identifies the top 10 initiators for each target product—ethylene,propylene,and butadiene.The relative error between predicted and simulated values is less than 7%.Additionally,reaction pathway analysis elucidates the mechanisms by which initiators influence the distribution of cracking products.The proposed framework provides a practical and efficient approach for the rapid identification and evaluation of high-performance cracking initiators.展开更多
The critical wear rate,surface damage,deformation layer,crack initiation and propagation of U76CrRE heavy rail steel samples of two different cooling conditions(rolled rail,and heat-treated rail)under intermittent loa...The critical wear rate,surface damage,deformation layer,crack initiation and propagation of U76CrRE heavy rail steel samples of two different cooling conditions(rolled rail,and heat-treated rail)under intermittent load were measured and observed by friction and wear tester,laser confocal microscope,scanning electron microscope and EBSD.The experimental results show that when the same kind of rail is matched with wheel steel with low hardness and high hardness successively,the critical wear rate of rail moves to the right.Moreover,when the rolled rail is matched with wheel steel with low hardness,the deformation layer and wear amount are larger than those of heat-treated rail are,while when it is matched with wheel steel with high hardness,the deformation layer and wear amount are smaller than those of heat-treated rail.When the rolled rail and heat-treated rail are matched with the same kind of wheel steel successively,the critical wear rate moves down,and the wear deformation layer of heat-treated rail is smaller than that of rolled rail.The failure life of the heat-treated rail is better than that of the rolled rail,which is due to the increase of the hardness of the heat-treated rail and the refinement of pearlite lamellae.With the increase of the distance from the surface layer,the proportion of large-angle grain boundaries of ferrite grains gradually increases of rolled rail and heat-treated rail,but the rolled rail presents the characteristics of large crack angle,deep depth and small length,mainly due to wear failure.While the heat-treated rail has the characteristics of small crack angle,shallow depth and long length,the crack propagation trend is obvious,and the failure form of the heat-treated rail is mainly fatigue failure.Moving the critical wear rate to the right and down is beneficial to inhibit the formation and propagation of fatigue cracks.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
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
Mg-Li alloys hold significant potential for applications in aerospace,automotive manufacturing,military weaponry,and biomedical implants,due to their excellent recyclability,high specific strength,biocompatibility,and...Mg-Li alloys hold significant potential for applications in aerospace,automotive manufacturing,military weaponry,and biomedical implants,due to their excellent recyclability,high specific strength,biocompatibility,and superior electromagnetic shielding properties.However,their poor corrosion resistance and high susceptibility to environmentally assisted cracking(EAC)significantly limit broader application.In recent years,growing attention has been directed toward understanding the corrosion and EAC behavior of Mg-Li alloys,as localized corrosion areas and hydrogen generated during the corrosion process can serve as crack initiation points and promote crack propagation.A comprehensive understanding of these mechanisms is essential for enhancing the reliability and performance of Mg-Li alloys in practical environments.This paper presents a detailed review of corrosion and EAC in Mg-Li alloys,focusing on corrosion behavior,crack initiation and propagation mechanisms,and the key factors influencing these processes.It summarizes recent advances in alloying,heat treatment,mechanical processing,microstructural control,environmental influences,mechanical loading,and surface treatments.In addition,the paper explores future research directions,highlights emerging trends,and proposes strategies to improve the durability and service performance of Mg-Li alloys.展开更多
The dynamic mechanical response and deformation mechanism of magnesium-yttrium alloy at high strain rate were investigated using split-Hopkinson pressure bar(SHPB)impact,and the microstructure evolution and crack form...The dynamic mechanical response and deformation mechanism of magnesium-yttrium alloy at high strain rate were investigated using split-Hopkinson pressure bar(SHPB)impact,and the microstructure evolution and crack formation mechanism were revealed.The yield strength and work hardening rate increase significantly with increasing impact strain rate.Deformation twinning and non-basal dislocation slip are the primary deformation mechanisms during testing.Contrary to crack initiation mechanism facilitated by adiabatic shear bands,we find that high-density co-axial nanocrystalline grains form near cracks,which leads to local softening and promotes crack initiation and rapid propagation.Most grains have similar<1^(-)21^(-)0>orientations,with unique misorientation of 24°,32°,62°,78°and 90°between adjacent grains,suggesting that these grains are primarily formed by interface transformation,which exhibits distinct differences from recrystallized grains.Our results shed light upon the dynamic mechanical response and crack formation mechanism in magnesium alloys under impact deformation.展开更多
Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectiv...Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectively impede the movement of basal dislocations,has prompted concerns about their very high cycle fatigue(VHCF)performance.In this study,an extruded Mg-Gd-Zn-Zr alloy was developed,showcasing exceptional VHCF resistance due to its bimodal structure comprisingfine grains and coarse grains consisting of curved LPSO lamellae.The investigation on the crack initiation mechanism revealed that slip-induced cracking predominantly occurs infine-grained regions rather than in the interior of coarse grains.The extrusion process aligns the basal planes of most coarse grains parallel to the axial direction,and the presence of curved LPSO lamellae acts as barriers to the movement of basal dislocations,thereby effectively increasing the threshold for slip-cracking along the basal plane.Consequently,fatigue damage manifests in the form of slip bands and micro-cracks within the interior offine grains,ultimately resulting in fatigue crack initiation,propagation andfinal fracture.展开更多
The evolution of cracks in shale directly affects the efficient production of shale gas.However,there is a lack of research on the characteristics of crack initiation in deep dense shale under different stress conditi...The evolution of cracks in shale directly affects the efficient production of shale gas.However,there is a lack of research on the characteristics of crack initiation in deep dense shale under different stress conditions.In this work,considering the different combinations of confining pressure and bedding plane inclination angle(α),biaxial mechanical loading experiments were conducted on shale containing circular holes.The research results indicate that the confining pressure and inclination angle of the bedding planes significantly influence the failure patterns of shale containing circular holes.The instability of shale containing circular holes can be classified into five types:tensile failure along the bedding planes,tensile failure through the bedding planes,shear slip along the bedding planes,shear failure through the bedding planes,and block instability failure.Furthermore,the evolution of strain and stress fields around the circular holes was found to be the fundamental cause of variations in the initiation characteristics and locations of shale cracks.The crack initiation criterion for shale containing circular hole was established,providing a new method for evaluating the trajectory of shale hole wall fractures.This study holds significant importance for evaluating the evolution and stability of fracture networks within shale reservoirs.展开更多
文摘This paper deals with the mechanism of the formation of initial cracks in the yttria partially stabilized zirconia thermal barrier coatings prepared by EB-PVD method.The microcracks were only recognized inside the ceramic top coat of the thermalcycled TBCs. SEM/EDS observations indicated that some special oxides exist in the area just below the cracks.It seems that the formation of the initial cracks can result from the oxidation stress as well as the thermal stress.
基金Project(11102224)supported by the National Natural Science Foundation of ChinaProject(201206370124)supported by the China Scholarship Council,China
文摘The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.
文摘Based on the theory of hydrogen enhanced localized plasticity of the hydrogen induced cracking and the consideration of the effect of the residual stress produced by eliminated stress heat-treatment, a fractal model of hydrogen induced cracking was presented, and the relationships among the effective surface energy (H), fractal dimension D and stress intensity factor of hydrogen induced cracking, KIH, for welding pipeline under hydrogen environment was set up, from which the relationship of D and KISCC was obtained. The model has been verified experimentally to be correct.
文摘A new ideological and theoretical model—a technology to control weld hot cracks by transverse compressive pre-stress in the welding of aluminum alloy was put forward,which was further proved by the subsequent self-designed test setup.Experiments are conducted on the fishbone shaped specimen under conventional welding and welding with various pre-stress values.The experimental results turn out that,the initiation rate of the weld hot cracks decreases with increasing values of the compressive pre-stress.When the pre-stress reaches 0.3-0.4 of the yield stress,the cracks even disappear.In mechanical viewpoint,the researches here develop a new way to control weld cracks.
基金This research was funded by Natural Science Foundation of Shandong Province(Grant No.ZR2019MEE076)National Natural Science Foundation of China(Grant No.52075305).
文摘Transverse crack often occurs in the trailing edge region of the bladewhen subjected to the excessive edgewise fatigue load.In this paper a refined model was established through local mesh refinement methods in order to investigate the initiation mechanism of crack and its extension in blade trailing edge.The material stress around the crack in trailing edge region under different thicknesses is calculated based on the fracture mechanics theory.The factors affecting the fatigue robustness of blade trailing edge are concluded by investigating the results of finite element analysis and coupons test.Compared with the laminate,the lower fatigue strength of the adhesive is the cause of the transverse crack of the adhesive joint at the trailing edge.The increase of the adhesive thickness at the adhesive joint will significantly increase the stress concentration factor at the crack region and accelerate the crack extension of the laminate.In final,a practical design scheme to prevent crack initiation is given for the manufacture of the wind turbine blade.
文摘The behavior of part-through and through short cracks in single edge blunt notched specimens of a medium carbon steel was investigated by a replication method. It is found that the fatigue failure of these notched specimens is caused mainly by the growth of short surface cracks originating from the surface of notch root. More than 70% of the fatigue life is spent in the regimes in which short surface cracks initiate and propagate as part-through cracks before joining up to form a single through-thickness crack. The effect of original crack profile which is formed through the coalescence of multiple part-through cracks is the main reason causing the 'anomalous' propagation behavior of the through-thickness crack in its early stage.
文摘Rotary bending fatigue tests were carried out using smooth specimens of a medium-carbon steel with two different grain sizes.The process of early crack development was observed by the replica method,and the effects of grain size and microstructure on short crack development were studied.It was shown that the initiation process of fatigue cracks is that the damaged region is intensified gradually,and the growth of short cracks relates to the coalescence of cracks.The grain size and microstructure have a great influence upon the initiation and growth of short cracks.A reasonable definition was given in relation to short fatigue crack initiation and propagation.
基金Projects(U19A2098,1210021843)supported by the National Natural Science Foundation of ChinaProject(2021SCU12130)supported by Fundamental Research Funds for the Central Universities,China+1 种基金Project(2021YJ0511)supported by the Sichuan Science and Technology Program,ChinaProjects(DESEYU202205,DESE202005)supported by the Open Fund of Key Laboratory of Deep Earth Science and Engineering,China。
文摘In order to simultaneously measure the initiation toughness of pure mode Ⅰ and mode Ⅱ cracks in one specimen,a large-size double-cracked concave-convex plate(DCCP)specimen configuration was proposed.Impacting tests were implemented in the drop plate impact device.Strain gauges were employed to measure impact loads and crack initiation time.The corresponding numerical model was established by using the dynamic finite difference program AUTODYN,and the experimental-numerical method and ABAQUS code were utilized to obtain the initial fracture toughness of the crack.Using experiments and numerical research,we concluded that the DCCP specimen is suitable for measuring the initial fracture toughness of pure mode Ⅰ and mode Ⅱ cracks at the same time;the dynamic initiation toughness increases with the increase of loading rate and the crack initiation time decreases with increasing loading rate;the initiation toughness of mode Ⅱ crack is 0.5 times that of mode Ⅰ crack when subjected to the same loading rate.For the pre-crack in the vicinity of the bottom of a sample,when its length increases from 20 to 100 mm,the dynamic initiation toughness of the pure mode Ⅰ crack gradually decreases,and the longer the lower crack length is,the easier the crack would initiate,but the dynamic initiation toughness of pure mode Ⅱ crack varies little.
文摘In order to clarify the physical background of the scatter in fatigue behaviour, rotary bending fatigue tests are carried out using smooth speciments of a medium carbon steel with two kinds of grain sizes. The statistical characteristics of short carck initiation and growth lives are investigated by a new definition of short fatigue crack initiation. Detailed analysis reveals that the distribution of short crack initiation life can be expressed by two or threeparameter Weibull distribution, and the threeparameter Weibull distribution is well fitted to the distribution of short crack growth life, and the grain size and stress level have a great influence on the statistical characteristics of crack initiation and growth life.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.U20A20266 and 12302503)Scientific and technological research projects in Sichuan province(Grant No.2024NSFSC0973).
文摘The loaded rock experiences multiple stages of deformation.It starts with the formation of microcracks at low stresses(crack initiation,CI)and then transitions into unstable crack propagation(crack damage,CD)near the ultimate strength.In this study,both the acoustic emission method(AEM)and the ultrasonic testing method(UTM)were used to examine the characteristics of AE parameters(b-value,peak frequency,frequency-band energy ratio,and fractal dimension)and ultrasonic(ULT)properties(velocity,amplitude,energy attenuation,and scattering attenuation)of bedded shale at CI,CD,and ultimate strength.The comparison involved analyzing the strain-based method(SBM),AEM,and UTM to determine the thresholds for damage stress.A fuzzy comprehensive evaluation model(FCEM)was created to describe the damage thresholds and hazard assessment.The results indicate that the optimal AE and ULT parameters for identifying CI and CD stress are ringing count,ultrasonic amplitude,energy attenuation,and scattering attenuation of the S-wave.Besides,damage thresholds were detected earlier by AE monitoring,ranging from 3 MPa to 10 MPa.CI and CD identified by UTM occurred later than SBM and AEM,and were in the range of 12 MPa.The b-value,peak frequency,energy ratio in the low-frequency band(0e62.5 kHz),correlation dimension,and sandbox dimension showed low values at the peak stress,while the energy ratio in a moderate-frequency band(187.5e281.25 kHz)and amplitude showed high values.The successful application of FCEM to laboratory testing of shales has demonstrated its ability to quantitatively identify AE/ULT precursors of seismic hazards associated with rock failure.
基金supported by the National Natural Science Foundation of China(Grant No.52404155)State Key Laboratory of Mining Disaster Prevention and Control(Shandong University of Science and Technology)+1 种基金Ministry of Education(Grant No.JMDPC202402)supported by the opening project of State Key Laboratory of Explosion Science and Safety Protection(Beijing Institute of Technology).The opening project number is KFJJ24-20M.
文摘Aiming at mitigating the high risks associated with conventional explosive blasting,this study developed a safe directional fracturing technique,i.e.instantaneous expansion with a single fracture(IESF),using a coal-based solid waste expanding agent.First,the mechanism of directional fracturing blasting by the IESF was analyzed,and the criterion of directional crack initiation was established.On this basis,laboratory experiments and numerical simulations were conducted to systematically evaluate the directional fracturing blasting performance of the IESF.The results indicate that the IESF presents an excellent directional fracturing effect,with average surface undulation differences ranging from 8.1 mm to 22.7 mm on the fracture surfaces.Moreover,during concrete fracturing tests,the stresses and strains in the fracturing direction are measured to be 2.16-3.71 times and 8 times larger than those in the nonfracturing direction,respectively.Finally,the IESF technique was implemented for no-pillar mining with gob-side entry retaining through roof cutting and pressure relief in an underground coal mine.The IESF technique effectively created directional cracks in the roof without causing severe roadway deformation,achieving an average cutting rate and maximum roadway deformation of 94%and 197 mm,respectively.These on-site test results verified its excellent directional rock fracturing performance.The IESF technique,which is safe,efficient,and green,has considerable application prospects in the field of rock mechanics and engineering.
基金National Natural Science Foundation of China,Grant/Award Numbers:51708016,12172036R&D Program of Beijing Municipal Education Commission,Grant/Award Number:KM202110016014+1 种基金Government of Perm Krai,Research Project,Grant/Award Numbers:СED-26-08-08-28,С-26/628Graduate Innovation Program of Beijing University of Civil Engineering and Architecture,Grant/Award Number:PG2024035。
文摘Microcrack growth during progressive compressive failure in brittle rocks strongly influences the safety of deep underground engineering.The external shear stressτxy on brittle rocks greatly affects microcrack growth and progressive failure.However,the theoretical mechanism of the growth direction evolution of the newly generated wing crack during progressive failure has rarely been studied.A novel analytical method is proposed to evaluate the shear stress effect on the progressive compressive failure and microcrack growth direction in brittle rocks.This model consists of the wing crack growth model under the principal compressive stresses,the direction correlation of the general stress,the principal stress and the initial microcrack inclination,and the relationship between the wing crack length and strain.The shear stress effect on the relationship between y-direction stress and wing crack growth and the relationship between y-direction stress and y-direction strain are analyzed.The shear stress effect on the wing crack growth direction during the progressive compressive failure is determined.The initial crack angle effect on the y-direction peak stress and the wing crack growth direction during the progressive compressive failure considering shear stress is also discussed.A crucial conclusion is that the direction of wing crack growth has a U-shaped variation with the growth of the wing crack.The rationality of the analytical results is verified by an experiment and from numerical results.The study results provide theoretical support for the evaluation of the safety and stability of surrounding rocks in deep underground engineering.
基金The financial support provided by the Project of the National Natural Science Foundation of China (22308314,U22A20415)the Natural Science Foundation of Zhejiang Province (LQ24B060001)+1 种基金the "Pioneer" and "Leading Goose" Research & Development Program of Zhejiang (2022C01SA442617)the SINOPEC Technology Development Project (224244)
文摘This study leverages machine learning to perform high-throughput computational screening of n-hexane cracking initiators.Artificial neural networks are applied to predict the chemical performance of initiators,using simulated pyrolysis data as the training dataset.Various feature extraction methods are utilized,and five neural network architectures are developed to predict the co-cracking product distribution based on molecular structures.High-throughput screening of 12946 molecules outside the training dataset identifies the top 10 initiators for each target product—ethylene,propylene,and butadiene.The relative error between predicted and simulated values is less than 7%.Additionally,reaction pathway analysis elucidates the mechanisms by which initiators influence the distribution of cracking products.The proposed framework provides a practical and efficient approach for the rapid identification and evaluation of high-performance cracking initiators.
基金Funded by the Inner Mongolia Autonomous Region Science and Technology Program(No.2023YFHH0036)the 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,and 2024YXXS039)。
文摘The critical wear rate,surface damage,deformation layer,crack initiation and propagation of U76CrRE heavy rail steel samples of two different cooling conditions(rolled rail,and heat-treated rail)under intermittent load were measured and observed by friction and wear tester,laser confocal microscope,scanning electron microscope and EBSD.The experimental results show that when the same kind of rail is matched with wheel steel with low hardness and high hardness successively,the critical wear rate of rail moves to the right.Moreover,when the rolled rail is matched with wheel steel with low hardness,the deformation layer and wear amount are larger than those of heat-treated rail are,while when it is matched with wheel steel with high hardness,the deformation layer and wear amount are smaller than those of heat-treated rail.When the rolled rail and heat-treated rail are matched with the same kind of wheel steel successively,the critical wear rate moves down,and the wear deformation layer of heat-treated rail is smaller than that of rolled rail.The failure life of the heat-treated rail is better than that of the rolled rail,which is due to the increase of the hardness of the heat-treated rail and the refinement of pearlite lamellae.With the increase of the distance from the surface layer,the proportion of large-angle grain boundaries of ferrite grains gradually increases of rolled rail and heat-treated rail,but the rolled rail presents the characteristics of large crack angle,deep depth and small length,mainly due to wear failure.While the heat-treated rail has the characteristics of small crack angle,shallow depth and long length,the crack propagation trend is obvious,and the failure form of the heat-treated rail is mainly fatigue failure.Moving the critical wear rate to the right and down is beneficial to inhibit the formation and propagation of fatigue cracks.
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supported by the National Natural Science Foundation of China Projects under Grant(nos.52301112,52331004,U21A2049,and 51871211)Guangdong Basic and Applied Basic Research Foundation(Grant no.2024A1515030065)+4 种基金Basic and Applied Basic Research Project of Guangzhou(Grant no.2024A04J6299)LiaoNing Revitalization Talents Program(XLYC1907062,and XLYC2403026)Shenyang Young and Middle-aged Science and Technology Innovation Talent Support Program(RC231178)the Fundamental Research Funds for the Central Universities(N25GFY002)the Innovation Fund of Institute of Metal Research(IMR),Chinese Academy of Sciences(CAS).
文摘Mg-Li alloys hold significant potential for applications in aerospace,automotive manufacturing,military weaponry,and biomedical implants,due to their excellent recyclability,high specific strength,biocompatibility,and superior electromagnetic shielding properties.However,their poor corrosion resistance and high susceptibility to environmentally assisted cracking(EAC)significantly limit broader application.In recent years,growing attention has been directed toward understanding the corrosion and EAC behavior of Mg-Li alloys,as localized corrosion areas and hydrogen generated during the corrosion process can serve as crack initiation points and promote crack propagation.A comprehensive understanding of these mechanisms is essential for enhancing the reliability and performance of Mg-Li alloys in practical environments.This paper presents a detailed review of corrosion and EAC in Mg-Li alloys,focusing on corrosion behavior,crack initiation and propagation mechanisms,and the key factors influencing these processes.It summarizes recent advances in alloying,heat treatment,mechanical processing,microstructural control,environmental influences,mechanical loading,and surface treatments.In addition,the paper explores future research directions,highlights emerging trends,and proposes strategies to improve the durability and service performance of Mg-Li alloys.
基金support from the National Natural Science Foundation of China(Grant Nos.52301137,51974097,52364050)the Natural Science Special Foundation of Guizhou University(No.(2023)20)+1 种基金Guizhou Province Science and Technology Project(Grant Nos.[2023]001,[2019]2163)Guiyang city Science and Technology Project(Grant No.[2023]48-16).
文摘The dynamic mechanical response and deformation mechanism of magnesium-yttrium alloy at high strain rate were investigated using split-Hopkinson pressure bar(SHPB)impact,and the microstructure evolution and crack formation mechanism were revealed.The yield strength and work hardening rate increase significantly with increasing impact strain rate.Deformation twinning and non-basal dislocation slip are the primary deformation mechanisms during testing.Contrary to crack initiation mechanism facilitated by adiabatic shear bands,we find that high-density co-axial nanocrystalline grains form near cracks,which leads to local softening and promotes crack initiation and rapid propagation.Most grains have similar<1^(-)21^(-)0>orientations,with unique misorientation of 24°,32°,62°,78°and 90°between adjacent grains,suggesting that these grains are primarily formed by interface transformation,which exhibits distinct differences from recrystallized grains.Our results shed light upon the dynamic mechanical response and crack formation mechanism in magnesium alloys under impact deformation.
基金supported by the National Natural Science Foundation of China(grant nos.12072212,12372185 and 12102280)the Sichuan Province Science and Technology Project(grant nos.2023NSFSC1918,and 2022JDJQ0011).
文摘Magnesium alloys with long-period stacking ordered(LPSO)structures are known for their impressive static mechanical strength,but the consistent occurrence of slip-cracking along the LPSO lamellae,which do not effectively impede the movement of basal dislocations,has prompted concerns about their very high cycle fatigue(VHCF)performance.In this study,an extruded Mg-Gd-Zn-Zr alloy was developed,showcasing exceptional VHCF resistance due to its bimodal structure comprisingfine grains and coarse grains consisting of curved LPSO lamellae.The investigation on the crack initiation mechanism revealed that slip-induced cracking predominantly occurs infine-grained regions rather than in the interior of coarse grains.The extrusion process aligns the basal planes of most coarse grains parallel to the axial direction,and the presence of curved LPSO lamellae acts as barriers to the movement of basal dislocations,thereby effectively increasing the threshold for slip-cracking along the basal plane.Consequently,fatigue damage manifests in the form of slip bands and micro-cracks within the interior offine grains,ultimately resulting in fatigue crack initiation,propagation andfinal fracture.
基金Projects(52104143,52109135,52374099)supported by the National Natural Science Foundation of ChinaProject(2025YFHZ0323)supported by the Natural Science Foundation of Sichuan Province,China。
文摘The evolution of cracks in shale directly affects the efficient production of shale gas.However,there is a lack of research on the characteristics of crack initiation in deep dense shale under different stress conditions.In this work,considering the different combinations of confining pressure and bedding plane inclination angle(α),biaxial mechanical loading experiments were conducted on shale containing circular holes.The research results indicate that the confining pressure and inclination angle of the bedding planes significantly influence the failure patterns of shale containing circular holes.The instability of shale containing circular holes can be classified into five types:tensile failure along the bedding planes,tensile failure through the bedding planes,shear slip along the bedding planes,shear failure through the bedding planes,and block instability failure.Furthermore,the evolution of strain and stress fields around the circular holes was found to be the fundamental cause of variations in the initiation characteristics and locations of shale cracks.The crack initiation criterion for shale containing circular hole was established,providing a new method for evaluating the trajectory of shale hole wall fractures.This study holds significant importance for evaluating the evolution and stability of fracture networks within shale reservoirs.
