Understanding the probabilistic nature of brittle materials due to inherent dispersions in their mechanical properties is important to assess their reliability and safety for sensitive engineering applications.This is...Understanding the probabilistic nature of brittle materials due to inherent dispersions in their mechanical properties is important to assess their reliability and safety for sensitive engineering applications.This is all the more important when elements composed of brittle materials are exposed to dynamic environments,resulting in catastrophic fatigue failures.The authors propose the application of a non-intrusive polynomial chaos expansion method for probabilistic studies on brittle materials undergoing fatigue fracture when geometrical parameters and material properties are random independent variables.Understanding the probabilistic nature of fatigue fracture in brittle materials is crucial for ensuring the reliability and safety of engineering structures subjected to cyclic loading.Crack growth is modelled using a phase-field approach within a finite element framework.For modelling fatigue,fracture resistance is progressively degraded by modifying the regularised free energy functional using a fatigue degradation function.Number of cycles to failure is treated as the dependent variable of interest and is estimated within acceptable limits due to the randomness in independent properties.Multiple 2D benchmark problems are solved to demonstrate the ability of this approach to predict the dependent variable responses with significantly fewer simulations than the Monte Carlo method.This proposed approach can accurately predict results typically obtained through 105 or more runs in Monte Carlo simulations with a reduction of up to three orders of magnitude in required runs.The independent random variables’sensitivity to the system response is determined using Sobol’indices.The proposed approach has low computational overhead and can be useful for computationally intensive problems requiring rapid decision-making in sensitive applications like aerospace,nuclear and biomedical engineering.The technique does not require reformulating existing finite element code and can perform the stochastic study by direct pre/post-processing.展开更多
BACKGROUND Subchondral fatigue fracture of the femoral head(SFFFH)mainly occurs in young military recruits and might be confused with osteonecrosis of the femoral head.However,less research focuses on the risk factor ...BACKGROUND Subchondral fatigue fracture of the femoral head(SFFFH)mainly occurs in young military recruits and might be confused with osteonecrosis of the femoral head.However,less research focuses on the risk factor for SFFFH.AIM To evaluate the intrinsic risk factors for SFFFH in young military recruits.METHODS X-ray and magnetic resonance imaging data were used for analysis.Acetabular anteversion of the superior acetabulum,acetabular anteversion of the center of the acetabulum(AVcen),anterior acetabular sector angle(AASA),posterior acetabular sector angle,superior acetabular sector angle,neck-shaft angle(NSA),inferior iliac angle(IIA),and ischiopubic angle were calculated.Then,logistic regression,receiver operating characteristic curve analysis,and independent samples t-test were performed to identify the risk factors for SFFFH.RESULTS Based on the results of logistic regression,age[odds ratio(OR):1.33;95%confidence interval(95%CI):1.12-1.65;P=0.0031]and treatment timing(OR:0.86;95%CI:0.75-0.96;P=0.015)could be considered as the indicators for SFFFH.AVcen(P=0.0334),AASA(P=0.0002),NSA(P=0.0007),and IIA(P=0.0316)were considered to have statistical significance.Further,AVcen(OR:1.41;95%CI:1.04-1.95)and AASA(OR:1.44;95%CI:1.21-1.77),especially AASA(area under curve:66.6%),should be paid much more attention due to the higher OR than other indicators.CONCLUSION We have for the first time unveiled that AASA and age could be key risk factors for SFFFH,which further verifies that deficient anterior coverage of the acetabulum might be the main cause of SFFFH.展开更多
Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 6...Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 66.72, 39.00, 24.38 and 24.40 MPa, respectively. The crack propagation behavior of the alloy was analyzed by optical microscopy. The AZ31B magnesium alloy base metal has a smooth crack propagation macroscopic path. However, the microscopic path is twisted and some cracks have two forks, and the crack propagation is transgranular. The crack initiates in the weld toe and the crack propagates along the HAZ for the BJ and TJ; for the LJ crack initiates in the fillet weld leg. The fatigue fracture mechanisms were analyzed by SEM. The fatigue fracture surface consists of quasi-cleavage patterns or cleavage step and a brittle fracture occurs. Numerous secondary cracks are observed; some fatigue striations exist in butt joint and its size is about 5 μm.展开更多
The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were furt...The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were further analyzed using optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）; phase analysis of these alloys was conducted with an X-ray diffraction （XRD）. The results show that when Sn is included, growth of the recrystallization grains in the solution-treated A1-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone （PFZ） of the overaged A1-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.展开更多
Rupture and safety of perilous rock are dominated by control fissure behind perilous rock block. Based on model-Ⅰ and model-Ⅱ stress strength factors of control fissure under acting of weight of perilous rock, water...Rupture and safety of perilous rock are dominated by control fissure behind perilous rock block. Based on model-Ⅰ and model-Ⅱ stress strength factors of control fissure under acting of weight of perilous rock, water pressure in control fissure and earth- quake forces, method to calculate critical linking length of control fissure is established. Take water pressure in control fissure as a variable periodic load, and abide by P-M criterion, when control fissure is filled with water, establish the method to calculate fatigue fracture life of control fissure in critical status by contributing value of stress strength factor stemming from water pressure of control fissure in Paris＇s fatigue equation. Further, parameters （C and m） of sandstone with quartz and feldspar in the area of the Three Gorges Reservoir of China are obtained by fatigue fracture testing.展开更多
Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM...Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.展开更多
The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour i...The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions展开更多
This laboratory was designated as Na-tional Laboratory in 1988 and is subordi-nate to the Institute of Metal Research(IMR),Academia Sinica.It is nowwell-equipped after rebuilding under a spe-cial grant-in-aid program ...This laboratory was designated as Na-tional Laboratory in 1988 and is subordi-nate to the Institute of Metal Research(IMR),Academia Sinica.It is nowwell-equipped after rebuilding under a spe-cial grant-in-aid program from the centralgovernment.According to the policy of“Opening,Flowing and Serving the WholeCountry”for the national laboratories,vis-iting research fellows at home and fromabroad are welcome to join common re-search projects in this lab.展开更多
High-speed milling of titanium alloys is widely used in aviation and aerospace industries for its high efficiency and good quality.In order to optimize the machining parameters in high-speed milling TB6 titanium alloy...High-speed milling of titanium alloys is widely used in aviation and aerospace industries for its high efficiency and good quality.In order to optimize the machining parameters in high-speed milling TB6 titanium alloy,experiments of high-speed milling and fatigue were conducted to investigate the effect of parameters on 3D surface topography and fatigue life.Based on the fatigue fracture,the effect mechanism of surface topography on the fatigue crack initiation was proposed.The experiment results show that when the milling speed ranged from 100 m/min to 140 m/min,and the feed per tooth ranged from 0.02 mm/z to 0.06 mm/z,the obtained surface roughness were within the limit（0.8 μm）.Fatigue life decreased sharply with the increase of surface equivalent stress concentration factor.The average error of fatigue life between the established model and the experimental results was 6.25%.The fatigue cracks nucleated at the intersection edge of machined surface.展开更多
The fatigue crack behavior of U75V rail under different quenching rates was studied using SEM and CLSM.The results show that the pearlite interlayer spacing and fatigue crack growth rate decrease,and the deflection pa...The fatigue crack behavior of U75V rail under different quenching rates was studied using SEM and CLSM.The results show that the pearlite interlayer spacing and fatigue crack growth rate decrease,and the deflection path of cracks and the number of branching cracks of U75V rail increases with an increase of cooling rate.The tearing edge of unstable region transites from pearlite lamellae to dimple.The fatigue crack growth path is closely related to the orientation of lamellae in pearlite microstructure.There are three modes direction of fatigue crack growth path and pearlite lamellar,which are parallel,vertical and 45°angle,and most of the branch cracks occur at an angle of 45°between the lamellar direction and the crack direction.More specifically,the deflection paths of cracks and branching crack have a relaxation effect on the stress intensity at the crack tip,which restrains the fatigue crack growth rate.展开更多
Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing（LSP） was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstr...Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing（LSP） was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstructure and fracture morphology were characterized by using an optical microscope（OM）, a scanning electron microscope（SEM）, and a transmission electron microscope（TEM）. The results indicated that the maximum residual compressive stress was at about 1 mm from the shocking spot center, where the residual compressive stress was slightly lower. High density tangling dislocations, dislocation walls, and dislocation cells in the microstructure of the specimens treated with LSP effectively prevented fatigue cracks propagation. The fatigue life was roughly twice as long as that of the specimens without LSP. The fatigue crack initiation（FCI） in specimens treated with LSP was observed in the lateral section and the subsurface simultaneously. The fatigue striation in the fracture treated with LSP was narrower than that in the untreated specimens. Moreover, dimples with tear ridges were found in the fatigued zones of the LSP treated specimens, which would be caused by severe plastic deformation.展开更多
The fatigue properties of friction stir welded （FSW） butt joint and base metal of MB8 magnesium alloy were investigated. The comparative fatigue tests were carried out using EHF-EM2OOK2-070-IA fatigue testing machin...The fatigue properties of friction stir welded （FSW） butt joint and base metal of MB8 magnesium alloy were investigated. The comparative fatigue tests were carried out using EHF-EM2OOK2-070-IA fatigue testing machine for both FSW butt joint and base metal specimens. The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063LA type. The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased. The conditional fatigue limit （2 x 106） of base metal and welded butt joint is about 77.44 MPa and 49. 91 MPa, respectively. The conditional fatigue limit （2 x 106 ） of the welded butt joint is 64.45% of that of base metal. The main reasons are that the welding can lead to stress concentration in the flash area, tensile welding residual stress in the welded joint（ The residual stress value was 30. 5 MPa）, as well as the grain size is not uniform in the heat-affected zone. The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface, the fracture type of the FSW butt joint belongs to a brittle fracture.展开更多
The fatigue behaviors of 2E12 aluminum alloy in T3 and T6 conditions at room temperature in air were investigated.The microstructures and fatigue fracture surfaces of the alloy were examined by transmission electron m...The fatigue behaviors of 2E12 aluminum alloy in T3 and T6 conditions at room temperature in air were investigated.The microstructures and fatigue fracture surfaces of the alloy were examined by transmission electron microscopy(TEM) and scanning electron microscopy(SEM).The results show that the alloy exhibits higher fatigue crack propagation(FCP) resistance in T3 condition than in T6 condition,the fatigue life is increased by 54% and the fatigue crack growth rate(FCGR) decreases significantly.The fatigue fractures of the alloy in T3 and T6 conditions are transgranular.But in T3 condition,secondary cracks occur and fatigue striations are not clear.In T6 condition,ductile fatigue striations are observed.The effect of aging conditions on fatigue behaviors is explained in terms of the slip planarity of dislocations and the cyclic slip reversibility.展开更多
The alloy 5052 was welded by friction stir welding （FSW） and tungsten inert gas （TIG） welding. The effect of welding processes （FSW and TIG） on the fatigue properties of 5052 aluminum-welded joints was analyzed ...The alloy 5052 was welded by friction stir welding （FSW） and tungsten inert gas （TIG） welding. The effect of welding processes （FSW and TIG） on the fatigue properties of 5052 aluminum-welded joints was analyzed based on fatigue testing, and the S-N curve of the joints were established. The results show that the fatigue properties of FSW welded joints are better than those of TIG welded joints. The fatigue strength is determined as 65 MPa under 106 cycling of fatigue life. The microstructure of joints is fine grains and narrow HAZ zone in FSW welds, which inhibit the growth of cracks and produce high fatigue life compared with that of TIG welds. Fracture morphologies also show that the fatigue fracture results from weld defects.展开更多
1.IntroductionMetal matrix composite materials exhibitquite a good combination of strength andstiffness of the reinforcing phase withductility and toughness of the matrix.Aluminium alloys reinforced with SiC parti-cle...1.IntroductionMetal matrix composite materials exhibitquite a good combination of strength andstiffness of the reinforcing phase withductility and toughness of the matrix.Aluminium alloys reinforced with SiC parti-cles and whiskers are receiving a greatdeal of attention from researchers andengineers,because the engineering properties展开更多
BACKGROUND Stress fractures of the femoral neck are not common in clinical practice,and simultaneous stress fractures of the femoral neck and proximal femur of the unilateral femur are even more rare.We introduce a ca...BACKGROUND Stress fractures of the femoral neck are not common in clinical practice,and simultaneous stress fractures of the femoral neck and proximal femur of the unilateral femur are even more rare.We introduce a case of this type of fracture that was treated in our department,analyze the causes,and review similar stress fractures reported in the literature to provide references for the diagnosis and treatment of such conditions.CASE SUMMARY A 62-year-old female,with a free medical history,was admitted to the hospital mainly due to pain in the right hip and worsening pain in the right thigh.The patient had no obvious history of trauma.X-ray and computed tomography showed fracture of the femoral neck and proximal femur.The patient had undergone surgery 1 year prior to address a fracture of the left proximal femur that had occurred in a traffic accident.Our first consideration was stress fracture of the femoral neck;however,simultaneous stress fractures of the femoral neck and proximal femur of the unilateral femur were seen.The femoral neck stress fracture was a tension fracture,with obvious displacement and varus deformity of the hip.Considering that the patient was an elderly female,we performed total hip arthroplasty.Follow-up X-rays showed that the stress fracture of the proximal femur had mostly healed after 3 mo.CONCLUSION Muscle fatigue and hip varus deformity provide an anatomical basis for the occurrence of femoral neck stress fractures.展开更多
In the present study, an aero pneumatic fatigue testing machine for complete dentures was designed, fabricated, and tested for the evaluation of the fatigue life of reinforced complete upper denture (CUD). On completi...In the present study, an aero pneumatic fatigue testing machine for complete dentures was designed, fabricated, and tested for the evaluation of the fatigue life of reinforced complete upper denture (CUD). On completion and testing, it was observed that the machine has the potential of generating reliable number of cyclic data. The machine’s performance was evaluated using test specimens of identical CUDs that were machined in conformity with standard procedures. The fatigue machine compressed the lower dental arch over the upper denture-specimen in centric occlusion, in the same way that the two masticatory muscles pull the lower jaw over the upper jaw during chewing. The incorporation of glass fibres into the CUD using a sandwich technique quadruples the lifespan of the denture (<em>P</em> = 0.004). The low standard deviation, along with the low coefficient of variation (CV) of the group of unreinforced dentures shows the repeatability of the results and the reliability of the machine. The high standard deviation and coefficient of variation of reinforced dentures was expected, since a high variation of results is usually recorded in fibre reinforcement cases. This research confirmed the view that the crack during denture fracture initiates in the anterior palatal area and propagates to the posterior.展开更多
This review presents the progress and current status of the investigation on electromechanical deformation and fracture of piezo electric/ferroelectric materials. An attempt is made to summarize a few fundamental aspe...This review presents the progress and current status of the investigation on electromechanical deformation and fracture of piezo electric/ferroelectric materials. An attempt is made to summarize a few fundamental aspects, which include electromechanical constitutive relations, piezoelectric micromechanics and electric fracture and fatigue, instead of describing all technological backgrounds, basic physics, experimental findings, and theoretical developments. A number of open questions and future prospective are presented. It is hoped that this review will encourage people to join the exploration of this important and interesting field.展开更多
Fatigue is usually the cause for the cracks identified at bridge elements in service. With an increase in the introduction of corrugated steel web girders in recent highway bridge construction, the understanding of th...Fatigue is usually the cause for the cracks identified at bridge elements in service. With an increase in the introduction of corrugated steel web girders in recent highway bridge construction, the understanding of the fatigue behaviour of welded details in such structures becomes an important issue for the design. The typical welded details were represented as welded joints assembled by longitudinal corrugated plates. All the experiments were performed under fatigue loading using a servo-control testing machine. The test results from the failure mode observation with the aid of infrared thermo-graph technology show that the failure manner of these welded joints is comparable to that of the corrugated steel web beams reported previously. It is indicated from the stiffness degradation analysis that the welded joints with larger corrugation angle have higher stiffness and greater stiffness degradation in the notable stiffness degradation range. It is shown from the test S-N relations based on the free regression and forced regression analyses that there is a good linear dependence between lg(N) and lg(ΔS). It is also demonstrated that the proposed fracture mechanics analytical model is able to give a prediction slightly lower but on the safe side for the mean stresses at 2 million cycles of the test welded joints.展开更多
Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled t...Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1 Hz and 10 Hz in air. The microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）.When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10 Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at these two frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.展开更多
文摘Understanding the probabilistic nature of brittle materials due to inherent dispersions in their mechanical properties is important to assess their reliability and safety for sensitive engineering applications.This is all the more important when elements composed of brittle materials are exposed to dynamic environments,resulting in catastrophic fatigue failures.The authors propose the application of a non-intrusive polynomial chaos expansion method for probabilistic studies on brittle materials undergoing fatigue fracture when geometrical parameters and material properties are random independent variables.Understanding the probabilistic nature of fatigue fracture in brittle materials is crucial for ensuring the reliability and safety of engineering structures subjected to cyclic loading.Crack growth is modelled using a phase-field approach within a finite element framework.For modelling fatigue,fracture resistance is progressively degraded by modifying the regularised free energy functional using a fatigue degradation function.Number of cycles to failure is treated as the dependent variable of interest and is estimated within acceptable limits due to the randomness in independent properties.Multiple 2D benchmark problems are solved to demonstrate the ability of this approach to predict the dependent variable responses with significantly fewer simulations than the Monte Carlo method.This proposed approach can accurately predict results typically obtained through 105 or more runs in Monte Carlo simulations with a reduction of up to three orders of magnitude in required runs.The independent random variables’sensitivity to the system response is determined using Sobol’indices.The proposed approach has low computational overhead and can be useful for computationally intensive problems requiring rapid decision-making in sensitive applications like aerospace,nuclear and biomedical engineering.The technique does not require reformulating existing finite element code and can perform the stochastic study by direct pre/post-processing.
文摘BACKGROUND Subchondral fatigue fracture of the femoral head(SFFFH)mainly occurs in young military recruits and might be confused with osteonecrosis of the femoral head.However,less research focuses on the risk factor for SFFFH.AIM To evaluate the intrinsic risk factors for SFFFH in young military recruits.METHODS X-ray and magnetic resonance imaging data were used for analysis.Acetabular anteversion of the superior acetabulum,acetabular anteversion of the center of the acetabulum(AVcen),anterior acetabular sector angle(AASA),posterior acetabular sector angle,superior acetabular sector angle,neck-shaft angle(NSA),inferior iliac angle(IIA),and ischiopubic angle were calculated.Then,logistic regression,receiver operating characteristic curve analysis,and independent samples t-test were performed to identify the risk factors for SFFFH.RESULTS Based on the results of logistic regression,age[odds ratio(OR):1.33;95%confidence interval(95%CI):1.12-1.65;P=0.0031]and treatment timing(OR:0.86;95%CI:0.75-0.96;P=0.015)could be considered as the indicators for SFFFH.AVcen(P=0.0334),AASA(P=0.0002),NSA(P=0.0007),and IIA(P=0.0316)were considered to have statistical significance.Further,AVcen(OR:1.41;95%CI:1.04-1.95)and AASA(OR:1.44;95%CI:1.21-1.77),especially AASA(area under curve:66.6%),should be paid much more attention due to the higher OR than other indicators.CONCLUSION We have for the first time unveiled that AASA and age could be key risk factors for SFFFH,which further verifies that deficient anterior coverage of the acetabulum might be the main cause of SFFFH.
基金Project(50675148)supported by the National Natural Science Foundation of China
文摘Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 66.72, 39.00, 24.38 and 24.40 MPa, respectively. The crack propagation behavior of the alloy was analyzed by optical microscopy. The AZ31B magnesium alloy base metal has a smooth crack propagation macroscopic path. However, the microscopic path is twisted and some cracks have two forks, and the crack propagation is transgranular. The crack initiates in the weld toe and the crack propagates along the HAZ for the BJ and TJ; for the LJ crack initiates in the fillet weld leg. The fatigue fracture mechanisms were analyzed by SEM. The fatigue fracture surface consists of quasi-cleavage patterns or cleavage step and a brittle fracture occurs. Numerous secondary cracks are observed; some fatigue striations exist in butt joint and its size is about 5 μm.
基金Project(2010CB731706) supported by the National Basic Research Program of China
文摘The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were further analyzed using optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）; phase analysis of these alloys was conducted with an X-ray diffraction （XRD）. The results show that when Sn is included, growth of the recrystallization grains in the solution-treated A1-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone （PFZ） of the overaged A1-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.
基金Project supported by the National Natural Science Foundation of China (No.50678182)the Chunhui International Fundation of China (No.Z2005155002)
文摘Rupture and safety of perilous rock are dominated by control fissure behind perilous rock block. Based on model-Ⅰ and model-Ⅱ stress strength factors of control fissure under acting of weight of perilous rock, water pressure in control fissure and earth- quake forces, method to calculate critical linking length of control fissure is established. Take water pressure in control fissure as a variable periodic load, and abide by P-M criterion, when control fissure is filled with water, establish the method to calculate fatigue fracture life of control fissure in critical status by contributing value of stress strength factor stemming from water pressure of control fissure in Paris＇s fatigue equation. Further, parameters （C and m） of sandstone with quartz and feldspar in the area of the Three Gorges Reservoir of China are obtained by fatigue fracture testing.
文摘Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.
文摘The effects of microstructure on the deformation and fracture behaviour of two-phase TiAl alloys were investjgated under monotonic and cyclical loading conditions, over a range of temperatu res.The tensile behaviour is analyzed for deformation temperatures between RT and 950℃, Fracture resistance behaviour and toughening mechanisms at RT and 800℃ are analyzed. and the inverse relationship botween ductility and toughness is explained using the crack initiation toughness. The preliminary results of load-controlled fatigue behaviour at 800℃ are interpreted using the tensile behaviour because deformation structure and fracture modes are similar under these two loading conditions
文摘This laboratory was designated as Na-tional Laboratory in 1988 and is subordi-nate to the Institute of Metal Research(IMR),Academia Sinica.It is nowwell-equipped after rebuilding under a spe-cial grant-in-aid program from the centralgovernment.According to the policy of“Opening,Flowing and Serving the WholeCountry”for the national laboratories,vis-iting research fellows at home and fromabroad are welcome to join common re-search projects in this lab.
基金Projects(50975237,51005184) supported by the National Natural Science Foundation of China
文摘High-speed milling of titanium alloys is widely used in aviation and aerospace industries for its high efficiency and good quality.In order to optimize the machining parameters in high-speed milling TB6 titanium alloy,experiments of high-speed milling and fatigue were conducted to investigate the effect of parameters on 3D surface topography and fatigue life.Based on the fatigue fracture,the effect mechanism of surface topography on the fatigue crack initiation was proposed.The experiment results show that when the milling speed ranged from 100 m/min to 140 m/min,and the feed per tooth ranged from 0.02 mm/z to 0.06 mm/z,the obtained surface roughness were within the limit（0.8 μm）.Fatigue life decreased sharply with the increase of surface equivalent stress concentration factor.The average error of fatigue life between the established model and the experimental results was 6.25%.The fatigue cracks nucleated at the intersection edge of machined surface.
基金Inner Mongolia Science and Technology Major Project(No.ZDZX2018024)Natural Science Foundation of Inner Mongolia(No.2019LH05016)+1 种基金Research Program of Science at Universities of Inner Mongolia Autonomous Region of China(No.NJZY20089)Innovation Fund of Inner Mongolia University of Science and Technology(No.2019QDL-B06)。
文摘The fatigue crack behavior of U75V rail under different quenching rates was studied using SEM and CLSM.The results show that the pearlite interlayer spacing and fatigue crack growth rate decrease,and the deflection path of cracks and the number of branching cracks of U75V rail increases with an increase of cooling rate.The tearing edge of unstable region transites from pearlite lamellae to dimple.The fatigue crack growth path is closely related to the orientation of lamellae in pearlite microstructure.There are three modes direction of fatigue crack growth path and pearlite lamellar,which are parallel,vertical and 45°angle,and most of the branch cracks occur at an angle of 45°between the lamellar direction and the crack direction.More specifically,the deflection paths of cracks and branching crack have a relaxation effect on the stress intensity at the crack tip,which restrains the fatigue crack growth rate.
基金Funded by the Key Program Project of National Natural Science Foundation of China(NSFC)(No.51641102)Natural Science Foundation of Jiangsu Province(No.16KJB430035)+1 种基金Qing Lan Project of Jiangsu Province in ChinaNational Key Laboratory of Science and Technology on Power Beam Processes of Beijing Aeronautical Manufacturing Technology Research Institute
文摘Low-cycle fatigue behavior of Ni-based superalloy GH586 with laser shock processing（LSP） was investigated. The residual stress of the specimens treated with LSP was assessed by X-ray diffraction method. The microstructure and fracture morphology were characterized by using an optical microscope（OM）, a scanning electron microscope（SEM）, and a transmission electron microscope（TEM）. The results indicated that the maximum residual compressive stress was at about 1 mm from the shocking spot center, where the residual compressive stress was slightly lower. High density tangling dislocations, dislocation walls, and dislocation cells in the microstructure of the specimens treated with LSP effectively prevented fatigue cracks propagation. The fatigue life was roughly twice as long as that of the specimens without LSP. The fatigue crack initiation（FCI） in specimens treated with LSP was observed in the lateral section and the subsurface simultaneously. The fatigue striation in the fracture treated with LSP was narrower than that in the untreated specimens. Moreover, dimples with tear ridges were found in the fatigued zones of the LSP treated specimens, which would be caused by severe plastic deformation.
基金supported by the National Natural Science Foundation(No.51265013)the Natural Science Foundation of Jiangxi Province(No.20151BAB206007)
文摘The fatigue properties of friction stir welded （FSW） butt joint and base metal of MB8 magnesium alloy were investigated. The comparative fatigue tests were carried out using EHF-EM2OOK2-070-IA fatigue testing machine for both FSW butt joint and base metal specimens. The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063LA type. The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased. The conditional fatigue limit （2 x 106） of base metal and welded butt joint is about 77.44 MPa and 49. 91 MPa, respectively. The conditional fatigue limit （2 x 106 ） of the welded butt joint is 64.45% of that of base metal. The main reasons are that the welding can lead to stress concentration in the flash area, tensile welding residual stress in the welded joint（ The residual stress value was 30. 5 MPa）, as well as the grain size is not uniform in the heat-affected zone. The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface, the fracture type of the FSW butt joint belongs to a brittle fracture.
基金Project(2005CB623705) supported by the National Basic Research Program of China
文摘The fatigue behaviors of 2E12 aluminum alloy in T3 and T6 conditions at room temperature in air were investigated.The microstructures and fatigue fracture surfaces of the alloy were examined by transmission electron microscopy(TEM) and scanning electron microscopy(SEM).The results show that the alloy exhibits higher fatigue crack propagation(FCP) resistance in T3 condition than in T6 condition,the fatigue life is increased by 54% and the fatigue crack growth rate(FCGR) decreases significantly.The fatigue fractures of the alloy in T3 and T6 conditions are transgranular.But in T3 condition,secondary cracks occur and fatigue striations are not clear.In T6 condition,ductile fatigue striations are observed.The effect of aging conditions on fatigue behaviors is explained in terms of the slip planarity of dislocations and the cyclic slip reversibility.
文摘The alloy 5052 was welded by friction stir welding （FSW） and tungsten inert gas （TIG） welding. The effect of welding processes （FSW and TIG） on the fatigue properties of 5052 aluminum-welded joints was analyzed based on fatigue testing, and the S-N curve of the joints were established. The results show that the fatigue properties of FSW welded joints are better than those of TIG welded joints. The fatigue strength is determined as 65 MPa under 106 cycling of fatigue life. The microstructure of joints is fine grains and narrow HAZ zone in FSW welds, which inhibit the growth of cracks and produce high fatigue life compared with that of TIG welds. Fracture morphologies also show that the fatigue fracture results from weld defects.
文摘1.IntroductionMetal matrix composite materials exhibitquite a good combination of strength andstiffness of the reinforcing phase withductility and toughness of the matrix.Aluminium alloys reinforced with SiC parti-cles and whiskers are receiving a greatdeal of attention from researchers andengineers,because the engineering properties
基金Supported by Suzhou Gusu Health Talent Program Training Program,No.GSWS2020121。
文摘BACKGROUND Stress fractures of the femoral neck are not common in clinical practice,and simultaneous stress fractures of the femoral neck and proximal femur of the unilateral femur are even more rare.We introduce a case of this type of fracture that was treated in our department,analyze the causes,and review similar stress fractures reported in the literature to provide references for the diagnosis and treatment of such conditions.CASE SUMMARY A 62-year-old female,with a free medical history,was admitted to the hospital mainly due to pain in the right hip and worsening pain in the right thigh.The patient had no obvious history of trauma.X-ray and computed tomography showed fracture of the femoral neck and proximal femur.The patient had undergone surgery 1 year prior to address a fracture of the left proximal femur that had occurred in a traffic accident.Our first consideration was stress fracture of the femoral neck;however,simultaneous stress fractures of the femoral neck and proximal femur of the unilateral femur were seen.The femoral neck stress fracture was a tension fracture,with obvious displacement and varus deformity of the hip.Considering that the patient was an elderly female,we performed total hip arthroplasty.Follow-up X-rays showed that the stress fracture of the proximal femur had mostly healed after 3 mo.CONCLUSION Muscle fatigue and hip varus deformity provide an anatomical basis for the occurrence of femoral neck stress fractures.
文摘In the present study, an aero pneumatic fatigue testing machine for complete dentures was designed, fabricated, and tested for the evaluation of the fatigue life of reinforced complete upper denture (CUD). On completion and testing, it was observed that the machine has the potential of generating reliable number of cyclic data. The machine’s performance was evaluated using test specimens of identical CUDs that were machined in conformity with standard procedures. The fatigue machine compressed the lower dental arch over the upper denture-specimen in centric occlusion, in the same way that the two masticatory muscles pull the lower jaw over the upper jaw during chewing. The incorporation of glass fibres into the CUD using a sandwich technique quadruples the lifespan of the denture (<em>P</em> = 0.004). The low standard deviation, along with the low coefficient of variation (CV) of the group of unreinforced dentures shows the repeatability of the results and the reliability of the machine. The high standard deviation and coefficient of variation of reinforced dentures was expected, since a high variation of results is usually recorded in fibre reinforcement cases. This research confirmed the view that the crack during denture fracture initiates in the anterior palatal area and propagates to the posterior.
基金The project supported by the National Natural Science Foundation of China (10025209)
文摘This review presents the progress and current status of the investigation on electromechanical deformation and fracture of piezo electric/ferroelectric materials. An attempt is made to summarize a few fundamental aspects, which include electromechanical constitutive relations, piezoelectric micromechanics and electric fracture and fatigue, instead of describing all technological backgrounds, basic physics, experimental findings, and theoretical developments. A number of open questions and future prospective are presented. It is hoped that this review will encourage people to join the exploration of this important and interesting field.
基金Projects(51308363,11327801)supported by the National Natural Science Foundation of ChinaProject(2013-1792-9-4)supported by the Scientific Research Foundation for the Returned Overseas Chinese ScholarsProject(YJ201307)supported by the Start-up Research Fund for Introduced Talents of Sichuan University,China
文摘Fatigue is usually the cause for the cracks identified at bridge elements in service. With an increase in the introduction of corrugated steel web girders in recent highway bridge construction, the understanding of the fatigue behaviour of welded details in such structures becomes an important issue for the design. The typical welded details were represented as welded joints assembled by longitudinal corrugated plates. All the experiments were performed under fatigue loading using a servo-control testing machine. The test results from the failure mode observation with the aid of infrared thermo-graph technology show that the failure manner of these welded joints is comparable to that of the corrugated steel web beams reported previously. It is indicated from the stiffness degradation analysis that the welded joints with larger corrugation angle have higher stiffness and greater stiffness degradation in the notable stiffness degradation range. It is shown from the test S-N relations based on the free regression and forced regression analyses that there is a good linear dependence between lg(N) and lg(ΔS). It is also demonstrated that the proposed fracture mechanics analytical model is able to give a prediction slightly lower but on the safe side for the mean stresses at 2 million cycles of the test welded joints.
基金the National Natural Science Foundation of China(No.50901042)NUST Research Funding(No.2011YBXM156)
文摘Samples prepared from as-extruded magnesium alloy Mg-3%Al-1%Zn （AZ31） billets were utilized in low-cycle fatigue tests in order to investigate the frequency-dependent fatigue life. Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1 Hz and 10 Hz in air. The microstructures were examined by optical microscopy （OM） and scanning electron microscopy （SEM）.When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10 Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at these two frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.
