Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw(F-T)weathering and dynamic mixed-mode loading,posing a serious threaten to the safety and stability of geotechnical e...Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw(F-T)weathering and dynamic mixed-mode loading,posing a serious threaten to the safety and stability of geotechnical engineering.In this study,a series of dynamic fracture tests were conducted on notched semi-circular bend(NSCB)granite specimens subjected to different mixed-mode loading and F-T cycles using a split Hopkinson pressure bar(SHPB)test system.The effects of F-T treatment and dynamic mixed-mode loading on the fracture properties of granite,including effective fracture toughness,progressive fracture process,and macroscopic morphology of fracture surface,were comprehensively revealed.The experimental results suggest that the dynamic effective fracture toughness of NSCB specimens is dependent on the loading rate,particularly when the mode I loading is dominant.Additionally,the fracture toughness decreases as the number of F-T cycles increases,with an inflection point at 30 F-T cycles.All granite specimens subjected to mixed-mode loading exhibit a curved fracture path,with faster crack propagation speed and more fine cracks in specimens exposed to higher F-T cycles.Macroscopic morphology of fracture surface obtained using three-dimensional(3D)scanning indicates that the fractal dimension of the fracture surface increases with increasing F-T cycles,and the increment is more pronounced for specimens subjected to a higher mode II loading component.Moreover,this study compared the fracture resistance of F-T treated granite subjected to dynamic mixed loading using the maximum tangential stress(MTS)criterion and the generalized maximum tangential stress-based semi-analytical(SA-GMTS)criterion.Compared with the MTS criterion,the SA-GMTS criterion shows a more reasonable consistency with the experimental results,with a root mean square error within±7%.展开更多
The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact...The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.展开更多
The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabric...The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.展开更多
An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form ...An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form the Moire fringes, then the COD versus time could be monitored from the movement of the Moire fringes, and finally the critical time could be determined from the velocity of COD. This method was also compared with another one.It could be concluded that the critical time determined by Dynamic Moire method corresponds with that of the Transmitted Wave method at the loading rates from 103 to 104 MPa·m(1/2)·S(-1).展开更多
This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-paramete...This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-parameter, the atom radius, is introduced into the criterion.Meanwhile a characteristic time concept is taken into account for describing the inertia effect of material. The criterion reveals The criterion reveals the effects of loading rate, defect and sample geometry,material constants including the micro-structure parameter.展开更多
The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a nove...The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.展开更多
A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid ...A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional （3D） transient numerical simulations were performed by a finite element （FE） computer program. Macroscopical plastic deformation was observed at the loading and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Therefore, elustic-viscoplustic analysis was performed on the specimen by adopting the Johnson-Cook （J-C） model to describe the rate-dependent plastic flow behavior of the material. The material heterogeneity in the mismatched welded joints, induced by the difference in the base metal （BM） and the weld metal （WM） in yield stress, has also been taken into consideration by using the J-C models separately. Good accordance was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 108 MPa.m^1/2.s^-1.展开更多
An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1....An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.展开更多
A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method...A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.展开更多
The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a...The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa.m1/2 and 32.6 m/s, respectively. SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA 15ELI alloy in dynamic mode-II fracture. Shear localization was observed in the vicinity of the crack initiation area.展开更多
The plots of load vs deflection(P-D)for two ultra-strength steels 30CrMnSiNi2A and 300M has been examined on a Charpy impact tester with digital memory and microcomputer. A criterion was found to availably represent t...The plots of load vs deflection(P-D)for two ultra-strength steels 30CrMnSiNi2A and 300M has been examined on a Charpy impact tester with digital memory and microcomputer. A criterion was found to availably represent the dynamic fracture toughness,K_(Ic)~D ,of the steels under impact loading.The K_(Ic)~D,.can be calculated with suggesting that the turning point of change in compliance on the P-D plot is the point of destabilized crack propagation.The K_(Ic)~D of steel 300M,quenched at 870℃ ,separately tempered at 300℃ and 450℃,was esti- mated to be 55—70 MN/m^(3/2)and 54—67 MN/m^(3/2)respectively.The K_(Ic)calculated with accuracy about 20% seems to be available to the engineering evaluation.展开更多
The method to detect the crack initiation point of Charpy pre-cracked specimen under dy- namie loading was studied using strain gauge.The load-time curve and nominal strain-time curve at the crack tip for impact testi...The method to detect the crack initiation point of Charpy pre-cracked specimen under dy- namie loading was studied using strain gauge.The load-time curve and nominal strain-time curve at the crack tip for impact testing specinens may be shnultaneously measured by twin-channel oscilloscope with high speed sampling and diskette storing.Based on the dynam- ic finite element simulation of impact response of Charpy specimen,the measuring method of dynamic fracture toughness was analysed and some problems in previous dynamic fracture toughness measurement were discussed.展开更多
The dynamic fracture behavior of a new near-beta Ti-5Al-5Mo-5V-3Cr-1Fe（Ti-5553）alloy under a high strain rate loading was investigated systemically using the Taylor impact test,over the impact velocity ranging from1...The dynamic fracture behavior of a new near-beta Ti-5Al-5Mo-5V-3Cr-1Fe（Ti-5553）alloy under a high strain rate loading was investigated systemically using the Taylor impact test,over the impact velocity ranging from156 ms-1 to 256 ms-1.An optical microscope（OM）and a scanning electron microscope（SEM）were used to characterize the microstructure evolution.The experimental results have demonstrated that the velocity from deformation to fracture is 256 ms-1 for the alloy with anα＋βduplex microstructure including more primaryαphase,while the velocity is 234 ms-1 for the alloy with a duplex microstructure including less primaryα phase.From the impact fracture morphologies,smooth and smeared surfaces and ductile dimple areas can be observed.The failure mode of the titanium alloy with both microstructures is adiabatic shear banding.According to the fracture analysis,the ductile fracture area with the dimple area in the alloy with much more primaryαphase were more than that with less primaryαphase.Compared to the duplex microstructure with less primaryα phase,Ti-5553 alloy with more primaryαphase exhibited a better capability to resist an adiabatic shear damage.展开更多
This paper deals with an experimental study on dynamic fracture speed of three point bending specimen and cylindrical specimen made of epoxy resin by means of silver conductive painting grid. According to the test res...This paper deals with an experimental study on dynamic fracture speed of three point bending specimen and cylindrical specimen made of epoxy resin by means of silver conductive painting grid. According to the test results ,the relations between fracture load ,fracture speed and loading speed are discussed and the features of final fracture surfaces are analysed in detail.展开更多
The influence of temperature on the resistance to rapid crack propagation of a semi-crystalline bio-based polymer was studied. The experimental results described in this study allow to initiate a first discussion on t...The influence of temperature on the resistance to rapid crack propagation of a semi-crystalline bio-based polymer was studied. The experimental results described in this study allow to initiate a first discussion on the role of viscosity and its link with the fracture behaviour and a heterogeneous microstructure such as the semi-crysalline polymer. Dynamic fracture tests on pipes were carried out. It would appear that a temperature decrease of approximately 40℃ relative to ambient has no significant influence on the average crack propagation velocity (≈0.6<em>c<sub>R</sub></em><sub>)</sub>, fracture energy and surface roughness. On the contrary, crack propagation paths seem to vary with temperature. The difference in fracture behaviour between the amorphous and crystalline phase varies significantly as a function of temperature. The difference between the initiation resistance and the rapid propagation also varies. This difference seems to be significantly reduced by lowering the temperature. The mechanisms of cavitation damage and plastic flow are increasingly limited by the decrease in temperature (and therefore in macromolecular mobility). Crack propagation in the pipe is more extensive and therefore more critical. This is emphasised in particular by the probability of the material to be macro-branched as the temperature decreases.展开更多
Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for...Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production.In this study,using the well-known notched semi-circular bend(NSCB)sample and the novel double-edge notched flattened Brazilian disc(DNFBD)sample combined with a split Hopkinson pressure bar(SHPB),various dynamic anisotropic fracture properties of Lushan shale,including failure characteristics,fracture toughness,energy dissipation and crack propagation velocity,are comprehensively compared and discussed under mode Ⅰ and mode Ⅱ fracture scenarios.First,using a newly modified fracture criterion considering the strength anisotropy of shale,the DNFBD specimen is predicted to be a robust method for true mode Ⅱ fracture of anisotropic shale rocks.Our experimental results show that the dynamic mode Ⅱ fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode Ⅰ fracture.The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation,while the maximal ones occur in the Divider orientation.In addition,it is interesting to find that the mode Ⅱ fracture toughness anisotropy index decreases more slowly than that in the mode Ⅰ fracture scenario.These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique.展开更多
The semi-circular bend(SCB) dynamic fracture toughness test is simulated using discrete element models. The influence of the frictional boundary condition, constitutive law and specimen thickness on the test measureme...The semi-circular bend(SCB) dynamic fracture toughness test is simulated using discrete element models. The influence of the frictional boundary condition, constitutive law and specimen thickness on the test measurements is investigated. It is found that friction between loading plates and the rock specimen affects the test results. Therefore, friction must be carefully considered to obtain accurate measurements. The simulation results also show that in contrast to the 2D model in which a rate-dependent cohesive law must be introduced, 3D models with a rate-independent law can produce good results. Furthermore, the study suggests that test measurements are seriously affected by specimen thickness; thus, full 3D modeling is required for simulation of the SCB test.展开更多
To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature condit...To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.展开更多
The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also th...The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.展开更多
The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are rec...The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.展开更多
基金support from the National Natural Science Foundation of China(Grant Nos.52225904 and 52039007)the Natural Science Foundation of Sichuan Province(Grant No.2023NSFSC0377).
文摘Rock masses in high-elevation or cold regions are vulnerable to the combined effects of freeze-thaw(F-T)weathering and dynamic mixed-mode loading,posing a serious threaten to the safety and stability of geotechnical engineering.In this study,a series of dynamic fracture tests were conducted on notched semi-circular bend(NSCB)granite specimens subjected to different mixed-mode loading and F-T cycles using a split Hopkinson pressure bar(SHPB)test system.The effects of F-T treatment and dynamic mixed-mode loading on the fracture properties of granite,including effective fracture toughness,progressive fracture process,and macroscopic morphology of fracture surface,were comprehensively revealed.The experimental results suggest that the dynamic effective fracture toughness of NSCB specimens is dependent on the loading rate,particularly when the mode I loading is dominant.Additionally,the fracture toughness decreases as the number of F-T cycles increases,with an inflection point at 30 F-T cycles.All granite specimens subjected to mixed-mode loading exhibit a curved fracture path,with faster crack propagation speed and more fine cracks in specimens exposed to higher F-T cycles.Macroscopic morphology of fracture surface obtained using three-dimensional(3D)scanning indicates that the fractal dimension of the fracture surface increases with increasing F-T cycles,and the increment is more pronounced for specimens subjected to a higher mode II loading component.Moreover,this study compared the fracture resistance of F-T treated granite subjected to dynamic mixed loading using the maximum tangential stress(MTS)criterion and the generalized maximum tangential stress-based semi-analytical(SA-GMTS)criterion.Compared with the MTS criterion,the SA-GMTS criterion shows a more reasonable consistency with the experimental results,with a root mean square error within±7%.
基金Project (51001014) supported by the Young Scientists Fund of the National Natural Science Foundation of China
文摘The dynamic fracture behaviors of Ti-6Al-4V alloy at high strain rate loading were investigated systemically through Taylor impact test, over the range of impact velocities from 145 m/s to 306 m/s. The critical impact velocity of fracture ranges from 217 m/s to 236 m/s. Smooth surfaces and ductile dimple areas were observed on the fracture surfaces. As the impact velocity reached 260 m/s, the serious melting regions were also observed on the fracture surfaces. Self-organization of cracks emerges when the impact velocity reaches 260 m/s, while some special cracks whose ＂tips＂ are not sharp but arc and smooth, and without any evidence of deformation or adiabatic shear band were also observed on the impact end surfaces. Examination of the sections of these special cracks reveals that the cracks expand along the two maximum shear stress directions respectively, and finally intersect as a tridimensional ＂stagger ridge＂ structure.
基金The Ph.D.Programs Foundation of Ministry of Education of China(No.20120023120020)the National Natural Science Foundation of China(No.51404273)
文摘The reflected optical caustics method is applied to study dynamic fracture problems in hardened cement paste. First both the unreinforced cement paste and the glass fibres reinforced cement paste specimens were fabricated and the reflective coating on the surface of the specimen was prepared. Secondly the crack path and the shadow spot patterns during the crack propagation process for the two specimens were recorded by using a multi-spark high speed camera.Thirdly some dynamic parameters of two cement paste specimens including crack onset time the dynamic stress intensity factor and crack growth velocity were determined and analyzed comparatively.This indicates that the glass fibres can improve the fracture resistance and delay fracture time.These results will play an important role in evaluating the dynamic fracture properties of cement paste.
文摘An effective Dynamic Moire method was presented to determine the critical time of crack instable propagation in rock dynamic fracture. Two pieces of grating were installed near the notch of Short Rod specimen to form the Moire fringes, then the COD versus time could be monitored from the movement of the Moire fringes, and finally the critical time could be determined from the velocity of COD. This method was also compared with another one.It could be concluded that the critical time determined by Dynamic Moire method corresponds with that of the Transmitted Wave method at the loading rates from 103 to 104 MPa·m(1/2)·S(-1).
文摘This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-parameter, the atom radius, is introduced into the criterion.Meanwhile a characteristic time concept is taken into account for describing the inertia effect of material. The criterion reveals The criterion reveals the effects of loading rate, defect and sample geometry,material constants including the micro-structure parameter.
文摘The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.
基金111 project(No.B07050)the National Natural Science Foundation of China(No.90405016).
文摘A novel method was proposed for the evaluation of Mode I dynamic fracture toughness （DFT） under plane stress and small scale yielding conditions for welded joints of stainless steel （SS）, 0Cr18Ni10Ti. In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional （3D） transient numerical simulations were performed by a finite element （FE） computer program. Macroscopical plastic deformation was observed at the loading and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Therefore, elustic-viscoplustic analysis was performed on the specimen by adopting the Johnson-Cook （J-C） model to describe the rate-dependent plastic flow behavior of the material. The material heterogeneity in the mismatched welded joints, induced by the difference in the base metal （BM） and the weld metal （WM） in yield stress, has also been taken into consideration by using the J-C models separately. Good accordance was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 108 MPa.m^1/2.s^-1.
基金supported by the 111 Project (B07050)the National Natural Science Foundation of China (10932008)
文摘An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness （DFT） of high strength metals always increases with the loading rate on the order of TPa.m1/2.s-1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar （SHPB） apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMnSiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor （DSIF） and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experimentsneed to be performed in a comparatively large striker velocity range.
基金supported by the 111 Project (B07050)the National Natural Science Foundation of China (10932008)
文摘A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.
文摘The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa.m1/2 and 32.6 m/s, respectively. SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA 15ELI alloy in dynamic mode-II fracture. Shear localization was observed in the vicinity of the crack initiation area.
文摘The plots of load vs deflection(P-D)for two ultra-strength steels 30CrMnSiNi2A and 300M has been examined on a Charpy impact tester with digital memory and microcomputer. A criterion was found to availably represent the dynamic fracture toughness,K_(Ic)~D ,of the steels under impact loading.The K_(Ic)~D,.can be calculated with suggesting that the turning point of change in compliance on the P-D plot is the point of destabilized crack propagation.The K_(Ic)~D of steel 300M,quenched at 870℃ ,separately tempered at 300℃ and 450℃,was esti- mated to be 55—70 MN/m^(3/2)and 54—67 MN/m^(3/2)respectively.The K_(Ic)calculated with accuracy about 20% seems to be available to the engineering evaluation.
文摘The method to detect the crack initiation point of Charpy pre-cracked specimen under dy- namie loading was studied using strain gauge.The load-time curve and nominal strain-time curve at the crack tip for impact testing specinens may be shnultaneously measured by twin-channel oscilloscope with high speed sampling and diskette storing.Based on the dynam- ic finite element simulation of impact response of Charpy specimen,the measuring method of dynamic fracture toughness was analysed and some problems in previous dynamic fracture toughness measurement were discussed.
文摘The dynamic fracture behavior of a new near-beta Ti-5Al-5Mo-5V-3Cr-1Fe（Ti-5553）alloy under a high strain rate loading was investigated systemically using the Taylor impact test,over the impact velocity ranging from156 ms-1 to 256 ms-1.An optical microscope（OM）and a scanning electron microscope（SEM）were used to characterize the microstructure evolution.The experimental results have demonstrated that the velocity from deformation to fracture is 256 ms-1 for the alloy with anα＋βduplex microstructure including more primaryαphase,while the velocity is 234 ms-1 for the alloy with a duplex microstructure including less primaryα phase.From the impact fracture morphologies,smooth and smeared surfaces and ductile dimple areas can be observed.The failure mode of the titanium alloy with both microstructures is adiabatic shear banding.According to the fracture analysis,the ductile fracture area with the dimple area in the alloy with much more primaryαphase were more than that with less primaryαphase.Compared to the duplex microstructure with less primaryα phase,Ti-5553 alloy with more primaryαphase exhibited a better capability to resist an adiabatic shear damage.
文摘This paper deals with an experimental study on dynamic fracture speed of three point bending specimen and cylindrical specimen made of epoxy resin by means of silver conductive painting grid. According to the test results ,the relations between fracture load ,fracture speed and loading speed are discussed and the features of final fracture surfaces are analysed in detail.
文摘The influence of temperature on the resistance to rapid crack propagation of a semi-crystalline bio-based polymer was studied. The experimental results described in this study allow to initiate a first discussion on the role of viscosity and its link with the fracture behaviour and a heterogeneous microstructure such as the semi-crysalline polymer. Dynamic fracture tests on pipes were carried out. It would appear that a temperature decrease of approximately 40℃ relative to ambient has no significant influence on the average crack propagation velocity (≈0.6<em>c<sub>R</sub></em><sub>)</sub>, fracture energy and surface roughness. On the contrary, crack propagation paths seem to vary with temperature. The difference in fracture behaviour between the amorphous and crystalline phase varies significantly as a function of temperature. The difference between the initiation resistance and the rapid propagation also varies. This difference seems to be significantly reduced by lowering the temperature. The mechanisms of cavitation damage and plastic flow are increasingly limited by the decrease in temperature (and therefore in macromolecular mobility). Crack propagation in the pipe is more extensive and therefore more critical. This is emphasised in particular by the probability of the material to be macro-branched as the temperature decreases.
基金supported by the National Natural Science Foundation of China(Grant No.12302500)the National Key Research and Development Program of China(Grant No.2020YFA0710503)Postdoctoral Fellowship Program(Grade B)of China Postdoctoral Science Foundation(Grant No.GBZ20230022).
文摘Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation.Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production.In this study,using the well-known notched semi-circular bend(NSCB)sample and the novel double-edge notched flattened Brazilian disc(DNFBD)sample combined with a split Hopkinson pressure bar(SHPB),various dynamic anisotropic fracture properties of Lushan shale,including failure characteristics,fracture toughness,energy dissipation and crack propagation velocity,are comprehensively compared and discussed under mode Ⅰ and mode Ⅱ fracture scenarios.First,using a newly modified fracture criterion considering the strength anisotropy of shale,the DNFBD specimen is predicted to be a robust method for true mode Ⅱ fracture of anisotropic shale rocks.Our experimental results show that the dynamic mode Ⅱ fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode Ⅰ fracture.The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation,while the maximal ones occur in the Divider orientation.In addition,it is interesting to find that the mode Ⅱ fracture toughness anisotropy index decreases more slowly than that in the mode Ⅰ fracture scenario.These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique.
基金supported by the Australian Research Council(Grant No.DE130100457)State Key Laboratory of Hydraulics and Mountain River Engineering(SKHL)(Grant No.SKHL1407)the National Natural Science Foundation of China(Grant Nos.41202207,51204112 and 2015JY0045)
文摘The semi-circular bend(SCB) dynamic fracture toughness test is simulated using discrete element models. The influence of the frictional boundary condition, constitutive law and specimen thickness on the test measurements is investigated. It is found that friction between loading plates and the rock specimen affects the test results. Therefore, friction must be carefully considered to obtain accurate measurements. The simulation results also show that in contrast to the 2D model in which a rate-dependent cohesive law must be introduced, 3D models with a rate-independent law can produce good results. Furthermore, the study suggests that test measurements are seriously affected by specimen thickness; thus, full 3D modeling is required for simulation of the SCB test.
基金support from the National Natural Science Foundation of China(No.41972283)。
文摘To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock,dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system.Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves.A high-speed camera was used to check the fracture characteristics of the specimens.The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave.The dynamic force balance is achieved even when the specimens are at a temperature of 400°C.The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25-400°C,and high temperature has a strengthening effect on the dynamic fracture toughness.
基金Armament Research Board (ARMREB),New Delhi for funding this project work (Project No MAA/03/41)
文摘The armour grade quenched and tempered steel joints fabricated using low hydrogen ferritic steel （LHF） filler exhibited superior joint efficiency owing to preferential ferrite microstructure in the welds and also they offered required resistance to HIC. However, the combat vehicles used in military operations will be required to operate under a wide range of road conditions ranging from first class to cross country. Structural components in combat vehicles are subjected to dynamic loading with high strain rates during operation. Stress loadings within the vehicle hull of these vehicles are expected to fluctuate considerably and structural cracking especially in welds during the service life of these vehicles can lead to catastrophic failures. Under these conditions, fracture behaviour of high strain rate sensitive structural steels can be better understood by dynamic fracture toughness （K1d）. Hence, an attempt was made to study dynamic fracture toughness of the armour grade quenched and tempered steel and their welds fabricated using LHF consumables. The experimental results indicate that the K1d values of the joints fabricated by shielded metal arc welding （SMAW） are higher than those of the joints fabricated using flux cored arc welding （FCAW） process.
基金supported by the National Basic Research Program of China(973 Program)(No.2010CB731503)
文摘The purpose of this work is to investigate the dynamic fracture properties of glassy polycarbonate （PC） with different aging times. The optical method of caustics is adopted in which the shadow spot patterns are recorded by a high speed camera during the dynamic fracture process. Then, the dynamic crack propagation, the stress intensity factor （SIF） and the dynamic fracture toughness of aged PC are obtained through an analysis of the characteristic size of caustic pattern. Moreover, by combining with the investigation of the fracture surface and the energy release rate analysis, the influence of aging time on the dynamic fracture behavior is discussed. Results show that the dynamic fracture toughness and critical energy release rate of PC decreases with aging time for short aging times, whereas they have little change or even increase for longer aging times. Therefore, aging modifies the mechanical properties especially the dynamic fracture properties of PC nonlinearly, not linearly as generally thought of.
