The micro-cracking behaviors of two high-entropy alloys(HEAs) of the FeMnCoCrNi family prepared by selective laser melting were systematically studied. Residual stresses were also analyzed by X-ray diffraction techniq...The micro-cracking behaviors of two high-entropy alloys(HEAs) of the FeMnCoCrNi family prepared by selective laser melting were systematically studied. Residual stresses were also analyzed by X-ray diffraction technique. Results show that the equiatomic FeMnCoCrNi HEAs with a relatively stable single-phase face-centered cubic(FCC) structure suffered from micro-cracking with residual tensile stress after laser melting. In contrast, the metastable non-equiatomic Fe MnCoCr HEAs with reduced stacking fault energy are free of micro-cracks with residual compressive stress at various volumetric energy densities(VEDs). The displacive transformation from the FCC matrix to the hexagonal close-packed(HCP) phase during cooling prevents the micro-cracking via consuming thermal stress related internal energy. Further, the displacive transformation during tensile deformation contributes to the higher strength and ductility of the metastable dual-phase HEA compared to that of the stable single-phase HEA. These findings provide useful guidance for the design of strong, ductile, and crack-free alloys for additive manufacturing by tuning phase stability.展开更多
Grain size effect on rock strength is a topic of great interest in geotechnical engineering.A consensus obtained from earlier laboratory tests is that rock strength generally decreases with the increase of grain size ...Grain size effect on rock strength is a topic of great interest in geotechnical engineering.A consensus obtained from earlier laboratory tests is that rock strength generally decreases with the increase of grain size for both silicate and carbonate rocks;however,some recent numerical results conflict with such laboratory test results.To address this intriguing issue,the effect of grain size on strength of polymineralic crystalline rock with low porosity is investigated numerically using the grain-based modeling(GBM)approach in discrete element method(DEM)by interpreting micro-cracking process in response to loading.In agreement with some previous DEM simulation results,the simulated rock strength is found to increase with increasing grain size for both homogeneous and heterogeneous models,even when the number of assembled disks in one mineral grain changes.The mechanism of strength increase with increasing grain size is mainly associated with the number of assembled smooth-joint contacts along grain interfaces and the generation of grain boundary cracks in response to loading.The grain interfaces significantly weaken the integrity of the rock model,which is similar to effects of inherent defects in real rock.As the grain size increases,fewer grain interfaces are built in the model and the rock strength becomes much higher.Hence,by solely changing the mineral grain size in a model,the mechanism of grain size effect as observed in laboratory tests cannot be replicated.To address this issue,a method of degradation of grain boundary strength parameters is used to mimic the possible mechanism of grain size effect.The simulated strength using the method becomes comparable with those obtained from laboratory tests when the heterogeneity in the rock is considered.Degradation of grain boundary parameters with increasing grain size provides a plausible explanation for the grain size effect on rock strength.展开更多
In this study,a novel testing method is proposed to characterize the dynamic shear property and failure mechanism of rocks by introducing the short beam compression(SBC)specimen into the split Hopkinson pressure bar(S...In this study,a novel testing method is proposed to characterize the dynamic shear property and failure mechanism of rocks by introducing the short beam compression(SBC)specimen into the split Hopkinson pressure bar(SHPB)system.Firstly,the stress distribution of SBC specimen is comprehensively analyzed by finite element method(FEM),and the results show that the optimal notch separation ratio of SBC specimen is C/H?0.2 to achieve successful dynamic simple-shear tests.Then,dynamic shear tests are conducted on sandstone using the SBC-SHPB method.Via careful pulse shaping technique,the dynamic force balance is guaranteed for SBC specimens,and the testing results show that the dynamic shear strength of sandstone is significantly rate-dependent.Combining the results of dynamic compression and tension tests,the failure envelopes of sandstone under different loading rates are obtained in the principle stress plane.It is found that the failure envelope of sandstone constantly expands outwards with increasing loading rate.Moreover,the energy partition of SBC specimen is quantified by virtue of high-speed digital image correlation(DIC)technique.The results show that the kinetic energy portion is non-negligible,and the shear fracture energy increases with increasing loading rate.In addition,the microscopic shear cracking mechanism of SBC specimen is analyzed by the thin section observation:the intra-granular(TG)fracture of minerals dominates the dynamic shear failure of sandstone,and the portion of TG fracture increases with increasing loading rate.This study provides a convenient and reliable method to investigate the dynamic shear property and failure mechanism of rocks.展开更多
An efficient and promising approach for effectively dispersing multi-walled carbon nanotubes(MWCNTs)in cementitious composites has been investigated.The naturally occurring organic extracts from species of indigenousl...An efficient and promising approach for effectively dispersing multi-walled carbon nanotubes(MWCNTs)in cementitious composites has been investigated.The naturally occurring organic extracts from species of indigenously known‘Keekar’trees scattered along tropical and sub-tropical regions;is found as an exceptional replacement to the non-natural commercial surfactants.In the initial phase of investigation,ideal surfactant’s content required for efficient dispersion of MWCNTs in solution was determined using ultra-violet spectroscopy.The experimental investigations were then extended to five different cement composite formulations containing 0.0,0.025,0.05,0.08 and 0.10%MWCNTs by weight of cement.It was observed that the natural surfactant produced efficient dispersion at much reduced cost(approx.14%)compared with the commercial alternate.The estimated weight efficiency factor f was found 6.5 times higher for the proposed sustainable replacement to the conventional along with remarkable increase of 23%in modulus of rupture on 0.08 wt%addition of MWCNTs.Besides strength enhancement,the dispersed MWCNTs also improved the first crack and ultimate fracture toughness by 51.5%and 35.9%,respectively.The field emission scanning electron microscopy of the cryofractured samples revealed efficient dispersion of MWCNTs in the matrix leading to the phenomenon of effective crack bridging and crack branching in the composite matrix.Furthermore,the proposed scheme significantly reduced the early age volumetric shrinkage by 39%.展开更多
基金financial support of the National Natural Science Foundation of China (51505166,51971248)the Huxiang Young Talents Project (2018RS3007,2019RS1001)+1 种基金the Innovation-Driven Project of Central South University,China (2020CX023)Science and Technology Project of Hunan Province (2020GK2031)。
文摘The micro-cracking behaviors of two high-entropy alloys(HEAs) of the FeMnCoCrNi family prepared by selective laser melting were systematically studied. Residual stresses were also analyzed by X-ray diffraction technique. Results show that the equiatomic FeMnCoCrNi HEAs with a relatively stable single-phase face-centered cubic(FCC) structure suffered from micro-cracking with residual tensile stress after laser melting. In contrast, the metastable non-equiatomic Fe MnCoCr HEAs with reduced stacking fault energy are free of micro-cracks with residual compressive stress at various volumetric energy densities(VEDs). The displacive transformation from the FCC matrix to the hexagonal close-packed(HCP) phase during cooling prevents the micro-cracking via consuming thermal stress related internal energy. Further, the displacive transformation during tensile deformation contributes to the higher strength and ductility of the metastable dual-phase HEA compared to that of the stable single-phase HEA. These findings provide useful guidance for the design of strong, ductile, and crack-free alloys for additive manufacturing by tuning phase stability.
基金in part supported by the National Natural Science Foundation of China(Grant Nos.41877217 and 51609178)the General Research Fund of the Research Grants Council(Hong Kong,China)(Grant No.17303917)the Singapore Academic Research Fund Tier 1 Grant(RG112/14).
文摘Grain size effect on rock strength is a topic of great interest in geotechnical engineering.A consensus obtained from earlier laboratory tests is that rock strength generally decreases with the increase of grain size for both silicate and carbonate rocks;however,some recent numerical results conflict with such laboratory test results.To address this intriguing issue,the effect of grain size on strength of polymineralic crystalline rock with low porosity is investigated numerically using the grain-based modeling(GBM)approach in discrete element method(DEM)by interpreting micro-cracking process in response to loading.In agreement with some previous DEM simulation results,the simulated rock strength is found to increase with increasing grain size for both homogeneous and heterogeneous models,even when the number of assembled disks in one mineral grain changes.The mechanism of strength increase with increasing grain size is mainly associated with the number of assembled smooth-joint contacts along grain interfaces and the generation of grain boundary cracks in response to loading.The grain interfaces significantly weaken the integrity of the rock model,which is similar to effects of inherent defects in real rock.As the grain size increases,fewer grain interfaces are built in the model and the rock strength becomes much higher.Hence,by solely changing the mineral grain size in a model,the mechanism of grain size effect as observed in laboratory tests cannot be replicated.To address this issue,a method of degradation of grain boundary strength parameters is used to mimic the possible mechanism of grain size effect.The simulated strength using the method becomes comparable with those obtained from laboratory tests when the heterogeneity in the rock is considered.Degradation of grain boundary parameters with increasing grain size provides a plausible explanation for the grain size effect on rock strength.
基金The authors thank the financial support from the National Natural Science Foundation of China(Grant.Nos.52039007 and 52225904)the Youth Science and Technology Innovation Research Team Fund of Sichuan Province(Grant.No.2020JDTD0001).
文摘In this study,a novel testing method is proposed to characterize the dynamic shear property and failure mechanism of rocks by introducing the short beam compression(SBC)specimen into the split Hopkinson pressure bar(SHPB)system.Firstly,the stress distribution of SBC specimen is comprehensively analyzed by finite element method(FEM),and the results show that the optimal notch separation ratio of SBC specimen is C/H?0.2 to achieve successful dynamic simple-shear tests.Then,dynamic shear tests are conducted on sandstone using the SBC-SHPB method.Via careful pulse shaping technique,the dynamic force balance is guaranteed for SBC specimens,and the testing results show that the dynamic shear strength of sandstone is significantly rate-dependent.Combining the results of dynamic compression and tension tests,the failure envelopes of sandstone under different loading rates are obtained in the principle stress plane.It is found that the failure envelope of sandstone constantly expands outwards with increasing loading rate.Moreover,the energy partition of SBC specimen is quantified by virtue of high-speed digital image correlation(DIC)technique.The results show that the kinetic energy portion is non-negligible,and the shear fracture energy increases with increasing loading rate.In addition,the microscopic shear cracking mechanism of SBC specimen is analyzed by the thin section observation:the intra-granular(TG)fracture of minerals dominates the dynamic shear failure of sandstone,and the portion of TG fracture increases with increasing loading rate.This study provides a convenient and reliable method to investigate the dynamic shear property and failure mechanism of rocks.
文摘An efficient and promising approach for effectively dispersing multi-walled carbon nanotubes(MWCNTs)in cementitious composites has been investigated.The naturally occurring organic extracts from species of indigenously known‘Keekar’trees scattered along tropical and sub-tropical regions;is found as an exceptional replacement to the non-natural commercial surfactants.In the initial phase of investigation,ideal surfactant’s content required for efficient dispersion of MWCNTs in solution was determined using ultra-violet spectroscopy.The experimental investigations were then extended to five different cement composite formulations containing 0.0,0.025,0.05,0.08 and 0.10%MWCNTs by weight of cement.It was observed that the natural surfactant produced efficient dispersion at much reduced cost(approx.14%)compared with the commercial alternate.The estimated weight efficiency factor f was found 6.5 times higher for the proposed sustainable replacement to the conventional along with remarkable increase of 23%in modulus of rupture on 0.08 wt%addition of MWCNTs.Besides strength enhancement,the dispersed MWCNTs also improved the first crack and ultimate fracture toughness by 51.5%and 35.9%,respectively.The field emission scanning electron microscopy of the cryofractured samples revealed efficient dispersion of MWCNTs in the matrix leading to the phenomenon of effective crack bridging and crack branching in the composite matrix.Furthermore,the proposed scheme significantly reduced the early age volumetric shrinkage by 39%.
