In the metallic components fabricated by the emerging selective laser melting(SLM)technology,most strategies used for strengthening the materials sacrifice the ductility,leading to the so-called strengthductility trad...In the metallic components fabricated by the emerging selective laser melting(SLM)technology,most strategies used for strengthening the materials sacrifice the ductility,leading to the so-called strengthductility trade-off.In the present study,we report that the strength and ductility of materials can be enhanced simultaneously by introducing nanoparticles,which can break the trade-off of the metallic materials.In the case of in-situ nano-TiB_(2)decorated AlSi10Mg composites,the introduced nanoparticles lead to columnar-to-equiaxed transition,grain refinement and texture elimination.With increasing content of nanoparticles,the strength increases continually.Significantly,the ductility first increases and then decreases.Our results show that the ductility is controlled by the competition between the crack-induced catastrophic fracture and ductile fracture associated with dislocation activities.The first increase of ductility is mainly attributed to the suppression of crack-induced catastrophic fracture when TiB_(2)nanoparticles present.With the further increase of TiB_(2)nanoparticles,the subsequent decrease of ductility is mainly controlled by dislocation activities.Thus,the materials will exhibit the optimum strength and ductility combination in a certain range of TiB_(2)nanoparticles.This study clarifies the physical mechanism controlling ductility for nano-TiB_(2)decorated Al Si10Mg composites,which provides the insights for the design of structural materials.展开更多
A damage prediction method based on FE simulation was proposed to predict the occurrence of hot shortness crocks and surface cracks in liquid-solid extrusion process. This method integrated the critical temperature cr...A damage prediction method based on FE simulation was proposed to predict the occurrence of hot shortness crocks and surface cracks in liquid-solid extrusion process. This method integrated the critical temperature criterion and Cockcroft ＆ Latham ductile damage model, which were used to predict the initiation of hot shortness cracks and surface cracks of products, respectively. A coupling simulation of deformation with heat transfer as well as ductile damage was carried out to investigate the effect of extrusion temperature and extrusion speed on the damage behavior of Csf/AZ91D composites. It is concluded that the semisolid zone moves gradually toward deformation zone with the punch descending. The amplitude of the temperature rise at the exit of die from the initial billet temperature increases with the increase of extrusion speed during steady-state extrusion at a given punch displacement. In order to prevent the surface temperature of products beyond the incipient melting temperature of composites, the critical extrusion speed is decreased with the increase of extrusion temperature, otherwise the hot shortness cracks will occur. The maximum damage values increase with increasing extrusion speed or extrusion temperature. Theoretical results obtained by the Deform^TM-2D simulation agree well with the experiments.展开更多
Background:Osteoarthritis(ΟΑ)is characterized by cartilage breakdown and subchondral sclerosis.Micro-fractures of the calcified tissues have been,also,detected,but their exact role has not been elucidated yet.This s...Background:Osteoarthritis(ΟΑ)is characterized by cartilage breakdown and subchondral sclerosis.Micro-fractures of the calcified tissues have been,also,detected,but their exact role has not been elucidated yet.This study was to examine the frequency of cracks during OA progression and to correlate them with the underlying cellular modifications and matrix metalloproteinase-2(MMP-2)expression using histological/immunohistological methods.Methods:Overall,20 patients and 3 controls(9 specimens per patient),aged 60-89 years,diagnosed with hip/knee OA were included.The development of cracks was examined in 138 sections,whereas the expression of MMP-2 was examined in 69 additional sections.Results:Based on Mankin score,three groups of OA severity were analyzed:Group I(mild)was constituted of sections with score 1-5 while Groups II(moderate)and III(severe)with score 6-7 and greater or equal to 8,respectively.Demographic characteristics did not reveal any association between the number of microdefects and age or body mass index(BMI).Cartilage micro-cracks were increased during moderate and severe OA,while bone cracks were increased during mild and severe OA.In knee OA,cartilage cracks were not correlated with Mankin score,whereas in hip OA they appeared association with severity score.Bone cracks were positively correlated with matrix apoptotic osteocytes and osteoblastic cells,but not with osteoclasts.MMP-2 immunostaining was increasing by OA severity in the osteochondral unit.Similarly,MMP-2 was expressed on the microcracks’wall mainly in Group III.Conclusion:Our data displayed that bone cracks during primary OA stages,represent an early adaptative mechanism aiming to maintain cartilage integrity.Accumulation of bone defects and concomitant increase of apoptotic osteocytes activated an abnormal remodeling due to osteoblastic activity,in which MMP-2 played a pivotal role,leading to subchondral sclerosis promoting further osteochondral deformities.展开更多
Based on the Schapery three-dimensional viscoelastic constitutive relationship with growing damage, a damage model with transverse matrix cracks for the unidirectional ?bre rein- forced viscoelastic composite ...Based on the Schapery three-dimensional viscoelastic constitutive relationship with growing damage, a damage model with transverse matrix cracks for the unidirectional ?bre rein- forced viscoelastic composite plates is developed. By using Karman theory, the nonlinear dynamic governing equations of the viscoelastic composite plates under transverse periodic loading are es- tablished. By applying the ?nite di?erence method in spatial domain and the Newton-Newmark method in time domain, and using the iterative procedure, the integral-partial di?erential gov- erning equations are solved. Some examples are given and the results are compared with available data.展开更多
基金financially supported by the National Key Research and Development Program of China(No.2018YFB1106302)。
文摘In the metallic components fabricated by the emerging selective laser melting(SLM)technology,most strategies used for strengthening the materials sacrifice the ductility,leading to the so-called strengthductility trade-off.In the present study,we report that the strength and ductility of materials can be enhanced simultaneously by introducing nanoparticles,which can break the trade-off of the metallic materials.In the case of in-situ nano-TiB_(2)decorated AlSi10Mg composites,the introduced nanoparticles lead to columnar-to-equiaxed transition,grain refinement and texture elimination.With increasing content of nanoparticles,the strength increases continually.Significantly,the ductility first increases and then decreases.Our results show that the ductility is controlled by the competition between the crack-induced catastrophic fracture and ductile fracture associated with dislocation activities.The first increase of ductility is mainly attributed to the suppression of crack-induced catastrophic fracture when TiB_(2)nanoparticles present.With the further increase of TiB_(2)nanoparticles,the subsequent decrease of ductility is mainly controlled by dislocation activities.Thus,the materials will exhibit the optimum strength and ductility combination in a certain range of TiB_(2)nanoparticles.This study clarifies the physical mechanism controlling ductility for nano-TiB_(2)decorated Al Si10Mg composites,which provides the insights for the design of structural materials.
基金Project(50972121) supported by the National Natural Science Foundation of China
文摘A damage prediction method based on FE simulation was proposed to predict the occurrence of hot shortness crocks and surface cracks in liquid-solid extrusion process. This method integrated the critical temperature criterion and Cockcroft ＆ Latham ductile damage model, which were used to predict the initiation of hot shortness cracks and surface cracks of products, respectively. A coupling simulation of deformation with heat transfer as well as ductile damage was carried out to investigate the effect of extrusion temperature and extrusion speed on the damage behavior of Csf/AZ91D composites. It is concluded that the semisolid zone moves gradually toward deformation zone with the punch descending. The amplitude of the temperature rise at the exit of die from the initial billet temperature increases with the increase of extrusion speed during steady-state extrusion at a given punch displacement. In order to prevent the surface temperature of products beyond the incipient melting temperature of composites, the critical extrusion speed is decreased with the increase of extrusion temperature, otherwise the hot shortness cracks will occur. The maximum damage values increase with increasing extrusion speed or extrusion temperature. Theoretical results obtained by the Deform^TM-2D simulation agree well with the experiments.
文摘Background:Osteoarthritis(ΟΑ)is characterized by cartilage breakdown and subchondral sclerosis.Micro-fractures of the calcified tissues have been,also,detected,but their exact role has not been elucidated yet.This study was to examine the frequency of cracks during OA progression and to correlate them with the underlying cellular modifications and matrix metalloproteinase-2(MMP-2)expression using histological/immunohistological methods.Methods:Overall,20 patients and 3 controls(9 specimens per patient),aged 60-89 years,diagnosed with hip/knee OA were included.The development of cracks was examined in 138 sections,whereas the expression of MMP-2 was examined in 69 additional sections.Results:Based on Mankin score,three groups of OA severity were analyzed:Group I(mild)was constituted of sections with score 1-5 while Groups II(moderate)and III(severe)with score 6-7 and greater or equal to 8,respectively.Demographic characteristics did not reveal any association between the number of microdefects and age or body mass index(BMI).Cartilage micro-cracks were increased during moderate and severe OA,while bone cracks were increased during mild and severe OA.In knee OA,cartilage cracks were not correlated with Mankin score,whereas in hip OA they appeared association with severity score.Bone cracks were positively correlated with matrix apoptotic osteocytes and osteoblastic cells,but not with osteoclasts.MMP-2 immunostaining was increasing by OA severity in the osteochondral unit.Similarly,MMP-2 was expressed on the microcracks’wall mainly in Group III.Conclusion:Our data displayed that bone cracks during primary OA stages,represent an early adaptative mechanism aiming to maintain cartilage integrity.Accumulation of bone defects and concomitant increase of apoptotic osteocytes activated an abnormal remodeling due to osteoblastic activity,in which MMP-2 played a pivotal role,leading to subchondral sclerosis promoting further osteochondral deformities.
基金Project supported by the National Natural Science Foundation of China (No.10272024).
文摘Based on the Schapery three-dimensional viscoelastic constitutive relationship with growing damage, a damage model with transverse matrix cracks for the unidirectional ?bre rein- forced viscoelastic composite plates is developed. By using Karman theory, the nonlinear dynamic governing equations of the viscoelastic composite plates under transverse periodic loading are es- tablished. By applying the ?nite di?erence method in spatial domain and the Newton-Newmark method in time domain, and using the iterative procedure, the integral-partial di?erential gov- erning equations are solved. Some examples are given and the results are compared with available data.
