Residual coal pillars play an important role in mining the adjacent coal seam safely,managing the gobs and maintaining the stability of abandoned coal mines.The height to diameter ratio(H/D)affects the stability of re...Residual coal pillars play an important role in mining the adjacent coal seam safely,managing the gobs and maintaining the stability of abandoned coal mines.The height to diameter ratio(H/D)affects the stability of residual coal pillars.In this study,uniaxial compressive tests of coal specimens with five H/D(2.0,1.5,1.0,0.8 and 0.6)were performed,and the stress,strain and acoustic emission(AE)were monitored.Results show that the uniaxial compressive strength(UCS)and peak strain increase with H/D decreasing.An empirical equation is proposed to calculate the UCS based on the H/D.The AE activities during coal failure process can be separated into four periods.The span of quiet period and rapid decline period shorten with H/D decreasing.The smaller the H/D is,the more complicated the failure characteristics of coal will be.The failure form of coal with H/D of 2.0,1.5,and 1.0 is primarily shear failure,while splitting failure along the axial direction is the mainly mode when H/D is 0.8 or 0.6.The initiation,expansion,aggregation and connection of micro-cracks can be reflected by the real-time spatial evolution of AE event points.展开更多
In this paper, the factors of affecting surface roughness and profiles accuracy of the machined larege depth diamter ratio aspheric surfaces in ultra-precision grinding process are analyzed theoretically. An ultra-pre...In this paper, the factors of affecting surface roughness and profiles accuracy of the machined larege depth diamter ratio aspheric surfaces in ultra-precision grinding process are analyzed theoretically. An ultra-precision aspheric grinding system is then designed and manufactured. Aerostatic form is adopted to build the spindle of the workpiece, transverse guideway, longitudinal guideway and the spindle of the grinder in this system. The following specification is achieved, such as the turning accuracy of the spindle of the workpiece is 0.05 μm, radial rigidity of the spindle is GE 220N/μm, axial rigidity is GE 160 N/μm, radial rigidity of the guideway is GE 200N/μm, the highest rotational speed of the grinder is 80 000 rev/min and its turning accuracy is 0.1 μm, the resolution of linear displacement of the transverse and longitudinal guideway is 4.9 nm. Adjusting range of this adjusting mechanism is 2 mm in the Y direction, the adjusting accuracy of the precise adjusting mechanism is 0.1 μm. Micro displacement measuring system of this ultra-precision aspheric grinding adopts two-backfeed strategy, and angle displacement back-feed is realized by photoelectric encoder, it’s resolution is 655 360 pulse/rev. after 4 frequency multiplication, it’s angle displacement resolution is achieved 2 621 440 pulse/rev. Straight-line displacement is monitored by single frequency laser interferometer (DLSTAX LTM-20B, made in Japan). This CNC system adopts inimitable bi-arc step length flex CN interpolation algorithm, it’s CN system resolution is 5 nm.So this aspheric grinding system ensures profile accuracy of the machined part. The resolution of this interferometer is 5 nm. Finally, lots of ultra-precision grinding experiments are carried out on this grinding system. Some optical aspheric parts, with profiles accuracy of 0.3 μm, surface roughness less than 0.01 μm, are obtained.展开更多
为探究高强高延性混凝土(high-strength and high-ductility concrete,HSHDC)的单轴拉伸性能,设计了12组试件进行了薄板单轴拉伸试验,研究了水胶比(0.14、0.16和0.18)、PE纤维体积率(1.5%、2.0%、2.5%)、纤维种类(PE和PVA)及PE纤维直径...为探究高强高延性混凝土(high-strength and high-ductility concrete,HSHDC)的单轴拉伸性能,设计了12组试件进行了薄板单轴拉伸试验,研究了水胶比(0.14、0.16和0.18)、PE纤维体积率(1.5%、2.0%、2.5%)、纤维种类(PE和PVA)及PE纤维直径(22μm、35μm和40μm)对薄板单轴拉伸行为的影响.并利用峰值前应变能密度分析了上述因素对HSHDC能量吸收能力的影响.结果表明:随着水胶比的增大,HSHDC的抗拉强度呈先升高后降低的趋势,而极限拉伸应变呈增大趋势;HSHDC的抗拉强度随着纤维体积率的增加而增大,而HSHDC的极限拉伸应变在水胶比较小时随着纤维体积率的增加而减小,在水胶比较大时则相反;与0.14水胶比相比,在0.16和0.18水胶比下,HSHDC的峰值前应变能密度较大;与掺入PVA纤维的HSHDC试件相比,掺入PE纤维的HSHDC的拉伸性能和能量吸收能力更优,且小直径的PE纤维更适合制备HSHDC.最后,利用扫描电子显微镜(scanning electron microscope,SEM)研究了HSHDC致密的基体和粗糙的PE纤维表面.展开更多
基金Projects(51974192,52004172)supported by the National Natural Science Foundation of ChinaProject(51925402)supported by the Distinguished Youth Funds of National Natural Science Foundation of ChinaProject(U1710258)supported by the Joint Funds of National Natural Science Foundation of China and Shanxi Province,China。
文摘Residual coal pillars play an important role in mining the adjacent coal seam safely,managing the gobs and maintaining the stability of abandoned coal mines.The height to diameter ratio(H/D)affects the stability of residual coal pillars.In this study,uniaxial compressive tests of coal specimens with five H/D(2.0,1.5,1.0,0.8 and 0.6)were performed,and the stress,strain and acoustic emission(AE)were monitored.Results show that the uniaxial compressive strength(UCS)and peak strain increase with H/D decreasing.An empirical equation is proposed to calculate the UCS based on the H/D.The AE activities during coal failure process can be separated into four periods.The span of quiet period and rapid decline period shorten with H/D decreasing.The smaller the H/D is,the more complicated the failure characteristics of coal will be.The failure form of coal with H/D of 2.0,1.5,and 1.0 is primarily shear failure,while splitting failure along the axial direction is the mainly mode when H/D is 0.8 or 0.6.The initiation,expansion,aggregation and connection of micro-cracks can be reflected by the real-time spatial evolution of AE event points.
文摘In this paper, the factors of affecting surface roughness and profiles accuracy of the machined larege depth diamter ratio aspheric surfaces in ultra-precision grinding process are analyzed theoretically. An ultra-precision aspheric grinding system is then designed and manufactured. Aerostatic form is adopted to build the spindle of the workpiece, transverse guideway, longitudinal guideway and the spindle of the grinder in this system. The following specification is achieved, such as the turning accuracy of the spindle of the workpiece is 0.05 μm, radial rigidity of the spindle is GE 220N/μm, axial rigidity is GE 160 N/μm, radial rigidity of the guideway is GE 200N/μm, the highest rotational speed of the grinder is 80 000 rev/min and its turning accuracy is 0.1 μm, the resolution of linear displacement of the transverse and longitudinal guideway is 4.9 nm. Adjusting range of this adjusting mechanism is 2 mm in the Y direction, the adjusting accuracy of the precise adjusting mechanism is 0.1 μm. Micro displacement measuring system of this ultra-precision aspheric grinding adopts two-backfeed strategy, and angle displacement back-feed is realized by photoelectric encoder, it’s resolution is 655 360 pulse/rev. after 4 frequency multiplication, it’s angle displacement resolution is achieved 2 621 440 pulse/rev. Straight-line displacement is monitored by single frequency laser interferometer (DLSTAX LTM-20B, made in Japan). This CNC system adopts inimitable bi-arc step length flex CN interpolation algorithm, it’s CN system resolution is 5 nm.So this aspheric grinding system ensures profile accuracy of the machined part. The resolution of this interferometer is 5 nm. Finally, lots of ultra-precision grinding experiments are carried out on this grinding system. Some optical aspheric parts, with profiles accuracy of 0.3 μm, surface roughness less than 0.01 μm, are obtained.
文摘为探究高强高延性混凝土(high-strength and high-ductility concrete,HSHDC)的单轴拉伸性能,设计了12组试件进行了薄板单轴拉伸试验,研究了水胶比(0.14、0.16和0.18)、PE纤维体积率(1.5%、2.0%、2.5%)、纤维种类(PE和PVA)及PE纤维直径(22μm、35μm和40μm)对薄板单轴拉伸行为的影响.并利用峰值前应变能密度分析了上述因素对HSHDC能量吸收能力的影响.结果表明:随着水胶比的增大,HSHDC的抗拉强度呈先升高后降低的趋势,而极限拉伸应变呈增大趋势;HSHDC的抗拉强度随着纤维体积率的增加而增大,而HSHDC的极限拉伸应变在水胶比较小时随着纤维体积率的增加而减小,在水胶比较大时则相反;与0.14水胶比相比,在0.16和0.18水胶比下,HSHDC的峰值前应变能密度较大;与掺入PVA纤维的HSHDC试件相比,掺入PE纤维的HSHDC的拉伸性能和能量吸收能力更优,且小直径的PE纤维更适合制备HSHDC.最后,利用扫描电子显微镜(scanning electron microscope,SEM)研究了HSHDC致密的基体和粗糙的PE纤维表面.
