A new linear yield criterion expressed by the geometric midline of error triangle between Tresca and Twin shear stress yield loci on the π-plane in Haigh-Westergaard space was introduced. The criterion was written in...A new linear yield criterion expressed by the geometric midline of error triangle between Tresca and Twin shear stress yield loci on the π-plane in Haigh-Westergaard space was introduced. The criterion was written in terms of the values of principal stress deviator and called GM yield criterion for short. Together with a Cartesian coordinate velocity field instead of the Avitzur＇s, the GM criterion was used to obtain an analytical solution for strip drawing. With a working example of the strip drawing through wedge-shaped die, the results of relative drawing stress calculated by the GM criterion were compared with those calculated by Mises＇ criterion from Avitzur formula. It indicated that the calculated results according to analytical solution were in good agreement with the numerical solution obtained from Avitzur formula.展开更多
Soil erosion induced by inappropriate tillage remains a serious problem on many agricultural fields in the humid tropics. Studies were conducted between 2004 and 2006, on an Alfisol in Ogbomoso in the Southern Guinea ...Soil erosion induced by inappropriate tillage remains a serious problem on many agricultural fields in the humid tropics. Studies were conducted between 2004 and 2006, on an Alfisol in Ogbomoso in the Southern Guinea Savanna of Nigeria to evaluate the effectiveness of Vetiver Grass(Vetiveria nigritana) Strips(VGS) under different tillage systems. The experiment was split-plot laid out in a randomized complete block design with two replications on 6% slope with 18 runoff plots. Main plot treatments were tillage systems; Manual Clearing(MC), Ploughing(P) and Ploughing plus Harrowing(PH). Subplot treatments were VGS spaced at intervals of 5 m(eight strips) and 10 m(four strips) with the control(no-vetiver). Runoffs and soil losses were collected after each major storm. Chemical analyses of eroded sediments and runoff were determined. Data were analyzed using ANOVA at p<0.05. The results showed that tillage had no significant reduction in runoffs and soil losses, but they were reduced with MC compared with P and PH. Mean total runoff on 5 and 10 m VGS plots were significantly(p<0.05) lower than that of the control by 74.4% and 45.0%, respectively. Corresponding soils loss on 5 and 10 m VGS plots were 27.1% and 53.5%, respectively. Mean NO3-N levels in runoff water were lower under PH plots than those under MC plots by 79.0% and 66.5%, respectively in 2004 and 2006 growing seasons. VGS spaced at 5 m significantly(p<0.05) reduced NO3-N loss than the control by 108.8% in 2004. Nutrients loads of eroded sediments were consistently higher for the control(no-vetiver) plots and least for 5 m VGS plot. Carbon, nitrogen and phosphorus contents of eroded sediments were 90%-92.4%, 83%-83.6% and 97%-97.8%, respectively, and were lower on 5 m than other treatments. Maize grain yield was significantly(p<0.05) affected by both tillage and VGS spacing only in 2005 growing season. P plot produced higher grain yield than MC and PH by 79.9% and 99.1%, respectively. Also, grain yield on VGS plot was significantly(p<0.05) higher on 5 and 10 m VGS plots than the control by 82.2% and 85.4%, respectively. The significant beneficial effect of PH in producing higher yields was dwarfed by the potential danger of soil erosion in the absence of a soil erosion control measure. The results showed that a balance needed to be struck between mechanical clearance and protective measure against soil erosion.展开更多
为探明玉米秸秆带状覆盖对黄土高原冬小麦农田土壤水热和产量的影响规律,寻求适宜该地区冬小麦种植的秸秆带状覆盖方式,以冬小麦“康庄974”作为供试材料,设置秸秆带状覆盖种植3行(S3,覆盖度58.8%)、4行(S4,覆盖度50.0%)、5行(S5,覆盖度...为探明玉米秸秆带状覆盖对黄土高原冬小麦农田土壤水热和产量的影响规律,寻求适宜该地区冬小麦种植的秸秆带状覆盖方式,以冬小麦“康庄974”作为供试材料,设置秸秆带状覆盖种植3行(S3,覆盖度58.8%)、4行(S4,覆盖度50.0%)、5行(S5,覆盖度41.7%)3种秸秆覆盖方式,以地膜覆盖(PM,覆盖度100.0%)和露地无覆盖(CK)作为对照。结果表明:与CK相比,秸秆带状覆盖处理显著增加全生育期0~200 cm全土层土壤含水量14.1%,增幅S3>S4>S5;不同时期增幅以成熟期S3最大,为32.5%,播种期S5最小,为1.8%;不同土层0~20 cm S3增幅最大,为32.8%,180~200 cm S5最小,为4.5%。0~200 cm土层贮水量在成熟期增幅最大,S3、S4和S5分别增加31.1%、21.5%和9.2%;不同土层增幅以180 cm处S4最大,为10.3%,S5最小,为1.1%。PM全生育期0~200 cm土层土壤含水量和贮水量分别提高20.3%和19.7%。秸秆带状覆盖显著降低冬小麦全生育期0~25 cm土壤平均温度1.67~3.14℃,土壤总有效积温12.6%~30.8%,S3降温幅度最大,S4次之,S5最小,PM在冬小麦全生育期表现为增温效应。S5增加产量、水分利用效率(WUE)、穗粒数和单位面积穗数分别为9.8%、13.8%、1.8%和22.0%,降低千粒重11.7%;PM提高产量和WUE分别为26.5%和47.9%。因此,综合考虑产量形成、WUE、保墒效果等,S5是黄土高原旱作冬小麦适宜种植覆盖方式。展开更多
基金Item Sponsored by National Natural Science Foundation of China(50474015)
文摘A new linear yield criterion expressed by the geometric midline of error triangle between Tresca and Twin shear stress yield loci on the π-plane in Haigh-Westergaard space was introduced. The criterion was written in terms of the values of principal stress deviator and called GM yield criterion for short. Together with a Cartesian coordinate velocity field instead of the Avitzur＇s, the GM criterion was used to obtain an analytical solution for strip drawing. With a working example of the strip drawing through wedge-shaped die, the results of relative drawing stress calculated by the GM criterion were compared with those calculated by Mises＇ criterion from Avitzur formula. It indicated that the calculated results according to analytical solution were in good agreement with the numerical solution obtained from Avitzur formula.
文摘Soil erosion induced by inappropriate tillage remains a serious problem on many agricultural fields in the humid tropics. Studies were conducted between 2004 and 2006, on an Alfisol in Ogbomoso in the Southern Guinea Savanna of Nigeria to evaluate the effectiveness of Vetiver Grass(Vetiveria nigritana) Strips(VGS) under different tillage systems. The experiment was split-plot laid out in a randomized complete block design with two replications on 6% slope with 18 runoff plots. Main plot treatments were tillage systems; Manual Clearing(MC), Ploughing(P) and Ploughing plus Harrowing(PH). Subplot treatments were VGS spaced at intervals of 5 m(eight strips) and 10 m(four strips) with the control(no-vetiver). Runoffs and soil losses were collected after each major storm. Chemical analyses of eroded sediments and runoff were determined. Data were analyzed using ANOVA at p<0.05. The results showed that tillage had no significant reduction in runoffs and soil losses, but they were reduced with MC compared with P and PH. Mean total runoff on 5 and 10 m VGS plots were significantly(p<0.05) lower than that of the control by 74.4% and 45.0%, respectively. Corresponding soils loss on 5 and 10 m VGS plots were 27.1% and 53.5%, respectively. Mean NO3-N levels in runoff water were lower under PH plots than those under MC plots by 79.0% and 66.5%, respectively in 2004 and 2006 growing seasons. VGS spaced at 5 m significantly(p<0.05) reduced NO3-N loss than the control by 108.8% in 2004. Nutrients loads of eroded sediments were consistently higher for the control(no-vetiver) plots and least for 5 m VGS plot. Carbon, nitrogen and phosphorus contents of eroded sediments were 90%-92.4%, 83%-83.6% and 97%-97.8%, respectively, and were lower on 5 m than other treatments. Maize grain yield was significantly(p<0.05) affected by both tillage and VGS spacing only in 2005 growing season. P plot produced higher grain yield than MC and PH by 79.9% and 99.1%, respectively. Also, grain yield on VGS plot was significantly(p<0.05) higher on 5 and 10 m VGS plots than the control by 82.2% and 85.4%, respectively. The significant beneficial effect of PH in producing higher yields was dwarfed by the potential danger of soil erosion in the absence of a soil erosion control measure. The results showed that a balance needed to be struck between mechanical clearance and protective measure against soil erosion.
文摘为探明玉米秸秆带状覆盖对黄土高原冬小麦农田土壤水热和产量的影响规律,寻求适宜该地区冬小麦种植的秸秆带状覆盖方式,以冬小麦“康庄974”作为供试材料,设置秸秆带状覆盖种植3行(S3,覆盖度58.8%)、4行(S4,覆盖度50.0%)、5行(S5,覆盖度41.7%)3种秸秆覆盖方式,以地膜覆盖(PM,覆盖度100.0%)和露地无覆盖(CK)作为对照。结果表明:与CK相比,秸秆带状覆盖处理显著增加全生育期0~200 cm全土层土壤含水量14.1%,增幅S3>S4>S5;不同时期增幅以成熟期S3最大,为32.5%,播种期S5最小,为1.8%;不同土层0~20 cm S3增幅最大,为32.8%,180~200 cm S5最小,为4.5%。0~200 cm土层贮水量在成熟期增幅最大,S3、S4和S5分别增加31.1%、21.5%和9.2%;不同土层增幅以180 cm处S4最大,为10.3%,S5最小,为1.1%。PM全生育期0~200 cm土层土壤含水量和贮水量分别提高20.3%和19.7%。秸秆带状覆盖显著降低冬小麦全生育期0~25 cm土壤平均温度1.67~3.14℃,土壤总有效积温12.6%~30.8%,S3降温幅度最大,S4次之,S5最小,PM在冬小麦全生育期表现为增温效应。S5增加产量、水分利用效率(WUE)、穗粒数和单位面积穗数分别为9.8%、13.8%、1.8%和22.0%,降低千粒重11.7%;PM提高产量和WUE分别为26.5%和47.9%。因此,综合考虑产量形成、WUE、保墒效果等,S5是黄土高原旱作冬小麦适宜种植覆盖方式。
