Boulder-induced form roughness and skin shear stresses in a gravel-bed stream

Boulder spacing in mountain rivers and near-wake flow zones within the boulder array is very useful for fish habitat and growth of aquatic organisms.The present study aims to investigate how the boulder array and spacing influence the near-bed flow structures in a gravel-bed stream.Boulders are staggered over a gravel-bed stream with three different inter-boulder spacing namely(a)large(b)medium and(c)small spacing.An acoustic Doppler velocimeter was used for flow measurements in a rectangular channel and the results were compared with those acquired from numerical simulation.The time-averaged velocity profiles at the near-wake flow zones of boulders experience maximum flow retardation which is an outcome of the boulder-induced form roughness.The ratio of velocity differences associated to form and skin roughness and its positive magnitude reveals the dominance of form roughness closest to the boulders.Form roughness computed is 1.75 to 2 times higher than the skin roughness at the near-wake flow region.In particular,the collective immobile boulders placed at different inter-boulder spacings developed high and low bed shear stresses closest to the boulders.The low bed shear stresses characterised by a secondary peak developed at the trough location of the boulders is attributed to the skin shear stress.Further,the spatial averaging of time-averaged flow quantities gives additional impetus to present an improved illustration of fluid shear stresses.The formation of form-induced shear stress is estimated to be 17%to 23%of doubleaveraged Reynolds shear stress and partially compensates for the damping of time-averaged Reynolds shear stress in the interfacial sub-layer.The quadrant analysis of spatial velocity fluctuations depicts that the form-induced shear stresses are dominant in the interfacial sub-layer and have no significance above the gravel-bed surface.

Exogenous Nitric Oxide Involved in Subcellular Distribution and Chemical Forms of Cu^(2+) Under Copper Stress in Tomato Seedlings

Nitric oxide（NO）,a bioactive signaling molecule,serves as an antioxidant and anti-stress agent under abiotic stress.A hydroponics experiment was conducted to investigate the effects of sodium nitroprusside（SNP）,a NO donor,on tomato seedlings exposed to 50 μmol L-1CuCl 2.The results show that copper is primarily stored in the soluble cell sap fraction in the roots,especially after treatment with Cu＋SNP treatment,which accounted for 66.2% of the total copper content.The copper concentration gradually decreased from the roots to the leaves.In the leaves,exogenous NO induces the storage of excess copper in the cell walls.Copper stress decreases the proportion of copper integrated with pectates and proteins,but exogenous NO remarkably reverses this trend.The alleviating effect of NO is blocked by hemoglobin.Thus,exogenous NO is likely involved in the regulation of the subcellular copper concentrations and its chemical forms under copper stress.Although exogenous NO inhibited the absorption and transport of excess copper to some extent,the copper accumulation in tomato seedlings significantly increased under copper stress.The use of exogenous NO to enhance copper tolerance in some plants is a promising method for copper remediation.

Research on flow stress of spray formed 70Si30Al alloy under hot compression deformation

The flow stress of spray formed 70Si30Al alloy was studied by hot compression on a Gleeble- 1500 test machine. The experimental results indicated that the flow stress depends on the strain rate and the deformation temperature. The flow stress increases with an increase in strain rate at a given deformation temperature. The flow stress decreases with the deformation temperature increasing at a given strain rate. The relational expression among the flow stress, the swain rate, and the deformation temperature satisfies the Arrhenius equation. The deformation activation energy of 70Si30Al alloy during hot deformation is 866.27 kJ/mol from the Arrhenius equation.

Determination and application of stress-based forming limit diagram in aluminum tube hydroforming

The stress-based forming limit diagram(FLSD) established with limit stress is independent of the strain paths. Compared with traditional strain-based forming limit diagram(FLD),it is more convenient and practical to use as the criterion of forming limit under complex strain paths. The forming limit of 3A21 aluminum alloy sheet was tested and its forming limit diagram(FLD) was determined. Then the FLSD of 3A21 was constituted by transformation formulas between limit strain and limit stress. This FLSD was used in conjunction with LS-DYNA finite element simulations to predict the onset of fracture and limit forming pressure in tube hydroforming. The results indicate that the fracture often occurs in the transition region between corner and straight side of the tube,and the limit forming pressure is 46.4 MPa. The simulation result agrees with the experimental result,and the FLSD is able to predict the forming limit of tube hydroforming with remarkable accuracy.

Analysis of the Stress Distribution Pattern of Anatomic and Non-Anatomic Tooth Forms on Maxillary and Mandibular Edentulous Ridges—A Photoelastic Study

Aim: To compare the type of stress distribution pattern occurring with anatomic and non-anatomic tooth forms beneath a complete denture in both maxillary and mandibular arch. Methodology: A photoelastic model of the edentulous maxillary and mandibular ridge was prepared meticulously to simulate the human mandible and maxilla. Two sets of acrylic teeth with anatomic and non-anatomic occlusal forms were used to fabricate upper and lower dentures. A vertical static load of 100 N was applied through the mandibular model to the maxillary model. After load application on the dentures the photoelastic model as well as the upper and lower complete dentures were sectioned in the midline. The sectioned photoelastic model was viewed through a polariscope to observe the fringe pattern indicating varying amounts of stress distribution. In this study, a two-dimensional photoelastic stress analysis technique was utilized. Results: Force per unit area was observed more in anatomic teeth than the non-anatomic counterpart. Hence anatomic tooth forms may increase the possibility of bone resorption rate over a period of time. However, in non-anatomic lower teeth, a decrease in value was observed from posterior to anterior region. Conclusion: Stress of greater magnitude was observed with cuspal teeth whereas non-anatomic (0°) showed slightly less magnitude of stress. Depending upon the clinical situation the clinician needs to choose the type of occlusal tooth forms for edentulous patients.

Zinc chemical forms and organic acid exudation in non-heading Chinese cabbages under zinc stress

As an essential element, zinc also is a heavy metal. Non-heading Chinese cabbage showed obvious tolerance to Zn stress in former research. To further understand the mechanisms involved in Zn adaptability and detoxification, two genotypes Suzhouqing and Aijiaohuang were selected to investigate the chemical forms of Zn and root exudation. Zinc stress obvious strained the plant growth, and Aijiaohuang was more injured than Suhouqing under Zn stress. Under normal Zn levels, the chemical forms of Zn were diverse in three organs between genotypes. Results showed extractions of 2% HAc, 80% ethanol and 1 M NaCl were separately dominant in roots, petioles and leaves. However, under Zn stress (13 mg·L–1 and 52 mg·L–1) most of the Zn was extracted by 1M NaCl, and the subdominant amount of Zn was extracted by 80% ethanol. In the control only four types of organic acid could be detected. While under Zn stress, oxalic acid, tartaric acid, malic acid, lactic acid, acetic acid, citric acid and amber acid were all detected, so it could be speculated Zn detoxification with organic ligands or integrated with pectates and proteins in cells might be responsible for the adaptation of Zn stress in Chinese cabbage.

Comprehensive Analysis of Stress-Strain in Typical Metal Forming Processes

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Experimental and Numerical Analysis of Forming Limit Diagram (FLD) and Forming Limit Stress Diagram (FLSD)

In this work, forming limit diagram for aluminum alloy 3105 is performed experimentally and forming limit based on stress (FLSD) calculated from strains that resulted from experimental procedure. In addition, numerical prediction by ductile fracture criteria using simulation is considered and it is shown that they are well suited with the experimental results. The strain paths from finite element simulations are found fairly acceptable to represent both sides of the FLD.

Closed form solution of stress intensity factors for cracks emanating from surface semi-spherical cavity in finite body with energy release rate method

In this paper, a new semi-analytical and semi-engineering method of the closed form solution of stress intensity factors （SIFs） of cracks emanating from a surface semi-spherical cavity in a finite body is derived using the energy release rate theory. A mode of crack opening displacements of a normal slice is established, and the normal slice relevant functions are introduced. The proposed method is both effective and accurate for the problem of three-dimensional cracks emanating from a surface cavity. A series of useful results of SIFs are obtained.

NaCl胁迫下pH值和氮素形态对狗牙根生长及钠钾离子调控的影响

以狗牙根〔Cynodon dactylon(Linn.)Pers.〕品种‘阳江’(‘Yangjiang’)为实验材料、2-(N-吗啡啉)乙磺酸(MES)为pH缓冲剂,采用水培法研究NaCl胁迫下pH值和氮素形态(铵态氮和硝态氮)对狗牙根生长和钠钾离子调控的影响。结果表明:处理1周后,若不添加MES,铵态氮处理组培养液的pH值下降到pH 4.0左右,而硝态氮处理组培养液的pH值则上升到pH 9.0左右;添加20 mmol·L^(-1)MES的铵态氮处理组培养液的pH值也下降到pH 4.0左右,而硝态氮处理组培养液的pH值则接近pH 7.0。与0 mmol·L^(-1)NaCl处理组相比,300 mmol·L^(-1)NaCl处理组狗牙根的枝条长度和枝条干质量显著(P<0.05)下降,而根长度、根干质量、根和叶中Na^(+)含量和Na^(+)/K^(+)比、钠钾选择性转运系数、叶Na^(+)和K^(+)的分泌量及分泌物Na^(+)/K^(+)比总体上显著升高。300 mmol·L^(-1)NaCl胁迫下,硝态氮处理组狗牙根的枝条长度、根长度、根干质量、根冠比、根中Na^(+)含量总体上显著高于铵态氮处理组,而叶中Na^(+)和K^(+)含量及Na^(+)/K^(+)比、叶Na^(+)和K^(+)的分泌量明显低于铵态氮处理组。300 mmol·L^(-1)NaCl胁迫下,20 mmol·L^(-1)MES处理组狗牙根的多数生长指标显著高于0 mmol·L^(-1)MES处理组,而根和叶中Na^(+)和K^(+)含量及Na^(+)/K^(+)比、钠钾选择性转运系数在0和20 mmol·L^(-1)MES处理组间的差异不显著。三因素方差分析结果表明:3个因子的单一和交互作用对多数生长指标、叶中Na^(+)含量和泌盐量相关指标的影响具有统计学意义。研究结果显示:硝态氮能缓解NaCl胁迫对狗牙根的伤害,而用MES缓冲根际酸碱环境对NaCl胁迫下狗牙根生长有一定的促进作用,因此,施用硝态氮或添加MES均有利于提高狗牙根的抗盐性。

Effect of Retrogression and Reaging on Stress Corrosion Cracking of Spray Formed Al Alloy

Stress corrosion cracking (SCC) resistance of a spray formed Al-Zn-Mg-Cu alloy underwent retrogression and reaging (RRA) was studied by slow strain rate tests in dry air and 3.5 wt% NaCl solution. The results showed that after RRA treatment, interrupted η phases at grain boundaries and slightly wide precipitate free zones could decrease SCC susceptibility of the alloy. Lots of reticular dislocations appeared in deformation process could prevent hydrogen induced cracking, and then SCC. Abundance transgranular dispersive η' phases separated out again promoted tensile strength to 759.4 MPa. The fracture ways of the specimens were dimple fracture in dry air and sub-cleavage fracture in 3.5% NaCl solution.

成形磨齿齿面残余应力测量与预测方法研究

成形磨齿通常是齿轮切削加工的最后一道工序,磨后齿面残余应力对齿轮的服役性能与寿命影响显著。传统基于X射线衍射方法的齿轮残余应力测量方法并未考虑齿面特殊几何特性,导致测量精度低。此外,磨齿工艺参数与磨前齿面状态对磨齿残余应力的影响规律不清。因此,本文提出一种考虑齿轮特殊几何特性的齿面残余应力测量方法与辅助工装夹具,并建立基于磨齿工艺参数的齿面残余应力预测模型,最后探讨了磨前齿面残余应力对磨后残余应力的影响。本文为非平面残余应力的测量提出了新方法,所提的齿面残余应力预测方法能够指导实际生产。

“Giant pressure shadow” structure and ore-finding method of tectonic stress field in the Tongchang Cu-Au polymetallic orefield,Shaanxi,China:Ⅱ.Dynamics of tectonic ore-forming processes and prognosis of concealed ores

The Tongchang orefield is located in the central part of the Mianxian-Lueyang-Yangpingguan area that is celebrated as a 'gold triangle' area,at the juncture of the latitudinal tectonic zone of South Qinling,the Longmenshan Cathysian tectonic zone and the Sichuan-Yunnan longitudinal tectonic zone,where there are distributed Cu-Au polymetallic ore deposits(occurrences) including the Tongchang,Chenjiaba,Qinjiabian,Hongtushi,Yinshangou and Xiakouyi ore deposits(mineralization).Based on the "giant pressure shadow" structure put forward and demonstrated by numerical modeling of the tectonic stress field and the static photoelasticity experiments on the basis of tectonic ore-controlling laws in the orefield,tectonic metallogenesis driven by orefield tectonic stress has been discussed in terms of its geological setting,orefield geomechanics,and tectonic stress field.It is thought that the dynamic evolution model of the tectonic stress field controls the whole process of formation of the polymetallic ore deposits(mineralization) in the orefield,as well as the deformation field.As a result,it controls the emplacement of rockbodies and the transformation of ore-source bodies,and provides both the channel-ways for ore-forming fluids and ore-hosting space.Furthermore,it controls the migration potential field of fluids and,thereafter,its flow direction,rate and volume;the tectonic stress field also controls the energy field and hence controls the position of occurrence of ore deposits and their scale.The method of tectonic stress field has been applied to ore prognosis in the orefield.The rules of magmatic emplacement and metallogenic fluid migrating and concentrating under the control of the structural stress field were expounded,hence providing the theoretical basis for the prognosis of concealed ores.In addition,a number of important target areas have been defined.

残余应力对冷弯厚壁压弯构件抗震性能影响分析

冷弯型钢构件因其成本效益和多功能性而在结构工程中得到广泛应用。然而,由制造过程中产生的残余应力可能会显著影响其在地震荷载下的行为。因此,通过数值分析研究残余应力对冷弯厚壁压弯构件抗震性能的影响。研究结果表明,残余应力会影响构件的峰值强度、延性和能量耗散能力,从而影响其整体抗震响应。研究结果为改进冷弯型钢在地震作用下的设计规范和指南提供了参考。

基于数值模拟的选区激光熔化成形残余应力演化研究

目的以选区激光熔化成形(SLM)试件的残余应力为研究对象,研究残余应力对成形质量的影响,为SLM成形的产业化应用提供理论依据。方法以316L不锈钢粉末为原材料,利用AltairInspire软件的Print3D模块分析SLM成形中支撑、成形角对残余应力的影响及残余应力的演化规律,并进行实验验证。结果SLM成形最大残余应力出现在零件与基板结合面,添加支撑可减小残余应力。零件不同位置残余应力的演化规律不同。顶部残余应力呈先增大后减小的趋势,底部两侧残余应力呈缓慢上升的趋势;底部中间残余应力的演化规律较为复杂:起先残余应力随温度的降低而增大,当温度降到最低点时达最大值;随后在热累积作用下,残余应力先减小后增大,当达到去应力退火温度时,残余应力又减小并在一定范围内波动。残余应力随着成形角的增大呈先增大后减小再增大的趋势,当成形角为60°时,残余应力较小。结论在SLM成形时,在零件底部添加支撑可将最大残余应力位置转移到支撑上,从而减小成形件内部的残余应力,提高成形质量。成形零件不同位置残余应力的演化规律不同,成形角对残余应力的影响也不同,成形时应根据零件工况制定合适的打印策略。

基于有限元模型的不同入射角弹丸冲击残余应力模拟

建立弹丸冲击有限元模型研究弹丸入射角度对板材残余应力场和板材变形的影响,并对模拟结果进行拟合。利用ABAQUS软件建立了单弹丸冲击板材的三维对称模型,模拟了弹丸在不同速度下以90°入射角冲击板材所产生的残余应力场,以及在100 m/s的冲击速度下以3个不同入射角度冲击板材产生的残余应力场。结果显示:最大残余应力位置的深度与弹丸撞击速度呈正相关;弹丸速度一定的情况下,随着入射角的增大最大残余应力也不断增加,并且在入射角度为90°时达到最大。

某空间螺旋塔斜拉桥合理结构体系研究

某斜拉桥方案采用反对称螺旋形桥塔,空间四索面,造型较为新颖。由于桥梁结构较为复杂,为选用适宜的结构体系,基于桥梁结构的受力形态,在综合比较了漂浮体系、半漂浮体系、半固结体系的优劣后,确定半漂浮体系为最佳方案,并进一步讨论了地震作用下该桥合理的约束体系。这一流程对解决类似复杂结构桥梁合理结构体系的问题有一定参考价值。

铌微合金化和淬火速率对热成形钢组织与力学性能的影响

基于传统22MnB5钢设计了一种新型含Nb热成形钢,研究在不同淬火速率下Nb对热成形钢显微组织与力学性能的影响。新型Nb微合金化热成形钢与广泛应用的22MnB5商业钢种均经900℃保温3 min的固溶处理后分别水淬和油淬至室温,检测两种钢在两个淬火条件下的力学性能,并通过扫描电镜、背散射电子衍射仪、X射线衍射仪和透射电镜等分析合金组织。结果表明:油淬时Nb微合金化热成形钢与22MnB5钢能发生较明显的自回火,但前者的屈服强度高于后者约130 MPa,且伸长率也略有改善,对强化机制的定量计算表明这是由于含Nb钢晶粒细化形成的细晶强化以及位错强化和沉淀强化的共同作用;而在水淬条件下,两种钢的屈服强度与伸长率均相似,推测是由于冷却速率高抑制了自回火,使得马氏体相变产生的残余应力成为影响屈服强度的主导因素,而当水淬样品在170℃回火减轻内应力后,此时含Nb钢屈服强度再次高于22MnB5钢。

基于“自气成积”探讨气郁在肿瘤发病中的始动作用

肿瘤是以正气亏虚为根本因素的本虚标实之病,已经成为了中医界认识肿瘤的主流观点,在查阅文献和结合临床经验后,作者认为气郁才是肿瘤发病的始动因素。该文以朱丹溪“自气成积,自积成痰,痰挟瘀血,遂成窠囊”理论为立足点,从中医和现代医学两方面阐述了肿瘤发病的首要及必要条件是气郁这一观点,并以三焦为划分依据,分别论述了气郁在上、中、下三焦肿瘤中引发痰饮、瘀血,并最终演化为肿瘤的过程,以期在临床上为肿瘤的治疗及预防肿瘤的复发提供新的思路。

海底管线管全流程成形过程中残余应力演化

为了研究海底管线管全流程成形过程中残余应力的演化,考虑成形工序中应力的继承,采用ABAQUS软件开展了管线管JCO、焊接和多向缩径校准全流程成形过程有限元模拟。首先进行了JCO成形过程模拟,然后基于高斯面热源子程序对JCO成形后的管线管进行了焊接过程模拟,最后对焊接后的管线管进行了多向缩径校准成形过程模拟,探讨了多向缩径校准成形工艺参数对管线管残余应力的影响。模拟结果表明:多向缩径校准可以实现管线管几何形状和残余应力的双重调控;相对传统的2瓣模具缩径,采用4瓣模具和8瓣模具校准后的管线管残余应力分布更加均匀;摩擦因数对残余应力的影响不明显;采用4瓣模具、 0.04摩擦因数和1.6%缩径率校准后的管线管椭圆度可达0.05%,除焊接热影响区,管线管内外表面的最大周向残余应力分别为28和-45 MPa,截面Mises残余应力均值为(120±20) MPa,焊缝区和非焊缝区的Mises残余应力均值分别降低了79%和30%。