石墨烯及其复合材料纳米力学研究进展

石墨烯是一种由碳原子构成的二维晶体,是目前已知最薄但却有着极高强度的纳米材料。由于在强度、导热性、电子输运和光学上显示出不同寻常的特性,石墨烯迅速成为材料科学、物理、化学和力学等学科的研究热点。与此同时,石墨烯复合材料的研究也迅速兴起。本文综述了近年来石墨烯及其复合材料的力学特性的研究进展。根据力学行为的差异,我们主要阐述了石墨烯面内力学特性、离面力学特性、原子尺度修饰和石墨烯复合材料力学特性的研究进展：石墨烯的面内拉伸力学特性通过纳米压痕等技术得到了测量,其断裂行为在微纳尺度下不能完全使用连续介质力学模型进行解释,在多层石墨烯情况下会出现超润滑现象;石墨烯的可控离面位移对于改变其物理特性有重要的意义,石墨烯上的屈曲受到手性和尺度的影响,在高频器件中存在着非连续性的离面响应;适当的原子尺度修饰可以改善石墨烯的拉伸和扭转力学特性;石墨烯可以改善复合材料的力学特性,如提高强度、韧性等,其主要强化效应是通过与基体材料的离面、面内力学行为结合产生的。最后,本文对石墨烯及其复合材料的力学研究进行了总结和展望。     

坡面道路上地面形貌对人体步进摩擦的影响

人体行走经常发生滑摔事故,而坡面道路是人体行走的典型路况之一. 当前通过改变鞋底和地板的材料、鞋底和地板的表面形貌以提高步进摩擦系数的研究多是基于水平路面,对坡面道路上的人体行走,特别是地面形貌变化对坡面道路上的人体行走研究较少. 本文中以45钢作为材料,制备出表面波纹度相同而表面形貌不同的地板,利用步进摩擦试验平台改变行走路面的坡度研究了地面形貌变化对坡面道路上人体步进摩擦的影响. 结果表明:坡度对有效摩擦系数的影响高于表面形貌;随着坡度的增加,安全行走所需的必要摩擦系数增大,地面提供的有效摩擦系数减小,步频则先增大后减小;上坡启动时脚掌与路面的有效接触面积和下坡制动时脚跟的有效接触面积随坡度的增加而减小,导致有效摩擦系数降低,滑摔倾向增大. 人体行走姿态为了保持平衡而缩短步长以降低必要摩擦系数.      

复杂空间力系的简化

<正>题目在铅锤面内的平面机构由四杆AB,BC,CD,AD铰接而成,如图1所示.A处为固定铰,C处为滑动铰,并通过弹簧连接到竖直墙壁的E点,ACE共线且水平,AE长度为20,DB与铅垂线平行.螺纹方向相反的螺母D和B对称安装在有反向螺纹的螺杆BD两边,两种螺纹的螺距同为h,螺母D分别与AD和CD两杆在D处铰接,螺母B分别     

沟谷坡面风沙起动规律研究

根据坡面风沙受力情况,以滚动模式建立了坡面风沙起动的风速公式.计算分析了相同风向条件下不同坡度风沙的起动风速和相同坡度条件下不同风向风沙的起动风速,并与平面风沙起动计算风速和实测风速进行了比较.计算分析表明,在风向一定时,坡面风沙更容易起动,风沙起动速率随着坡度的变陡而变小.在坡度一定时,风沙起动风速受到风向的影响很大,起动风速随风力与水平面垂线夹角的增大而增大.与平面相比,当风力与水平面垂线夹角小于90°时,坡面风沙起动流速比平面为小.在当风力与水平面垂线夹角大于90°时,起动风速比平面为大.当坡面为平面时,风沙起动公式与平面拜格诺起动公式完全一致.     

应用杆束系统研究水泥砂浆节理面的压剪动特性

基于分离式Hopkinson压杆系统提出一套Hopkinson束杆装置,研究水泥砂浆节理面在压剪复合加载下的动态界面滑移特性。应用三根小杆作为接收杆,分别采用单杆和三杆两种入射方式,对含有节理面的水泥砂浆试件进行冲击加载。在节理面发生滑动时,由三根接收杆测试得到水泥砂浆试件不同位置滑移状态,进而得出节理面的整体滑移速度。此研究初步揭示了节理面在发生滑移时的局部滑移状态以及整体滑移状态,为节理面的动摩擦特性研究提供了较可靠的实验技术。     

高温分离式Hopkinson压杆技术及其应用

本文介绍了在分离式Hopkinson压杆装置上通过使用一种气动同步机构,实现对试样进行高温高应变率加载的技术。利用此技术仅对试样加高温度而保持入射杆和透射杆与试样脱离且处在较低温度,到预定温度时,借助气动同步机构使入射杆、透射杆与试样接触并同时实现对试样加载。利用波形抑制技术,仅对试样实现一次加载,入射杆中的后续二次加载波通过反作用质量块吸收。通过这些技术的结合,1)可以进行材料在高温高应变率下应力应变测试;2)可以测试材料在高应变率不同温度下的等温曲线;3)可以间接对材料的塑性功热转换系数进行测试;4)可以进行不同温度高应变率下的中断跳跃试验等。在文中给出了一些典型的试验曲线和结果,并对测试方法和结果进行了分析讨论。     

光滑曲面点接触的理想约束推导

分析力学在讲述形状光滑的两刚性面保持点接触时,如果无摩擦或者纯滚动,使用了两个接触点的可能微小位移在接触点法向分量相等,或者接触点的相对速度为零的条件推导出接触为理想约束。本文从两曲面保持点接触时满足共点和相切的几何条件出发,获得这一结论,前提条件更加明确。     

基于PVDF薄膜传感器的猪肝动态压缩力学性能测试

瞬态冲击载荷作用下肝脏的力学响应是损伤生物力学的重要研究内容.本文提出了一种可用于软组织动态压缩力学特性测试的改进SHPB(分离式霍普金森压杆,Split Hopkinson Pressure Bar)方法.该方法采用PVDF(聚偏氟乙烯,Polyvinylidene Fluor)压电薄膜传感器测量实验过程中试件两端面...     

材料力学实物演示教学培养创新能力的实践

通过课堂实物演示分析帮助学生掌握力学基本概念,基本规律,引导学生运用力学原理分析力学现象,解决力学问题,是在课堂教学环节培养学生创新能力的有效途径.通过钢卷尺演示分析开口薄壁杆件中心压杆失稳,侧立弯曲失稳及水平布置弯曲失稳现象.分析了钢卷尺水平布置弯曲时,卷尺拉出长度相同的条件下,截面弧形凸出一侧向上时失稳,凸出一侧向下时不失稳的现象.     

小问题

正问题:在铅锤平面内三根均质细杆以铰链相连,A端和B端另以铰链连接在固定水平直线AB上,如图1所示.已知各杆的重量与其长度成正比,AC=a,CD=DB=2a,AB=3a.假设铰链为理想约束,求平衡时α,β,γ之间的关系.(清华大学航天航空学院,钱学森力学班2014级,郝育昆供稿)     

伺服控制土石混合体压力渗透仪研究

土石混合体渗流特性的研究对于工程建设、土石混合体滑坡的防治有重要意义。总结了前人对于堆石料渗透试验仪的研究,其特点在于渗透筒的大尺寸、供水系统、供水压力的控制;透水板、制样方法、筒壁防渗对于试验的影响。周中等自制的常水头渗透仪,主要对土石混合体渗透系数及其影响因素进行研究。本文研制的伺服控制土石混合体压力渗透仪能够准确伺服控制供水压力和供水速度,提供不超过2MPa的高水压,调节试样渗流速度,操作方便,能够有针对性的研究土石混合体的渗流特性。     

Feasibility of operating air cushion vehicle in wetland paddy field

The problem of sinkage of machinery, poor trafficability and compaction of soil in wetland paddy fields has stimulated new interest to make an investigation of a new device for performing the field operations. Since an Air Cushion Vehicle (ACV) has the ability to travel unimpeded over a variety of difficult surfaces, whose load-bearing properties make it difficult for other vehicles to move, this may be a suitable device for overcoming the above problems. The air cushion concept has been considered for application to off-road transport ever since the early days of its development, but so far it has not been employed for wetland paddy field operations. Therefore, a study of this type was undertaken at the Asian Institute of Technology, Bangkok. This paper describes the feasibility of using an ACV in wetland paddy fields. It shows that there is a reduction of drag force of about 60%, which is quite significant.     

估计迹长概率分布函数的新方法及其应用

本文提出一种估计迹长概率分布函数的新方法。该方法的优点是不需要有节理在测窗上出露长度的数据, 有效的减少了野外工作量。     

常德-张家界高速公路某大桥桥基砂土液化评价

结合常德—张家界高速公路某大桥桥基工程,在DSD—160型电磁式振动三轴试验仪上,通过往返加荷三轴试验,对饱和砂土进行了液化试验研究,探讨了基于动三轴液化试验结果判断饱和砂土液化的方法。并尝试了这种室内研究—反应分析的液化可能性估计方法与地震剪应力时程相结合的综合判断方法。在该高速公路大桥桥基的饱和砂土液化评价中,采用这种综合判断方法,对大桥桥基砂土液化进行了判断。在判断场地是否液化后,对其液化危害程度进行了等级划分,给出了该工程场地在未来遭受到不同超越概率下的地震作用时发生液化的危害程度,得到了一些有工程实用意义的结果。     

不同类型水平弹簧组合刚度非线性吸振器的性能分析及稳定性研究

动力吸振器作为一种振动控制单元被广泛运用于各种工程场合,但传统的线性吸振器只能实现窄带振动控制.文章在线性吸振器的基础上引入对称水平弹簧构建线性刚度与非线性刚度相结合的组合刚度非线性吸振器,以提升吸振器的吸振性能.考虑实际工程中可能的安装方式,分别建立水平弹簧接地安装和不接地安装的组合刚度非线性吸振器模型,利用谐波平衡法结合弧长延拓法解析求解动力学响应,并与数值结果相互验证,证明了求解结果的准确性.随后分析比较两种组合刚度非线性吸振器与线性吸振器以及非线性能量阱之间的吸振性能,发现水平弹簧接地安装类型的组合刚度非线性吸振器在保留线性吸振器优势的同时又改善其吸振频带窄的缺点,且与非线性能量阱相比在主共振频率附近的较宽频内吸振性能更优.在此基础上,讨论了水平弹簧参数以及吸振器阻尼对主结构振动幅频响应和稳定性的影响,最后观察分析主结构幅频响应曲线不稳定区内的复杂动力学行为.研究结果表明合适的设计参数能够使得主结构振动峰值较低的同时,频响曲线不稳定运动区域的范围也较小.     

Using Carrier Fringes to Study the High Temperature Deformation Behavior of a BGA Package Under Extended Dwell Times

There is a compelling need to experimentally understand solder joint deformation behavior at high temperatures over an extended period of time. Accordingly, the deformation behavior of solder joints in a Ceramic Ball Grid Array (CBGA) package mounted on an organic FR4 board under extended dwell time at a high temperature has been studied using laser moiré interferometry. The warpage and the in-plane horizontal deformation of the ceramic substrate and the organic board as a function of time were determined. The variation of the normal strains and shear strains in the solder joints with time were also investigated. It was found that increased sensitivity was necessary to accurately determine the strains in the small sized solder joints. A new method utilizing carrier fringes to increase the sensitivity of the moiré interferometry system is proposed and has been used to determine the strains in the small sized solder joints. Increased sensitivity can be obtained merely by changing the incident angle of the laser light on the surface of the specimen, thereby making it unnecessary to use expensive phase shifting apparatus with the traditional laser moiré system.     

线形?拱形组合梁式三稳态压电俘能器动力学特性研究

利用振动能量俘获技术将设备工况振动能转化为电能, 为实现煤矿井下无线监测节点自供电提供了新的思路. 通过引入非线性磁力设计了一种线形?拱形组合梁式三稳态压电俘能器, 分析了磁铁水平间距、垂直间距和激励加速度对动力学特性的影响规律. 利用磁偶极子法建立磁力模型, 通过实验测量线形?拱形组合梁的恢复力, 并采用多项式拟合得到恢复力模型, 基于欧拉?伯努利梁理论和拉格朗日方程建立系统的动力学模型, 从时域角度仿真分析了磁铁水平间距、垂直间距和激励加速度对系统动力学特性的影响规律. 研制线形?拱形组合梁式三稳态压电俘能器样机并搭建实验平台进行实验研究, 通过采集组合梁末端响应速度数据, 验证了理论分析的正确性. 研究表明: 引入非线性磁场能够使系统势能呈现单势阱、双势阱或三势阱, 激励一定时, 调整磁铁水平间距和垂直间距能够使系统实现单稳态、双稳态或三稳态运动, 且在三稳态运动时响应位移较大, 增大激励水平有利于系统越过势垒实现大幅响应. 研究为线形?拱形组合梁式三稳态压电俘能器的设计提供了理论指导.      

Test fixture for eccentricity and stiffness of corrugated board

Of particular importance for the mechanical response and load-bearing capacity of corrugated board boxes is the manner in which load is introduced in the vertical panels through the horizontal flaps and the adjoining creases (folding lines). This study reports on a test fixture developed to measure the effective eccentricity moment acting on the vertical panels of corrugated board boxes. The axial compressive response of the creased board can also be measured using the new test fixture. The deformation mechanism of the board was monitored by visual inspection and photographically recorded and related to the measured eccentricity moment. The results show that significant eccentricity of the load path exists, which depends on the degree of lateral constraint and amount of axial deformation in a quite complex manner. The axial compressive response was found to be independent of the degree of lateral constraint and unsupported length of the specimen, which is evidence for the majority of the axial deformation occurring in the creased region. Photographic recording of the deformation mechanism of the creased region during compressive loading verified that the board is loaded unevenly as evidenced by kink zone formations appearing at opposite sides at different axial deformation levels.     

Effet de l'orientation d'un champ magnétique horizontal sur la stabilité de l'écoulement de HadleyEffet of the orientation of a horizontal magnetic field on the stability of Hadley flow

A numerical study based on the linear stability analysis is undertaken, in order to determine the influence of a horizontal magnetic field on the marginal modes occuring in a fluid layer subjected to a horizontal temperature gradient. A particular interest is devoted to the influence of the magnetic field orientation on both nature and critical values of the unstable modes. Calculations show, that when it is subjected to such a magnetic field, this type of flow, known as Hadley flow, can present oblique waves, hitherto non-existent when no magnetic field is applied and even when a vertical, a transverse or a longitudinal magnetic field is imposed. A new asymptotic behavior is also observed for the stabilizing effects. To cite this article: S. Kaddeche et al., C. R. Mecanique 331 (2003).     

Coordination learning in stag hunt games with application to emergency management

Response teams for natural disaster emergencies require coordinated and self-organized efforts for rescue, medical services, damage containment, and evacuation. The unfolding of events depends on initial conditions of specific time, location, and preparedness of the response teams. This study considered a new range of emergency response situations where the perpetrator is a sentient attacker that learns with repeated exchanges. The strategy is conceptualized as a stag hunt coordination game on the part of the defense agents and a strictly competitive game with respect to the attacking adversary. Participants were 28 university students who played an iterative board game (The Creature that Ate Sheboygan) wherein a team of three Humans represented military and civil resources, against one Godzilla-type monster. The Monster gained points by destroying buildings and human combat power. The Humans gained points by wearing down the Monster's defenses and containing damage caused by the Monster. Experimental manipulations and empirical analysis showed the following: Communication outages among the Humans assisted the Monster, but the ability to communicate only equalized the Humans' chances rather than providing them with an advantage. Coordination among Humans was instant, but it fluctuated greatly as a result of the Monster's progress. Nonlinear analysis showed an asymptotic decline in coordination to a non-zero level in response to outcome uncertainty. Learning effects were noted for Humans and Monsters, but there were significant interactions with communication blackout conditions