Seven Conformations of the Macrocycle Cyclododecanone Unveiled by Microwave Spectroscopy

The physicochemical properties and reactivity of macrocycles are critically shaped by their conformations. In this work, we have identified seven conformations of the macrocyclic ketone cyclododecanone using chirped-pulse Fourier transform microwave spectroscopy in combination with ab initio and density functional theory calculations. Cyclododecanone is strongly biased towards adopting a square configuration of the heavy atom framework featuring three C–C bonds per side. The substitution and effective structures of this conformation have been determined through the observation of its ¹³C isotopologues. The minimisation of transannular interactions and, to a lesser extent, HCCH eclipsed configurations drive conformational preferences. Our results contribute to a better understanding of the intrinsic forces mediating structural choices in macrocycles.     

基于颗粒离散元法的连接键应变软化模型及其应用

基于颗粒间的有限接触假设,提出了可表述颗粒间力、力矩传递的连接键模型. 为了表征连接键的塑性、损伤及断裂过程,在连接键中引入了考虑应变软化效应的Mohr-Coulomb 准则及最大拉应力准则. 单一连接键的单向拉伸测试及直剪测试表明了上述连接键应变软化模型的计算精度. 研究了颗粒体系的宏观应变能与颗粒平均配位数的对应关系. 通过计算发现,对于二维颗粒体系,当平均配位数为5 时,颗粒体系的宏观应变能与相同参数下连续介质方法(如有限元等) 计算获得的应变能基本一致. 利用上述连接键应变软化模型对岩石的单轴压缩过程进行了模拟,计算结果表明:岩石单轴压缩的应力应变曲线经历了线性上升段、非线性上升段、非线性下降段及缓变段等4 个阶段,并给出了上述4 个阶段与岩石内部损伤破裂状态的内在联系. 计算结果还表明,随着断裂应变的增大,岩石的破裂模式逐渐由拉剪复合型破裂向单一压剪型破裂转化;随着断裂应变的增大,峰值应力及达到峰值应力时的应变均逐渐增大,但峰值时的破裂度及终态时的破裂度将逐渐减小.      

单杠结构差分离散系统的振动反问题

研究组合结构中的杆-梁-杆组合单支结构,或称单杠结构的模态反问题,从数学上证明单杠结构差分离散系统固有振动的刚度矩阵的符号振荡性,并导出其频率和振型的一些定性性质.在此基础上,探讨由两组位移或应变振型及相应频率构造物理参数的条件和方法,给出两个计算实例.讨论结构参数关于其几何对称轴具有对称性的条件下的模态反问题,并给出计算实例.计算结果表明,上述反问题提法合理,解法正确.     

利用压杆技术确定与验证一些结构材料的塑性模型(英文)

提出了一种关于材料力学特性的综合研究方法,包括获取材料在宽大的应变率及温度范围下的必要的力学特性,确定或辨别材料的变形及破坏模型的必要参数,并通过特别的实验及数字模拟实验来检验或验证模型的适当性.通过对一些结构金属及合金材料的研究,展现了该方法的应用潜力.     

考虑屋盖柔度的开孔两空间结构风致内压研究

基于合理假定, 推导了迎风面单开孔两空间结构在其屋 盖以动力和准静态两种方式响应时的内压控制方程组. 通过数值算 例, 分析了各种屋盖柔度下由动力模型与准静态模型计算得到的两子空间内压增益函数、内压系数功率谱以及内压系数均方根值. 结果表明: 当屋盖较柔时, 须采用动力模型来计算内压响应, 反之, 可采用简化的准静态模型. 随着屋盖自振频率的增大, “内压-屋盖”系统的三阶共振频率都增大, 前二阶共振频率处的峰值内压响应也随之增大, 但第三阶共振频率处的峰值内压响应却越来越小, 由两模型计算得到的内压系数均方根值均随之增大, 但它们的差值却越来越小.     

RRR-PRP平面六杆Ⅱ级机构的运动学仿真

以机构的组成原理为出发点,以应用最为广泛的RRR-PRP平面六杆机构为分析对象,采用复数向量推导出曲柄、RRRⅡ级杆组、PRPⅡ级杆组三个基本模组的运动学数学模型,利用其组成机构杆组并搭建平面连杆机构的运动学仿真模型,充分利用MATLAB的S imu link仿真模型数据可视化的特点观察     

Thermo-mechanical response of Al 6061 with and without equal channel angular pressing (ECAP)

The thermo-mechanical responses of Al 6061 before and after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. Al 6061 was solution heat treated before ECAP pressing at room temperature and subjected to up to three passes. After pressing, the billets were aged at 100 °C for 2 days. An as-received Al 6061-T651 was studied similarly to investigate the differences between processed and non-processed specimens. The responses of ECAP material were determined at −30, 22, 125 and 250 °C, and at strain rates from 10⁻⁵ to 2530 s⁻¹; the 6061-T651 specimens were subjected to uniaxial compressive loading at −31, 22, 85, 150, 230 and 315 °C, and strain rates ranging from 10⁻⁵ to 2200 s⁻¹. It was found that, the ECAP process increases the strength versus the T651 condition. Additionally, the Al 6061 ECAP is not sensitive to strain rate at room and lower temperatures, but the sensitivity increases as the number of passes and/or temperature are increased and this is the same for the non-processed material. Increasing the number of passes increases the flow stress at room and lower temperatures, has almost no effect at 125 °C and decreases at 250 °C. For both materials, the dynamic flow stress is higher than the stress at quasi-static strain rates even when the quasi-static strain rate regime is insensitive to strain rate. The Al 6061 has strong texture after one pass but steadily increases as the number of passes are increased. This is the first study that reports on the thermo-mechanical responses of ECAP and non-ECAP Al 6061 at such a wide range of strain rates, including dynamic, and temperatures.     

高g值加速度计的设计与冲击特性分析

针对特殊场合的测试需求,设计了四端全固支的高过载梁岛结构加速度计;利用Hopkinson杆冲击校准装置对该加速度计的动态特性进行了测试。测试结果表明,传感器的灵敏度为1.2V/g,线性度5%左右;结构受到2105g冲击后完好且输出信号正常,能有效满足高冲击、强烈振动场合的特殊测试要求,可以应用于侵彻系统。 更多还原     

用于Hopkinson压杆装置的电磁驱动技术

为提高分离式Hopkinson压杆装置的测试效率与精度,通过对电磁驱动技术的分析,设计并进行了电磁线圈驱动导体杆的原理性实验。以微型分离式Hopkinson压杆装置为基础,将电磁线圈驱动原理用于撞击杆的驱动。通过单级线圈驱动不同长度撞击杆,获得储能电量与不同长度撞击杆的速度为线性关系。结合对镁合金材料的动态应力应变关系测试结果,证明此系统速度容易控制、重复性好、可靠性高和实用性强、电磁干扰并不影响信号采集。借助此原理,通过提高储能电量或采用多级同轴线圈驱动方法,可以实现各种规格的Hopkinson压杆装置中撞击杆的有效驱动,使Hopkinson杆测试装置简化。     

Rate dependent critical strain energy density factor of Huanglong limestone

Critical strain energy density of rock can be defined as a fundamental parameter in rock fracture mechanics, an intrinsic material property related to resistance to crack initiation and propagation. By means of the three-point bending experiments, the critical strain energy density factor of Huanglong limestone was measured over a wide range of loading rates from 8.97 × 10⁻⁴ MPam^1/2 s⁻¹ to 1.545 MPam^1/2 s⁻¹. According to the approximate relationship between static and dynamic critical strain energy density factor of Huanglong limestone, relationship between the growth velocity of crack and magnitude of load is obtained. The main conclusions are summarized as follows: (1) when the loading rate is higher than 0.0279 MPam^1/2 s⁻¹, the critical strain energy density factor of rock increased markedly with increasing loading rate. However, when loading rate is lower than 0.0279 MPam^1/2 s⁻¹, the critical strain energy density factor slightly increased with an increase in loading rate. It is found from experimental results that the critical strain energy density factor is linear proportional to the exponential expression of loading rate, (2) for Huanglong limestone, when the growth velocity of crack is lower than 100 m/s, value of the maximum load was nearly a constant. However, when the growth velocity of crack is higher than 1000 m/s, value of the maximum load dramatically increases with increasing the crack growth velocity, and (3) the critical SED of Huanglong limestone is higher as the loading rate is higher.