正态应力和正态强度结构可靠性的经典精确限

结构可靠性的估计问题是机械可靠性的主要问题之一.强度和应力均为正态变量的结构模式是一种较常见的模式.研究这一模式结构可靠性的区间估计问题.提出了用正态分布变差系数的区间估计结果来处理此模式结构可靠性的区间估计的新方法,给出了正态强度和正态应力的分布参数均未知时,结构可靠性的具有简单表达式的经典精确置信下限,它是此模式结...     

一种新的小口径炮弹紧口装置

针对小口径炮弹紧口的特点、技术要求以及生产过程中存在的问题,提出一种新的自动紧口方式。通过控制紧口槽的深度和紧口槽的位置,间接地控制小口径炮弹拔弹力的大小,确保紧口后炮弹的拔弹力的一致性;并使用气液增力缸来提供紧口的驱动力,既保证了紧口节拍又减小了设备体积。使用结果证明,该新型紧口装置设计先进、上下料方便,紧口力便于自动调整,拔弹力一致性好,偏差在允许范围内,提高了弹药的品质,保证了杀伤力,有一定的推广价值。     

工程机械用800 MPa低合金高强度钢焊接接头低周疲劳性能研究

针对大型工程机械用800 MPa低合金高强度钢及其焊接接头,进行低周疲劳性能研究。试验在常温下进行,对称三角波加载。试验结果表明,母材两个轧制方向的应变疲劳寿命没有明显差别,焊接接头的疲劳寿命偏低。焊接接头的局部软化是应变集中,导致应变疲劳寿命降低的主要原因。应变疲劳断口有明显的条纹特征,母材疲劳断口上伴有与条纹平行的二次裂纹,焊接接头的疲劳断口条纹清晰且宽度较细。试验确定的循环应力应变特性参数和应变疲劳寿命参数有明显的相关关系。     

The Effect of Nanotube Content and Orientation on the Mechanical Properties of Polymer–Nanotube Composite Fibers: Separating Intrinsic Reinforcement from Orientational Effects

We have measured the mechanical properties of coagulation‐spun polymer–nanotube composite fibers. Both the fiber modulus, Y, and strength, σ_B, scale linearly with volume fraction, V_f, up to V_f ～10%, after which these properties remain constant. We measured dY/dV_f = 254 GPa and dσ_B/dV_f = 2.8 GPa in the linear region. By drawing fibers with V_f < 10% to a draw ratio of ～60%, we can increase these values to dY/dV_f = 600 GPa and dσ_B/dV_f = 7 GPa. Raman measurements show the Herman's orientation parameter, S, to increase with drawing, indicating that significant nanotube alignment occurs. Raman spectroscopy also shows that the nanotube effective modulus, Y_Eff, also increases with drawing. We have calculated an empirical relationship between the nanotube orientation efficiency factor, η_o, and S. This allows us to fit the data for Y_Eff versus η_o, showing that the fiber modulus scales linearly with η_o, as predicted theoretically by Krenchel. From the fit, we estimate the nanotube modulus to be; Y_NT = 480 GPa. Finally, we show that the fiber strength also scales linearly with η_o, giving an effective interfacial stress transfer of τ = 40 MPa and a nanotube critical length of l_c=1250 nm. This work demonstrates the validity of the Cox‐Krenchel rule of mixtures and shows that continuum theory still applies at the near‐molecular level.     

烧结温度对漂珠/ZrO2多孔轻质材料性能的影响

以漂珠为成孔材料,以ZrO2为结合剂,采用凝胶注模的方法制备出多孔轻质材料。通过分析材料的微观形貌、物相组成、显气孔率、体积密度、抗弯强度等性能参数,研究了烧结温度1200℃~1500℃对材料性能的影响。结果得出,烧结温度达到1400℃时,漂珠外壳与氧化锆的界面结合不再明显,ZrO2颗粒由点接触扩展到面接触;烧结温度1400℃的试样中锆英石的峰最强,说明漂珠和氧化锆已烧结熔融;1400℃下制备试样的显气孔率最大(68%),体积收缩率(20.8%)最低,而抗弯强度可达7.5MPa;烧结温度控制在1400℃,可使材料处于烧结中期,保证材料的各项性能满足其应用需求。     

Adhesion strength of leadframe/EMC interfaces

Cu-based leadframe sheets were oxidized in alkaline solutions to produce brown and/or black oxide on the surfaces, and molded with epoxy molding compound (EMC). The adhesion strength of leadframe/EMC interface was measured using sandwiched double-cantilever beam (SDCB) specimens and pull-out specimens. Results showed that the adhesion strength of leadframe/EMC interface was inherently very poor but could be increased drastically with the nucleation of acicular CuO precipitates. The presence of smooth-faceted Cu₂O on the surface of the leadframe gave close to zero fracture toughness (G_C) and suitable pull strength (PS). A direct correlation between G_C and PS showed that PS can be a measure of G_C only in a limited range.     

Effect of thermal cycles on the mechanical strength of quad flat pack leads/Sn-3.5Ag-X (X=Bi and Cu) solder joints

Quad Flat Pack (QFP) Leads/Sn-3.5Ag-X (X=Bi and Cu) joint was thermally cycled between 243 K and 403 K or 273 K and 373 K, and both metallographic examination and mechanical pull test were performed to evaluate thermal fatigue damage of the joint. The addition of bismuth drastically degrades the thermal fatigue resistance of Sn-3.5Ag solder. On the other hand, the pull strength of Sn-3.5Ag-Cu solder joints slightly decreased with increasing number of thermal cycles, though it still remains higher in comparison to that for conventional Sn-37Pb or bismuth containing solder joint. The behavior observed here reflects the isothermal fatigue properties of bulk solder, because thermal fatigue crack initiates at the surface of solder fillet and propagates within the fillet in an early stage of fatigue damage. Furthermore, the lead phases lying at the interface between lead-frame and bismuth containing solder joint may promote the crack propagation at the interface, resulting in the extremely low thermal fatigue resistance of the joint.     

Bulky rigid substitutions: A route to high electron mobility and high solid-state luminescence efficiency of perylene diimide

In this paper, we report that a kind of perylene diimide derivative with bulky rigid substituents, 1,7-bis(p-tert-butylphenoxy)-N,N′-dicyclohexyl-perylene-3,4,9,10-tetracarboxylic diimide (TBPCHPDI) possesses both high electron mobility (1.8 cm² V⁻¹s⁻¹) and high fluorescence quantum yield (0.32) in the solid state. Through X-ray diffraction (XRD), UV–Vis absorption and fluorescence spectra, and differential scanning calorimetry (DSC) measurements, it is demonstrated that the above phenomenon can be ascribed to the unique crystal structure of TBPCHPDI: due to steric hindrance of bulky rigid substituents, the intermolecular π–π actions are neglectable, providing high luminescence efficiency; in the mean time, the spacing between perylene chromophores is still very short (3.47 Å), which is favorable for the hopping transportation of charge carrier from one molecule to neighboring molecule. Therefore, our finding would help design and synthesize novel organic semiconductive materials with potential applications in electrically pumped lasers which require high emission efficiency when large current density is applied.     

Gel Polymer Electrolyte toward Large-Scale Application of Aqueous Zinc Batteries

Aqueous zinc batteries are promising candidates for energy storage and conversion devices in the “post-lithium” era due to their high energy density, high safety, and low cost. The electrolyte plays an important role in zinc batteries by conducting and separating the positive and negative electrodes. However, the issues of zinc dendrites growth, corrosion, by-product formation, hydrogen evolution and leakage, and evaporation of the aqueous electrolytes affect the commercialization of the batteries. Moreover, the widely used aqueous electrolytes result in large battery sizes, which are not conducive to the emerging smart devices. The intrinsic properties of gel polymer electrolytes (GPEs) can solve the above problems. In order to promote the wider application of GPEs-based zinc batteries, in this review, the working principle and the current problems of zinc batteries are first introduced, andthe merits of GPEs compared to aqueous electrolytes are then summarized. Subsequently, a series of challenges and corresponding strategies faced by GPE is discussed, and an outlook for its future development is finally proposed.     

基于射线跟踪的小区信号强度估计

基于经验模型的无线信号强度估计方法无法针对具体物理场景,估计结果精度低,无法满足移动通信网络规划和优化的需要。射线跟踪技术能依据场景信息跟踪电波传播路径,但现有的反向跟踪方法在进行室外成片区域信号强度估计时复杂度高,无法实用。为提高估计结果的精度,降低估计方法的复杂度,提出了一种正向跟踪信号强度估计方法,将基站天线处发射的电磁波模拟成离散的射线,在考虑建筑物形状、分布信息的基础上采用基于网格的搜索算法跟踪射线路径,在接收点处对反射、绕射射线接收合并,最终得到基站覆盖区域的信号强度分布。仿真结果表明该方法的场强估计结果和实测数据吻合程度远大于经验模型估计方法,并且80%左右的估计结果和实测数据的误差在12 dBm以内,可应用于移动通信网络规划和优化。