A review on mechanisms and models for very-high-cycle fatigue of metallic materials

在循环载荷作用下, 合金材料发生裂纹萌生、扩展直至断裂的周次在10⁷以上的过程被称为超高周疲劳 (very-high-cycle fatigue, VHCF).本综述将从30年前超高周疲劳的研究起源讲起, 直到近年的最新进展.引言之后的内容包括: 超高周疲劳研究的起源, 超高周疲劳的主要特征, 超高周疲劳裂纹萌生特征区和特征参量, 裂纹萌生特征区的形成机理与模型, 超高周疲劳性能预测模型. 在叙述中, 试图回答下列问题: 什么是超高周疲劳?为什么要研究超高周疲劳?超高周疲劳的关键科学问题是什么?超高周疲劳的S-N曲线趋势为什么发生变化?超高周疲劳裂纹为什么萌生于材料 (试样) 内部?裂纹内部萌生的过程和机理是什么? 上述问题有的可以给出明确的回答, 有的则是现阶段的最新结果, 并有待于对问题的继续探索.     

测定疲劳强度的双细节串联升降法

指定寿命下的疲劳强度是无法直接测定的,自从测定疲劳强度的小子样升降法被提出以后,这种方法被广泛地应用于测定材料疲劳强度的随机特征。本文提出一种用于疲劳强度测定的双细节串联试件,给出了一个新的升降法流程。试件两个细节的疲劳强度在统计上是相互独立的,一个细节的破坏不会影响另一个细节,当一个细节破坏后,另一个细节的疲劳试验仍可以继续进行。为验证方法的有效性,本文进行了一系列的数值模拟。同时,这种方法还被用于飞机结构模拟件的疲劳强度试验。数值模拟结果和实际试验过程均表明,串联双细节升降法可以显著地节约试验时间。     

A mathematical model of external electrostatic field of a special collector for electrospinning of nanofibers

This article focuses on the analysis of electrostatic field generated by a special collector consisting of two parallel cylindrical conductors used for electrospinning. Computed values of critical electrical potential obtained by an analytical model are compared with measurements. Here we show that the gap distance between the special collector and a spinning electrode, i.e., a syringe needle, has not substantial effect on the critical potential if the special collector distance from the syringe needle is higher than the gap distance between cylindrical conductors. The presented model is a good analytical approximation of electric field created by the used special collector.     

Castor Oil-Based Polyurethane/Epoxy Intercross-linked Polymer Network Adhesives for Metal Substrates

Castor oil based polyurethane (CO-PU) was first synthesized from castor oil and 4, 4’-diphenyl-methane-diisocyanate (MDI). Then, a series of CO-PU/epoxy (EP) intercross-linked polymer network (ICPN) adhesives for metal substrates were prepared by a sequential method. The functional groups, tack -free time, mechanical properties, adhesive properties, and thermal stability were studied. Fourier transform infrared spectroscopy analysis indicated that an ICPN structure was formed through the introduction of CO-PU into EP. Results of adhesive measurements showed that the maximal value of lap shear strength was achieved at the CO-PU content of 20%. Thermogravimetric analysis results indicated that thermal stability of the adhesive film decreased with increased CO-PU content.     

一种基于声散射特性的有鳔鱼特征获取方法

鱼分类技术在渔业资源评估中的重要性日益凸显,而分类中能够得到不同鱼种的本质声散射特征尤为重要。文章基于鱼的声散射特性,提出一种有鳔鱼种特征获取新方法。鱼的目标强度反映了鱼的本质声散射特征,其主要取决于鱼鳔的大小、形状及声波入射角度、频率,文章基于基尔霍夫模式模型和图像处理方法进行建模,不仅可对鱼体和鱼鳔进行更真实的近似,而且可以得到精确的目标强度仿真结果。通过图像识别方法,对鱼体及鱼鳔图像进行灰度变换、二值化、边缘识别、轮廓提取等处理,得到鱼体及鱼鳔边缘坐标,利用得到的信息进行建模,并给出了相应的计算机仿真结果。最后进行了水池试验,对仿真数据与试验数据进行了对比分析,并提取得到有鳔鱼的目标强度特征,为鱼分类特征获取提供了一条新的途径。     

基于分形理论的超音速等离子喷涂层界面结合行为研究

为研究结合界面形貌与涂层体系结合强度之间的关系,采用超音速等离子喷涂设备制备Fe基合金涂层,选用(Ni,Al)涂层作为黏接底层.通过改变Ar气流量,获得不同粗糙表面的黏接底层.采用对偶件拉伸法测量复合合金涂层体系的结合强度,同时引入分形理论对结合界面形貌进行定量表征,结果表明:黏接底层能显著提高涂层的结合强度,随着Ar气流量的升高,黏接底层表面分形维数不断降低,涂层体系的结合强度则呈现出先增大后减小的趋势.     

利用源强密度声辐射模态重建声场

为了利用声场中少量测点声压数据精确重建复杂结构的辐射声场,提出了源强密度声辐射模态分析理论和声场重建公式.在结构表面定义的空间上,利用以源强密度分布函数为参量的结构辐射声功率泛函表达式定义了一个线性自伴正辐射算子,该算子的特征函数为结构的源强密度声辐射模态.然后通过对矩形平板和带有半球帽的圆柱体的源强密度声辐射模态的分析,证明了源强密度声辐射模态具有空间滤波特性,并利用该性质建立了声场重建公式.球体仿真和平板实验验证了所提出的声场重建方法的可行性和稳健性.基于源强密度声辐射模态的声场重建方法简单,利用较少测点数据就可以获得较高的声场重建精度,特别适合于复杂结构的低频声场重建.     

Carbon nanofiber reinforced syntactic foams: Degradation mechanism for long term moisture exposure and residual compressive properties

Hollow particle filled polymer matrix composites, called syntactic foams, are challenging for studies related to environmental exposure and degradation mechanisms due to the possible role of matrix, particle–matrix interface, and particle material and wall thickness. In the current study, syntactic foams reinforced with 1 wt.% vapor grown carbon nanofibers (CNFs) are subjected to water immersion and characterized for residual compressive properties under quasi-static and high strain rates. The testing is conducted on four different types of syntactic foams, fabricated with glass hollow particles of two different densities: 220 and 460 kg/m³ in 30 and 50 vol.% quantities. After a period of 6 months, a maximum of 7% weight gain is observed in the worst performing syntactic foam. The exposed specimens are tested for residual compressive properties and the results are compared with the properties of dry specimens. The quasi-static compressive strength of CNF reinforced syntactic foams is found to decrease and the modulus remained unaffected due to the moisture exposure. The high strain rate compressive strength was 1.3–2.2 times higher for wet and dry specimens compared to the quasi-static strength of the same type of syntactic foams.     

Desorption of Asphaltenes from Silica-Coated Quartz Crystal Surfaces in Low Saline Aqueous Solutions

The quartz crystal microbalance (QCM) was used to study desorption of asphaltenes from silica-coated quartz crystals upon exposure to various aqueous low saline solutions of different salt concentrations and cationic valency. Ultraviolet (UV) spectroscopy measurements confirmed desorption in selected experiments. The amount of desorption was related to the type and concentration of electrolyte and the sequence of injecting the electrolyte solutions. Initial desorption upon exposure to solutions with high ionic strength was likely due to repulsion between negatively charged sites acquired at the silica and the asphaltenes. During the injection of low saline aqueous solutions, a critical expansion of the diffuse double layer was required for desorption to occur. Comparatively lower desorption of asphaltenes was observed in the CaCl₂ solutions than in NaCl and seawater solutions.