Temperature dependent first matrix cracking stress model for the unidirectional fiber reinforced ceramic composites

Based on a kind of equivalence between heat energy and fracture energy, assuming that there is a constant maximum storage of energy that includes both heat energy and fracture energy, a new temperature dependent fracture surface energy model is developed. Using the new model and the classical ACK theory, a temperature dependent first matrix cracking stress model is obtained for the fiber reinforced ceramic composites. According to the model, the temperature dependent first matrix cracking stress of materials can be easily predicted using some basic material parameters such as matrix fracture surface energy and Young’s modulus. The model is verified by comparison with experimental data of SiC fiber reinforced reaction-bonded Si₃N₄ composites at different temperatures. Good agreement is obtained between predicted and experimental data of first matrix cracking stress. The dependency of first matrix cracking stress on fracture surface energy and interfacial shear strength is systematically analyzed.     

考虑热效应复合材料典型壁板结构模态演变规律

以复合材料加筋板和连接板为研究对象,进行了考虑材料物性变化、热应力及热变形影响因素的跨越屈曲温度热模态分析,研究了壁板结构的模态演变规律。结果表明：热屈曲前,结构各阶频率因受材料物性变化和热应力的影响逐渐减小,而热屈曲后引起的大变形起到增加结构刚度的作用,频率转而增大。热效应会导致结构模态发生相互演变的现象,且高温阶段具有模态密集特征;加筋板的初始挠度、加筋尺寸和方式不仅改变结构热屈曲温度,也会使模态形式发生变化并呈现局部化特点;连接板结构均匀受热后产生的热变形,会"刚化"与其相似的模态,使该阶模态随着温升跃迁至高阶位置。     

一种新型三相Buck-Boost光伏逆变器漏电流抑制研究

漏电流问题是限制非隔离型光伏逆变器广泛使用的关键因素之一。为解决该问题,提出一种可应用于光伏系统的新型三相Buck-Boost逆变器。首先介绍该逆变器的拓扑结构和工作原理,然后推导该拓扑在任意开关时刻共模电压的表达式,从而研究正弦脉宽调制（SPWM）和系统共模电压及漏电流之间的关系,并通过数学模型分析逆变器的升降压能力。最后通过实验平台对上述方案进行验证,实验结果证明了上述方案的有效性和可行性。     

Correlation between failure mechanism and rupture lifetime of 2D-C/SiC under stress oxidation condition based on acoustic emission pattern recognition

Creep tests of 2D-C/SiC in a wet oxidizing atmosphere were implemented for six samples. The loading process was monitored by acoustic emission (AE). Principal component analysis and a fuzzy clustering algorithm were used to perform pattern recognition of the AE data. All of the AE events were divided into four clusters and labelled as matrix cracking, interfacial damage, fiber breakage and fiber-bundle breakage respectively, according to their physical origin. It was found C/SiC has very scattered rupture lifetimes even under the same test conditions, and the evolution of AE events corresponding to fiber failure is quite different. With increasing rupture lifetime, the AE energy of fiber-bundle breakage is higher, while the number of these events is less. Thus, it is concluded that local oxidation and damage development is the controlling failure mechanism for short-lived specimens and uniform oxidation and damage development is the controlling failure mechanism for long-lived specimens.     

The use of initial imperfection approach in design process and buckling failure evaluation of axially compressed composite cylindrical shells

Thin-walled cylindrical shells are susceptible to buckling failures caused by the axial compressive loading. During the design process or the buckling failure evaluation of axially-compressed cylindrical shells, initial geometric and loading imperfections are of important parameters for the analyses. Therefore, the engineers/designers are expected to well understand the physical behaviours of shell buckling to prevent unexpected serious failure in structures. In particular, it is widely reported that no efficient guidelines for modelling imperfections in composite structures are available. Knowledge obtained from the relevant works is open for updates and highly sought. In this work, we study the influence of imperfections on the critical buckling of axially compressed cylindrical shells for different geometries and composite materials (Glass Fibre Reinforced Polymer (GFRP), Carbon Fibre Reinforced Polymer (CFRP)) and aluminium using the finite element (FE) analysis. Two different imperfection techniques called eigenmode-affine method and single perturbation load approach (SPLA) were adopted. Validations of the present results with the published experimental data were presented. The use of the SPLA for introducing an imperfection in axially compressed composite cylindrical shells seemed to be desirable in a preliminary design process and an investigation of a buckling failure. The knockdown factors produced by the SPLA were becoming attractive to account for uncertainties in the structure.     

Digital image correlation based high-speed crack tip locating method and its application

Based on a digital image correlation (DIC) method with the measurements of a high speed crack''s displacement and strain fields, a technique to accurately and automatically locate its crack tip has been developed. The crack tip is identified by finding the abrupt jump on the opening (or dislocation) curve of a point on the trace of the crack propagation, while the opening is measured through a virtual extensometer technique and the abrupt jump is identified by finding the peak on the curve. The method was verified using a specially designed experiment and applied to measure the critical crack tip opening angle of a rock sample. Because the involvement of analytical models has been avoided and then the good performance could be ensured for low resolution speckle images, this technique is expected to be very useful in the quantitative study of high speed cracks in experiments using high speed cameras.     

岩石破坏动态变形场观测系统及应用

基于高速图像采集设备和数字散斑相关方法（DSCM）,建立了一套岩石破坏动态变形场观测实验系统,介绍了系统的组成和原理,重点阐述了用于岩石动态变形测量时光源的选择、采集速率的设置和图像采集的触发等关键技术。用此动态变形场观测实验系统可完成动（静）态载荷作用下岩石破坏瞬间试件表面图像的准确高速采集,并进一步完成变形场的定量分析。用设计的系统完成了岩石巴西圆盘实验和岩石I型裂纹动态断裂实验,对动态变形场观测实验系统的可行性和有效性进行了验证。观测系统获得的脆性岩石失稳破坏过程中动态、定量的变形场数据可为深入研究岩石失稳破坏机理提供基础的数据和认识。     

Postbuckling behaviour of a blade-stiffened composite panel loaded in uniaxial compression

The postbuckling behaviour of a panel with blade-stiffeners incorporating tapered flanges was experimentally investigated. A new failure mechanism was identified for this particular type of stiffener. Failure was initiated by mid-plane delamination at the free edge of the postbuckled stiffener web at a node-line. This was consistent with an interlaminar shear stress failure and was calculated from strain gauge measurements using an approximate analysis based on lamination theory and incorporating edge effects. The critical shear stress was found to agree well with the shear strength obtained from a three-point bending test of the web laminate.     

天然气循环钻井系统可靠性研究

天然气循环钻井是一种新型的气体钻井工艺,可以使钻井用天然气得到循环使用,有效节约成本,提高钻井效率,保护环境;但该工艺技术复杂,对系统安全性要求较高.为此,运用故障树模型与蒙特卡罗仿真相结合的方法,首先分析工艺过程存在的危险源以及可能发生的事故;建立基于故障树模型的工艺过程仿真逻辑关系,采用蒙特卡罗仿真方法对系统进行事...     

伸展运动悬臂梁的稳定性分析

对匀速伸展运动悬臂梁的稳定性进行了研究。采用数值方法求解梁的运动微分方程,研究自由端处位移随时间变化的情况。提出了两种判断这类时变系统失稳的方式,一是瞬态特征值方法,通过分析特征值的实部和虚部来判断梁的失稳特性;二是刚度判定方法,由刚度随时间的符号变化判断梁的失稳特性。讨论了特征值及刚度随梁长度的变化的趋势,进而判定时变系统的稳定性。用这两种方法对运动微分方程分别取一阶截断和五阶截断进行了计算,并将两种方法进行对比,结果取得了很好的一致性。