快速三维MR血管壁成像评估急性脑梗死及颈动脉粥样硬化的研究

本研究探讨在脑梗死及颈动脉粥样硬化斑块评估中,快速三维MR血管壁成像技术的应用价值。选取颈动脉粥样硬化斑块患者78例,共计斑块112个,其中发生脑梗死患者36例,未发生脑梗死患者42例。与病理结果比较,快速三维MR血管壁成像评估颈动脉斑块性质一致性Kappa值为0.790(P<0.05);不稳定性斑块MR积分明显高于稳定性斑块(P<0.05);脑梗死患者甘油三酯(TG)、低密度脂蛋白胆固醇(LDL-C)和平均MR积分明显高于无脑梗死患者(P<0.05),而高密度脂蛋白胆固醇(HDL-C)明显低于无脑梗死患者(P<0.05);平均MR积分预测脑梗死的ROC曲线下面积为0.852(P<0.05)。因此,快速三维MR血管壁成像技术能有效评估颈动脉粥样硬化斑块性质,对预测脑梗死有一定应用价值。     

Multi-frequency sonoreactor characterisation in the frequency domain using a semi-empirical bubbly liquid model

Recently, multi-frequency systems were reported to improve performance in power ultrasound applications. In line with this, digital prototyping of multi-frequency sonoreactors also started gaining interest. However, the conventional method of simulating multi-frequency acoustic pressure fields in the time-domain led to many challenges and limitations. In this study, a multi-frequency sonoreactor was characterised using frequency domain simulations in 2-D. The studied system consists of a hexagonal sonoreactor capable of operating at 28, 40 and 70 kHz. Four frequency combinations were studied: 28–40, 28–70, 40–70 and 28–40–70 kHz. A semi-empirical, modified Commander and Prosperetti model was used to describe the bubbly-liquid effects in the sonoreactor. The root-mean-squared acoustic pressure was compared against experimental validation results using sonochemiluminescence (SCL) images and was noted to show good qualitative agreement with SCL results in terms of antinode predictions. The empirical phase speed calculated from SCL measurements was found to be important to circumvent uncertainties in bubble parameter specifications which reduces error in the simulations. Additionally, simulation results also highlighted the importance of geometry in the context of optimising the standing wave magnitudes for each working frequency due to the effects of constructive and destructive interference.     

CHARACTERISTICS OF FATIGUE SURFACE MICROCRACK GROWTH IN VICINAL INCLUSION FOR POWDER METALLURGY ALLOYS

Inclusion flaw is one of the worst flaws of powder metallurgy. The inclusion flaw plays an important role in the failure of high temperature turbine materials in aircraft components and automotive parts, especially fatigue failure. In this paper, an experimental investigation of fatigue microcrack propagation in the vicinal inclusion were carried out by the servo-hydraulic fatigue test system with scanning electron microscope (SEM). It has been found from the SEM images that the fatigue surface microcrack occurs in the matrix and inclusion. According to the SEM images, the characteristics of fatigue crack initiation and growth in vicinal inclusion for powder metallurgy alloys are analyzed in detail. The effect of the geometrical shape and material type of surface inclusions on the cracking is also discussed with the finite element method (FEM).     

三角形夹芯板夹心层的等效弹性常数

推导了三角形夹芯板夹心层的等效弹性常数,为验证所推导的弹性常数的合理性,用NASTRAN有限元计算程序对两种模型进行了静力计算比较,一种是用所推导的弹性常数将夹心等效后,以层合板的形式进行计算的模型,另一种是对整个结构直接采用板单元进行计算的模型.计算结果表明:两种模型结果吻合良好.     

缺口件表象疲劳极限的定量化分析

测定了淬火、高温回火35CrMo钢光滑试样和三种缺口试样的三点弯曲疲劳极限,并利用ANSYS有限元软件计算了缺口试样加载时缺口截面上的应力分布.用“疲劳源形成的微细观过程理论”对试验结果进行了分析,认为,疲劳源的形成虽然发生在个别薄弱晶粒内部,是其中位错往返运动及交互作用的结果,但还必须满足一定的形变协调条件和概率条件,因而必须形成由相当多晶粒组成的“细观屈服区”.对试验结果的分析表明,如取“细观屈服区”临界尺寸(xW)等于11个奥氏体晶粒平均直径,则计算得到的缺口试样的表象疲劳极限,与实测值相比,其综合误差最小.据此计算的三种缺口试样的表象疲劳极限,其误差都小于5％.该xW可认为是材料的特征参量.     

带槽平板的三维应力集中效应

本文提供了用电阻应变片方法测量一组不同厚度的带有对称圆槽的拉伸平板在应力集中部位的应力分布情况。实验结果表明,应力集中系数不仅与板平面的几何尺寸有关,而且与板的厚度有关。     

Chater梁屈曲传播的动态分析

本文从非线性有限元的一般方法出发,结合应用加权残数技术和ALE边界元法,对Chater梁的屈曲传播进行了完全的动态分析,抛弃了前人一直采用的准静态和动稳态假设;另外,基于势流理论和非线性液面条件,还考虑了结构充液后由于流固耦合作用对屈曲传播所造成的影响,大量算例揭示了结构的菲线性、作用载荷、屈曲传播速度以及液体介质之间的内在关系,从而对屈曲传播有了更深入的认识。     

计算机辅助工程(CAE)及其发展

 CAD/CAM/CAE三者中，前两者概念较直观，CAE概念则是发展的，涵盖范围 广，比较灵活. 当前，CAE已由数值计算发展到仿真，并向CAD/CAM全面融合的虚拟现实拓 展. 随着有限元技术的成熟以及神经网络、遗传算法等智能算法的引入，计算机辅助工程分 析不仅服务于详细设计，而且支持总体方案设计. 基于产品研发过程，对CAE本身进行 了分析，全面阐述了CAE的目标、物理内容、算法，说明了有限元/人工智能在方案设计中 的应用. 对优化技术的发展历程进行了回顾，并展望了未来CAE的发展动向及若干重要问题.     

弹性与弹塑性问题的有限元与边界元耦合解法

本文提出了一种新的弹性与弹塑性问题的对称耦合解法。根据分区广义变分原理,直接导出问题的求解方程式。通过典型算例,验证了该方法的有效性,本文建议的方法与超单元形式的耦合法相比,在理论上比较直接,在计算上更为经济。     

A Three-Dimensional Hybrid LES-Acoustic Analogy Method for Predicting Open-Cavity Noise

A three-dimensional (3D) hybrid LES-acoustic analogy method for computational aeroacoustics (CAA) is presented for the prediction of open-cavity noise. The method uses large-eddy simulation (LES) to compute the acoustic source while the Ffowcs Williams-Hawkings (FW-H) acoustic analogy is employed for the prediction of the far-field sound. As a comparison, a two-dimensional (2D) FW-H analogy is also included. The hybrid method has been assessed in an open-cavity flow at a Mach number of 0.85 and a Reynolds number of Re=1.36×10⁶, where some experimental data are available for comparison. The study has identified some important technical issues in the application of the FW-H acoustic analogy to cavity noise prediction and CAA in general, including the proper selection of the integration period and the modes of sound sources in the frequency domain. The different nature of 2D and 3D wave propagation is also highlighted, which calls for a matching acoustic solver for each problem. The developed hybrid method has shown promise to be a feasible, accurate and computationally affordable approach for CAA.