图像重建算法中的窗函数研究

对广角扇束扫描模式下的图像重建算法进行了研究．在引入实验的七种典型窗函数中，有限带宽、三角形窗以及余弦窗是卷积窗函数比较理想的选择．在图像重建，选择有限带宽窗时有较好的空间分辨率，三角形窗时有较好的密度分辨率。选择余弦窗时是空间分辨率与密度分辨率两者的折衷；从窗函数的频谱角度考虑。可用于图像重建的一个良好的卷积窗函数应该具备以下条件：（1）最小的3dB带宽B，即最小的主瓣宽度；（2）最小的旁瓣最大峰值A；（3）最大的旁瓣峰值衰减速度D．通过实验，验证了基于窗函数的卷积反投影图像重建算法可以有效地提高重建图像的质量．     

钢管初应力对钢管混凝土偏压构件承载力的影响

对有钢管初应力的钢管混凝土偏压构件承载力的影响因数进行了探讨,介绍了目前国内对该类构件承载力计算的两种方法,并将两种方法的计算结果进行了比较,分析了两者的计算误差,以进一步加深对钢管混凝土这一复杂结构的认识.     

混凝土中变形与光圆钢筋腐蚀速率试验研究

为了比较含氯离子的混凝土中变形钢筋及光圆钢筋腐蚀特征和腐蚀速度的不同，制作了同时含有变形和光圆2种钢筋并掺加了质量为水泥质量3％的盐ω（NaCl）的混凝土试件，然后进行了近一年的室外自然气候暴露试验．试验期间利用电化学线性极化法和试件腐蚀失重称量方法测试了2种钢筋的腐蚀速度；并分析和总结了它们的腐蚀特征．试验结果表明光圆钢筋的腐蚀机理主要是微电池腐蚀，腐蚀形态比较均匀，平均腐蚀失重率比变形钢筋大；而变形钢筋腐蚀机理主要是宏电池腐蚀，腐蚀主要发生在钢筋表面某几个局部区域，局部最大坑蚀深度约为光圆钢筋最大坑蚀深度的1．7倍．     

早强微膨胀泵送C50钢管混凝土的研制

将硼酸、葡萄糖酸钠和聚羧酸盐减水剂按一定比例复合,研制出一种用于硫铝酸盐水泥的新型缓凝减水剂M,实验表明,该外加剂对硫铝酸盐水泥具有较强的缓凝及减水作用。采用该外加剂和42.5级快硬硫铝酸盐水泥,配制出具有良好工作性的C50早强微膨胀钢管混凝土:初始坍落度为24 cm,3 h后仍达18 cm以上,不离析、不泌水,满足泵送施工要求;其3 d抗压强度达50 MPa以上6、0 d自由膨胀率3.41×10-4,能有效解决钢管混凝土中普遍存在的脱粘问题。     

Estimating the stresses in cantilever beam loaded by a parabolically distributed load with Airy stress functions

This study presents three mathematical methods namely the polynomial stress function approach, the Fourier series form approach and the approximated equations form approach for finding the stress distribution in a cantilever beam with rectangular cross section loaded by a parabolically distributed load. By taking the stress function as a polynomial of the seventh degree, it is attempted to find the coefficients either in complete or in full shape of the polynomial. In the Fourier series approach, the load is discreted to unlimited series of harmonic loads and superposing resultant stresses is the affect of parabolically distributed load on the beam. The resultant stresses are compared with some approximated stress formulas which have been provided before. Finite element analysis are done for square, short, medium and long cantilever beams and the mathematical results of stress distribution in five different height of the beam was compared with FEM results. It was found good results for τ _yy and τ _xy in all cross section of the beams and acceptable results for τ _xx only in y = 0. It is found that discreting loads to even a limit number of harmonic loads and superposing the resultant stresses can give the distribution of τ _yy and τ _xy with the acceptable precision in medium and long cantilever beams with rectangular cross section.     

Material and bonding characteristics for dimensioning and modelling of textile reinforced concrete (TRC) elements

The collaborative research center “Textile Reinforced Concrete (TRC) – Development of a New Technology” (SFB 532) established at Aachen University (RWTH Aachen) is investigating the basic mechanisms of this new composite material. The use of technical textiles as reinforcement material in cementitious binder systems allows the production of thin-structured elements as will be dimensioned, modelled, and produced within the research project. For this reason the material properties of the single components have to be known and will be integrated in analytical and numerical simulations of textile reinforced structures. Thus key parameters on the meso-level are introduced. These are on the one hand the tensile strength and elastic modulus of filaments and rovings, on the other hand mechanical and fracture mechanical parameters of the matrix, and finally the bonding characteristics of filaments as well as rovings embedded in the cement based matrix.     

Ductility of FRP plated flexural members

The design of reinforced concrete (RC) flexural members such as beams, slabs and columns is intrinsically based on the inherent ductility of the member. In reinforced concrete beams and slabs, ductility is generally achieved by using ‘under-reinforced’ sections and generally governed by the neutral axis depth parameter k_u which requires ultimate failure by concrete crushing at a specified strain ε_c. As the plates of fibre reinforced polymer (FRP) plated RC beams can fracture or debond before the concrete crushes at ε_c, the k_u approach is not directly applicable. Hence, new fundamental approaches and a deeper understanding of ductility are required which are the subjects of this paper.     

Active damping of rotating composite thin-walled beams using MFC actuators and PVDF sensors

The object of this research is to enhance the damping performance for vibration suppression of rotating composite thin-walled beams using MFC actuators and PVDF sensors. The formulation is based on single cell composite beam including a warping function, centrifugal force, Coriolis acceleration and piezoelectric effect. Adaptive capability of the beam is acquired through the use of a negative velocity feedback control algorithm. Numerical analysis is performed using finite element method and Newmark time integration method is used to calculate the time response of the model. It is observed that the feedback control gain has an effect on damping performance. The paper continues with an investigation into influences of parameters such as the rotating speed and the fiber orientation in host structures. Also, it is confirmed that effective damping performance is achievable through the suitable arrangement and distributed size of sensor and actuator pair using case study.     

Magnetic properties and microstructure of FePt–SiO2 nanocomposite films fabricated by nanocluster beam technology

Nanocluster beam technology combined with conventional sputtering was used to fabricate FePt–SiO₂ nanocomposite films in this present work. The post-deposition annealing affected the final particle size and size distribution of FePt nanoclusters. The effects of both volume fraction of SiO₂ matrix and annealing temperature on magnetic and microstructural properties were studied. Partial ordering, grain growth and agglomeration of FePt particles in FePt–SiO₂ nanocomposite films occurred during annealing. A higher volume fraction of SiO₂ matrix was effective in suppressing diffusion of atoms and magnetic exchange coupling of FePt grains. Excessive SiO₂ however resulted in a lower degree of FePt ordering and thus lower coercivity.     

Deep levels in GaInAs grown by molecular beam epitaxy and their concentration reduction with annealing treatment

X-ray diffraction (XRD), current–voltage (IV), capacitance–voltage (CV), deep-level transient Fourier spectroscopy (DLTFS) and isothermal transient spectroscopy (ITS) techniques are used to investigate the thermal annealing behaviour of three deep levels in Ga_0.986In_0.014As heavily doped with Si (6.8 × 10¹⁷ cm⁻³) grown by molecular beam epitaxy (MBE). The thermal annealing was performed at 625 °C, 650 °C, 675 °C, 700 °C and 750 °C for 5 min. XRD study shows good structural quality of the samples and yields an In composition of 1.4%. Two main electron traps are detected by DLTFS and ITS around 280 K, with activation energies of 0.58 eV and 0.57 eV, capture cross sections of 9 × 10⁻¹⁵ cm² and 8.6 × 10⁻¹⁴ cm² and densities of 2.8 × 10¹⁶ cm⁻³ and 9.6 × 10¹⁵ cm⁻³, respectively. They appear overlapped and as a single peak, which divides into two smaller peaks after annealing at 625 °C for 5 min.

Annealing at higher temperatures further reduces the trap concentrations. A secondary electron trap is found at 150 K with an activation energy of 0.274 eV, a capture cross section of 8.64 × 10⁻¹⁵ cm² and a density of 1.38 × 10¹⁵ cm⁻³. The concentration of this trap level is also decreased by thermal annealing.