基于激光超声场检测的材料常数测量方法

由于采用干涉仪的激光超声技术造价较高,对环境噪声极为敏感,很难在工业中得到广泛应用,为此开发了基于压电传感器的激光超声场检测仪(LUFT)。该系统采用激光激发、压电传感接收的方式,既保留了激光高分辨率的优势,又兼具了系统造价低、使用方便的特点。本文中,利用该检测仪对标准钢试样及不同厚度的铝试样进行了测量,获得其表面波和纵波波速,并根据固体力学理论得到铝的泊松比和杨氏模量。计算结果与文献参考值误差较小,说明该方法的可行性,也说明了该激光超声场检测仪的可靠性。     

Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers(EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave's mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT's meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT's geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated(strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.     

Na/Bi配比对0.94(Bi0.5+xNa0.5-x)TiO3-0.06BaTiO3压电陶瓷组织结构与性能的影响

采用传统固相烧结方法制备出0.94(Ri0.5+xNa0.5-x)TiO3-0.06BaTiO3(BNBT6)二元系无铅压电陶瓷(x分别为0,0.08;,0.12;,0.16;,0.24;和0.50;摩尔分数),系统地研究了不同Na/Bi配比对BNT基陶瓷材料物相结构、显微组织和压电、介电性能的影响.结果表明:添加不同的Na/Bi,所制备的BNBT6压电陶瓷组织分布均匀、致密度高,存在三方-四方共存的准同型相界结构,且不同的Na/Bi配比不影响陶瓷的相结构,但其烧结性能及电性能与Na/Bi配比密切相关,当x=0.16;时,BNBT6陶瓷样品的性能最佳,相对密度达到97;,在1 kHz的测试频率下,BNBT6陶瓷样品的压电常数d33为138 PC/N、介电常数εr为1486、介电损耗tanδ为2.1;、机械品质因数Qm为217.     

Exact analytical solutions for the local and global buckling of sandwich beam-columns under various loadings

Sandwich structures are widely used in many industrial applications, due to the attractive combination of a lightweight and strong mechanical properties. This compromise is realized thanks to the presence of different parts in the composite material, namely the skins and possibly core reinforcements or thin-walled core structure which are both thin/slender and stiff relative to the other parts, namely the homogeneous core material, if any. The buckling phenomenon thus becomes mainly responsible for the final collapse of such sandwiches. In this paper, classical sandwich beam-columns (with homogeneous core materials) are considered and elastic buckling analyses are performed in order to derive the critical values and the associated bifurcation modes under various loadings (compression and pure bending). The two faces are represented by Euler–Bernoulli beams, whereas the core material is considered as a 2D continuous solid. A set of partial differential equations is first obtained from a general bifurcation analysis, using the above assumptions. Original closed-form analytical solutions of the critical loading and mode of a sandwich beam-column are then derived for various loading conditions. Finally, the proposed analytical formulae are validated using 2D linearized buckling finite element computations, and parametric analyses are performed.     

Vibration analysis and active control of nearly periodic two-span beams with piezoelectric actuator/sensor pairs

The piezoelectric materials are used to investigate the active vibration control of ordered/disordered periodic two-span beams. The equation of motion of each sub-beam with piezoelectric patches is established based on Hamilton's principle with an assumed mode method. The velocity feedback control algorithm is used to design the controller. The free and forced vibration behaviors of the two-span beams with the piezoelectric actuators and sensors are analyzed. The vibration properties of the disordered two-span beams caused by misplacing the middle support are also researched. In addition, the effects of the length disorder degree on the vibration performances of the disordered beams are investigated. From the numerical results, it can be concluded that the disorder in the length of the periodic two-span beams will cause vibration localizations of the free and forced vibrations of the structure, and the vibration localization phenomenon will be more and more obvious when the length difference between the two sub-beams increases. Moreover, when the velocity feedback control is used, both the forced and the free vibrations will be suppressed. Meanwhile, the vibration behaviors of the two-span beam are tuned.     

厚度振动模式下背衬结构PZT的声能特性

为了充分利用埋入混凝土中压电陶瓷所激励的声能,采用了一种在压电陶瓷圆片结构的一面设置金属背衬的方法将压电陶瓷双侧的声能集中到一侧,提高了声指向性和声能。实验表明:该方法能使同距离下压电陶瓷圆片轴向声能提高10倍以上,并且得到了声能的相对增量随背衬厚度和声激励频率之积变化的特性曲线。声信号的谱分析还表明:背衬没有改变声信号的主频和主峰区间的频率带宽;当声激励频率为谐振频率79kHz时,最优背衬厚度为2.4mm;当声激励频率f为60kHz～100kHz时,接收端声能相对较大,适合用于实际的压电埋入式超声无损检测。     

Basic solutions of multiple parallel symmetric mode-III cracks in functionally graded piezoelectric/piezomagnetic material plane

The Schmidt method is adopted to investigate the fracture problem of multiple parallel symmetric and permeable finite length mode-III cracks in a functionally graded piezoelectric/piezomagnetic material plane. This problem is formulated into dual integral equations, in which the unknown variables are the displacement jumps across the crack surfaces. In order to obtain the dual integral equations, the displacement jumps across the crack surfaces are directly expanded as a series of Jacobi polynomials. The results show that the stress, the electric displacement, and the magnetic flux intensity factors of cracks depend on the crack length, the functionally graded parameter, and the distance among the multiple parallel cracks. The crack shielding effect is also obviously presented in a functionally graded piezoelectric/piezomagnetic material plane with mul- tiple parallel symmetric mode-III cracks.     

THE BASIC SOLUTION OF A 3-D RECTANGULAR PERMEABLE CRACK IN A PIEZOELECTRIC/PIEZOMAGNETIC COMPOSITE MATERIAL

The solution of a 3-D rectangular permeable crack in a piezoelectric/piezomagnetic composite material was investigated by using the generalized Almansi’s theorem and the Schmidt method.The problem was formulated through Fourier transform into three pairs of dual integral equations,in which the unknown variables are the displacement jumps across the crack surfaces.To solve the dual integral equations,the displacement jumps across the crack surfaces were directly expanded as a series of Jacobi polynomials.Finally,the relations between the electric filed,the magnetic flux field and the stress field near the crack edges were obtained and the efects of the shape of the rectangular crack on the stress,the electric displacement and magnetic flux intensity factors in a piezoelectric/piezomagnetic composite material were analyzed.     

Determination of displacement with a piezoelectric transducer using phase-shift algorithms

This investigation accurately measures the relationship between applied voltage and the displacement of a piezoelectric transducer device both by analyzing phase shifting interferometry algorithms and using a Twyman-Green interferometer. Simulations are performed to calculate unknown phase shifts, especially around π/2 and 3π/2. It is indicated that the Hariharan algorithm is indeed an effective phase shifting interferometry algorithm.