Measuring Fracture Energy in a Brittle Polymeric Material: Application of a High-Speed Optical Extensometer

The brittle fracture of polymethyl methacrylate (PMMA) was studied using a high-speed extensometer, which consisted of an optical fiber and a position-sensing detector (PSD). Single-edge-cracked tensile specimens were pin-loaded with a special jig so that they could split and fly in the loading direction after fracture. The flying height and residual deformation of the split specimen were measured to estimate the elastic energy E _e and non-elastic energy E _n, respectively. By subtracting E _e and E _n from the external work U _ex applied to the specimen, the fracture energy E _f for creating a new fracture surface was evaluated. The results showed that E _e, E _n, and E _f increased with U _ex, and the ratio E _f/U _ex was about 45% over a wide range of U _ex. Energy release rate was also estimated using U _ex or E _f, and the results suggested that it was overestimated if E _e and E _n were included.     

Crack in functionally graded piezoelectric strip bonded to elastic surface layers under electromechanical loading

Solved is the problem of a crack in a functionally graded piezoelectric material (FGPM) bonded to two elastic surface layers. It is assumed that the elastic stiffness, piezoelectric constant, and dielectric permittivity of the FGPM vary continuously along the thickness of the strip. The outside layers are under antiplane mechanical loading and in-plane electric loading. The solution involves solving singular integral equations by application of the Gauss–Jacobi integration formula. Numerical calculations are carried out to obtain the energy density factors. Their variations with the geometric, loading and material parameters are shown graphically.     

Material plasticity dependence of mixed mode fatigue crack growth in commonly used engineering materials

In this paper, an investigation of fatigue crack propagation in rectangular plates containing an inclined surface crack is presented. A criterion for the three-dimensional stress state is proposed to predict fatigue crack initiation angles. It is assumed that the direction of crack initiation coincides with the direction of the minimum radius of the plastic zone defined by the von Mises yield criterion. The maximum energy release rate criterion, i.e., G_max criterion, is extended to study the fatigue crack growth characteristics of mixed mode cracks. A modification has been made to this criterion to implement the consideration of the plastic strain energy. Subsequently, this concept is applied to predict crack growth due to fatigue loads. Experiments for checking the theoretical predictions from the proposed criterion have been conducted. The results obtained are compared with those obtained using the commonly employed fracture criteria and the test data.     

随机载荷作用下浅曲结构首次发生突变时间的概率分析

本文用能量包线随机平均法,导出了随机载荷作用下浅曲结构首次发生突变的时间的各阶矩的解析表达式,并通过用臆式差分法数字求解条件可靠性函数所满足的后向柯尔英哥洛夫方程,给出了发生首次突变的时间的概率密度.与数字模拟结果比较表明,在结构阻尼不大时该法给出令人满意的结果.文中还研究了各种参数对结构首次发生突变的时间统计量的影响.     

Strain energy release rate for interfacial cracks in hybrid beams

The finite element modeling and fracture mechanics concept were used to study the interfacial fracture of a FRP-concrete hybrid structure. The strain energy release rate of the interfacial crack was calculated by the virtual crack extension method. It is shown that the crack growth has three phases, namely, cracking initiation, stable crack growth and unstable crack propagation. The effects of geometric and physical parameters of the hybrid beam on the energy release rate were considered. These parameters include Young’s moduli of the FRP, the concrete and the adhesive, thickness of the FRP plate and adhesive, and the distance of FRP plate end from the beam end. The numerical results show that the energy release rate of the interfacial crack is influenced considerably by these parameters. The present investigation can contribute to the mechanism understanding and engineering design of the hybrid structures.     

Spatio-spectral analyses of electromagnetic wave energy absorption and heating effect

This study presents a theoretical analysis method to calculate electromagnetic (EM) wave power absorption spectrum of materials by using attenuation coefficients. The heating effect of EM waves is modeled to analyze spectral distribution of temperature rises inside material body as a result of EM wave power absorption. These analyses are very useful for the investigation of electromagnetic wave-material interaction on the bases of electro-physical material parameters (permittivity, permeability and conductivity). An illustrative analysis of spatio-spectral distribution of EM wave energy absorption and resulting heating effect were conducted for muscle tissues and the results are discussed.     

A FEM-based method to determine the complex material properties of piezoelectric disks

Numerical simulations allow modeling piezoelectric devices and ultrasonic transducers. However, the accuracy in the results is limited by the precise knowledge of the elastic, dielectric and piezoelectric properties of the piezoelectric material. To introduce the energy losses, these properties can be represented by complex numbers, where the real part of the model essentially determines the resonance frequencies and the imaginary part determines the amplitude of each resonant mode. In this work, a method based on the Finite Element Method (FEM) is modified to obtain the imaginary material properties of piezoelectric disks. The material properties are determined from the electrical impedance curve of the disk, which is measured by an impedance analyzer. The method consists in obtaining the material properties that minimize the error between experimental and numerical impedance curves over a wide range of frequencies. The proposed methodology starts with a sensitivity analysis of each parameter, determining the influence of each parameter over a set of resonant modes. Sensitivity results are used to implement a preliminary algorithm approaching the solution in order to avoid the search to be trapped into a local minimum. The method is applied to determine the material properties of a Pz27 disk sample from Ferroperm. The obtained properties are used to calculate the electrical impedance curve of the disk with a Finite Element algorithm, which is compared with the experimental electrical impedance curve. Additionally, the results were validated by comparing the numerical displacement profile with the displacements measured by a laser Doppler vibrometer. The comparison between the numerical and experimental results shows excellent agreement for both electrical impedance curve and for the displacement profile over the disk surface. The agreement between numerical and experimental displacement profiles shows that, although only the electrical impedance curve is considered in the adjustment procedure, the obtained material properties allow simulating the displacement amplitude accurately.     

荧光光谱法研究胆红素与牛血清白蛋白的相互作用

在模拟生理条件下,利用荧光光谱法研究了胆红素BR和牛血清白蛋白的相互作用。实验结果表明胆红素BR对牛血清白蛋白有较强的荧光猝灭作用,两者形成了新的复合物,属于静态荧光猝灭,发生分子内的非辐射能量转移。根据荧光猝灭法得到胆红素与牛血清白蛋白的结合常数和结合位点数。根据Foster非辐射能量传递理论确定了胆红素BR和牛血清白蛋白间的结合距离。此外,利用同步荧光光谱分析胆红素BR对牛血清白蛋白构象的影响。     

Infrared-to-visible up-conversion luminescence and energy transfer of RE3+/Yb3+(RE = Ho,Tm) co-doped SrIn2O4

Near-infrared excited up-conversion phosphors of RE³⁺/Yb³⁺(RE = Ho, Tm) co-doped SrIn₂O₄ were synthesized by a solid-state reaction method. X-ray diffraction analysis revealed the phase composition of those samples, and the up-conversion spectroscopic properties were studied in terms of up-conversion emission spectra. Under 980 nm near-infrared laser excitation, strong green emission with the peak at 546 nm was observed in SrIn₂O₄: Ho³⁺/Yb³⁺, which can be assigned to the characteristic ⁵S₂(⁵F₄) → ⁵I₈ transition of Ho³⁺. Furthermore, SrIn₂O₄: Tm³⁺/Yb³⁺ showed bright blue emission with the peak at 486 nm, which is associated with the ¹G₄ → ³H₆ transition of Tm³⁺. The UC power studies indicated that the luminescence of SrIn₂O₄: Ho³⁺/Yb³⁺ and SrIn₂O₄: Tm³⁺/Yb³⁺ are attributed to two-photon and three-photon process, respectively. The possible UC luminescence mechanism and energy transfer in SrIn₂O₄: RE³⁺/Yb³⁺ were discussed.     

六方相LaOF纳米体系中Sm3+/Eu3+的能量转移效应

为了探讨Sm3+/Eu3+共掺体系的荧光光谱特性和能量转移机理,采用水热-烧结法制备了Sm3+/Eu3+共掺LaOF纳米晶体颗粒,并用X射线衍射和透射电子显微镜对纳米晶体颗粒进行了表征.结果显示,所制备的纳米晶体颗粒呈六方相,均匀性和分散性良好,平均粒径为70 nm左右.通过442 nm连续光激发,在六方相LaOF:Sm3+/Eu3+纳米体系中实现了Sm3+离子向Eu3+离子的能量转移,观测到了因能量转移效应而产生的Eu3+离子5D0能级的荧光发射谱线.光谱学研究发现,能量转移源于Sm3+离子4G5/2能级向Eu3+离子5D0能级的弛豫过程,并且随着受主离子Eu3+浓度的增加,能量转移效率也随之提高.