基于小波有限元法的连续梁移动荷载识别

将待识别车载简化为移动荷载投影到小波空间;车载所作用的连续梁模型以小波尺度函数为插值函数,以小波有限元法为建模方法,并通过单元转换矩阵实现了小波空间向物理空间的变换;采用部分加速度信号作为测得动响应数据,据此积分求出速度与位移响应并用于识别移动荷载。识别方法则借助了动态规划法与正则化法,避免了时程分析中的振荡现象。仿真算例验证了小波有限元用于连续梁模型移动荷载识别的可行性,且单元数较少;识别过程中所用一阶Tikhonov正则化法具有平滑去噪能力。     

Analysis of geometric features of quasi-brittle profiles of iron fractures with various levels of wavelet filtration in relation to the grain size of the ferritic structure

In this paper a relationship between metal structures described by equivalent grain size D and the average distance between local maximum and minimum points (LMMD—Local Maximum Minimum Distance) of fracture profiles for brittle fractures of commercially pure iron (99.65% Fe) samples is examined. The ferrite grain size was varied by cold upsetting and recrystallization annealing of the specimens. Impact loads, at liquid nitrogen temperature (Charpy V test), were used to break the samples. Fracture profiles were recorded using a contact profilometer. The profiles were subjected to wavelet filtration and a linear dependence LMMD = f(D) was revealed, and observed irrespective of the filtration level range of 2–7. Profiles were examined at low-level frequency wavelet approximations of raw profiles and with high-level frequencies to obtain details of these same profiles.     

Geometric surface relief and the allotropic transformation in iron

Geometric surface relief effects have been observed in association with the allotropic transformation in iron. The selected-area electron channelling pattern technique has been used to establish that tent shaped geometric relief effects, common to single precipitate plates in Fe-C and Fe-C-X alloys, are also associated with similar single crystal morphologies in pure iron. These experimental observations extend to the allotropic transformation in iron the same rationale for the relationship of interfacial structure to surface relief effects that is known to apply in the instance of precipitation reactions. In addition it provides support for an earlier proposal that massive transformations can be subject to the same crystallographic constraints as precipitation processes.     

Analyzing laser plasma interferograms with a continuous wavelet transform ridge extraction technique: the method

Laser plasma interferograms are currently analyzed by extraction of the phase-shift map with fast Fourier transform (FFT) techniques [Appl. Opt. 18, 3101 (1985)]. This methodology works well when interferograms are only marginally affected by noise and reduction of fringe visibility, but it can fail to produce accurate phase-shift maps when low-quality images are dealt with. We present a novel procedure for a phase-shift map computation that makes extensive use of the ridge extraction in the continuous wavelet transform (CWT) framework. The CWT tool is flexible because of the wide adaptability of the analyzing basis, and it can be accurate because of the intrinsic noise reduction in the ridge extraction. A comparative analysis of the accuracy performances of the new tool and the FFT-based one shows that the CWT-based tool produces phase maps considerably less noisy and that it can better resolve local inhomogeneties.     

Determination of the fracture energy of a composite by the method of digital speckle correlation

We have developed a computational-and-experimental procedure for evaluating the fracture energy of composite materials, which lies in the determination of surface displacements in a neighborhood of the tip of a stress concentrator under static tension by the method of digital speckle correlation. Using the known displacement distribution, we calculate strains in the prefracture zone as well as the opening and shear of crack lips for given loading force and measuring base. The static tensile stress-strain diagram, together with the established specimen strain, enables us to find stresses in the prefracture zone. Using these data, we find the fracture energy of the composite. We also present some results of investigations for a three-layer composite material based on an epoxy-phenol plastic reinforced chaotically with disperse particles.     

Application of an indentation method to the fracture mechanics study of a polycrystalline graphite

None of the conventional indentation techniques are applicable to carbon and graphite materials for determining fracture mechanics parameters because of the difficulty in introducing well-defined median/radial cracks. A novel indentation method is proposed in this work for fracture mechanics studies and then applied to a polycrystalline graphite fracture. The most prominent advantage of the indenter designed is that the residual stresses beneath the indentation impression, which prevail in conventional indentation methods (Knoop and Vickers indentations) and lead to crucial difficulties in fracture mechanics analysis, are negligibly small. This makes possible a quantitative study on the microstructural interaction between the indentation-induced micro-flaw and the natural intrinsic flaws of the material. The dependence of flexural strength of a polycrystalline graphite on the indentation-induced surface flaw size is also discussed by examining the microstructural scaling transition of fracture origin from the indentation-induced to the intrinsic flaws with diminishing indentation surface flaw. An important role of the Mrozowsky micro-crack system in the scaling transition is emphasized.     

Grain size dependence of dielectric properties of ultrafine BaTiO3 prepared by a sol-crystal method

Ultrafine BaTiO3 prepared by a decomposition of an organometallic crystal with unity of Ba/Ti ratio (sol-crystal method) has been characterized. While the as-prepared product resulting from the decomposition of the organometallic crystal at room temperature was BaTiO3 with pseudo-cubic structure, the well-crystallized tetragonal polymorph was obtained by firing the as-prepared product above 1000°C. Residual organic compounds, CO2-3 and OH- ions in the samples prevent the grain growth and tetragonal distortion of BaTiO3. We obtained quite higher room temperature permittivity (3700) at 1 kHz for the sample fired at 1200°C than that (630) prepared by conventional solid-state reaction starting from BaCO3 and TiO2. Such a high value was probably due to the accomplishment of homogeneous cation stoichiometry, which was achieved by this preparation method via the organometallic crystal with stoichiometric Ba/Ti ratio. This revised version was published online in November 2006 with corrections to the Cover Date.     

Grain size effect on the R-phase transformation of nanocrystalline NiTi shape memory alloys

Development of nanoscale actuators and sensors in recent years calls for functional materials with small dimensions and high strengths. High strength nanocrystalline NiTi alloys which experience the R-phase transformation with a small thermal hysteresis are ideal candidates for these applications. To facilitate the application of the R-phase transformation in nanocrystalline NiTi alloys, this study investigated the effect of grain size on the R-phase transformation of a nanocrystalline Ti-50.2at.%Ni alloy. The nanometric grain size was created by severe cold deformation and low temperature anneal. It was found that in the recrystallized state, achieving nanoscale grain sizes (<100 nm) was effective in suppressing the B2→B19’ martensitic transformation and revealing the B2?R transformation. The B2?R transformation temperature was found to increase with the decreasing grain size within the range of 22–155 nm. The suppression of the B19’ martensite in nanograins is attributed to the limited space within the grains to allow the formation of self-accommodation structures to contain the large lattice distortion of the martensite.     

Correlation of geometry effects with fracture toughness by damage equivalence

The near crack tip porosity fields in different fracture specimens, single edge notched, single edge notched loaded in centre of ligament, three point bending specimens, and small scale yielding (SSY) mode have been studied by the finite deformation finite element method. The presence and subsequent growth of smaller-scale voids were taken into account by using Gurson's model to describe the constitutive behaviour of the material. Based on damage equivalence at a characteristic position in the SSY mode and actual fracture specimens, the ratio of scaling parameters,J values, in both modes was obtained, and was used to eliminate the geometry dependence of fracture toughness through correlations to the small scale yielding mode.On leave, Department of Mechanical and Intelligent Systems Engineering, Tokyo Institute of Technology, Tokyo, Japan.     

Demodulation of single interferograms using a sliding 2-D continuous wavelet transform method

In this paper, we present an alternative technique for the demodulation of single interferograms using a sliding 2-D continuous wavelet transform (2-D CWT) method. The sliding strategy proposed in this work is used with two purposes: to reduce the processing time when the 2-D CWT is applied, and to solve the problem of the phase ambiguity when closed fringes are present. Experimental results with real and simulated interferograms show that the proposed multiresolution method is a proper alternative for many applications of interferogram demodulation with closed fringes.     

基于加速度响应连续小波变换的线性时变结构瞬时频率识别

推导了函数积分运算的连续小波变换计算方法。借助此法,假设线性时变结构的质量系数为时不变常数,或者其随时间变化的规律已被经验掌握,仅利用线性时变结构自由振动的加速度响应信号,就可以计算出速度响应和位移响应信号的连续小波变换值。将一小段时刻点的线性代数方程组构造成最小二乘问题,求解最小二乘解识别出结构的时变阻尼和时变刚度,从而确定时变结构的瞬时频率。通过对5层剪切梁楼房模型和3自由度密集模态时变结构瞬时频率的识别,验证了识别方法的正确性、有效性和抗噪声能力.     

Simulation of iron fracture caused by a blast wave

A numerical simulation is made of the fracture of iron caused by a blast wave. The numerical method is described and results of calculations using a one-dimensional formulation of three qualitatively different explosion regimes are presented. Pis’ma Zh. Tekh. Fiz. 23, 33–39 (May 12, 1997)     

Relieving micro-strain by introducing macro-strain in a polycrystalline metal surface by cavitation shotless peening

Peening using cavitation impact is called “cavitation shotless peening CSP”, since there is no requirement for shot in the process. Micro- and macro-strain of polycrystalline metal peened by CSP were evaluated using X-ray diffraction methods, as the full width at half maximum (FWHM) of the X-ray diffraction profile from the peened surface was decreased, although compressive residual stress was introduced. It was found that CSP reduced the micro-strain in the surface, but simultaneously introduced compressive residual stress, i.e., a macro-strain. The results demonstrate that the micro-strain is relieved by CSP without the need for heat treatment, and is, therefore, a sort of annealing. Thus, CSP can renew the metallic material while the shape itself is maintained.