Subcritical statistics in rupture of fibrous materials: experiments and model

We study experimentally the slow growth of a single crack in a fibrous material and observe stepwise growth dynamics. We model the material as a lattice where the crack is pinned by elastic traps and grows due to thermally activated stress fluctuations. In agreement with experimental data we find that the distribution of step sizes follows subcritical point statistics with a power law (exponent 3/2) and a stress-dependent exponential cutoff diverging at the critical rupture threshold.     

Failure of a welded pressure vessel due to creep: damage initiation,evolution and reheat cracking

Continuum damage mechanics (CDM)-based constitutive equations are reviewed and presented for austenitic AISI 316H stainless steels, which undergo creep deformation and damage at 550°C. Two multi-axial stress rupture functions/criteria have been utilised. Equations and the finite element computer code, DAMAGE XX, were used to carry out CDM studies of the weldment in an axi-symmetric equivalent of the flank section of a pressurised superheater header at 550°C. Damage and lifetime predictions were made using finite and small deformation solution techniques; an insignificant difference was observed. It is shown that, using appropriate mesh refinement techniques and selecting suitable weld material ductility, CDM can be used to predict damage initiation and growth and, subsequent creep crack growth due to reheat cracking. Predicted damage initiation and growth modes are shown to correlate well with experimental and in-service observations. The results highlight the importance of multi-axial stress rupture criteria and underscore the need for more accurate materials data, under the relevant multi-axial stress states, on well-characterized material batches. Also, ductilities of the different material phases of the weld, strain affected zone, fusion boundary and weld materials, are shown to play an important role and, therefore, require more accurate determination. It is concluded that a more precise determination of the mechanical properties is necessary if the governing physical mechanisms of reheat cracking and the conditions for possible crack arrest are to be understood.     

CRACK IDENTIFICATION IN BEAM STRUCTURES USING MECHANICAL IMPEDANCE

The influence of a transverse open crack on the mechanical impedance of cracked beams under various boundary conditions is investigated both analytically and experimentally. It is shown that the driving-point impedance changes substantially due to the presence of the crack and the changes depend on the location and the size of the crack and on the force location. Monitoring the change of the first antiresonance as a function of the measuring location along the beam, a jump in the slope of the plot in the vicinity of the crack occurs. The magnitude of the jump depends on the crack size. Experimental results are presented from tests on Plexiglas beams damaged at different locations and different magnitudes and are found to be in good agreement with theoretical predictions. Based on the results of the present work, an efficient prediction scheme for accurate crack localization is proposed.     

A method for determining the location and depth of cracks in double-cracked beams

The influence of two transverse open cracks on the antiresonances of a double cracked cantilever beam is investigated both analytically and experimentally. It is shown that there is a shift in the antiresonances of the cracked beam depending on the location and size of the cracks. These antiresonance changes, complementary with natural frequency changes, can be used as additional information carrier for crack identification in double cracked beams. Experimental results from tests on plexiglas beams damaged at different locations and different magnitudes are found to be in good agreement with theoretical predictions. Based on the results of the present work, an efficient prediction scheme for crack localization and characterization in double cracked beams is proposed.     

The linear noise approximation for reaction-diffusion systems on networks

Stochastic reaction-diffusion models can be analytically studied on complex networks using the linear noise approximation. This is illustrated through the use of a specific stochastic model, which displays travelling waves in its deterministic limit. The role of stochastic fluctuations is investigated and shown to drive the emergence of stochastic waves beyond the region of the instability predicted from the deterministic theory. Simulations are performed to test the theoretical results and are analyzed via a generalized Fourier transform algorithm. This transform is defined using the eigenvectors of the discrete Laplacian defined on the network. A peak in the numerical power spectrum of the fluctuations is observed in quantitative agreement with the theoretical predictions.     

强激光辐照下窗口材料靶传热及热应力破裂的解析分析

用有限Ｈａｎｋｅｌ积分变换法对轴对称强激光束辐照下平面薄靶（或薄膜）传热及热应力分布进行了解析推导，得到了随时间演变的温度场及相应的热应力场的解析解。对比有关实验，对强激光束辐照窗口材料（例如ＭｇＦ２晶体、Ｋ９玻璃）靶引起其脆性热应力破裂进行了分析，详细讨论了激光强度、激光－靶能量耦合系数、辐照时间及光斑尺寸等因素对靶的破坏的影响。     

Experimental investigation of the travel-time variance of an acoustic wave propagating through the grid-generated turbulence

An experimental technique for the investigation of the behaviour of acoustic wave propagation through a turbulent medium is discussed. The present study utilizes the ultrasonic travel-time technique to diagnose a grid-generated turbulence. Travel-time variance is studied versus mean flow velocity, travel distance and outer turbulence scale. The effect of thermal fluctuations, which result in fluctuations of sound speed, is studied using a heated-grid experiment. Experimental data obtained using ultrasonic technique confirm numerical and theoretical predictions of nonlinear increase of the travel-time variance with propagation distance, which could be connected to the occurrence of caustics. The effect of turbulent intensity on the travel-time variance and appearance of caustics is studied. It is demonstrated experimentally that the higher turbulence intensity leads to the shorter distance, at which the first caustic occurs. The probability density for caustics appearance is analysed against the measured wave amplitude fluctuations. The analysis reveals that the region of high-amplitude fluctuations corresponds to the region where the probability of formation of random caustics differs from zero. Experimental results are in very good agreement with theoretical and numerical predictions.     

Periodic adhesive fingers between contacting cells

We study in detail the properties of fingers, a particular type of cell-cell adhesive structures appearing in adherens junctions. These periodic patterns break the symmetry of cell-cell contacts. We show that finger formation is driven by cadherin interactions and actin growth. A theoretical model is introduced in which the growth of fingers is limited by membrane tension. The steady shape and formation kinetics of fingers are experimentally measured and compared with the theoretical predictions.     

Crack tip dislocation emission and nanoscale deformation fields in silicon

A micro-crack in silicon was experimentally investigated by using a combination of transmission electron microscopy and geometric phase analysis. The strain fields of the crack tip, with scales of a few tens of nanometers, were mapped. The crack tip dislocation emission and stress relief by dislocation generation around a crack tip can be proved. And, the strain field of an edge dislocation was compared with the Peierls–Nabarro dislocation model at the scale of a dislocation width. We show that the Peierls–Nabarro model is the appropriate theoretical model to describe the deformation fields of the dislocation core.     

Nucleation, Growth, and Scaling in Slow Combustion

We study the nucleation and growth of flame fronts in slow combustion. This is modeled by a set of reaction-diffusion equations for the temperature field, coupled to a background of reactants and augmented by a term describing random temperature fluctuations for ignition. We establish connections between this model and the classical theories of nucleation and growth of droplets from a metastable phase. Our results are in good agreement with theoretical predictions.     

Suppression of noise in a noisy optical trap

We have experimentally explored and clarified the concept of noise suppression in intrinsically noisy systems by adding noise at the input using a microscopic bead held in a moving and intensity-modulated optical trap. By coupling the stiffness of the optical trap to its position, we have explicitly constructed an experimental model system in which added fluctuations in the trap position result in reduced variance of the observed bead position as compared to a stationary trap. This reduction in variance and the spectral properties of the observed output noise agree with theoretical predictions. Our experiment demonstrates that the essential aspect of noise reduction in such a system is that the added fluctuations drive the system into states with a reduced intensity of intrinsic noise sufficiently often.     

Thermal and kinetic surface roughening studied by scanning tunneling microscopy and atomic force microscopy

Recent studies of thermal roughening on Si surfaces and kinetic roughening of some growing films, copper and tungsten, by using scanning tunneling microscopy and atomic force microscopy are reviewed. A logarithmic divergence of the surface height fluctuations of Si(111) vicinal surfaces is confirmed, in agreement with the theoretical prediction of rough surface in thermal equilibrium. For the kinetically formed rough surfaces, power law dependences of the interface width on the system size are clearly observed. Furthermore, the tungsten films show a short-range scaling regime and a long-range “smooth” regime. The roughness exponents α are compared with theoretical predictions: for the typical Cu electrode position condition (α=1/2), the exponent appears to be close to that found for local growth models, and for tungsten films (0.7～0.8), it is consistent with recent predictions for growth where surface diffusion is predominant.     

Band-structure determination for finite 3-D photonic crystals

The partial band structure from a finite photonic crystal is determined using a model based on light diffraction and the transfer-matrix formalism. The predictions from such a model are compared to an experimental measurement of the bands in the LU direction of a face centered cubic colloidal crystal. Then, both the theoretical predictions and the experimental measurements are compared with the usual band-structure calculation based on a plane-wave expansion with perfectly periodic boundary conditions. As in measurements performed in the past, discrepancies between the predictions of this later model and the experimentally determined bands are observed. On the contrary, using the model presented based on light propagation through a finite crystal, where no periodicity is imposed in the direction perpendicular to any of the set of planes considered to determine a specific branch of the band structure, we found a very good agreement between the experimentally determined and the predicted bandwidths.     

相干梯度敏感干涉测量技术及在静态断裂力学实验中的应用

介绍了相干梯度敏感(CGS)干涉测量技术的基本原理及其在静态Ⅰ型断裂实验中的应用,验证了该方法对裂纹尖端局部变形场和断裂特性进行定量研究的可行性。给出了代表静态Ⅰ型裂纹尖端奇异场光力学信息的CGS控制方程,模拟并分析了Ⅰ型裂纹尖端的CGS条纹模式,对静态Ⅰ型裂纹尖端变形场和断裂特性进行了三点弯曲的CGS试验,并提取了应力强度因子KⅠ。结果表明,试验结果与理论分析结果相吻合。     

Distribution of eigenfrequencies for vibrating plates

Acoustic spectra of free plates with a chaotic billiard shape have been measured, and all resonance frequencies in the range 0-500 kHz have been identified. The spectral fluctuations are analyzed and compared to predictions of the Gaussian Orthogonal Ensemble (GOE) of random matrices. The best agreement is found with a superposition of two independent GOE spectra with equal density which indicates that two types of eigenmodes contribute to the same extent. To explain and predict these results a detailed theoretical analysis is carried out below the first cut-off frequency where only flexural and in-plane vibrations are possible. Using three-dimensional plate dispersion relations and two-dimensional models for flexural and in-plane vibrations we obtained two first terms of the asymptotic expansion of the counting function of these eigenmodes. The contribution of edge modes is also discussed. The results are in a very good agreement with the experimentally measured number of modes. The analysis shows that the two types of modes have almost equal level density in the measured frequency interval, and this explains the observed spectral statistics. For a plate with broken symmetry in the up-down direction (where flexural and in-plane modes are strongly coupled) experimentally observed spectral fluctuations correspond to a single GOE spectrum. Above the first cut-off frequency a greater complexity of the spectral fluctuations is expected since a larger number of types of modes will contribute to the spectrum. Received 5 January 1999 and Received in final form 5 September 1999     

Finite element analysis of Rayleigh wave interaction with finite-size,surface-breaking cracks

The interaction of surface acoustic waves with finite-size, surface-breaking, semi-circular cracks is studied numerically, and experimentally. We focus on the behavior of the reflection coefficient of the Rayleigh wave from such cracks in the far field of the crack, when the depth of the crack is comparable to the wavelength of the interrogating surface wave. The cases in which the depth of the crack is much smaller or much larger compared to the wavelength have been extensively investigated by many authors and are not considered here except for validating the numerical and experimental results in these regimes. The theoretical, finite element, and experimental results presented are in very good agreement over the range were the crack depth is much smaller or much larger compared to the wavelength of the incident Rayleigh wave. In the transition regime, between these two limiting cases, only the finite element and experimental data show good agreement since the theoretical predictions are no longer applicable. In the high crack depth to wavelength ratio (a/lambda(R)) regime, the finite element and experimental results close to the crack approach the limiting value of the reflection coefficient from a 90 degrees corner.     

Energy–density functional plus quasiparticle–phonon model theory as a powerful tool for nuclear structure and astrophysics

During the last decade, a theoretical method based on the energy–density functional theory and quasiparticle–phonon model, including up to three-phonon configurations was developed. The main advantages of themethod are that it incorporates a self-consistentmean-field and multi-configuration mixing which are found of crucial importance for systematic investigations of nuclear low-energy excitations, pygmy and giant resonances in an unified way. In particular, the theoretical approach has been proven to be very successful in predictions of new modes of excitations, namely pygmy quadrupole resonance which is also lately experimentally observed. Recently, our microscopically obtained dipole strength functions are implemented in predictions of nucleon-capture reaction rates of astrophysical importance. A comparison to available experimental data is discussed.     

深度学习在核物理中的应用

深度学习是目前最好的模式识别工具,预期会在核物理领域帮助科学家从大量复杂数据中寻找与某些物理最相关的特征。本文综述了深度学习技术的分类,不同数据结构对应的最优神经网络架构,黑盒模型的可解释性与预测结果的不确定性。介绍了深度学习在核物质状态方程、核结构、原子核质量、衰变与裂变方面的应用,并展示如何训练神经网络预测原子核质量。结果发现使用实验数据训练的神经网络模型对未参与训练的实验数据拥有良好的预测能力。基于已有的实验数据外推,神经网络对丰中子的轻原子核质量预测结果与宏观微观液滴模型有较大偏离。此区域可能存在未被宏观微观液滴模型包含的新物理,需要进一步的实验数据验证。     

Finite element modeling of heating phenomena of cracks excited by high-intensity ultrasonic pulses

A three-dimensional thermo-mechanical coupled finite element model is built up to simulate the phenomena of dynamical contact and frictional heating of crack faces when the plate containing the crack is excited by high-intensity ultrasonic pulses.In the finite element model,the high-power ultrasonic transducer is modeled by using a piezoelectric thermal-analogy method,and the dynamical interaction between both crack faces is modeled using a contact-impact theory.In the simulations,the frictional heating taking place at the crack faces is quantitatively calculated by using finite element thermal-structural coupling analysis,especially,the influences of acoustic chaos to plate vibration and crack heating are calculated and analysed in detail.Meanwhile,the related ultrasonic infrared images are also obtained experimentally,and the theoretical simulation results are in agreement with that of the experiments.The results show that,by using the theoretical method,a good simulation of dynamic interaction and friction heating process of the crack faces under non-chaotic or chaotic sound excitation can be obtained.