Dynamics of microcracks and time dependences of surface deformation of a heterogeneous body (granite) under an impact

The dynamics of fractoluminescence flashes and the time dependences of surface deformation of granite with different sizes of feldspar grains under an impact on samples by a metal pin have been studied with a 10 ns resolution. A band at ??1.9 eV has been observed in the fractoluminescence spectra, which means that, under the influence of mechanical stresses, Si-O-Si bonds are broken and ?? SiO^?? free radicals are formed. The fractoluminescence has the form of flashes with a duration of ??10 ns. It has been assumed that each of them corresponds to the nucleation of a microcrack. From the flash intensities and the elastic wave velocity, the linear size of microcracks has been estimated to be from ??8 to 30 ??m. Microcracks are mainly generated during passage of a deformation wave through feldspar grains. An impact causes the appearance of eigenvibrations of the entire sample, and cracking of grains gives rise to eigenvibrations of grains.     

Creep ruptures in heterogeneous materials

We present creep experiments on fiber composite materials with different controlled heterogeneity. All samples exhibit a power-law relaxation of the strain rate in the primary creep regime (Andrade's law) followed by a power-law acceleration up to rupture. We discover that the rupture time is proportional to the duration of the primary creep regime, showing the interplay between the two regimes and offering a method of rupture prediction. These experimental results are rationalized by a mean-field model of representative elements with nonlinear viscoelastic rheology and with a large heterogeneity of strengths.     

Slow crack propagation in heterogeneous materials

Statistics and thermally activated dynamics of crack nucleation and propagation in a two-dimensional heterogeneous material containing quenched randomly distributed defects are studied theoretically. Using the generalized Griffith criterion we derive the equation of motion for the crack tip position accounting for dissipation, thermal noise, and the random forces arising from the defects. We find that aggregations of defects generating long-range interaction forces (e.g., clouds of dislocations) lead to anomalously slow creep of the crack tip or even to its complete arrest. We demonstrate that heterogeneous materials with frozen defects contain a large number of arrested microcracks and that their fracture toughness is enhanced to the experimentally accessible time scales.     

Magnetic-field behavior of heterogeneous magnetic materials

Results on the investigation of the magnetic properties of nanocrystalline Co/Ni/Fe and Fe/Zr/Fe thin-film systems are presented. The study of the magnetic properties of the examined samples was carried out employing magneto-optical micromagnetometer with a surface sensitivity about of 20 nm of the thickness depth and a vibrating sample magnetometer. The examined samples were revealed to exhibit hysteresis loops of complicated forms. These data were explained by the magnetostatic and exchange interactions between the layers in heterogeneous magnetic materials.     

Continuous wave MRI of heterogeneous materials

A prototype continuous wave MRI system operating at 7T has been used successfully to study a variety of heterogeneous materials exhibiting T2 relaxation values ranging from 10 micros to 50 ms. Two-dimensional images of a poly(methly methacrylate) (PMMA) resolution phantom (T2=38 micros) exhibited a spatial resolution of approximately 1mm at a magnetic field gradient strength of 200 mT/m. The technique was used to study the hydration, drying, and subsequent water penetration properties of cement samples made from ordinary Portland cement, and revealed inhomogeneities arising from the cure conditions. Sandstone samples from an oil reservoir in the North Sea were also studied; structure within these materials, arising from the sedimentary bed layering in the reservoir, was found to have an effect on their water transport properties. A section from a confectionery bar (T2* approximately 50-60 ms) was also imaged, and its internal structure could be clearly discerned.     

Mechanoelectrical transformations in heterogeneous materials with piezoelectric inclusions

Mechanoelectrical transformations are studied on models with different geometries and piezoelectric inclusion concentrations. The electrical signal intensity is found to decrease with increasing depth of a piezoelectric source relative to the position of an electrical signal detector. The electrical signal is the difference between signals from the unlikely charged surfaces of the piezoelectric source being deformed by an acoustic excitation wave. The spectral amplitudes of electrical signals coming from different regions of one sample and from identical (in composition) samples containing a large amount of piezoelectric inclusions differ considerably. This difference is due to the random orientation of the quartz piezoelectric axes relative to the electrical detector. Therefore, nondestructive mechanoelectrical techniques for inspection of heterogeneous materials with piezoelectric inclusions must use amplitude-independent criteria.     

Ultrasonic characterization of granites obtained from industrial quarries of Extremadura (Spain)

The industry of ornamental rocks, such as granites, represents one of the most important industrial activities in the region of Extremadura, SW Spain. A detailed knowledge of the intrinsic properties of this natural stone and its environmental evolution is a required goal in order to fully characterize its quality. In this work, two independent NDT acoustic techniques have been used to measure the acoustic velocity of longitudinal waves in different prismatic granitic-samples of industrial quarries. A low-frequency transceiver set-up, based on a high-voltage BPV Steinkamp instrument and two 50 kHz probes, has been used to measure pulse travel times by ultrasonic through-transmission testing. In complementary fashion, an Erudite MK3 test equipment with an electromagnetic vibrator and two piezoelectric sensors has also been employed to measure ultrasonic velocity by means of a resonance-based method, using the same types of granite varieties. In addition, a comprehensive set of physical/mechanical properties have also been analyzed, according to Spanish regulations in force, by means of alternative methods including destructive techniques such as strength, porosity, absorption, etc. A large number of samples, representing the most important varieties of granites from quarries of Extremadura, have been analyzed using the above-mentioned procedures. Some results obtained by destructive techniques have been correlated with those found using ultrasonic techniques. Our experimental setting allowed a complementary characterization of granite samples and a thorough validation of the different techniques employed, thus providing the industry of ornamental rocks with a non-destructive tool that will facilitate a more detailed insight on the properties of the rocks under study.     

Simulation of crack growth during fracture of heterogeneous materials

A cellular automaton model for describing the fracture of mechanically loaded heterogeneous materials has been constructed. Two extreme scenarios of the fracture process have been revealed, i.e., the dispersion (percolation) scenario, according to which defects accumulate uniformly throughout the volume of the material, and the correlated scenario (growth of predominantly a single source), which have been observed during the fracture of real materials. It has been shown that, in the case of the correlated fracture, a crack grows through the mechanism of ejection of double kinks of its front. In the intermediate case, the process occurs according to both scenarios: first, the slow accumulating (percolation) fracture and, then, the rapid correlated fracture; by the time the latter process begins, a self-organized critical state with a power-law size distribution of cracks typical of it has been formed.     

Order and disorder in the microstructure of dielectrically nonlinear heterogeneous materials

The paper deals with the electrical characterisation of dispersions of pseudo-oriented ellipsoids of revolution. We are thus dealing with mixtures of inclusions of arbitrary eccentricity and arbitrary non-random orientational distributions. The analysis ranges from parallel spheroidal inclusions to completely random oriented inclusions. A unified theory covers all the orientational distributions between the random and parallel cases. Each ellipsoidal inclusion is made from an isotropic nonlinear dielectric material described by means of the so-called Kerr nonlinear relation.The electrical averaging inside the composite material is carried out by means of explicit results. We obtain closed form expressions for the macroscopic or equivalent dielectric properties of the overall composite materials. This study affirms that the nonlinear electrical behaviour of such a dispersion of pseudo-oriented particles is completely defined by two specific order parameters, which depends on the given angular distribution. The theory may be applied to characterise media with different shapes of the inclusions (i.e. spheres, cylinders or planar inhomogeneities) yielding a set of procedures describing several composite materials of great technological interest.     

Failure time and critical behaviour of fracture precursors in heterogeneous materials

The acoustic emission of fracture precursors, and the failure time of samples of heterogeneous materials (wood, fiberglass) are studied as a function of the load features and geometry. It is shown that in these materials the failure time is predicted with a good accuracy by a model of microcrack nucleation proposed by Pomeau. We find that the time interval δt between events (precursors) and the energy ɛ are power law distributed and that the exponents of these power laws depend on the load history and on the material. In contrast, the cumulated acoustic energy E presents a critical divergency near the breaking time τ which is E∼ . The positive exponent γ is independent, within error bars, on all the experimental parameters. Received 31 July 2001 and Received in final form 17 December 2001     

On the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials

This paper deals with the measurement of frequency-dependent ultrasonic attenuation in strongly heterogeneous materials, such as cementitious materials. To improve the measurement of this parameter on this kind of materials, a linear swept–frequency signal is used to drive an emitter transducer to conduct a through-transmission inspection in immersion. To filter out undesirable frequency content, time–frequency filtering and detection process are performed. The use of this method has been compared with two excitation techniques, the broadband and the narrowband pulses. The results obtained using the swept–frequency excitation together with the time–frequency filtering, allows the determination of the attenuation curves with high accuracy over a wide frequency range without the need for complicated equipment, and improves the effective bandwidth by using a unique pair of transducers.     

Statistics, Symmetry, and (In)Distinguishability in Bohmian Mechanics

This paper continues an earlier work by considering in what sense and to what extent identical Bohmian-mechanical particles in many-particle systems can be considered indistinguishable. We conclude that while whether identical Bohmian-mechanical particles ace considered to be statistically (in)distinguishable is a matter of theory choice underdetermined by logic and experiment, such particles are in any case physically distinguishable.     

X-ray diffraction and heterogeneous materials: An adaptive crystallography approach

Cultural heritage materials are often complex and heterogeneous, with a multi-scale architecture. Phases from a variety of crystalline forms co-exist in a wide grain size distribution, with each of these phases keeping in their structural arrangement a memory of the transformations that occurred to the material. Over the last two decades, X-ray diffraction has been applied successfully to the study of cultural heritage materials, with the use of synchrotron facilities offering new possibilities to describe the structural features of such complex materials. The long-range and/or short-range organization of the different crystallographic phases as well as their global position/dispersion in the material are closely related to the properties of the material (optical, mechanical…), its manufacturing process, functionality, or long-term conservation. In this paper, possible diffraction setups and data collection strategies are discussed in order to retrieve adequate data from crystalline and amorphous phases and to take into account single-crystal contributions.     

超快非均质电子转移的优化控制研究(Ⅱ)

本文用优化控制理论,以分子的基态作为目标态,考虑了在不同条件下,从二萘嵌苯到TiO2超快电子转移的动力学过程.发现在此目标态的作用下,电子经过一个电子激发-回落的过程从分子的基态激发到与半导体导带耦合的第一电子激发态,然后在给定目标态的作用下回落至电子基态,从而实现了较高的目标态产生率.由于二萘嵌苯的电子激发态与半导体导带能级间的强电子转移耦合,这一控制任务较电子激发态为目标态而言,更适合于实验研究.     

Destruction dynamics of a heterogeneous body (diorite) under friction

Physics of the Solid State - Triboluminescence bursts are observed in two heterogeneous (diorite) specimens under friction against each other. Triboluminescence appears upon the relaxation of...     

Dimension reduction in heterogeneous neural networks: Generalized Polynomial Chaos (gPC) and ANalysis-Of-VAriance (ANOVA)

We propose, and illustrate via a neural network example, two different approaches to coarse-graining large heterogeneous networks. Both approaches are inspired from, and use tools developed in, methods for uncertainty quantification (UQ) in systems with multiple uncertain parameters – in our case, the parameters are heterogeneously distributed on the network nodes. The approach shows promise in accelerating large scale network simulations as well as coarse-grained fixed point, periodic solution computation and stability analysis. We also demonstrate that the approach can successfully deal with structural as well as intrinsic heterogeneities.