破碎岩石渗透性的试验测定方法

传统的试验仪器和方法仅能测试岩石破坏前的渗透系数，文中介绍了一种自行设计并已获国家专利的破碎岩石压实渗透仪，依靠其与MTS815.02岩石力学伺服系统连接匹配。利用MTS的两套闭环系统施加轴压和孔压，可测试压力作用下岩石破碎后的渗透系数，对破碎砂岩进行高压下的渗透测试进一步说明该方法的可行性。利用该方法得出的测试结果。对破碎岩体中渗流问题研究及灾害防治具有参考价值。     

On the mechanics of mother-of-pearl: A key feature in the material hierarchical structure

Mother-of-pearl, also known as nacre, is the iridescent material which forms the inner layer of seashells from gastropods and bivalves. It is mostly made of microscopic ceramic tablets densely packed and bonded together by a thin layer of biopolymer. The hierarchical microstructure of this biological material is the result of millions of years of evolution, and it is so well organized that its strength and toughness are far superior to the ceramic it is made of. In this work the structure of nacre is described over several length scales. The tablets were found to have wavy surfaces, which were observed and quantified using various experimental techniques. Tensile and shear tests performed on small samples revealed that nacre can withstand relatively large inelastic strains and exhibits strain hardening. In this article we argue that the inelastic mechanism responsible for this behavior is sliding of the tablets on one another accompanied by transverse expansion in the direction perpendicular to the tablet planes. Three dimensional representative volume elements, based on the identified nacre microstructure and incorporating cohesive elements with a constitutive response consistent with the interface material and nanoscale features were numerically analyzed. The simulations revealed that even in the absence of nanoscale hardening mechanism at the interfaces, the microscale waviness of the tablets could generate strain hardening, thereby spreading the inelastic deformation and suppressing damage localization leading to material instability. The formation of large regions of inelastic deformations around cracks and defects in nacre are believed to be an important contribution to its toughness. In addition, it was shown that the tablet junctions (vertical junctions between tablets) strengthen the microstructure but do not contribute to the overall material hardening. Statistical variations within the microstructure were found to be beneficial to hardening and to the overall mechanical stability of nacre. These results provide new insights into the microstructural features that make nacre tough and damage tolerant. Based on these findings, some design guidelines for composites mimicking nacre are proposed.     

A rigorous assessment of the benefits of miniaturization in the Kolsky bar system

A rigorous experimental and numerical assessment is made of the benefits and limits of miniaturization in the Kolsky bar system. The primary issues that arise in very high strain rate testing (stress equilibration, inertial effects, wave dispersion, friction, and controllability of deformations) are addressed through experiments coupled with explicit finite element analyses. A miniaturized Kolsky bar system that includes the input bar is developed, together with the use of the laser occlusive radius detector to obtain local measurements of specimen strain during the very high rate deformations. It is demonstrated that this miniaturized Kolsky bar system can be used to provide fully validated results, including the explicit determination of equilibration, over a very wide range of strain rates (1×10³ to 5×10⁴ s⁻¹). The desired high strain rate can be achieved even at low accumulated strains, and the total strain developed can be controlled very effectively. Specific conditions are developed for determining the range of utility of the technique for a given material. The technique is applied to the characterization of 6061-T651 aluminum, and the results are compared with the results obtained using a conventional Kolsky bar.     

Determination of Bone Trabeculae Modulus—An Ultrasonic Scanning and MicroCT (μCT) Imaging Combination Approach

To date, there is no method to measure non-destructively the modulus of trabeculae within cancellous bone, whilst retaining its structural integrity. In this study ultrasonic scanning, coupled with microCT imaging, is employed to determine trabeculae modulus along the three major anatomical axes non-destructively. The proposed method allows cancellous bone specimens to remain intact, for possible use in subsequent studies. Volume rendering of the microCT images allows three-dimensional visualization of cancellous bone specimens to be tested. This facilitates trabeculae selection and accurate measurement of distance traveled by the ultrasonic wave, thus yielding a good degree of confidence in the acoustic velocity measured. For all the three principal anatomical directions, the measured acoustic speeds ranged from 2,115 to 3,077 m/s, giving an average of 2,505 m/s. Average wave velocities in the superior–inferior, medial–lateral and anterior–posterior anatomical directions were found to be 2,295, 2,469 and 2,754 m/s, respectively; the differences corresponding to the three directions do not appear to be significant. Subsequently, the modulus was then determined using elastic wave propagation theory.     

A Technique for Dynamic Tensile Testing of Human Cervical Spine Ligaments

An experimental study is undertaken to examine the dynamic stress–strain characteristics of ligaments from the human cervical spine (neck). Tests were conducted using a tensile split Hopkinson bar device and the engineering strain rates imposed were of the order of 10²∼10³/s. As ligaments are extremely soft and pliable, specialized test protocols applicable to Hopkinson bar testing were developed to facilitate acquisition of reliable and accurate data. Seven primary ligaments types from the cervical spines of three male cadavers were subjected to mechanical tests. These yielded dynamic stress–strain curves which could be approximated by empirical equations. The dynamic failure stress/load, failure stain/deformation, modulus/stiffness, as well as energy absorption capacity, were obtained for the various ligaments and classified according to their location, the strain rate imposed and the cadaveric source. Compared with static responses, the overall average dynamic stress–strain behavior foreach type of ligament exhibited an elevation in strength but reduced elongation.     

Dynamic dip testing as a method to assess the condensate drainage behavior from the air-side surface of compact heat exchangers

A new method to assess the condensate drainage behavior of the air-side surface of compact heat exchangers—dynamic dip testing—is introduced. The new method is shown to provide highly repeatable data for real-time drainage. Results from experiments with more than 20 flat-tube and round-tube-and-fin heat exchangers are presented, and the data clearly show geometrical effects such as the impact of the tube type on condensate drainage. By comparing the results from dip testing to wind-tunnel experiments for the same heat exchangers, we find dip testing can serve as a powerful tool for assessing the condensate retention behavior. The coils retaining the most and the least condensate in a steady-state wind-tunnel test, likewise held the most and the least in a dip test. However, different amounts of water are retained on the air-side surface during dip tests and wind-tunnel tests. A model based on gravity, surface tension and drag effects is developed to help understand and predict the drainage behavior of heat exchangers. The new model and experimental approach are useful in screening heat exchangers for condensate retention and for assessing off-cycle drainage behavior.     

Foam Self-Clarification Phenomenon: An Experimental Investigation

This paper is focused on the capabilities of gas–liquid foams to attenuate acoustic waves. It is postulated that the sound attenuation phenomenon in foams is largely governed by the hydrodynamic resistance of the Plateau-Gibbs channels (PGC) to the flow of liquid through them. It is shown that the addition of solid particles to gas–liquid foams has opposite effects depending on the concentration of the added solid particles. As long as the concentration of the added solid particles is smaller than a certain critical value the sound attenuation coefficient increases and as a result in the sound velocity decreases. However, if the concentration of the added solid particles becomes larger than this critical value the attenuation coefficient decreases and the sound velocity increases. When the concentration of the solid particles reaches some critical value, the particles block the Plateau-Gibbs channels and stop the filtration. As a result the attenuation coefficient of the sound wave decreases while the sound velocity, in such three-phase foams, increases. The point at which the sound wave stops attenuating and its velocity starts to increase is known as the point of self-clarification. Based on this postulate and on the results of our preliminary tests the present study provides a plausible explanation to the above-mentioned contradicting effect, and the self- clarification phenomenon.     

Effect of air gaps on ballistic resistance of targets for conical impactors

High velocity penetration of a rigid conical impactor into a ductile target with air gaps between the plates is studied using the cylindrical cavity expansion approximation describing impactor–target interaction. It is showed that the latter model predicts improvement of the ballistic performance of the target with the increase of air gaps. It is found analytically that the ballistic limit velocity of the target consisting of N plates with a fixed total thickness with large air gaps increases with the increase of N. The conditions are discussed when the predicted effects can be most pronounced.     

Exterior cloaking with active sources in two dimensional acoustics

We cloak a region from a known incident wave by surrounding the region with three or more devices that cancel out the field in the cloaked region without significantly radiating waves. Since very little waves reach scatterers within the cloaked region, the scattered field is small and the scatterers are for all practical purposes undetectable. The devices are multipolar point sources that can be determined from Green's formula and an addition theorem for Hankel functions. The cloaking devices are exterior to the cloaked region.     

Broadband acoustic cloaks based on the homogenization of layered materials

We present an analysis of acoustic cloaks based on the homogenization of two fluidlike materials, with an emphasis on periodically layered imperfect cloaks, by removing the singularities of the acoustic parameters required for ideal cloaks. The conditions that material layers should satisfy are systematically analyzed and critically discussed according to their feasibility.