A universal theory describing the wide range of mechanical and acoustic phenomena in solids with internal contacts such as rocks, concrete, ceramics and composites is quite complex to develop. The goal of this paper is to demonstrate the potential to deduce the macroscopic stress-strain constitutive equation for a material as a whole starting from the microscopic hysteretic force-displacement relationship of individual asperities in contact. The material considered in the proposed model contains a large number of isotropic oriented penny-shaped cracks with rough internal surfaces. The stress-strain relationship we obtained for such a material is based on physical principles and laws. Even so, it displays close resemblance to the phenomenological Preisach-Mayergoyz model adopted for mechanical hysteresis and nonlinearity. This constitutive relationship is then used to simulate an experiment with standing acoustic waves in a resonant bar, and to compare model predictions to actual observations. We show that the most important experimentally measurable nonlinear features of these materials, such as the typical classical and nonclassical shifting behavior of the resonant frequency, the dependencies of the amplitudes of the generated harmonics, the softening due to intensive straining, and the subsequent relaxation effect (slow dynamics) can be attributed and explained in terms of the mechanics and the statistics of the internal contacts. The present model bridges the gap between three scales: macroscopic (material as a whole), mesoscopic (structure of intergranular contacts and cracks) and microscopic scale (contacts of individual asperities). 相似文献
Motivated by the observations that natural materials such as bone, shell, tendon and the attachment system of gecko exhibit multi-scale hierarchical structures, this paper aims to develop a better understanding of the effects of structural hierarchy on flaw insensibility of materials from the viewpoint of multi-scale cohesive laws. We consider two idealized, self-similar models of hierarchical materials, one mimicking gecko’s attachment system and the other mimicking the mineral–protein composite structure of bone, to demonstrate that structural hierarchy leads to multi-scale cohesive laws which can be designed from bottom up to enable flaw tolerance from nanoscale to macroscopic length scales. 相似文献
An experimental track layer tractor, based on an Allis Chalmers 8070 tractor (141 kW) was tested on bitumen covered concrete and on cultivated sandy loam at 7.8%; 13% and 21% soil water content. The two articulated beam-type tracks (500 mm wide × 2000 mm soil contact length) were constructed out of 500 mm long and 70 mm wide rubber covered steel track elements, carried by five steel cables (36 mm diameter). The tracks resisted inward deflection but allowed outward articulation between two smooth rear driving and two smooth front pneumatic truck tires (1060 mm diameter) per track. The contact pressure and the tangential force on an instrumented track element, as well as the total torque input to one track, were simultaneously recorded during the drawbar pull/slip tests.
Different possible pressure distribution profiles under the tracks were considered and compared to the recorded data. Two possible traction models are proposed, one constant pressure model for minimal inward track deflection, and a deformable track model with inward deflection and a higher contact pressure at both the front free-wheeling and rear driving tires. For both models, the traction force was generated mainly by rubber/soil friction and adhesion and limited soil shear. A close agreement between the measured and predicted contact pressures and traction force for individual track elements, based on the deformable track model, was observed. The recorded and calculated coefficient of traction based on the summation of the force for the series of track elements were comparable, but were considerably lower than the predicted values, probably due to internal track friction rather than soil sinkage. The tractive efficiency for both a hard or soft surface was also unacceptably low, probably caused by internal track friction. 相似文献
Geckos and many insects have evolved elastically anisotropic adhesive tissues with hierarchical structures that allow these animals not only to adhere robustly to rough surfaces but also to detach easily upon movement. In order to improve our understanding of the role of elastic anisotropy in reversible adhesion, here we extend the classical JKR model of adhesive contact mechanics to anisotropic materials. In particular, we consider the plane strain problem of a rigid cylinder in non-slipping adhesive contact with a transversely isotropic elastic half space with the axis of symmetry oriented at an angle inclined to the surface. The cylinder is then subjected to an arbitrarily oriented pulling force. The critical force and contact width at pull-off are calculated as a function of the pulling angle. The analysis shows that elastic anisotropy leads to an orientation-dependent adhesion strength which can vary strongly with the direction of pulling. This study may suggest possible mechanisms by which reversible adhesion devices can be designed for engineering applications. 相似文献
Gecko and many insects have evolved specialized adhesive tissues with bottom-up designed (from nanoscale and up) hierarchical structures that allow them to maneuver on vertical walls and ceilings. The adhesion mechanisms of gecko must be robust enough to function on unknown rough surfaces and also easily releasable upon animal movement. How does nature design such macroscopic sized robust and releasable adhesion devices? How can an adhesion system designed for robust attachment simultaneously allow easy detachment? These questions have motivated the present investigation on mechanics of robust and releasable adhesion in biology. On the question of robust adhesion, we introduce a fractal gecko hairs model, which assumes self-similar fibrillar structures at multiple hierarchical levels mimicking gecko's spatula ultrastructure, to show that structural hierarchy plays a key role in robust adhesion: it allows the work of adhesion to be exponentially enhanced with each added level of hierarchy. We demonstrate that, barring fiber fracture, the fractal gecko hairs can be designed from nanoscale and up to achieve flaw tolerant adhesion at any length scales. However, consideration of crack-like flaws in the hairs themselves results in an upper size limit for flaw tolerant design. On the question of releasable adhesion, we hypothesize that the asymmetrically aligned seta hairs of gecko form a strongly anisotropic material with adhesion strength strongly varying with the direction of pulling. We use analytical solutions to show that a strongly anisotropic elastic solid indeed exhibits a strongly anisotropic adhesion strength when sticking on a rough surface. Furthermore, we perform finite element calculations to show that the adhesion strength of a strongly anisotropic attachment pad exhibits essentially two levels of adhesion strength depending on the direction of pulling, resulting in an orientation-controlled switch between attachment and detachment. These findings not only provide a theoretical foundation to understand adhesion mechanisms in biology but also suggest possible strategies to develop novel adhesive materials for engineering applications. 相似文献
Organosilicate glass (OSG) is a material that is used as a dielectric in advanced integrated circuits. It has a network structure similar to that of amorphous silica where a fraction of the Si-O bonds have been replaced by organic groups. It is well known from prior work that OSG is sensitive to subcritical crack growth as water molecules in the environment are transported to the crack tip and assist in rupturing Si-O bonds at the crack tip. In this study, we demonstrate that exposure of an OSG containing film stack to water prior to fracture results in degradation of the adhesion of the film stack. This degradation is the result of the diffusion of water into the film stack. We propose a quantitative model to predict adhesion degradation as a function of exposure time by coupling the results of independent subcritical crack growth measurements with diffusion concentration profiles. The model agrees well with experimental data and provides a novel method for measuring the water diffusion coefficient in film stacks that contain OSG. This study has important implications for the reliability of advanced integrated circuits. 相似文献
Pineapple leaf fiber (PALF), used as a reinforcing agent, does not have good adhesion to natural rubber (NR) due to the difference in their polarities. As a result, the degree of reinforcement of NR imparted by PALF remains low compared to that in a polar rubber like acrylonitrile butadiene (NBR). One of the factors that determines the adhesion between the rubber and the reinforcement is the rubber molecular weight. Thus, the aim of this paper is to demonstrate that the stress at very low strains of short pineapple leaf fiber (PALF) reinforced natural rubber (NR) can be significantly increased by lowering the matrix molecular weight. This can be achieved by increasing the matrix mastication time. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr). The PALF fibers were both untreated (UPALF) and sodium hydroxide treated (TPALF). Mastication times of 2, 4, 8 and 16 min were used. Stress-strain curves of PALF reinforced NR prepared with different mastication times were then compared. The most affected region of the curve is in the low strain region. The slopes of the stress-strain curves (moduli) increase with increasing mastication time, indicating better fiber-rubber interaction. The maximum stress achieved at 10% strain is almost 370% that obtained with the usual short mastication time (2 min). The effect remains up to very high strains, although becoming smaller as the strain is increased. Hence, we demonstrate that, by using long enough mastication time, stress-strain curves and stress at low strain of PALF reinforced NR can be improved without the need of any other adhesion promoters. 相似文献