Some notes on the study of fractals in fracture

In this paper, some exciting advances in the application of fractals in fracture over the last two decades are briefly reviewed and a few controversial issues are discussed. The main topics include fractality of fracture surfaces, relationships between fractal dimension and toughness, scaling and universality in fracture, fractal fracture mechanisms, and their implications for fabrication of novel materials. Also highlighted are several open problems and continuing challenges in this multidisciplinary field

Fracture statistics of dental ceramics: Discrimination of strength distributions

The Weibull distribution is the most widely used function in the reliability analysis and structural design of dental ceramics; however, it is still unclear whether Weibull distribution is always the most suitable one. With wide applications of dental ceramics, a special attention has been paid in discriminating their strength distributions. In this paper, three versatile functions, involving normal, log-normal and Weibull distributions, are applied to the analysis of ten strength data sets of dental ceramics with different compositions and the results are compared in terms of the Akaike information criterion and the Anderson-Darling test. It reveals that various microstructures and compositions in the investigated dental ceramics cause their strength distributions deviated from the Weibull distribution. The influence of microstructure induced fracture properties (multiple-modal flaw size distribution, R-curve behavior and subcritical crack growth) on strength distributions is discussed

Dexterity analysis and robot hand design

Understanding about a dexterous robot hand's motion ranges is important to the precision grasping and precision manipulation. A planar robot hand is studied for object orientation, including ranges of motion, measures with respect to the palm, position reaching of a point in the grasped object, and rotation of the object about the reference point. The rotational dexterity index and dexterity chart are introduced and an analysis procedure is developed for calculating these quantities. A design procedure for determining the hand kinematic parameters based on a desired partial or complete dexterity chart is also developed. These procedures have been tested in detail for a planar robot hand with two 2- or 3-link fingers. The derived results are shown to be useful to performance evaluation, kinematic parameter design, and grasping motion planning for a planar robot hand

Precursory deformation responses in different directions to catastrophic failure of uniaxially compressed sandstones

The precursory acceleration of deformation is widely validated as a method for predicting the failure time. Damage evolution inside rocks generally results in complex strain patterns in the vicinity of failure and various responses of deformation in different directions. However, it is still unclear what the differences and similarities are during the evolution of strain components. In this paper, we compare the evolving properties of strain components in different directions based on experiments of sandstones under uniaxial compression. It is shown that the temporal patterns of vertical strains are much more complex in spatial distributions than that of horizontal strains. The horizontal strain presents two kinds of time courses characterized by precursory accelerations in both the strain localized zone and its surrounding areas, and the evolution without accelerations in positions is far from the strain localized zone. However, the vertical strain components corresponding to loading direction present complex evolving patterns with five kinds of time courses. The final amplitudes of horizontal strains are much higher than vertical components. Horizontal strains follow the power law acceleration with the well-defined exponents, but the exponents for vertical components are more scattered. Thus, horizontal strains can be applied to predict the failure time

Time courses of strains that induce necking and fracturing in high-density polyethylene

High-density polyethylene is widely used in pressure pipe applications, but its necking and pre-cut effects are still poorly understood. Herein, we analyze the spatial distributions of the time courses of strains to highlight the strain field evolution to necking and the effect of pre-cutting on the strain field evolution in a high-density polyethylene material deformed under tension. Digital image correlation was used to measure the strain fields on two perpendicular surfaces of a specimen. Necking and its propagation in the tension direction dominate the failure behavior of an intact specimen. However, in the pre-cut specimen, crack propagation prevents neck propagation in the tension direction. Energy release outside the crack zone is observed as a decrease in strain at approximately the failure time. This leads to a macroscopic stress-strain curve that deviates from that of the intact specimen. These findings provide novel insights that are significant in the theoretical modelling and simulations of advanced polymeric materials and structures

Enhancing Water Resistance and Mechanical Properties of Cemented Soil with Graphene Oxide.

Although cemented soil as a subgrade fill material can meet certain performance requirements, it is susceptible to capillary erosion caused by groundwater. In order to eliminate the hazards caused by capillary water rise and to summarize the relevant laws of water transport properties, graphene oxide (GO) was used to improve cemented soil. This paper conducted capillary water absorption tests, unconfined compressive strength (UCS) tests, softening coefficient tests, and scanning electron microscope (SEM) tests on cemented soil using various contents of GO. The results showed that the capillary water absorption capacity and capillary water absorption rate exhibited a decreasing and then increasing trend with increasing GO content, while the UCS demonstrated an increasing and then decreasing trend. The improvement effect is most obvious when the content is 0.09%. At this content, the capillary absorption and capillary water absorption rate were reduced by 25.8% and 33.9%, respectively, and the UCS at 7d, 14d, and 28d was increased by 70.32%, 57.94%, and 61.97%, respectively. SEM testing results demonstrated that GO reduces the apparent void ratio of cemented soil by stimulating cement hydration and promoting ion exchange, thereby optimizing the microstructure and improving water resistance and mechanical properties. This research serves as a foundation for further investigating water migration and the appropriate treatment of GO-modified cemented soil subgrade

Failure prediction of high-capacity electrode materials in lithium-ion batteries

The large volume change during lithium-ion insertion/extraction leads to huge stress and even failure of active materials. To well understand such a problem, the two-phase lithiation process of film and hollow core-shell electrodes is simulated by using a non-linear diffusion lithiation model. The dynamic evolution of lithium-ion concentration and diffusion-induced stress are obtained. Based on the dimensional analysis, a phase diagram is determined to demonstrate the relationship between critical failure, structure dimensions and mechanical properties. As a case study, the critical state of charge in Sn films are measured and compared with theoretical results

On the intrinsic hardness of a metallic film/substrate system: Indentation size and substrate effects

To examine effects of indentation size and substrate on the hardness determination of thin films, two typical types of hard film/soft substrate (Ni/Fe) and soft film/hard substrate (Al/Si and Al/glass) systems are investigated. A simple model is proposed to predict the intrinsic hardness of thin films, which allows a more accurate fitting to empirical data and the estimation of ultimate film hardness. The model can be used to interpret indentation data and extrapolate the indentation depth-hardness curve to an important region where indentation depth lies between 1% to 5 times of film thickness. The results are well consistent with the evolving trend of composite hardness obtained from experiments and numerical results by finite element analysis

Effects of size and concentration on diffusion-induced stress in lithium-ion batteries

Capacity fade of lithium-ion batteries induced by chemo-mechanical degradation during charge-discharge cycles is the bottleneck in design of high-performance batteries, especially high-capacity electrode materials. Stress generated due to diffusion-mechanical coupling in lithium-ion intercalation and deintercalation is accompanied by swelling, shrinking, and even micro-cracking. In this paper, we propose a theoretical model for a cylindrical nanowire electrode by combining the bond-order-length-strength and diffusion theories. It is shown that size and concentration have a significant influence on the stress fields in radial, hoop, and axial directions. This can explain why a smaller electrode with a huge volume change survives in the lithiation/delithiation process