Cloud computing plays a vital role in processing a large amount of data. However, with the arrival of the Internet of Things, huge data are generated from these devices. Thus, there is the need for bringing characteristics of cloud closer to the request generator, so that processing of these huge data takes place at one-hop distance closer to that end user. This led to the emergence of fog computing with the aim to provide storage and computation at the edge of the network that reduces network traffic and overcomes many cloud computing drawbacks. Fog computing technology helps to overcome challenges of big data processing. The paper discusses the taxonomy of fog computing, how this is different from cloud computing and edge computing technologies, its applications, emerging key technologies (i.e., communication technologies and storage technologies) and various challenges involved in fog technology.

Fog computing: from architecture to edge computing and big data processing

Rock engineering is highly susceptible to cyclic loads resulting from earthquakes, quarrying or rockbursts. Acquiring the fatigue properties and failure mechanism of rocks is pivotal for long-term stability assessment of rock engineering structures. So far, significant progress has been gained on the mechanical characteristics of rocks subjected to cyclic loading. For providing a global insight of typical results and main features of rocks under cyclic loading conditions, this study comprehensively reviews the state-of-the-art of deformation and failure mechanism and fatigue constitutive relationship of rocks subjected to cyclic loading in the past 60 years. Firstly, cyclic tests on rocks are classified into different types based on loading paths, loading parameters, loading types and environment conditions. Secondly, representative results are summarized and highlighted in terms of the fatigue response of rocks, including the deformation degradation, energy dissipation, damage evolution and failure characteristics; both laboratory testing and numerical results are presented, and various measurement techniques such as X-ray micro-computed tomography (micro-CT) and digital image correlation (DIC) are considered. Thirdly, the influences of cyclic loads on the mechanical characteristics of rocks are discussed, including the cyclic stress, frequency, amplitude and waveform. Subsequently, constitutive relationships for rocks subjected to cyclic loading are outlined, in which typical fatigue constitutive models are compared and analyzed, regarding the elastoplastic model, the internal variable model, the energy-based damage model and the discrete element-based model. Finally, some ambiguous questions and prospective research are interpreted and discussed.

A review of experimental and theoretical research on the deformation and failure behavior of rocks subjected to cyclic loading

Effects of stress lower limit during cyclic loading and unloading on deformation characteristics of sandstones

Deformation characteristics of sandstones during cyclic loading and unloading with varying lower limits of stress under different confining pressures

In-situ TiC/Ti titanium matrix composites were successfully fabricated through a novel approach, utilizing the reaction of graphene and Ti mixture powders in spark plasma sintering. Microstructure, hardness and compressive properties of such composites were investigated. The layered graphene nanosheets as the carbon source were dispersed in the powders, and subsequently formed uniform TiC particles in Ti matrix during the rapid sintering. The resulting TiC particles exhibited the micro- and nano-sized equiaxial structures. Such composites possessed the significantly enhanced hardness and room-temperature compressive strength comparing with the as-cast and as-sintered pure Ti. The ultimate and yield compressive strengths of as-prepared 7.0 vol% TiC incorporated composite respectively reached up to 2.64 GPa and 1.93 GPa, beyond some advanced Ti materials reported to date. In-situ micro- and nano-sized TiC particles were believed to be beneficial to harden and strengthen Ti matrix. The relevant strengthening mechanisms of such composites were discussed.

Microstructural and mechanical characterization of in-situ TiC/Ti titanium matrix composites fabricated by graphene/Ti sintering reaction

Water saturation effects on dynamic behavior and microstructure damage of sandstone: Phenomena and mechanisms

A nonlinear parallel-bonded stress corrosion (NPSC) model is proposed to simulate the fatigue characteristics of artificial rock (concrete) during cyclic loading. Numerical simulations of fatigue tests replicate the main mechanical features of concrete specimens subjected to cyclic loading observed in the laboratory. A nonlinear reduction speed of the bond diameter between two bonded particles represents the damage rate induced by the fatigue load. The damage rate is proportional to the maximum cyclic load level when the minimum cyclic load level is fixed. Compared with laboratory data, a logarithmic function of bond diameter in the NPSC model resulted in the best fit to simulate the fatigue behaviour of concrete. The simulation includes acoustic emission (AE) monitoring during fatigue tests. The axial strain of the assembly is governed by the evolution of bond breakages. The sum of released bond strain energy is documented as value proportional to cumulative AE energy. The simulation results show very similar evolution compared with laboratory data, which verifies the effectiveness of AE energy simulation.

Zhengyang Song

Papers

Water saturation effects on dynamic behavior and microstructure damage of sandstone: Phenomena and mechanisms

Bonded-particle model-based simulation of artificial rock subjected to cyclic loading

Mechanical behavior of marble exposed to freeze-thaw-fatigue loading

Fracture behavior and damage mechanisms of sandstone subjected to wetting-drying cycles

Inhomogeneous mechanical behaviour of concrete subjected to monotonic and cyclic loading