Stress corrosion cracking of rock bolts in underground coal mines

Six service-failed rock bolts, all broken through the threaded region at the end of the bolt, were examined. All bolts were made from 840 Grade steel, a medium carbon manganese silicon vanadium hot rolled microalloyed steel, and had a diameter of 22 mm. All six bolts had been installed in a single mine which had a high-pH, sulphate-rich groundwater. Prior to investigating the failed bolts, some preliminary studies were undertaken to provide background information for the work. The first of these involved tensile testing of notched and un-notched rock bolts made from the same steel as the failed rock bolts. The notched rock bolt contained a 2 mm deep flat-bottomed V notch with a bottom width of 0.5 mm, while the full rock bolt profile was retained for the un-notched test. The un-notched rock bolt underwent substantial necking and failed by microvoid coalescence. The notched rock bolt showed no appreciable macroscopic plasticity and failed by cleavage. Additionally, a sample from the same rock bolt steel was loaded through the threads in bending until failure occurred. Fracture initiated at the base of one of the threads and propagated in a ductile manner by microvoid coalescence for the first 1.5 mm of the fracture, but thereafter by brittle cleavage. Stable cracks were also produced at the base of the adjoining threads. The effect of corrosion by mine water on the appearance of ductile (microvoid coalescence) and brittle (cleavage) fracture surfaces in the same steel was also examined. Corrosion substantially changed the appearance of both types of fracture surface, but the two different types of fracture were still quite different in appearance. Five of the six rock bolts were found to have failed by stress corrosion, with cracking initiating at the base of one of the threads. In two of these cases, the stress corrosion crack initiated from a 1-2 mm deep pre-existing crack, considered to have been induced by bending. One of the six bolts failed by overload. Laps were seen frequently at the root of the cold rolled threads but these did not appear to have played a significant role in the failures

The environmentally induced corrosion failure of cable bolts in underground coal mines

The failure of cable bolts, made from high carbon cold-drawn steel wires, is frequently observed in underground coal mines. Hydrogen-induced stress corrosion cracking (HISCC) is known to be the main mechanism of such a failure. The groundwater and geomaterials (mixture of coal and clay) collected from the affected mines have not been found to be corrosive. In this study, we examine the effect of sulfate-reducing bacteria (SRB), which exist in affected mines, on the failure of cable bolts. We make stressed coupons from cable bolt wires and test the coupons in different solutions containing SRB. We find that the hydrogen sulfide produced by SRB promote hydrogen diffusion into the steel and causes HISCC while the steel is under constant load. The fractures in failed coupons show similar features to those failed in underground coal mines. This study provides insights into the role of microorganisms in the failure of underground structures. We recommend future studies to develop prevention measures to stop hydrogen diffusion into steel or microbial activities around the bolts

Re-parameterizing Your Optimizers rather than Architectures

The well-designed structures in neural networks reflect the prior knowledge incorporated into the models. However, though different models have various priors, we are used to training them with model-agnostic optimizers such as SGD. In this paper, we propose to incorporate model-specific prior knowledge into optimizers by modifying the gradients according to a set of model-specific hyper-parameters. Such a methodology is referred to as Gradient Re-parameterization, and the optimizers are named RepOptimizers. For the extreme simplicity of model structure, we focus on a VGG-style plain model and showcase that such a simple model trained with a RepOptimizer, which is referred to as RepOpt-VGG, performs on par with or better than the recent well-designed models. From a practical perspective, RepOpt-VGG is a favorable base model because of its simple structure, high inference speed and training efficiency. Compared to Structural Re-parameterization, which adds priors into models via constructing extra training-time structures, RepOptimizers require no extra forward/backward computations and solve the problem of quantization. We hope to spark further research beyond the realms of model structure design. The code and models are publicly available at https://github.com/DingXiaoH/RepOptimizers.Comment: Under revie

Passive Non-line-of-sight Imaging for Moving Targets with an Event Camera

Non-line-of-sight (NLOS) imaging is an emerging technique for detecting objects behind obstacles or around corners. Recent studies on passive NLOS mainly focus on steady-state measurement and reconstruction methods, which show limitations in recognition of moving targets. To the best of our knowledge, we propose a novel event-based passive NLOS imaging method. We acquire asynchronous event-based data which contains detailed dynamic information of the NLOS target, and efficiently ease the degradation of speckle caused by movement. Besides, we create the first event-based NLOS imaging dataset, NLOS-ES, and the event-based feature is extracted by time-surface representation. We compare the reconstructions through event-based data with frame-based data. The event-based method performs well on PSNR and LPIPS, which is 20% and 10% better than frame-based method, while the data volume takes only 2% of traditional method

A Structural Gel Composite Enabled Robust Underwater Mechanosensing Strategy with High Sensitivity

One of the key challenges in developing gel-based electronics is to achieve robust sensing performance, by overcoming the intrinsic weaknesses such as unwanted swelling induced deformation, signal distortion caused by dehydration, large hysteresis in sensing signal, etc. In this work, we proposed a structural gel composite (SGC) approach by encapsulating the conductive hydrogel/MXene with a lipid gel (Lipogel) layer through an in situ polymerization. The hydrophobic Lipogel coating fulfils the SGC with unique anti-swelling property at an aqueous environment and excellent dehydration feature at an open-air, thus leading to long-term ultra-stability (over 90 days) and durability (over 2000 testing cycles) for underwater mechanosensing applications. As a result, the SGC based mechanoreceptor demonstrates a high and stable sensitivity (GF of 14.5). Moreover, several SGC based conceptual sensors with high sensitivity are developed to unveil their profound potentials in underwater monitoring of human motions, waterproof anti-counterfeiting application, and tactile trajectory tracking