Stress distribution and roof subsidence of surrounding strata considering in situ coal conversion and CO2 mineralization backfilling: Photoelastic experiments using 3D-printed models of mining faces and goafs.

Coal, a reliable and economical fuel, is expected to remain the primary energy source for power generation for the foreseeable future. However, conventional mining and utilization of coal has caused environmental degradation and infrastructure damage. An in situ coal conversion method has been proposed to mitigate environmental problems and reduce CO2 emissions resulting from coal extraction and utilization. This method involves the in situ conversion and utilization of coal, backfilling of waste rock, and CO2 mineralization to backfill the goaf. In this study, the impact of mining and conversion activities on the surrounding strata was evaluated to ascertain the effectiveness and advantages of the in situ coal conversion method. Transparent stope models were created using three-dimensional printing technology. The stress distribution and deformation characteristics of the surrounding strata were examined using photoelasticity and digital image correlation methods. The results were compared with those obtained using the traditional backfill mining method. The comparison revealed that the disturbance to the surrounding strata was 14.4 times less in the in situ conversion method than in the traditional backfill mining method. Additionally, the disturbance height at the roof and the disturbance depth at the floor were 4.2 and 2.1 times lower, respectively. The roof subsidence in the in situ conversion method was 1.97 times less than that in the traditional backfill mining method. These results confirm the advantages of minimizing the disturbance to surrounding rocks and controlling the subsidence of roof strata.

Design and Experimental Results of a Three-Dimensional Force Sensor for Shearer Cutting Pick Force Monitoring.

The main focus of this work is the design and development of a three-dimensional force sensor for the cutting pick of a coal mining shearer's simulated drum. This sensor is capable of simultaneously measuring the magnitude of force along three directions of the cutting pick during the cutting sample process. The three-dimensional force sensor is built based on the strain theory of material mechanics, and reasonable structural design is implemented to improve its sensitivity and reduce inter-axis coupling errors. The strain distribution of the sensor is analyzed using finite element analysis software, and the distribution of the strain gauges is determined based on the analysis results. In addition, a calibration test system is designed for the sensor, and the sensitivity, linearity, and inter-axis coupling errors of the sensor are calibrated and tested using loading experiments in three mutually perpendicular directions. Modal simulation analysis and actual cutting pick testing of the coal mining machine's simulated drum are conducted to study the dynamic characteristics and functionality of the sensor in practical applications. The experimental results depict sensitivities of 0.748 mV/V, 2.367 mV/V, and 2.83 mV/V for the newly developed sensor, respectively. Furthermore, the cross-sensitivity error was lower than 5.02%. These findings validate that the sensor's structure satisfies the measurement requirements for pick-cutting forces.

Kinematic Modeling of a Combined System of Multiple Mecanum-Wheeled Robots with Velocity Compensation.

In industry, combination configurations composed of multiple Mecanum-wheeled mobile robots are adopted to transport large-scale objects. In this paper, a kinematic model with velocity compensation of the combined mobile system is created, aimed to provide a theoretical kinematic basis for accurate motion control. Motion simulations of a single four-Mecanum-wheeled virtual robot prototype on RecurDyn and motion tests of a robot physical prototype are carried out, and the motions of a variety of combined mobile configurations are also simulated. Motion simulation and test results prove that the kinematic models of single- and multiple-robot combination systems are correct, and the inverse kinematic correction model with velocity compensation matrix is feasible. Through simulations or experiments, the velocity compensation coefficients of the robots can be measured and the velocity compensation matrix can be created. This modified inverse kinematic model can effectively reduce the errors of robot motion caused by wheel slippage and improve the motion accuracy of the mobile robot system.

A Review on the Research Progress of Nano Organic Friction Materials.

BACKGROUND: Traditional organic friction materials are difficult to adapt to people's growing technical requirements for stability, safety, comfort and environmental protection in the braking process. With the rapid development of nanotechnology, the brake's organic friction materials meet new opportunities. This article aims to review the research progress of organic friction materials that have applied nanotechnology. METHODS: The research progress of nano organic friction materials was reviewed from four aspects in this article. Firstly, this article outlined the development history of friction materials. Secondly, two preparation methods of the nano organic friction material were summarized as by nano modifying of matrix material and by adding nanoparticles into friction material. Thirdly, it was indicated that the nano organic friction material has generally better mechanical, physical properties and tribological performance than traditional organic friction materials. And the main factors that affect the friction and wear performance were analyzed. Finally, the main existing problems in this field were summarized. RESULTS: It was pointed out that the nano organic friction material may be an important developing trend of friction materials. It was also pointed out that the dispersion of nanoparticles must be a key process during preparation. What is more, the improvement mechanisms of performance by nano modifying were explained. And it was considered at the end that the functional friction material with magnetism or self-adsorption may be a leading developoing direction of nano organic friction materials in the future. CONCLUSION: The findings of this review confirm the excellent performance of nano organic friction materials. It is concluded that the development of a new functional friction material by using the special effect of nanoparticles will be an important developing trend. Few relevant patents to the topic have been reviewed and cited.

Contact damage failure analyses of fretting wear behavior of the metal stem titanium alloy-bone cement interface.

Although cemented titanium alloy is not favored currently in the Western world for its poor clinical and radiography outcomes, its lower modulus of elasticity and good biocompatibility are instrumental for its ability supporting and transforming physical load, and it is more suitable for usage in Chinese and Japanese populations due to their lower body weights and unique femoral characteristics. Through various friction tests of different cycles, loads and conditions and by examining fretting hysteresis loops, fatigue process curves and wear surfaces, the current study investigated fretting wear characteristics and wear mechanism of titanium alloy stem-bone cement interface. It was found that the combination of loads and displacement affected the wear quantity. Friction coefficient, which was in an inverse relationship to load under the same amplitude, was proportional to amplitudes under the same load. Additionally, calf serum was found to both lubricate and erode the wear interface. Moreover, cement fatigue contact areas appeared black/oxidative in dry and gruel in 25% calf serum. Fatigue scratches were detected within contact areas, and wear scars were found on cement and titanium surfaces, which were concave-shaped and ring concave/ convex-shaped, respectively. The coupling of thermoplastic effect and minimal torque damage has been proposed to be the major reason of contact damage. These data will be important for further studies analyzing metal-cement interface failure performance and solving interface friction and wear debris production issues.

The Relation of Three-dimensional Knee Kinematics between Walking and Squatting for Healthy Young and Elderly Adults.

[Purpose] The purpose of this study was to study the correlation of knee range of motion between walking and squatting for young and elderly populations. [Subjects] Sixteen young and eight elderly healthy subjects were recruited for this study. [Methods] Three-dimensional joint motions of each subject were captured while they performed walking and squatting exercises. [Results] Significant differences in the non-sagittal plane knee motions (peak adduction, and peak external and internal rotation) were revealed between the young and the elderly during squatting. Correlations of three-dimensional knee range of motion between walking and squatting were positive and high in all three planes for the young subjects (R(2)=0.70, 0.52, and 0.45, respectively), but not for the elderly subjects (R(2)=0.23, 0.0004, and 0.05, respectively). [Conclusion] We suggest that changes in secondary knee kinematics and poor correlations between walking and squatting for the elderly may result from degeneration of the sensory and neuromuscular systems. It could be injurious for the elderly to perform high flexion activities.

Bond strength analysis of the bone cement- stem interface of hip arthroplasties.

OBJECTIVE: To study and establish the preliminary linear and modified models for the interface shear mechanics performance between implant and bone cement and to explore its damage significance. METHOD: The loosening research between artificial hip joint prosthesis stem and bone cement interface performance can be evaluated by the push-in test. Based on the debonding performance test, the analytical expressions of the average load and displacement from the debonding failure and splitting failure process were deduced and determined. The correlations of the expressions of the average load-displacement and statistical experimental data were analyzed. RESULTS: It demonstrated that the interface debonding failure mechanical model could be characterized as interface bond strength mechanical performance. Based on analysis of models and experimental data by the three statistical analysis methods, the results indicated the modified model could be better represented by the interfacial debonding strength properties. The bond stress τ and relative sliding s distribution along the embedment regional were coupling affected by both pressure arch effect and shear lag effect in bone cement. Two stress peaks of implant have been found at the distance from 0.175La loading tip to 0.325La free tip, which also verified the early loosening clinical reports for the proximal and latter region. As the bone cement arch effect, the bond stress peak tend to move to the free tip when the debonding failure would be changed into the splitting failure, which presents a preliminary study on the mechanism of early debonding failure for the stem-cement interface. CONCLUSIONS: Functional models of the stem-bone cement interfacial debonding failure are developed to analyze the relevant mechanism. The different locational titanium alloy stress, and the interfacial bond stress and the relative slides are evaluated to acquire a guide of the different positions of interfacial damage. The coupling effect which is original from the pressure arch and the interfacial shear hysteresis cumulative effect has influence on the interfacial debonding and damage process.

Alterations in Three-dimensional Knee Kinematics and Kinetics during Neutral, Squeeze and Outward Squat.

The squat exercise was usually performed with varying feet and hip angles by different populations. The objective of this study was to compare and contrast the three-dimensional knee angles, moments, and forces during dynamic squat exercises with varying feet and hip angles. Lower extremity motions and ground reaction forces for fifteen healthy subjects (9 females and 6 males) were recorded while performing the squat with feet pointing straight ahead (neutral squat), 30º feet adduction (squeeze squat) and 30º feet abduction (outward squat). Nonparametric procedures were used to detect differences in the interested measures between the conditions. No significant difference in three-dimensional peak knee angles was observed for three squat exercises (p>0.05), however, the overall tendency of knee rotations was affected by varying feet and hip positions. During the whole cycle, the outward squat mainly displayed adduction moments, while the neutral and squeeze squat demonstrated abduction moments. Peak abduction moments were significantly affected by feet positions (p<0.05). Moreover, the tibiofemoral and patellofemoral joint forces progressively increased as knee flexed and decreased as knee extended, yet peak forces were not affected by varying feet positions (p>0.05). In conclusion, a neutral position is recommended to perform the squat exercise, while the squeeze squat and outward squat might contribute to the occurrence of joint pathologies.

Friction and durability of virgin and damaged skin with and without skin cream treatment using atomic force microscopy.

Skin can be damaged by the environment easily. Skin cream is an effective and rapid way to moisten the skin by changing the skin surface properties. Rat skin and pig skin are common animal models for studies and were used as skin samples in this study. The nano- and macroscale friction and durability of damaged skin were measured and compared with those of virgin (intact/undamaged) skin. The effect of skin cream on friction and durability of damaged and virgin skin samples is discussed. The effects of velocity, normal load, relative humidity and number of cycles were studied. The nanoscale studies were performed by using atomic force microscope (AFM), and macroscale studies were performed by using a pin-on-disk (POD) reciprocating tribometer. It was found that damaged skin has different mechanical properties, surface roughness, contact angle, friction and durability compared to that of virgin skin. But similar changes occur after skin cream treatment. Rat and pig skin show similar trends in friction and durability.

Microstructure analysis and wear behavior of titanium cermet femoral head with hard TiC layer.

Titanium cermet was successfully synthesized and formed a thin gradient titanium carbide coating on the surface of Ti6Al4V alloy by using a novel sequential carburization under high temperature, while the titanium cermet femoral head was produced. The titanium cermet phase and surface topography were characterized with X-ray diffraction (XRD) and backscattered electron imaging (BSE). And then the wear behavior of titanium cermet femoral head was investigated by using CUMT II artificial joint hip simulator. The surface characterization indicates that carbon effectively diffused into the titanium alloys and formed a hard TiC layer on the Ti6Al4V alloys surface with a micro-porous structure. The artificial hip joint experimental results show that titanium cermet femoral head could not only improve the wear resistance of artificial femoral head, but also decrease the wear of UHMWPE joint cup. In addition, the carburized titanium alloy femoral head could effectively control the UHMWPE debris distribution, and increase the size of UHMWPE debris. All of the results suggest that titanium cermet is a prospective femoral head material in artificial joint.

[A study of the influence of temperature and humidity on skin friction property].

To investigate the influence of temperature and humidity on skin friction property and to unveil the mechanism therein involved, a test of friction coefficient for four volunteers was carried out on a multi-specimen friction tester. The temperature and humidity of skin were measured with infrared temperature instrument and dermohygrometer. The results showed that the fluidity and ductility of skin were affected by the change of skin temperature. The skin temperature decreasing friction coefficient and the normal displacement decreased first, and then remained unchanged, deformation friction and adhesive friction being the major underlying mechanism. Humidity significantly affected the skin friction properties. The friction coefficient increased with the increasing of humidity. When skin humidity reached to 42% or so, the friction coefficient increased to 1.0 and higher. Meniscus effect was noted to be the major cause of moist skin surface with high friction coefficient.