Utilizing the Explicit Finite Element Method for Studying Granular Flows

Granular flows are systems of complex dry particulates whose behavior is difficult to predict during sliding contact. Existing computational tools used to simulate granular flows are particle dynamics, cellular automata (CA), and continuum modeling. In the present investigation, another numerical tool—the explicit finite element method (FEM)—is analyzed as a potential technique for simulating granular flow. For this purpose, explicit dynamic finite element models of parallel shear cells were developed. These models contained 52 particles and consisted of granules that are both round and multi-shaped (diamond, triangle, and rectangle). Each parallel shear cell consisted of a smooth stationary top wall and a rough bottom surface that was given a prescribed velocity of U = 0.7 in/sec (1.78 cm/s). The coefficient of friction (COF) between the particle–particle and particle–wall collisions was varied between 0.0 and 0.75. Utilizing the output of the simulations, results are presented for the shear behavior, particle kinetic energy, and particle stresses within the shear cell as a function of time. As a means of validating the explicit technique for granular flow, a 75 particle, zero roughness, couette shear cell model (solid fraction of 0.50) is subsequently presented for which direct comparisons are made to the results published by Lun. [Lun, C.K. et al.: Phys. Fluids 8, 2868–2883 (1996)] Overall, the results indicate that the explicit FEM is a powerful tool for simulating granular flow phenomena in sliding contacts. In fact, the explicit method demonstrated several advantages over existing numerical techniques while providing equivalent accuracy to the molecular dynamics (MD) approach. These advantages included being able to monitor the collision (sub-surface and surface) stresses and kinetic energies of individual particles over time, the ability to analyze any particle shape, and the ability to capture force chains during granular flow.     

MoS<Subscript>2</Subscript> Films Formed by <Emphasis Type="Italic">In-contact</Emphasis> Decomposition of Water-soluble Tetraalkylammonium Thiomolybdates

Synthesis and tribological evaluation of three tetraalkylammonium thiomolybdate (R₄N)₂MoS₄ (R = methyl, propyl, or ammonia) aqueous-based lubricant additives on a ball-on-disk tribometer was carried out for a steel–aluminum contact. Tests were performed at the same conditions of load, entrainment speed, sliding distance, temperature, and concentration of MoS₂ to compare the activity (lubrication effect) of the thiomolybdates prepared. A friction reduction is observed for the three salts compared to pure water; however, significant differences in friction coefficient are observed depending on the alkyl group. SEM/EDAX and Raman analysis of the wear tracks reveal the in-contact formation of a MoS₂-lubricating film, rich in molybdenum and sulfur.     

A New Phenomenon Observed in Determining the Wear-Corrosion Synergy During a Corrosive Sliding Wear Test

Corrosive wear of materials is a complex surface failure process, which involves both the wear-accelerated corrosion and the corrosion-accelerated wear. Separating contributions of these two processes to the total material loss is of significance for gaining better understanding of the wear-corrosion synergy. Previous studies have generated an electrochemical scratch technique for measuring the wear-accelerated corrosion, based on which other components involved in the wear-corrosion synergy can be determined. In this article, a new phenomenon in measuring the wear-accelerated corrosion is reported. It was observed that for non-passivation materials there existed a stirring effect (by pin), which significantly affected the measurement. The resultant mistake was serious. The previous technique was modified accordingly and applied to determine the corrosion-wear synergy of AISI 1045 steel in comparison with that of AISI 304 stainless steel.     

Model-based optimization of injection strategies for SI engine gas injectors

A mathematical model for the prediction of the mass injected by a gaseous fuel solenoid injector for spark ignition (SI) engines has been realized and validated through experimental data by the authors in a recent work [1]. The gas injector has been studied with particular reference to the complex needle motion during the opening and closing phases. Such motion may significantly affect the amount of injected fuel. When the injector nozzle is fully open, the mass flow depends only on the upstream fluid pressure and temperature. This phenomenon creates a linear relationship between the injected fuel mass and the injection time (i.e. the duration of the injection pulse), thus enabling efficient control of the injected fuel mass by simply acting on the injection time. However, a part of the injector flow chart characterized by strong nonlinearities has been experimentally observed by the authors [1]. Such nonlinearities may seriously compromise the air-fuel mixture quality control and thus increase both fuel consumption and pollutant emissions (SI engine catalytic conversion systems have very low efficiency for non-stoichiometric mixtures). These nonlinearities arise by the injector outflow area variation caused by needle impacts and bounces during the transient phenomena, which occur in the opening and closing phases of the injector. In this work, the mathematical model previously developed by the authors has been employed to study and optimize two appropriate injection strategies to linearize the injector flow chart to the greatest extent. The first strategy relies on injection pulse interruption and has been originally developed by the authors, whereas the second strategy is known in the automotive engine industry as the peak and hold injection. Both injection strategies have been optimized through minimum injection energy considerations and have been compared in terms of linearization effectiveness. Efficient linearization of the injector flow chart has been achieved with both injection strategies, and a similar increase in injector operating range has been observed. The main advantage of the pulse interruption strategy lies on its ease of implementation on existing injection systems because it only requires a simple engine electronic control unit software update. Meanwhile, the peak and hold strategy reveals a substantial lack of robustness and requires expressly designed injectors and electronic components to perform the necessary voltage commutation.     

Experimental evaluation of polycrystalline diamond tool geometries while drilling carbon fiber-reinforced plastics

Polycrystalline diamond (PCD) drills are commonly employed in carbon fiber-reinforced plastic (CFRP) drilling to satisfy hole quality conditions with an acceptable tool life and productivity. Despite their common use in industry, only a small number of studies have been reported on drilling CFRPs with PCD drills. In this study, drilling performances of three different PCD drill designs are investigated experimentally using thrust force, torque, and hole exit quality measurements. Results show that work material properties, drilling conditions, and drill design should all be considered together during the selection of process parameters, and the relationships among these factors are quite complex.     

一种基于线结构光的表面形貌测量方法

本文介绍了一种非接触测量方法,通过建立线结构光测量解析几何模型和视觉透视投影模型对该方法进行分析,确立实际物点与探测像点的数值关系,并搭建应用平台进行实际测量试验,取得了理想的效果。     

用于动态试验机的CPCI总线采集控制系统

本文基于CPCI机箱结构,提出了一种动态试验机可应用的采集控制系统解决方案,大大提高了产品的抗振动冲击能力,提升了系统的稳定性。由于其符合IEC上架机箱的标准,对日后多通道扩展奠定了基础。     

Design of a novel knee joint for an exoskeleton with good energy efficiency for load-carrying augmentation

When a linear actuator is used for rotation motion by a knee joint of an exoskeleton, the specifications of the joint range of motion (ROM) and joint torque change according to how the linear actuator are attached. Moreover, while the linear actuator generates a constant amount of force, the joint torque generated by the actuator changes according to the joint angle, which causes the torque contraction. This makes it difficult to meet the required torque and ROM for walk and stand-to-sit and sit-to-stand (STS) motions while carrying a load. To solve these problems we propose a novel knee joint for an exoskeleton with good energy efficiency during walk and STS motions while carrying a load. The mechanism is composed of a four-bar linkage and an elastic element. Based on an analysis of human motion, the design variables of the joint were optimized and the feasibility of the optimized variables was verified through the simulation. The findings from the simulation results suggest that combining a four-bar linkage with a linear actuator allows a large ROM and good torque performance of the knee joint for walk and STS motions. Moreover, the energy efficiency can be improved because the spring mounted parallel to the actuator can store the energy dissipated as negative work and recycle the energy as positive work.     

RETRACTED ARTICLE: Development and application of intensified envelope analysis for the condition monitoring system using acoustic emission signal

The application of the high-frequency acoustic-emission (AE) technique in the condition monitoring of rotating machinery has been increasing of late. It has a major drawback, though, the attenuation of the signal, and as such, the AE sensor has to be close to its source. Two signal-processing methods, envelope analysis and wavelet transform, were found to be useful for detecting faults in the rolling element bearing and gearboxes. These methods have a disadvantage, though: their application is focused only on a component of the assembled machine. For example, envelope analysis is a powerful method for detecting faults in the bearing system, but it is not proper for use in the gear system. Thus, these methods could not be used to detect combined faults in the common assembled machines. Therefore, we propose a signal-processing method consisting of envelope analysis and DWT (discrete wavelet transform). In addition, a novel mother function optimized for the AE signal for DWT was extracted through a fatigue crack growth test, and is also proposed herein. Then the proposed method, called intensified envelope analysis (IEA), was used to detect the faults in the rolling element bearing and rotating shaft. According to the results, IEA can be a better signal processing method for the condition monitoring system using AE technique.