A study on an equivalent model of the threaded fasteners in complex structures through tightening and loosening analysis

Journal of Mechanical Science and Technology - The threaded fasteners are typical machine components in tightening of the machine parts and structures. In addition, as threaded fasteners are easy...     

Simultaneous Formation of Ohmic Contacts on p +- and n +-4H-SiC Using a Ti/Ni Bilayer

In this work, Ti/Ni bilayer contacts were fabricated on both p ⁺- and n ⁺-4H-SiC formed by ion implantation, and the effects of the Ti interlayer on the contact resistance and interfacial microstructure were studied. Adoption of a thin (10 nm) Ti interlayer resulted in specific contact resistance of 4.8 μΩ cm² and 1.3 mΩ cm² on n ⁺- and p ⁺-4H-SiC, respectively, comparable to the values for contacts using only Ni. Moreover, contacts using Ti/Ni provide a flat and uniform interface between Ni₂Si and SiC, whereas discontinuous, agglomerated Ni₂Si islands are formed without the use of a Ti interlayer. In addition, the Ti interlayer was demonstrated to effectively dissociate the thin oxide film on SiC, which is advantageous for low-resistance, reliable ohmic contact formation. In summary, use of a Ti/Ni bilayer is a promising solution for one-step formation of ohmic contacts on both p ⁺- and n ⁺-4H-SiC, being especially suitable for SiC n-channel metal-oxide-semiconductor field-effect transistor (nMOSFET) fabrication.     

Generating interpretable fuzzy rules for adaptive job dispatching

Adaptive scheduling is an approach that selects and applies the most suitable strategy considering the current state of the system. The performance of an adaptive scheduling system relies on the effectiveness of the mapping knowledge between system states and the best rules in the states. This study proposes a new fuzzy adaptive scheduling method and an automated knowledge acquisition method to acquire and continuously update the required knowledge. In this method, the criteria for scheduling priority are selected to correspond to the performance measures of interest. The decisions are made by rules that reflect those criteria with appropriate weights that are determined according to the system states. A situated rule base for this mapping is built by an automated knowledge acquisition method based on system simulation. Distributed fuzzy sets are used for evaluating the criteria and recognizing the system states. The combined method is distinctive in its similarity to the way human schedulers accumulate and adjust their expertise: qualitatively establishing meaningful criteria and quantitatively optimizing the use of them. As a result, the developed rules may readily be interpreted, adopted and, when necessary, modified by human experts. An application of the proposed method to a job-dispatching problem in a hypothetical flexible manufacturing system (FMS) shows that the method can develop effective and robust rules.     

Mechanical property enhancement of non‐bonding electrospun mats via adhesive

In the present study, the effect of adhesive on the morphology of different electrospun polymeric mats was investigated. The modification of two polymers, poly(methyl methacrylate) and poly(vinyl chloride), was carried out by blending the polymers with different amounts of poly(butyl acrylate) (PBA) adhesive to investigate the effect of different amounts of adhesive with heat hardener in hybrid mats. The introduction of various concentrations of PBA into different polymer solutions led to the formation of point‐bonded electrospun fibrous mats. Scanning electron microscopy images indicated that point‐bonded polymer/adhesive fibers were uniformly distributed throughout the mats. Fourier transform infrared spectrometry, contact angle measurements and thermogravimetric analysis were used to study the different properties of the hybrid mats. The tensile strength of the blended fibrous electrospun mats was increased effectively. This enhancement of the mechanical properties of the mats due to the presence of adhesive increases the number of potential applications of the electrospun mats, especially for mechanically weak polymers. Copyright © 2012 Society of Chemical Industry     

Effect of boron addition on the microstructure and mechanical properties of 6.5% V-5% W high speed steel

The effect of adding 0.04% additional boron to 6.5% V-5% W high speed steel was investigated. The microstructure characterization, phase identification, and carbide identification of the materials were performed using SEM/EDS and XRD. The cell size and carbide volume fraction were examined using image analysis software. The boron distribution was observed by PTA Boron Tracking. The addition of 0.04% more boron to a HSS alloy exerted a cell refining effect on the sample. The cell refinement of dendritic structures in the alloy containing boron may be attributed to the constitutional supercooling effect associated with the fairly small distribution coefficient for boron in iron. The addition of boron increases the bending strength of the material by more than 10%, as well as increasing its hardness.     

An adaptive ARQ–HARQ interworking scheme in WiMAX systems

For reliable data transmissions, WiMAX systems support automatic repeat query (ARQ) that operates at the upper MAC and hybrid automatic repeat query (HARQ) that operates at the lower MAC and PHY. ARQ and HARQ schemes have their own weakness that results in low throughput and high delay in WiMAX systems. Although ARQ and HARQ schemes can complement with each other, they operate independently. Some studies focus on the benefits of the interaction between ARQ and HARQ schemes, but these studies have limitations. In this paper, we propose an adaptive ARQ and HARQ interworking scheme to provide reliable transmissions without performance degradation in WiMAX systems. The proposed scheme has five features that are designed to solve the weaknesses of the ARQ and HARQ schemes. We compare the proposed scheme with existing schemes that utilize the ARQ and HARQ interaction through simulations, and the simulation results show that the proposed scheme shows improved performance over existing schemes.     

Importance of initial aortic properties on the evolving regional anisotropy, stiffness and wall thickness of human abdominal aortic aneurysms

Complementary advances in medical imaging, vascular biology and biomechanics promise to enable computational modelling of abdominal aortic aneurysms to play increasingly important roles in clinical decision processes. Using a finite-element-based growth and remodelling model of evolving aneurysm geometry and material properties, we show that regional variations in material anisotropy, stiffness and wall thickness should be expected to arise naturally and thus should be included in analyses of aneurysmal enlargement or wall stress. In addition, by initiating the model from best-fit material parameters estimated for non-aneurysmal aortas from different subjects, we show that the initial state of the aorta may influence strongly the subsequent rate of enlargement, wall thickness, mechanical behaviour and thus stress in the lesion. We submit, therefore, that clinically reliable modelling of the enlargement and overall rupture-potential of aneurysms may require both a better understanding of the mechanobiological processes that govern the evolution of these lesions and new methods of determining the patient-specific state of the pre-aneurysmal aorta (or correlation to currently unaffected portions thereof) through knowledge of demographics, comorbidities, lifestyle, genetics and future non-invasive or minimally invasive tests.     

A Surface Tailoring Method of Ultrathin Polymer Gate Dielectrics for Organic Transistors: Improved Device Performance and the Thermal Stability Thereof

Tailoring the surface of the dielectric layer is of critical importance to form a good interface with the following channel layer for organic thin film transistors (OTFTs). Here, a simple surface treatment method is applied onto an ultrathin (<15 nm) organosilicon‐based dielectric layer via the initiated chemical vapor deposition (iCVD) to make it compatible with organic semiconductors without degrading its insulating property. A molecular‐thin oxide capping layer is formed on a 15 nm thick poly(1,3,5‐trimetyl‐1,3,5‐trivinyl cyclotrisiloxane) (pV3D3) by a brief oxygen plasma treatment. The capping layer greatly enhances the thermal stability of the dielectrics, without degrading the original mechanical flexibility and insulating performance of the dielectrics. Moreover, the surface silanol functionalities formed by the plasma treatment can also be utilized for the surface modification with silane compounds. The surface‐modified dielectrics are applied to fabricate low‐voltage operating (<5 V) pentacene‐based OTFTs. The highest field‐effect mobility of the device with the surface‐treated 15 nm thick pV3D3 is 0.59 cm² V^?1 s^?1, which is improved up to two times compared to the TFT with the pristine pV3D3. It is believed that the simple surface treatment method can widely extend the applicability of the highly robust, ultrathin, and flexible pV3D3 gate dielectrics to design the surface of the dielectrics to match well various kinds of organic semiconductors.