Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

A new understanding on the mechanism of fountain solution in the prevention of ink transfer to the non-image area in conventional offset lithography

In conventional offset lithographic printing, it has been well established that the existence of a continuous layer of fountain solution (FS) on the surface of the non-image area is an essential condition to ensure correct operation of lithography. However, the mechanistic function of FS in preventing the ink from being transferred onto the non-image area has not been fully understood. Several major mechanistic interpretations can be found in the literature, which are based either on comparing of static works of adhesion and cohesion of ink and FS, or on the splitting of the 'weaker' FS layer. Although the latter becomes more accepted, direct experimental evidence is difficult to find in the literature. On the other hand, confusing information found in the literature showed that the ink-transfer (or non-transfer) observations reported in many case studies correlate well with simple comparisons of works of adhesion, cohesion and spreading data of ink/FS, ink/plate and FS/plate obtained under the static condition. These results, therefore, imply that, in explaining the function of FS in preventing ink transfer to the non-image area, the ink/FS interfacial adhesion failure would be the dominant mechanism. The work presented in this study covered two specific areas in order to address and better understand the responses of ink and FS layers and their interface to forces encountered during ink transfer. Firstly, an analysis of lithographic plates contaminated with a cationic polymer revealed that the violation of the ink non-transfer condition of the plate non-image area due to contamination could be predicted by traditional criteria of plate wetting and works of adhesion and cohesion. However, these traditional criteria cannot reliably predict the non-transfer condition of the ink on the clean non-image area that was covered by FS. Secondly, in some novel experiments conducted in this study using ice or Teflon as a substrate, the works of adhesion and cohesion were not able to predict ink transfer in most cases. Direct experimental evidence from this work revealed that splitting of the FS layer was involved in the prevention of ink transfer to the non-image areas, and that the thickness of the FS layer was critical in allowing the splitting to occur.     

A study of paint adhesion to polymeric substrates

In order to explore the fundamental mechanism of paint adhesion to polymer substrates the surface of polypropylene- ethylene propylene rubber (PP-EPR) blends was modified by flame or plasma treatments. The changes in surface composition and properties were investigated and discussed in light of the results of simple adhesion tests. The topography and surface properties of the PP-EPR samples were studied by employing various surface sensitive techniques. Additionally, the surface properties of the pre-treated PP-EPR were compared with the model polymers poly(methyl methacrylate) (PMMA) and polycarbonate (PC) displaying a poor and an excellent paint adhesion, respectively. Differential scanning calorimetry (DSC) measurements showed that the miscibility of the polymer substrate with paint components was an essential factor for the understanding of the adhesion mechanism. A general model of paint adhesion to polymer surfaces is proposed, where the degree of interdiffusion of the polymer chains of the substrate and paint in the interphase determines the adhesion strength.     

Compact laparoscopic assistant robot using a bending mechanism

This paper presents the development of a compact laparoscopic assistant robot. The robot was designed to increase convenience and reduce possible interference with surgical staff by confining the majority of motions inside the abdomen. Its size was miniaturized as much as possible for convenient handling. A bending mechanism composed of several articulated joints was introduced to produce motions inside the abdomen. The proposed assistant robot can generate 3-DOF motion, including 2-DOF internal bending motion and 1-DOF external linear motion. Since the robot itself functions as a laparoscope, a small CCD camera module and a bundle of optical fibers were integrated as part of the system. For accurate control, mathematical modeling of the bending mechanism and a method of hysteresis compensation were introduced and implemented. For the control of the robot, a voice interface and a visual-servoing method were implemented. The performance of the developed system was tested through solo-surgery of in vivo porcine cholecystectomy. It was found that the views generated by the bending mechanism were sufficient throughout the surgery. Since the robot has functions comparable to the previously developed systems, while retaining its compactness, it is expected to be a useful device for human cholecystectomy.     

Development of a novel crawler mechanism with polymorphic locomotion

The design of a novel crawler mechanism with polymorphic locomotion is presented in this paper. The proposed mechanism, which is equipped with a planetary gear reducer, provides two kinds of outputs in different form only using one actuator. By determining the reduction ratio of two outputs in a suitable proportion, the crawler mechanism is capable of switching between two locomotion modes autonomously according to terrain. Using this property, robots equipped with the crawler mechanism can perform more efficient and adaptable locomotion or posture in irregular environments. Experimental tests showed that the developed crawler-driven module equipped with the proposed crawler mechanism cannot only move on moderately rugged terrain, but also perform a particular locomotion mode to negotiate high obstacles or adapt to different terrains without any sensors for distinguishing obstacles or any extra actuators or mechanisms for assistance.     

Effect of lanthanum rare earth addition on low temperature plasma nitriding

Abstract

The effect of lanthanum additions on the plasma nitriding of nanocrystalline steel 3J33 is reported. Nitriding was carried out in 20N₂ + 80H₂ at a pressure of 400 Pa at 350 and 410°C. Glow discharge spectrometry shows that although its atomic radius is larger than that of Fe, La can diffuse into the surface layer to a significant depth; the La addition increases the compound layer thickness. Based on the microhardness profile, the diffusion layer thickness also increases due to La addition. XRD analyses indicate that the presence of La did not change the phase structure of the surface, i.e. a compound layer of γ′-Fe₄N type. The incorporation of La not only enhances the microhardness but also improves the toughness of the nitrided layer. The atomic and electron structures of phases γ′-Fe₄N and γ′-(Fe,La)₄N were calculated using the plane wave pseudopotential method. The (Fe,La)₄N phase is more stable than Fe₄N and the shear/bulk modulus ratio G/K of (Fe,RE)₄N is much smaller. This finding provides a possible mechanism for the increased microhardness and improved toughness of nitrided layers containing La.     

Geometric Factors Impacting Adhesive Lap Joint Strength and Design

Too often adhesive thickness, adherend thickness and other geometric factors are not explicitly considered in adhesive joint design. This study includes experimental and computational research exploring the means of enhancing the engineering design process for adhesive lap joints to include such effects. It clearly demon-strated that both the cleavage stresses and the shear stresses, near the bond termini, play important roles in lap 'shear' joint failure. Finite Element and Fracture Mechanics analyses were used to examine the energy release rate applied to growth of cracks in adhesive lap joints. Lap joints with similar geometries to those analyzed were designed, fabricated and tested. In a separate set of experiments the bond termini were constrained in the direction normal to the uniaxial loading. If the strength of lap shear joints is dominated by the adhesive shear strength, then constraining the lateral motion of the bond termini should have little or no effect on the overall shear strength of the adhesive joint. This work clearly demonstrates that this is not the case. If cleavage stresses are important in lap joints then constraining the bond termini, in a direction normal to the bond area, should have a commensurate effect on the overall strength of the lap joint. None of the ASTM standardized 'lap shear tests' provide any insight into this premise. This paper also presents analyses and experimental results for lap joints to which several methods of lateral constraint were applied near the bond termini. The analytical and numerical methods described and used for explaining and predicting such effects might be a useful adhesive joint design tool.     

A novel reinforcing filler: application to addition-type liquid silicone rubber adhesive system

A novel addition-type liquid silicone rubber (AL-SR) adhesive has been prepared by incorporation of organic montmorillonite (OMMT). Physico-mechanical properties, such as adhesion, tensile strength and thermal stability, were investigated and compared to aerosilica (also known as pyrogenic silica) filled systems. Results showed that the properties of AL-SR containing 1 phr OMMT were nearly equal to that of the AL-SR containing 3 phr aerosilica, due to exfoliation of OMMT in the system. The structure of the AL-SR/OMMT adhesives was analyzed by wide angle X-ray diffraction (WAXD), Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM). Results showed that OMMT layers in the AL-SR adhesives were in the nanometer scale and the molecular structure of AL-SR was disturbed by the presence of OMMT, which was due to the reaction between the monomers and the intercalation agent. The nano-reinforcing effect caused by the exfoliated silicate layers may reduce the amount and size of voids during tensile drawing, and thus increase the length of the crack path in their vicinity, which is regarded as the adhesion mechanism for the OMMT-reinforced AL-SR systems.     

绷丝机上丝装置的自动控制

介绍了绷丝机上丝装置的一种自动控制系统，实践证明它结构简单、准确率高。     

Factors Affecting the Quality of Black-and-White Reflexion Prints

Reflexion-print quality has been shown to be predictable from a knowledge of objective tone-reproduction characteristics for black-and-white reflexion prints, based on a mathematical relationship between print quality and tone-reproduction data, quantitative data have been derived to show the dependence of print quality on several of the sensitometric variables in negative-positive reproduction systems.