Key Engineering Materials Vols. 345-346

Abstract: Fuel cladding tubes in nuclear fuel assembly are held up by supporting grids because the tubes are long and slender. Fluid flows of high-pressure and high-temperature in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration), which causes fretting wear in contact parts of tube-support. The fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research about the fretting wear characteristics of tube-support is required. The fretting wear tests were performed with supporting grids and cladding tubes, especially after corrosion treatment on tubes, in water. The tests were done using various applied loads with fixed amplitude. From the results of fretting tests, the wear amounts of tube materials can be predictable by obtaining the wear coefficient using the work rate model. Due to stick phenomena the wear depth was changed as increasing load and temperature. The maximum wear depth was decreased as increasing the water temperatures. At high temperatures there are the regions of some severe adhesion due to stick phenomena.

Abstract: Wear and scuffing tests were conducted using friction and wear measurement of piston rings and cylinder blocks for the low friction diesel engine. Scuffing, described as sudden catastrophic failure of lubricated sliding surfaces, usually characterized by a sudden rapid increase in friction, temperature and noise, is an important failure mode on sliding surfaces. In this study, the frictional forces, wear amounts and cycles to scuffing in boundary lubricated sliding condition were measured using the reciprocating wear tester. The cylinder blocks with several values of surface roughness were used as reciprocating specimens, and a piece of piston ring was used as fixed pin. As increasing load by several steps in lubricated sliding, the friction signals indicated the state of surface interactions, such as friction forces, changes of lubricating films and scuffing. There were some rapid increases in friction forces just before the scuffing would occur. It was found that there was the optimum value of initial surface roughness to prolong the wear life of sliding surfaces. As decreasing the surface roughness of cylinder blocks, the wear amounts were decreased due to increasing the contact area. There was also the optimum surface roughness to reduce the friction and to prolong the scuffing life.

Abstract: It is studied the deformation process by IN SITU surface morphology observation for Mg alloy AZ31 sheet in an SEM. It is found that the deformation starts phenomenologically from twinning process, making out of the first twinning in both intact crystallite grains and existing twinned bands by rolling process, and the secondary twinning in original twinned bands. As the plastic strain increases the twinned bands turn to transverse direction (TD) of the sheet in all three tensile directions: rolling direction (RD), TD and 45° to rolling direction (45). The turning characteristic of grain boundaries were dealt with as well.

Abstract: New laminate design for improved toughness in hexacelsian-alumina composite is introduced. The composite is based on crack deflection in a weak interphase in the alumina matrix and hexacelsian interphase. The strength and toughness of the laminated composite were studied both qualitatively by electronic microscopy and measuring flexure strength. The metastable hexacelsian interphases had partially microcracks to provide crack deflection in the composite, and the crack deflection noticeably proceeded along the meta-stable hexacelsian interphase. Load-deflection curve for the laminate showed improved work of fracture of 2.23 kJ/m2.

Abstract: This research aims to investigate strain rate effect on the out of plane shear strength of unidirectional fiber composites. Both glass/epoxy and graphite/epoxy composites were considered in this study. To demonstrate strain rate effect, composite brick specimens were fabricated and tested to failure in the transverse direction at strain ranges from 10-4/s to 700/s. Experimental observations reveal that the main failure mechanism of the specimens is the out of plane shear failure taking place on the plane oriented around 30 to 35 degree to the loading direction. The corresponding out-of-plane shear strength was obtained from the uniaxial failure stress through Mohr-Coulomb strength analysis. In addition, the associated shear strain rate on the failure plane was calculated through the coordinate transformation law. Results show that the out-plane shear strength increases with the increment of the shear train rates. A semi-logarithmic function expressed in terms of the normalized shear strain rate was employed to describe the rate dependence of the out-plane shear strength.

Abstract: A finite element model is proposed to determine the residual strength and the evolution of damage area of indented sandwiches structures with Nomex honeycomb core and metallic skins indented by a spherical indenter under longitudinal compression load (CAI). The honeycomb is represented by a grid of non-linear springs which its behavior law is obtained by performing simple transverse uniform compression test on a block of honeycomb alone.The comparisons between computation and test result show that the model can simulate accurately the form of damage geometry during indentation, its residual print when the load is relieved (relaxation) and the residual strength and the evolution of damage geometry during CAI.

Abstract: Pt-IrOx and Au-V2O5 thin films were created by magnetron co-sputtering from multiple targets in an Ar-O2 mixture. Successful Pt-IrOx production required high O2 partial pressure and slow deposition rate followed by post-annealing in pure O2. In contrast, deposition of Au-V2O5 films required relatively low O2 partial pressure, and did not need any post-anneal. These different strategies for forming oxide dispersion strengthened films in a multi-target reactive sputtering configuration are directly related to the thermodynamic characteristics of the two materials systems. The most important characteristics are the low equilibrium oxygen solubility in Pt and Au, and the different degrees of oxygen affinity by Ir and V.

Abstract: The quality of surface of the product of metal forming processes depends on physical processes in the vicinity of frictional interfaces between the material and tool. It is well known that material properties in a narrow layer in the vicinity of such interfaces are usually quite different from the properties in the bulk. It is therefore necessary to develop a special approach to account for this feature of the distribution of material properties. A possible approach is proposed in the present paper. It is based on the concept of strain rate intensity factor.

Abstract: Thin metal films often play an important role as structural elements or reflective surfaces in MEMS applications. Mechanical properties of the films are important due to their influence on the performance of MEMS devices that involve bending or stretching metal parts. In order to gain a better understanding of the mechanical behavior of thin metal films, we have developed a novel bulge system and measured mechanical properties of aluminum thin films. The thin films were prepared by e-beam evaporation of high purity Al onto 2 or 3mm ×12 mm rectangular silicon nitride membrane windows in silicon frames. N2 gas was used to pressurize and thus bulge the membranes. The bulge height was measured based on changes of capacitance between the membrane and a fixed, closely spaced electrode. This apparatus provides resolution of approximately 50 nm in bulge height at a data acquisition rate of 100/sec and provides strain rates in the membrane up to 10-5/sec. The stability of the apparatus allows stress relaxation measurements to be made to times of many hours. Time dependent elastic modulus changes of 1
m Al films were measured over periods of times under constant stress.

Abstract: In this paper, the pumping performance of a piezoelectric micropump is simulated with commercial finite element analysis (FEA) software COMSOL Multiphysics 3.2a. The micropump is composed of a 4-layer piezo-composite actuator (LIPCA), a polydimethylsiloxane (PDMS) pump chamber, and two diffusers. The piezoelectric domain, structural domain and fluid domain are coupled in the simulation. Water flow rates are numerically predicted for geometric parameters of the micropump. Based on this study, the micropump is optimally designed to obtain its better pumping performance.