Key Engineering Materials Vols. 326-328

Abstract: The CO2 was investigated as an alternative refrigerant to replace HFC (hydrofluorocarbons) refrigerant in air conditioning systems due to the environmental concerns. Because new compressors with CO2 are going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To develop new compressor, especially rotary type, the friction and wear characteristics of sliding surfaces between a vane and a roller in the rotary compressor were evaluated in this paper. Several hard coatings, such as TiN, CrN and WC/C, were applied on vane surfaces in order to improve the tribological characteristics, and their performances were evaluated experimentally. Two types of lubricants were used, namely POE (polyol ester) oil and PAG (polyalkylene glycol) oil. The vane-on-disk type sliding tests were performed with an exclusive high pressure wear tester. From the tests, wear volumes of vane surfaces applied various coatings were compared. Test results showed that WC/C and CrN coatings showed good tribological properties. Also, the results of the sliding tests showed that using PAG oil has an advantage over POE oil in CO2 environment.

Abstract: Friction and wear characteristics between two steel surfaces under fretting condition are investigated experimentally. The fretting damage caused by low-amplitude oscillatory sliding can be classified into three regimes of gross-slip, mixed-slip and partial-slip due to stick-slip phenomenon. One of the most important characteristics of fretting wear is the transition from gross-slip to mixed-slip. This study was focused on getting the degree of stick-slip out of the friction transition under fretting condition. Fretting wear is divided into three conditions of gross-slip/mixed-slip/ partial-slip. The criteria for the division are friction and displacement amplitude, wear scar morphology and dissipated energy. In this test, friction force and displacement were measured for detecting the transition from mixed-slip to gross-slip and qualitatively predicting the degree of the wear.

Abstract: This paper introduces the method that determines rock bridge intensity based on rock discontinuity and connectivity ratio in the multi-fields coupling action. First, the method is used to study the smallest anti-shear way of rock mass in the microscopic discontinuity and integrated rock. Then the smallest anti-shear stress is computed in the way. The connectivity ratio of discontinuity is taken as the weighting coefficient, and the synthetical anti-shear intensity is obtained with summing the indexes of the discontinuity parameters and connectivity ratio coefficient.

Abstract: This study was carried out to analyze the effect of wind load on the stability of an articulation type container crane using wind tunnel testing. This was done in order to furnish designers with data that can be used in the design of an articulation type container crane that is wind resistant, assuming an applied wind load of 75m/s velocity. Data acquisition conditions for this experiment were established in accordance with similarity. The scale of the articulation type container crane model, wind speed and time were chosen as 1/200, 1/13.3 and 1/15 respectively and this experiment was conducted using an Eiffel type atmospheric boundary-layer wind tunnel with 11.52m2 cross-sectional area. All directional drag and overturning moment coefficients were investigated and uplift forces due to wind load at each supporting point were analyzed.

Abstract: The magnetic coercivity of martensitic 12Cr steel was measured in order to evaluate its degree of isothermal aging. As the aging time increased, the lath width increased and the dislocations were recovered. Aging resulted in the coarsening of the as-tempered carbides (M23C6 and MX) followed by additional precipitation of Fe2W. The magnetic coercivity rapidly decreased during the initial aging period of about 1,000 hours and then decreased slightly thereafter. The decrease in the coercivity with increasing aging time was related to the decrease in the number of pinning sites, those associated with the dislocations, fine precipitates and coarsening of the martensite lath/subgrain.

Abstract: Noticeably in casting and heat treatment process, computational methods are commonly engaged to predict process and mechanical characteristics such as solidification time, cooling speed, hardness and residual stress, in which analyzing thermo-mechanical coupled process necessitates such heat transfer, microstructure transformation, and stress. This paper proposes a FDM/FEM hybrid method for thermal stress analysis in casting process; taking advantage of each computational method, we use both FDM and FEM to analyze thermal stress. We use FDM to predict temperature distribution and microstructure and then transfer the result data to FEM to calculate thermal stress distribution. In this process, a systematic field data conversion procedure is developed for a successful data transfer. For the validation of this proposed method, numerical examples are presented and compared with antecedent experiment results. The interface data conversion program developed in this study can be used for any other program as well as FDM and FEM.

Abstract: hybrid experimental-numerical method is presented for determining the stresses around a circular hole in a finite-width, tensile loaded plate. Calculated fringes obtained by FEA provided the information about the external boundary of the hybrid element, and those fringes on straight lines were used for hybrid analysis. In order to see the effects of varying stress field, different numbers of terms in a power-series representation of the complex type stress function were tested. Actual and reconstructed fringes were compared. The hybrid results were highly comparable with those predicted by FEA. The result showed that this approach is effective and promising because isochromatic data along the straight lines in photoelasticity can be conveniently measured by use of phase shifting photoelasticity.

Abstract: This study aimed to achieve optimization of grinding parameters for aluminum-based metal matrix composites using response surface model and genetic algorithm. Experiments were conducted in accordance with a preplanned orthogonal array. The effect of grinding parameters on surface roughness and grinding forces was evaluated and second-order response surface models were developed for predicting grinding outcomes. Optimal grinding parameters were determined from the genetic algorithm and the response surface models.

Abstract: Topology optimization of the inner reinforcement for a vehicle’s hood has been performed by evolutionary structural optimization (ESO) method. The purpose of this study is to obtain optimal topology of the inner reinforcement for a vehicle’s hood considering static stiffness and natural frequency simultaneously. To do this, the multiobjective design optimization technique was implemented. From several combinations of weighting factors, a Pareto-optimal solution was obtained. Optimal topologies were obtained by the ESO method, i.e., by eliminating the elements having the lowest efficiency from the structural domain. As the weighting factor of the elastic strain efficiency goes from 1 to zero, it is found that the optimal topologies transmits from the optimal topology of static stiffness problem to that of natural frequency problem. Therefore, it was concluded that ESO method is effectively applied to topology optimization of the inner reinforcement of a vehicle’s hood.

Abstract: Generally, a Johnsen-Rahbek (J-R) type electrostatic chuck (ESC) generates higher attractive force than a Coulomb type ESC. Attractive force in a J-R type ESC is caused by the high electrical resistance that occurs in the contact region between an object plate and a dielectric layer. This research tries the simple geometrical modeling of the contact surface and simulates the contact resistance, the attractive force and the response time according to the variation of contact surface shape. In the latter half of this research, the simulation for a pin-combined chuck is accomplished using a similar surface modeling and the comparison between the pin chuck and the general flat chuck is made in aspects of the attractive force and the response time.