Key Engineering Materials Vols. 353-358

Abstract: In this study, we performed thermal stress analyses on a ventilated disk brake with a 3D model for two cases (whether the pressure distribution on a contact surface is uniform or not). A pressure distribution analysis was performed to determine the pressure distribution on the contact surface. Then, by using the results that were obtained from the pressure distribution analyses, we performed thermal stress analyses. Finally, we have found the spots where the maximum thermal stresses occur. Also, for the life evaluation of a disk brake, we have conducted the fatigue test and obtained the S-N curve. From those results, we evaluated the life of a disk brake.

Abstract: The aim of this paper is to investigate the effect of ultra-violet (UV) ray irradiation on the crack formation of brittle ceramic coating on polymer substrate. It is well known that ultra-violet ray irradiation degrades the mechanical properties of polymer and polymer-based coating films. We carried out the tensile tests of PET/ITO film specimen after UV irradiation under the microscope and observed the crack formation on ITO surface. Also, we carried out nano-indentation tests of PET substrate after removing ITO layer to characterize the change of the mechanical properties near the interface between PET and ITO. The results show that the number of cracks vertical to loading direction and the rate of vertical crack formation decreased after UV irradiation. Hardness and Young’s modulus of PET substrate increase due to the oxidation of PET after UV irradiation. To explain the relationship between the crack formation and mechanical property change, the energy release rates of the thin film channeling cracks are considered.

Abstract: A-C: H (Hydrogenated amorphous carbon) diamond like carbon(DLC) film is the low stabilities of chemical and mechanical properties at high temperatures. Some researchers reported that Raman spectra shown significant conversion of DLC films to nano-crystalline graphite on heating in ambient air at temperatures above 300°C and conversion to nano-crystalline graphite was completed by 450~600°C In this study, DLC films(thickness : 1.5-m) were deposited on p-type (100) silicon films by using C6H6 and C6H14 plasma produced with a 13.6MHz RF source. To investigate the mechanical stability of DLC films below 250°C, we thermally aged the films for different aging temperatures (150°C, 200°C, 250°C) and times (0: virgin, 120, 240, 360 hours). By using indentation test and Raman spectra analyses, it has been found that the H3/E2 decreases with increasing I(D)/I(G) ratio. Also we confirmed that sample-2(precursor gas: C6H14) had better mechanical characteristic and thermal stability than the sample-1(precursor gas: C6H6).

Abstract: In order to improve the heat transfer efficiency, the fins are commonly used in the industrial boiler in Japan. In actual application, the thermal fatigue due to the cyclic change of temperature usually occurs in the fins. The thermal fatigue tests were carried out by using the thermal fatigue apparatus designed. The designed testing apparatus can apply thermal load to the fin by giving heating and cooling alternatively. The fatigue cracks can be observed in the vicinity of the toe in the thermal fatigue test. The heat transfer coefficient and the thermal stress were calculated by using Finite Element Analysis (FEA) method. The fatigue experiments of the fins were also conducted using electro-hydraulic servo fatigue test machine in the laboratory. The results of thermal fatigue experiments were discussed by comparing with those of the mechanical fatigue experiments.

Abstract: The present paper investigates distributions of α’ martensitic phase on fracture surfaces of plate specimens of SUS 304 stainless steel having through- and part-through-thickness fatigue cracks. The volume fraction Vα’ of the α’ phase transformed on the fracture surfaces including the two types of cracks was measured after specimens had been fractured due to fatigue at room temperature in air. The measurements were made with ferrite scope. The contour maps of Vα’ revealed crack shapes imprinted on the fracture surfaces of the specimens; i.e., nearly rectangular shapes on the fracture surfaces of through-thickness-cracked specimens, whereas semi-elliptical shapes on the fracture surfaces of part-through-thickness-cracked specimens. The values of Vα’ measured on crack surfaces can be correlated with the maximum stress intensity factor values Kmax calculated at the points of Vα’ measurements. The resultant Vα’ vs. Kmax plots fall within narrow bands whose upper and lower bounds are expressed by exponential curves. These results imply that quantitative post-fracture analysis can be made by measuring Vα’ values on the fracture surfaces of structural components with ferrite scope.

Abstract: A modified linear-elastic fracture mechanics approach proposed by McEvily has been applied to predict the effects of small defects on the fatigue limit and the threshold level. In the analysis, three modifications were taken into account (1) the effect of elastic-plastic behavior of small cracks, (2) the Kitagawa effect where in the very small crack regime the required stress for propagation is controlled by the fatigue limit of a smooth specimen rather than by the long-crack threshold condition, and (3) the effect of crack closure development from zero up to the macroscopic level as a newly formed crack extends. Three steels, a brass and an Al alloy were investigated. Good agreement between predicted and experimental results has been obtained and a rational basis for the area parameter model was shown.

Abstract: The fatigue performance of one widely used wrought magnesium alloy AZ31B in rolled form was studied using acoustic emission (AE). AE energy release during corrosion fatigue (CF) testing was measured at room temperature. It is found that AE energy release can express corrosion fatigue damage experienced by the material.

Abstract: There are three common empirical expressions used for the fatigue curves, which are power function, exponential function and three-parameter power function expression, respectively. The mathematical difference between the former two and the latter is whether there exists the constant term S0 in the equations. The S0 can be calculated to determine whether the two-paprameter expression or three-parameter expression should be used. If the two-parameter expression should be used, the power function and exponential function expressions can be compared to determine which one is the optimum one. Finally, the method has been validated by several groups of fatigue data.

Abstract: Rotational bending fatigue tests were carried out on a medium carbon steel with a thermally sprayed Co-based alloy coating. The effect of two different fusing treatments on the fatigue strength of fused specimens was investigated. Fusing treatment was performed using a vacuum furnace, and an induction heating system. When the specimens were treated in vacuum furnace at 1373 K for 4 h, the diffusion layer providing a strong adhesive force were formed at the interface between the coating and substrate. As a result, fatigue strength remarkably increased in comparison with the uncoated specimens. On the contrary, for the treated specimens with an induction heating system at 1373 K for 120 s, since these specimens had a lower adhesive force due to no formation of diffusion layer, leading to delamination of the entire coating.

Abstract: Metal dusting is a catastrophic corrosion phenomenon that leads to the disintegration of structural metals and alloys into dust composed of fine particles of the metal/alloy and carbon at high temperature. Considerable research efforts has been taken on the initiation of metal dusting in the alloy since it was observed. However, the previous researches were focused mostly on the initiation mechanisms of metal dusting without external applied loads. In fact, structural materials are mostly used with external applied loads. It is thus necessary to study the initiation and propagation mechanism of metal dusting in the stressed condition. In the present work pure bending testing of Cr-Mo alloy in metal dusting condition was performed. The laboratory experiments were performed at 560°C, using a synthesis gas mixture with composition 75%H2 + 25%CO. The maximum bending stresses were 100MPa, 50MPa, 10MPa and 0MPa, respectively. Post exposure metallographical examinations, SEM, EDX analysis were made in order to identify the difference between loaded and unloaded specimens. The effect of mechanical and chemical interaction on high temperature corrosion was preliminary discussed.