Key Engineering Materials Vols. 324-325

Abstract: A new method is proposed to estimate the parameters of probabilistic fatigue crack growth rate models, including the Paris equation and its’ improved type. To take the statistical characteristics of whole test data into account, the method inherits the idea from the general maximum likelihood method which is widely used in parameters estimation of fatigue S-N curves, ε-N curves, and da/dN-"K curves, and extends the conventional correlation coefficient optimization method into parameters evaluation not only for mean curve, but also for standard deviation curve and probabilistic curve. Analysis on the test data of 16MnR steel indicates that present method is available and feasible. Comparing to the general maximum likelihood method, present method has simpler algorithm, and can avoid constructing and solving the likelihood function, so it is speedier in calculation.

Abstract: The metal magnetic memory (MMM) technology, based on the magneto-elasticity and magneto-mechanical effect theory, has been applied to remaining fatigue life prediction. The correlation between fatigue life and MMM parameter has been investigated through rotary bending fatigue experiments. Steel X45 samples, with artificial cracks of different depth and breadth, are tested with MMM method. Based on the results of the metallographic examination, the feasibility of remaining fatigue life prediction is studied. A new remaining fatigue life MMM model of ferromagnetic material is presented. The proving experiments show the maximum error of remaining fatigue life is 4.58% between MMM model calculation and the actual life. The agreement of remaining fatigue life predicting values and testing values is found to be quite good.

Abstract: Understanding the dynamic behavior of concrete in rapid loading is an issue of great importance in civil engineering. In this study, an experimental program was performed to investigate the dynamic behavior of concrete subjected to different strain-rate loadings. From the test results the rate-dependent effect on the ultimate strength of concrete was confirmed, i.e., the strength increases with the increasing strain rate. The dynamic failure process of concrete in tension and physical mechanism were discussed based on the experimental observations.

Abstract: Edge closeout is one of the better methods for fastening composite materials to other structures. In this paper, fastened region of Carbon/Fabric composite plate edge closeout is mechanically tested. Finite element analysis and AE(acoustic emission) characteristics are utilized for comparison of test and analysis. The characteristics of AE signal used in this study are hit distributions, peak frequency of generated AE waveform and amplitude of signals. The frequency regions for failure of composite materials are under 100 kHz and 200-300 kHz for failure initiation and the ultimate failure occurred with frequency region of 300-400 kHz.

Abstract: The rotating components in the hot sections of land-based gas turbine are exposed to severe environment during several tens thousand hours at above 1100則 operating temperature. To protect such components from high temperature oxidation, an intermediate bond coat is applied, typical of a MCrAlY-type metal alloy. Various processing methods have been studied for bond coat deposition. This study is concerned with the thermal cyclic behavior of thermal barrier coatings. The MCrAlY bond coatings are deposited by a vacuum plasma, low vacuum plasma and highvelocity oxygen fuel spray on a nickel-based superalloy (GTD-111). Thermal cyclic tests at 1100則 in still air for various periods of time were used to evaluate the thermal cyclic resistance of the TBC coating with the various processing sprayed bond coating layer. The microstructure and morphology of as-sprayed and of thermal cycled coatings were characterized by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The order of thermal cyclic resistance was YSZ-VPS bond coat, YSZ-LVPS and YSZHVOF. The influence of bond coat spray methods on durability of TBCs is discussed.

Abstract: The purpose of this study is to understand the fundamental resistance mechanism and the shear strength of the frame with the reinforced concrete infill wall by comparing analytical with experimental results. For this, one-story and one-bay four specimens were manufactured with variables; Lightly Reinforced Concrete Frame (LRCF), monolith placing Shear Wall (SW), CIP Infill Wall (CIW-1) and CIP Infill Wall reinforced with diagonal rebar (CIW-2). The addition of the RC infill wall was significantly improved the strength and the stiffness. Compared with specimen LRCF, ultimate strength and initial stiffness of infills was improved 4 and 6 times, respectively. The case of specimen CIW-2, structural performance was improved remarkably by placing a diagonal rebar.

Abstract: The characteristics relating to high tensile ductility, High Performance Fiber Reinforced Cementitious Composites (HPFRCC) are being so that studied it’ll be adopted in repair and retrofit of buildings. A series of three shear wall specimens were tested under constant axial stress and reversed cyclical lateral loading in order to evaluate the seismic retrofit that had been proposed for the shear wall with the opening. The retrofit involved the use of ECC and MDF, both of which are sprayed through the high pressure pump, over the entire face of the wall. The results indicate that two different types of retrofitting strategies cause the different effects on the strength and ductility of each specimen.

Abstract: Cold forging has various advantages compared to other forming processing. Cracking phenomenon was taken place during cold forging of ML25Mn steel. In this study, microstructural analyses were made on the cracked regions of the steel. The reason of cracking phenomenon occurring during cold forging for ML25Mn steel was investigated based on SEM observation in detail. The results have shown that ML25Mn steel presents lower strength and toughness. A larger amount of inclusions which are composed of MnS and complex oxides containing Mg, Al, Mn, Fe, S, Ca and O are found, and the complex inclusion might be brought during the deoxidation and the solidification in smelting and casting. Non-metallic inclusions result in significant stress concentration, which cause cracking phenomenon occurring during cold forging of ML25Mn steel.

Abstract: Frequency tuning characteristics of the multi-layered micro-resonators have been extensively investigated by using thermal and piezoelectric actuations. Based on the layerwise displacement theory and geometric nonlinear formulation, the nonlinear deformation and its attendant vibration characteristics of un-symmetrically deposited camped-camped micro-beams under piezoelectric and thermal actuations have been analyzed. The effects of the eccentric piezoelectric actuation and uniform thermal loading on the large deflection and natural modes were discussed with respect to geometric nonlinear responses and initial imperfection. Present results show that both piezoelectric and thermal actuations can effectively tune the resonant frequencies as increasing and decreasing desired values by the alternative selection of the dominance between in-plane deformation and out-of-plane deformation.

Abstract: Welding deformations of real structures are complicated and the accurate prediction of welding deformations has been a difficult problem. The equivalent load method is presented in this paper. The method uses the FE method combining the inherent strain theory and the experimental results for accurate and efficient analysis. A simplified elastic plastic analysis model, bar-spring model is used. It is showed that the highest temperature and the degree of restraint change the distributions of the inherent strains in the welded region. Applying the equivalent loads induced by the inherent strain on the structure, the final deformations are calculated by FE analysis. The results in this paper have been supported by the results of C.H.LEE and the experimental data.