Key Engineering Materials Vols. 297-300

Abstract: The cyclic deformation in Cu and Cu-35Zn alloy were studied using ultrasonic nondestructive evaluation (NDE) technique to measure the ultrasonic velocity, attenuation coefficient and a nonlinear parameter. These materials were cyclically deformed under total strain amplitude control to investigate their ultrasonic reaction to different dislocation substructures, depending on the stacking fault energy difference. The microstructure evolution was observed using a transmission electron microscope, and the ultrasonic NDE parameter was measured after several cycles of fatigue deformation, in order to clarify the relationship between them. In both materials, the ultrasonic velocity was observed to decrease as the fatigue life fraction increased, which was attributed to the increasing dislocation density caused by the cyclic deformation. In the case of Cu, with its cell structure evolving during cyclic deformation, the rate of increase of the ultrasonic attenuation coefficient and the ultrasonic nonlinear parameter was higher than that observed in the case of the Cu-35Zn alloy with its planar array structure. This result implies that the dislocation cell structure is more sensitive to the ultrasonic parameter changes than the planar array structure formed during cyclic deformation.

Abstract: In modern society, we can see various large structures such as large buildings, ships, bridges that have been constructed using metal, polymers and polymer alloys. Many other new materials are being developed and invented in the world. These materials are made on automatic line systems in factories. These are tested under the rigorous and sever conditions, and shipped on demand. After rigorous testing, however, materials often contain cracks, holes or inclusions. The intensity stress generates on top of the crack and may cause accidents. Holes or inclusions in the material have negative effects on the rigidity and the safety. Many researchers use the high polymer materil for the purpose of the analyzing about cracks, holes and inclusions. The crack, the empty hole and the inclusion become of one of the cause of the fracture and the accident. The authors think that the various problems about cracks, holes and includes are not studied enough. They tried to examine the relation ship of the details of stress intensity factor of the high polymer materials by means of the caustic method and the photoelastic method. This paper discusses the causes and effects of mutual interference of cracks, holes and inclusions in the materials.

Abstract: A new method [1] to evaluate indentation flow curves using an instrumented indentation test has been applied to many materials for several years. Though the method produces relatively good results compared to uniaxial tensile tests, a few parameters had not been verified by theoretical or numerical methods. In this study, proportional constants of representative strain and representative stress were verified using finite element analysis and proven to be unaffected by the elastic property and strain level. The constants were generally dependent on the plastic property; however, one combination of the constants is independent of all properties. The values of this combination are consistent with early research and produced overlapping indentation flow curves with uniaxial curves.

Abstract: In this paper, a new Structural Health Monitoring (SHM) technique for composite laminates through the use of embedded FBG sensors is presented. This method monitors the ply stress states of a laminate and compares them with failure criteria continuously during structures’ service time. The ply stress state of each ply composing the composite laminate can be obtained by embedding three FBG sensors in the laminate based on the classical lamination theory. In this study FBG sensor embedded graphite/epoxy composite laminate specimens were fabricated. With ply stress states being monitored, tension and fatigue tests were performed until laminates’ failure. Experimental results show that laminates experience fracture when the ply stress states are beyond the boundaries of failure criteria. Embedded FBG sensors had good fracture strain and reliability. Therefore, critical damage can be detected by the ply stress states which are close to the boundaries of failure criteria.

Abstract: Most domestic fossil power plants have exceeded 100,000hours of operation with the severe operating condition. Also, cyclic operation of the plant subjects the piping system to mechanical and thermal fatigue mechanisms and poor of defective support assemblies can impose massive loads onto the piping system and induce unstable piping displacement. In order to prevent the serious damage and failure of the piping system in fossil power plants, 3-dimensional displacement measurement system was developed for the on-line monitoring. Displacement measurement system was developed with LVDT and rotary encoder type sensors. This system was installed and operated on a real power plant successfully.

Abstract: A preliminary study of the behavior of ultrasonic guided wave mode in a pipe using a comb transducer for maintenance inspection of power plant facilities has been verified experimentally. Guided wave mode identification is carried out in a pipe using time-frequency analysis methods such as wavelet transform (WT) and short time Fourier transform (STFT), compared with theoretically calculated group velocity dispersion curves for longitudinal and flexural mode. The results are in good agreement with analytical predictions and show the effectiveness of using the time-frequency analysis method to identify the individual guided wave modes. And, It was found out that longitudinal mode (0, 1) is affected by mode conversion less than the other modes. Therefore, L (0, 1) is selected as a optimal mode for evaluating location of the surface defect in a pipe.

Abstract: In photoelastic experimental method, until now, we have used the Newton-Raphson numerical method in analysis of photoelastic experimental data such as the non-linear least square method for the photoelastic expreriment. We used the Hook-Jeeves’ numerical method in stead of Newton-Raphson numerical method for the non-linear least square method for photoelastic experimental method. The new photoelastic experimental hybrid method, that is, the photoelastic experimental hybrid method with Hook-Jeeves’ numerical method has been developed in this research. Applying the new photoelastic experimental hybrid method to stress concentration problems and plane fracture problems, it’s validity was assured. The new photoelastic experimental hybrid method is more precise and stabler than the photoelastic experimental hybrid method with Newton- Raphson numerical method (the old photoelastic experimental hybrid method)