Key Engineering Materials Vols. 345-346

Abstract: Linear and nonlinear ultrasonic responses of contacting surfaces were studied based on the contact-pressure dependence of the interfacial stiffnesses. To this purpose, the reflection coefficients of longitudinal and transverse waves were measured for the contacting interface between polished aluminum blocks. The normal and tangential stiffnesses of the interface were then obtained as function of the contact pressure. The reflection coefficients were found to decrease, while the interfacial stiffnesses were found to increase, with the contact pressure. It was also found that the ratio of the tangential to normal stiffness as well as the nonlinearity parameters (ratio of the second harmonic to the squared fundamental amplitudes) could be used as a parameter to characterize the contact condition.

Abstract: An inverse technique based on vibration tests to characterise isotropic, orthotropic and viscoelastic material properties of advanced composites is developed. An optimisation using the planning of experiments and response surface technique to minimise the error functional is applied to decrease considerably computational expenses. The inverse technique developed is tested on aluminium plates and applied to characterise orthotropic material properties of laminated composites and viscoelastic core material properties of sandwich composites.

Abstract: In this study, elastic waves of ultrasonic and acoustic emission were used to evaluate the propagation characteristic of the wave in pipe, and study on mode conversion of the elastic wave due to the cracks in the pipe was also performed. An acoustic emission (AE) sensor was used to receive the propagated ultrasonic wave. AE technique has a merit that it can identify the received ultrasonic wave by the analysis of the AE parameters such as count, energy, frequency, duration time and amplitude. For transmitting and receiving of the wave, a wedge for universal angle was manufactured. The optimum angles for transmitting of ultrasonic wave and signal receiving at the attached AE sensor on the pipe were determined. Theoretical dispersion curve was compared with the results of the time-frequency analysis based on the wavelet transformation. The received modes showed a good agreement with theoretical one. The used ultrasonic sensor was 1MHz, and AE sensor was broadband (100kHz – 1200kHz). The artificial cracks were induced in the pipe to measure the propagation characteristics of the elastic wave for the cracks. AE parameters for the received signals were also varied with the crack types in the pipe. AE parameters of amplitude and duration time were more effective factors than the analysis of mode conversion for evaluation of the cracks in the pipe.

Abstract: The use of non destructive techniques for the elastic characterization of isotropic materials is continuously increasing and those based on the modal vibration testing of plate-like specimens is very widespread. In the present paper, an optimized search procedure is proposed which allows the material constants of isotropic plates to be non-destructively identified from vibration testing data and using finite element analyses. The identification process is performed by an optimizing algorithm in which the error function to be minimized depends on the difference between the natural frequencies obtained by finite element analyses and the measured ones. In order to verify the proposed identification procedure a comparison with the results reported in literature has been made.

Abstract: Flow Accelerated Corrosion (FAC) has become a hot issue because of aging of passive components. Ultrasonic Technique (UT) has been adopted to inspect the secondary piping. UT, however, covers only narrow region, which results in numerous detecting points and thus takes time. In this paper, we suggested a Wide Range Monitoring (WiRM) concept with Equipotent Switching Direct Current Potential Drop (S-DCPD) method to monitor the thickness of piping. Since the DCPD method covers area, not a point, it needs less monitoring points. We use the SDCPD method to screen the candidate area to monitor. Based on the monitoring results, we can determine the inspection area. To improve the applicability to the piping system, we suggested the Equipotent concept, which eliminates the leakage current. Finite element analysis results and developed resistance model are presented for the simple analysis to describe the wall thinning by DCPD signals. And also validation test results are presented, from which we can identify the consistency of the model and the experiment.

Abstract: In stress-controlled load increase and single step tests besides the plastic strain amplitude the changes of the temperature and the electrical resistance caused by cyclic plastic deformation were measured. Temperature and resistance measurements don’t require a defined gage length and can be applied also on components even under service loading to ensure economic and safe operation conditions. On the basis of Morrow and Basquin equations a physically based fatigue life calculation “PHYBAL” was developed. This new short-time procedure allows a rapid and precise calculation of Woehler curves with data of one load increase and two single step tests.

Abstract: A primary water stress corrosion cracking (SCC) and an outside diameter SCC have occurred in the steam generator (SG) tubes of nuclear power plants around the world. It is important to establish the repair criteria for the degraded tubes to assure a reactor integrity, and yet maintain the plugging ratio within the limits needed for an efficient operation. For assessment and management of the degradation, it became crucial to understand initial leak behaviors under a small pressure and leak rate evolution under a constant pressure of SCC flaws. Stress corrosion cracked tube specimens were prepared by using a room temperature cracking technique, and leak behaviors of these tubes were measured at room temperature. Water pressure inside the tube was increased slowly in a step like manner with a designated holding time. Water leak rates just after a ligament rupture were measured by collecting the leaked water in a plastic container for a designated time. A leak rate was calculated by dividing the amount of water by the time. Under 3.45 MPa, a small water droplet was formed, but it did not grow after a 10 minute holding period at a constant pressure of 3.45 MPa. A throughwall crack seemed to open at around 8.28 MPa (1200 psi). Some tubes with 100 % through wall cracks did not show a leakage at 10.8 MPa, which is a typical pressure difference of pressurized water reactors (PWRs) during a normal operation. The higher the pressure was applied, the larger the rates of increase with the time were. Axial cracks showed a lower leak pressure than that of the circumferential cracks, which might be from a higher hoop stress than the axial stress. A large open and long axial crack showed an increasing leak rate with the time at a constant pressure.