Key Engineering Materials Vols. 321-323

Abstract: This work aims at assessing the use of embedded fiber Bragg grating to monitor the internal damage evolution in polymeric composite during fatigue loading . The results suggested that the embedded FBGs have good potentials to detect the appearance of damage and to differentiate the damage types. The evolution in light spectrum during fatigue may be divided into four stages when compared against X-ray, C-scan and optical microscope (OM) results. These stages are associated with (1) matrix cracking in 90o lamina; (2) fiber splitting in the ±45o laminae; (3) fiber splitting in the 0o lamina ; and (4) delamination.

Abstract: This paper considers the use of on-line structural health monitoring in advanced nuclear power systems such as IRIS. The motivation for the on-line health monitoring is to prevent routine maintenance from interrupting long-term continuous reactor operation. However, the outcome of the on-line monitoring implementation has a broader impact, and amounts to a paradigm shift in maintenance strategy from outage-based maintenance to continuous real-time monitoring of operational and structural integrity. Indeed, on-line health monitoring data will provide a foundation for diagnostics and prognostics (i.e., predictive) capabilities that will detect component degradation prior to failure, thus allowing for proactive rather than reactive maintenance strategies. Specifically, this paper briefly reports on our studies on (1) on-line monitoring strategy and its benefits, (2) candidate reactor components where on-line monitoring provides maximum benefits, (3) applicable on-line NDE sensor methodologies and conceptual sensor designs, and (4) model-based sensor performance estimations.

Abstract: High-tension bolts have been used widely for the clamping of many kinds of large structures. In these bolts, the estimation of clamping force has been regarded as the main issue in the evaluation of clamping condition. This paper proposes a method using ultrasonic wave, which is based on the dependency of sound speed on the stress. In order to verify the usefulness of the proposed method, two kinds of experiments are carried out. The first one involves the measurement of sound speed when the bolt is stressed by the tension tester, and here, the relationship between the exact axial force and sound speed is calibrated. The result shows good agreement with the expected linear relationship between sound speed and axial stress. The second experiment involves the measurement of axial stress by the proposed method when the bolt is stressed by the torque wrench. The results are coincident to the strain gage measurement. From these results, we can conclude that the proposed method is indeed useful in evaluating clamping force in high-tension bolts.

Abstract: Reliability of structures is an important task to ensure the ease and safety of our life, and further development of non-destructive evaluation for structures such as bridges and tunnels is required. Some fatigue sensors that consist of sacrificed specimen have been developed to evaluate the fatigue damage of structures such as fatigue cyclic number and residual lifetime. However, these fatigue sensors can be used only when the applied stress amplitude is known. We tried to develop a new smart stress memory patch that measured both maximum stress and number of fatigue cycles simultaneously using Kaiser effect of Acoustic Emission (AE) and crack length. In this study, the characteristics of the smart patch was evaluated. Pure copper was used for this sensor because its good corrosion resistance, stable crack propagation and detectability of AE near yield point. Fatigue test was performed under the constant stress amplitude to evaluate the crack propagation behavior using the relationship between stress intensity factor and crack propagation rate. The obtained curve between crack length and number of fatigue cycles by these crack propagation behavior was in good agreement with experimental results. AE measurement after some fatigue tests was performed and AE was detected at the applied fatigue stress. These results demonstrated that number of fatigue cycles and the maximum stress could be measured by this fatigue sensor.

Abstract: Structural monitoring is concerned with the safety and serviceability of the users of structures, especially for the case of building structures and infrastructures. When considering the safety of a structure, the maximum stress in a member due to live load, earthquake, wind, or other unexpected loadings must be checked not to exceed the stress specified in a code. Although the steel will not fail at yield, excessively large deflections will deteriorate the serviceability of a structure. Therefore, to guarantee the safety and serviceability of steel beams, the maximum stress and deflection in a steel beam must be monitored. However, no practical method has been reported to monitor both the maximum stress and deflection. In this paper, assessment model for both safety and serviceability of a steel beam is proposed. The model was tested in an experiment by comparing stress level estimated by LiDAR system and stress level directly measured from electrical or fiber optic sensors. The maximum deflection measured from LiDAR system is also compared with the maximum deflection directly measured from LVDTs. In addition to displacement measurement, the proposed system can provide information on deformed shapes of steel beams.

Abstract: Both the acoustic emission (AE) and corrosion potential fluctuation were monitored for chloride stress corrosion cracking (SCC) of sensitized Type-304 stainless steel plate under bi-axial stress states. Branched SCCs were produced from rectangular-shaped corrosion pits initiated by falling-off of surface grains and filled with chromium oxide in 30
mass% MgCl2 solution (363K). Both the AE and potential fluctuation were simultaneously detected during pit formation and SCC growth. Two types of AE (Type-I and Type-II) were monitored. Type-I AEs with higher frequency components were detected during the pit growth and supposed to be produced by falling-off of surface grains due to intergranular attack, while a number of Type-II AEs (approximately 12,500 counts) with low frequency components were detected during SCC propagation and supposed to be produced by cracking of the chromium oxy-hydroxides. Though the AEs detected during SCC test are not always the primary AEs from the SCC itself, secondary AEs can be usefully utilized to monitor the SCC initiation and propagation as well as the corrosion potential fluctuation.

Abstract: We developed an advanced optical fiber Acoustic Emission (AE) monitoring system with a
phase compensation feed back circuit. Two lasers transmitted through the reference and sensing arms are combined by a 2x2 coupler and detected by two photodiodes as the intensity change of the interfered laser. Outputs of the photodiodes are then combined in a differential amplifier which extracts error signal and improves the S/N ratio. The developed system was demonstrated to detect weak guided wave AE signals such as zero-th order longitudinal cylindrical wave (Lo-mode) with the S/N ratio higher than that of PZT sensor. This system was utilized to monitor the oxidation resistance of various alloys during thermal cycles from 300 K to 1273 K. We detected weak AE signals from micro fractures of growing oxide films of boiler tube
and austenitic stainless steel, but no AE from Inconel 600
tubes. Here the optical fiber sensor was wound over the tube at 600 K. Both the AE counts and amplitudes during thermal cycles showed clear tendency depending on the oxidation resistances of the alloys.

Abstract: This study investigates an existing steel plate girder railroad bridge in order to monitor static and dynamic responses using fiber Bragg grating (FBG) sensors. This paper also presents an experimental technique to estimate the vertical deflection of the bridge using FBG sensors. Seven FBG sensors are multiplexed in a single optical fiber and installed in parallel pairs along the length of the bridge, with one set on the top flange and the other on the bottom flange. The train passes over the bridge at different speeds, ranging from 10 km/h to 90 km/h, so as to monitor the dynamic response of the bridge. The results show that the proposed instrumentation technique is capable of estimating the deflection of the bridge for various loading conditions, which is crucial in structural monitoring.

Abstract: This paper presents an effective method of FE model updating for health monitoring of structures by applying ambient vibration. And this method is experimented through damage detection and proved to be valid. Experiment about ambient vibration is performed on cantilever beam, and the dynamic characteristics are analyzed by NExT and ERA. The results of such experiments are compared to those of FE analysis, and this comparison enables us to overcome some errors in experiments and analysis. On the basis of improved results by the comparison, model updating is performed in order to construct a basic structure for health monitoring. For model updating, we employ direct matrix updating method (DMUM) and Error matrix method (EMM) in which ambient vibration is easily applied. The model updating by the methods are again evaluated in terms of error ratio of natural frequency, comparing each result before and after updating. Finally, we perform experiments on damage detection to verify the method of updating presented here, and evaluate its performance by eigen-parameter change method. The evaluation proves that the method of FE model updating using ambient vibration is effective for health monitoring of structure, and some further application of this method is suggested.

Abstract: The paper presents two algorithms for determining optimal accelerometer locations for structural health monitoring when structural condition is assessed by a system identification scheme in time-domain. The accelerometer locations are determined by ranking the components of an effective independent distribution vector computed from a Fisher information matrix. One of the proposed algorithms formulates a Fisher information matrix by multiplying acceleration matrix with its transpose and the other as a Gauss-Newton Hessian matrix composed of acceleration sensitivities with respect to structural parameters. Since the structural parameters cannot be known exactly in an actual application, a statistical approach is proposed by setting an error bound between the actual and the baseline values. To examine the algorithm, simulation studies have been carried out on a two-span planar truss. The results using locations selected by the two algorithms were compared.