Key Engineering Materials Vols. 297-300

Abstract: Through-wall axial cracks occurred by primary water stress corrosion are one of the serious defects in steam generator (SG) tubes (made of alloy 600) in pressurized water reactors. Therefore, it is necessary to detect and size them by eddy current testing (ECT) conducted during in-service inspection of SG tubes. To address this issue, it has been recently proposed an effective method, namely „M-shape profile“ approach, which relies on the difference in the amplitude between the pancake and plus point coils in a MRPC probe. Even though the M-shape curve approach is straightforward in principle, it requires time-consuming data processing if performed by human operators. In order to get rid of this tedious task, an automated system is developed in the present work. This paper addresses the principle of the M-shape approach together with the automated system and its performances for the detection of natural axial cracks in SG tubes. The results observed in the present work demonstrate the high potential of the developed system as a very promising tool for detecting through-wall cracks in many practical field applications.

Abstract: Ultrasonic guided waves have been widely employed for the long range inspection of structures such as plates and pipes. In ultrasonic guided waves, however, there are numerous modes with different wave velocities, so that the generation and detection of the appropriate wave mode of the guided wave is one of key techniques in the application of guided waves. In the present work, mode tuning using an array transducer was investigated with hardware implements. For this purpose, 8-channel ultrasonic pulser and their controller which enables sequential activation of each channels with given time delay were developed. Transmitter tuning, group velocity measurements, reciver tuning, mode identification and long range transmission testing were carried out. As a result, the selective tuning of wave mode can be achieved by changing the time interval between adjacent elements of an array transducer.

Abstract: Acoustic emission (AE) technique was used to investigate fatigue crack growth on compact tension specimens of aging materials at room temperature. Test materials have been sampled steam pipe serviced the actual operation conditions for a long time in fossil power plant. The compact tension test specimens were subjected to load stress ratios of 0.33, 0.5, and 0.66. All the fatigue tests were performed at a frequency of 1Hz. The test results indicate that acoustic emission counts show reasonable correlation with crack propagation rates for applied stress ratios. When the crack growth rates increase, AE’s counts and energies show increment. Also, the higher stress ratios, the faster crack propagation rates. Based on these relationships it may be possible to predict the remaining service life of fatigue-damaged steam pipes.

Abstract: Application of signal processing techniques to nondestructive evaluation (NDE) in general has become a standard tool in determining the frequency characteristics of the signals and relating these characteristics to the integrity of the structure under consideration. The joint time-frequency analysis techniques are applied to analyze ultrasonic signals in degraded austenite stainless 316 steel, to study the evaluation of damage in this material. It is demonstrated that the nonstationary characteristics of ultrasonic signals could be analyzed effectively by these methods. WVD was found to be more effective for analyzing the attenuation and frequency characteristics of degraded materials through ultrasonic. It is indicated that the time-frequency analysis methods should also be useful in evaluating various damages and defects in structural members.

Abstract: This study aims at identifying the structural dynamic characteristics using an ambient vibration, and developing a health monitoring system which adopts damage detecting algorithms. One of the main problems for this system design is to measure long-time dynamic response signals and simultaneously estimate the structural dynamic properties. In order to be suitable for a long-time monitoring, we conduct an ambient dynamic testing for a 3-floor moment resistance steel structure and analyze structural dynamic characteristics using time domain estimation techniques. Also, a damage detecting test is performed to evaluate damage state by various detecting algorithms (modal correlation method (MAC & COMAC), eigen-parameter change method). Finally, this paper suggests the optimal algorithms for the identification of the structural damage locations and damage quantities with all such comparisons. The algorithms presented in this paper prove to be applicable in structural health monitoring of structures.

Abstract: The layup of a CFRP (carbon-fiber reinforced plastics) composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. It is very important to detect ply error before the laminate is cured for both manual procedure and fiber placement procedure. An ultrasonic technique would be very beneficial, which could be used to test the part after and before curing laminates and requires less time than the optical test. Also cross-polarized scan is very sensitive to the layup errors and ply misorientations. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. Firstly, a manual scanner was built for making transmission measurements using a pair of normal-incidence shear wave transducers to find the effect of fiber misorientations of composite laminates. Also a method for nondestructively determining the ply layup in a composite laminate is presented. The method employs a normal-incidence longitudinal ultrasound to perform C-scan of ply interfaces of the laminate. And a ply-by-ply vector decomposition model has been utilized for evaluating layup errors in composite laminates fabricated from unidirectional plies. Test results between after and before curing laminates with model data were compared for a fiber orientation of the laminates.

Abstract: This paper describes a novel tip position sensor for a cantilever beam made of a triangularly shaped distributed piezoelectric PVDF (polyvinylidene fluoride) film. Due to the boundary condition of the cantilever beam and the spatial sensitivity function of the distributed PVDF sensor, the charge output of the PVDF sensor can be shown to be proportional to the tip position of the beam. Experimental result using the triangular PVDF sensor were compared with those using two commercially available position sensors: an inductive sensor and an accelerometer (after double integration). The resonance frequencies of the test beam, measured using the PVDF sensor, matched well with those measured with the two commercial sensors and the PVDF sensor also showed good coherence over wide frequency range, whereas the inductive sensor became poor above 300Hz. However, the measured response of the PVDF sensor showed a bit larger magnitude compared with the two commercial sensors at higher frequencies. The triangular PVDF sensor have a number of advantages over conventional position sensors and could be used as tip position sensors.

Abstract: The weld joints in power-plant pipelines have long been considered important sites for safety and reliability assessment. In particular, the residual stress in pipeline weldments induced by the welding process must be evaluated accurately before and during service. This study reports an indentation technique for evaluating welding residual stress nondestructively. Indentation load-depth curves were found to shift with the magnitude and direction of the residual stress. Nevertheless, contact depths in the stress-free and stressed states were constant at a specific indentation load. This means that residual stress induces additional load to keep contact depth constant at the same load. By taking these phenomena into account, welding residual stress was obtained directly from the indentation load-depth curve. In addition, the results were compared with values from the conventional hole-drilling and saw-cutting methods.

Abstract: Based on the functional characteristics of carbon fiber reinforced concrete (CFRC), an improved infrared nondestructive testing method, to detect crack in CFRC by using infrared thermography, is presented in this paper. The principle is that when a CFRC specimen is applied a low voltage, crack existing in the specimen will result in non-homogeneous surface temperature distribution due to the electro-thermal effect of the material. Monitoring the temperature difference on the surface, the crack under the observed surface can be inspected by using infrared thermography. In theory, the mechanism causing the temperature difference comes down to an unsteady heat transfer problem with internal energy sources. In the case of the thermo-physical property of CFRC as given, the sensitivity of this method to the depth of the crack is analyzed by numerical computation.