Key Engineering Materials Vols. 297-300

Abstract: Several nondestructive evaluation methods were attempted for the estimation of the creep damage of degraded 2.25Cr-1Mo steel. The specimens of three different aging periods were prepared by an isothermal heat treatment at 430°C, 482°C, and 515°C. The effect of probe configuration on the electrical resistivity was studied. Single configuration method and dual-configuration method were utilized for measuring electrical resistivity. The electrical resistivity was determined by a standard DC four-point probe method at 24±0.5°C. Indentation test, magnetic characteristics test and ultrasonic test were carried out to investigate the correlation between the major characteristics and aging parameter. Unlike the electrical resistivity characteristics, ultrasonic attenuation coefficient and indentation characteristics did not show a relation to Larson-Miller parameter. However, a correlation between the electrical resistivity and aging parameter was identified, which allows one to estimate the extent of material degradation.

Abstract: A new acoustic bending vibration method of determining the resonance frequency, Young’s modulus and loss factor applicable for both metallic and non-metallic materials is proposed. Measurements were conducted by using both conventional electromagnetic excitation method specified by ASTM E 756 and ISO 6721 and our method for conductive metallic specimen of stainless steel and non-metallic polypropylene specimen. Difference in the mechanical properties observed between both methods is attributed to the presence of a metal disk attached to the non-metallic sample for electromagnetic. However, the good agreement between the results obtained by both methods for the metallic sample ascertains the reliability and advantages of the method.

Abstract: This study was performed to classify the acoustic emission (AE) signal due to surface check and water movement of the flat-sawn boards of oak (Quercus Variablilis) during drying using the principle component analysis (PCA) and artificial neural network (ANN). To reduce the multicollinearity among AE parameters such as peak amplitude, ring-down count, event duration, ring-down count divided by event duration, energy, rise time, and peak amplitude divided by rise time and to extract the significant AE parameters, correlation analysis was performed. Over 96 % of the variance of AE parameters could be accounted for by the first and second principal components. An ANN was successfully used to classify the AE signals into two patterns. The ANN classifier based on PCA appeared to be a promising tool to classify the AE signals from wood drying.

Abstract: Metal materials especially steel or cast iron are used extensively in many types of construction such as buildings, bridges, cranes, vehicles and so on, so detection or prediction of metal failure is very important, but deformation of metal material often deliveries heat energy which can be detected by infrared imager. The test specimen is installed between the two grips of the testing machine and then loaded in tension and in compression. The IR913A infrared imager is used to observe the deformation of metal specimen. The high-sensitive infrared thermal images of metal specimen in the different phase of deformation are obtained. The paper theoretically analyzes the reason from the thermodynamics and plastic mechanics, the conclusions are drawn as follows: 1) When applying loads, temperature field on the surface of metal specimen is changing, the local rise of temperature is remarkable, this can be observed from the infrared thermal images. 2) From the infrared thermal images, rising temperatures are found in the regions of stress concentrations. 3) When the test specimen approaches to failure or appears fracture, there is a remarkable change that can be shown from the infrared thermal images with remarkable colors.

Abstract: In this work, a wood material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in wood using automated data acquisition software. It is desirable to perform nondestructive evaluation (NDE) to assess material properties and absorption of wood because wood is a natural composite and often inhomogeneity. Several ultrasonic techniques were applied to wood for the evaluation of variations material properties. The variation of ultrasonic velocity was measured and found to be somewhat consistent with those in infiltrated area in the course of nature absorption. Low frequency (1MHz) through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. These results were compared with those obtained by dry-coupling ultrasonics. A good correlation was found between ultrasonic velocity (a pulse overlap and peak-delay measurement method) and C-scan image signal based on peak-to-peak amplitude.

Abstract: The objective of this paper is to develop a fiber-optic acoustic emission(AE) sensor applicable to on-line monitoring systems which is suitable for long-distance signal transmission. An AE sensor was developed by use of a fiber-optic cantilever and an extrinsic Fabry-Perot interferometer(EFPI). The efficiency of signal processing was improved by driving the high frequency AE signals into the low frequency ones. In order to verify the developed sensor, the tensile and the pencil lead fracture(PLF) tests were performed including the experiment showing the Kaiser effect. From tests, AE signals were successfully detected in the elastic-plastic deformation range, especially higher signals at the crack propagation. The developed sensor is expected to be used for an on-line monitoring of crack propagation in mechanical systems.

Abstract: In this study we attempt to investigate the possibility of detecting sub-surface crack and the understanding of the propagating phenomena of transient stress waves due to impact in thick aluminum plate by the simultaneous measurement of longitudinal and shear creeping and Rayleigh wave resulted from the mode conversion of laser induced transient stress wave impact. The propagation of the transient stress wave generated by laser irradiation is affected by the sub-surface crack and the result is analyzed. It was observed that the longitudinal and shear creeping wave velocities are varied depending on the depth of sub-surface crack. In addition, the variation of amplitude ratio generated by propagating the stress wave is investigated. The longitudinal creeping wave velocity in the presence of the sub-surface crack is somewhat faster than in case of non-crack. And the shear creeping wave velocities represent large variations which are shown nearly 2nd order quadratic curve shape as the sub-surface crack depth increase under the same experimental condition. The results of this study are very useful for the nondestructive evaluation of the surface layer in thick structures by non-contact method and the opposing and the structures difficult to access.

Abstract: For the quality monitoring and reliable application of stationary gas turbine blade (vane), near surface damages in the isothermally degraded vein material (i.e., cobalt based superalloy) were characterized by ultrasonic surface wave technique. Surface wave velocity and attenuation were measured for the artificially degraded specimens at 1100°C, together with microstructural analysis and micro-hardness measurement. Surface wave velocity increased with thermal degradation time, which was attributed to the increasing depletion of solute chromium near the surface. Strong frequency dependence of surface wave velocity was observed in the specimens with surface depletion layer. Attenuation coefficient of surface wave increased with increasing degradation time. The potential of ultrasonic surface wave technique to assess near surface damages in vein material was discussed with an emphasis on the relationship between the microstructural damage and the governing principles of ultrasonic response.

Abstract: This paper describes an array sensor of piezoceramic PZT discs with flexible covering layers, which is designed to detect surface elastic waves in structures, such as Rayleigh waves. The sensor is distinguished from conventional “passive” sensors for elastic waves detection because it possesses an ability to actuate an “active” pulse. This indicates that the function of active pulse generation ensures the diagnosis of the structures, where the sensor attached or embedded into, can be accomplished in a smart way including an easy inspection of faults. Actually the sensor has been inspired from human sensory systems in order to apply for smart structure sensing. They utilized 6mm diameter and 0.25mm thick PZT discs for the active array sensor. The performance of the sensor was demonstrated at various experiments of some metal and composite structures that the sensor can be applied for the health monitoring of the advanced smart structure system.