Shiro Kubo

Applications of pulse heating thermography and lock-in thermography to quantitative nondestructive evaluations

This paper describes several experimental investigations of newly developed thermographic NDT techniques based on the transient temperature measurements. Several experimental results of the pulse heating thermographic NDT and the lock-in thermographic NDT are shown.

Development of a new non-destructive testing technique for quantitative evaluations of delamination defects in concrete structures based on phase delay measurement using lock-in thermography

A new quantitative NDT technique for delamination defects in concrete structures was developed based on the phase delay measurement using a lock-in infrared thermography under the application of periodical heating. It was found that the location and size of the defect can be estimated from the contrast change and that the depth can be estimated from the relationship between phase delay and heating period.

Electric Potential CT Method Based on BEM Inverse Analyses for Measurement of Three-Dimensional Cracks

Feasibility of the proposed electric potential CT method was studied for detecting and measuring three-dimensional cracks. This method uses techniques of inverse analysis, by which observed values of electric potential distribution on the surfaces of a cracked body are computer-processed to detect cracks. Two schemes, i.e., “Inverse Boundary-Integral-Equation Method” and “Least Residual Method” were examined. Numerical simulations of crack shape determination were made for a rectangular surface crack, a rectangular internal crack and an L-shaped surface crack. The results demonstrated the versatility of the proposed method.

Development of self-reference lock-in thermography and its application to remote nondestructive inspection of fatigue cracks in steel bridges

A new remote nondestructive evaluation technique, based on thermoelastic temperature measurement by the infrared thermography, was developed for evaluation of fatigue cracks propagated from welded joints in steel bridges. Fatigue cracks were detected from localized thermoelastic temperature change at crack tips due to stress singularity under wheel loading from traffics on the bridge. Self-reference lock-in data processing technique was developed for the improvement of signal-to-noise ratio of the thermal images obtained in the crack detection process. In this paper, experimental results of fatigue crack detection by the self-reference lock-in thermography are reviewed.

Development of a new crack identification method based on singular current field using differential thermography

A new thermographic NDT technique was proposed, in which singularity of the temperature field near crack tips under application of the periodically modulated electric current was measured using differential thermography based on lock-in data processing technique. Experimental investigation was made on the resolution and the applicability in the detection of through-thickness cracks embedded in steel plate samples. Modulated electric current was applied to the cracked sample by an induction coil. Differential thermal images synchronized to the reference current modulation signal were taken by the differential thermography. Significant singular temperature field was observed at the crack tips in the differential thermal images. The cracks were found to be sensitively detected by the proposed technique in good resolution compared with the singular method using a conventional thermographic temperature measurement.

Characterization of the Tikhonov regularization for numerical analysis of inverse boundary value problems by using the singular value decomposition

Publisher Summary The structure of the Tikhonov regularization for numerical analysis of an inverse boundary value problem is examined in this chapter. Singular value decomposition is applied to evaluate the amplitude of error magnification, which is called the condition number. It was found that the condition number in the Tikhonov regularization showed a V-shaped behavior with an increase in the regularization parameter α. For determining the optimum value of the Tikhonov regularization parameter α, the optimum condition number method is applied, in which the regularization parameter giving the optimum value of the condition number is employed as the effective one. Because of the V-shaped behavior of the condition number, the optimum condition number method has two or no candidates of the effective regularization parameter. To make the method applicable to the Tikhonov regularization, a new condition number is proposed. Numerical simulations are made for estimating boundary values in the Laplace filed from boundary values over described on a part of the boundary. It is found that the optimum condition number method with the new condition number is useful for estimating the effective regularization parameter. Good estimates of boundary values are obtained by applying the Tikhonov regularization with the estimated effective regularization parameter α.

Classification of Inverse Problems Arising in Field Problems and Their Treatments

An overview of inverse problems related to field problems and inverse approaches for their treatments is made. Discussion is made on the classification of inverse problems arising in field problems and it is shown that inverse problems can be classified into domain/boundary inverse problems, governing equation inverse problems, boundary value/initial value inverse problems, force inverse problems, and material properties inverse problems. Examples of inverse analyses conducted by the author’s group for dealing with these 5 categories of inverse problems are shown with special emphasis on the applicability of the representative inverse approaches and the regularization schemes.

A new full-field motion compensation technique for infrared stress measurement using digital image correlation

A new full-field motion compensation technique based on digital image correlation was developed for infrared thermoelastic stress measurements. Speckle patterns with variable infrared emissivity were applied to a test sample for motion analysis. Infrared images of the speckle pattern were acquired under the same loading conditions as for thermoelastic stress measurement. Displacements and deformations on the test sample were analysed using digital image correlation based on information on movement of the speckle patterns. Full-field motion compensation was performed in subsequent thermoelastic stress measurements based on the results of displacement measurements. The feasibility of the proposed motion compensation technique is demonstrated experimentally for thermoelastic stress measurement and identification of local plasticity at the stress-concentrated area in a plate specimen with a circular hole.

Passive Electric Potential CT Method Using Piezoelectric Material for Crack Identification

When the piezoelectric film is glued on the surface of a cracked material subjected to mechanical load, change in electric potential distribution is observed on the surface of the film. Based on this phenomenon, the passive electric potential CT (computed tomography) method can be developed, which does not require electric current application for identifying cracks. This method may be applied to develop an intelligent structure with a function of self-monitoring of flaws and defects. For the crack identification from electric potential distribution, an inverse method based on the least residual method was applied, in which square sum of residuals are evaluated between the measured electric potential distributions and those computed by using the finite element method. Numerical simulations were carried out on identification of a through-thickness transverse crack. It was found that the location and size of the crack can be quantitatively identified by the proposed passive electric potential CT method.

Fatigue crack identification using near-tip singular temperature field measured by lock-in thermography

Applicability of the newly developed two different lock-in thermographic NDT techniques is discussed. One of the proposed techniques is based on the lock-in measurement of the singular temperature field, which appears near crack tips under the application of periodically modulated electric current. Experimental study is made on the resolution and the applicability in the detection of through-thickness cracks embedded in steel and aluminum alloy plate samples. Modulated electric current is applied to the cracked sample by an induction coil. Temperature amplitude and phase delay thermal images synchronized to the reference current modulation signal are taken by the lock-in thermography. Significant temperature rise related to singular temperature field is observed at the crack tips in the lock-in thermal images. It is found that the cracks are sensitively detected by the lock-in thermography technique combined with near-tip singular temperature field measurement. The other technique is based on the lock-in measurement of the surface temperature under the application of periodical xenon light heating. Experimental study is made on the applicability to the detection of flat objective body. In-phase and out-of-phase temperature amplitude images are taken by the lock-in thermography, synchronized to the reference signal of the electric shutter operation. It is found that the location and size of the defects can be identified by the localized contrast change in the out-of- phase images. Further, the depths of the defects can be identified from the heat penetration depth, which is changed by the frequency of thermal wave stimulation.