Development of a Global NDT Technique by Infrared Thermography for Strategic Maintenance System of Aging Structures.

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In recent years aging and deterioration of infra-structures before their design life has become a serious problem. The importance of appropriate maintenance for aging structures is emphasized. Introduction of a new condition based maintenance system based on the evaluation of fitness for service is required, since the conventional time-based maintenance system has a lot of problems in efficiency, cost, saveenergy, etc. The present project developed a new nondestructive testing (NDT) technique based on the infrared thermographic temperature measurement for structural integrity assessment of aging infra-structures. Global diagnosis system was constructed based on infrared thermography, active heating equipment and processing system of sequential infrared data. For concrete structures, thermographic NDT techniques based on the Fourier analyses and the lock-in analyses of the transient temperature distribution under step heating were developed for the identification of delamination defects … More . Experimental investigations were made on the resolution and the applicability in the identification of defects in concrete samples. It was found that defect parameters were quantitatively determined from Fourier coefficients or phase values. For steel structures, a new NDT technique was developed for detecting and sizing of fatigue cracks. A self-reference lock-in infrared thermography was developed, in which a reference signal was constructed by using the same sequential data on thermoelastic temperature change. Temperature change in a region of interest was correlated with that in a remote area for reference signal construction. The lock-in algorithm based on the least squares method was employed for random loading. Proposed technique was applied for fatigue crack identification based on the detection of significant thermoelastic temperature change due to the singular stress field in the vicinity of crack tips. It was found that significant contrast change was observed at the crack tip in the self-reference lock-in thermal image, demonstrating the feasibility of the proposed technique. Less