<title>Application of lock-in data processing for thermographic NDT of concrete structures</title>

Abstract

A new quantitative nondestructive testing 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. The lock-in thermography technique was developed based on the thermal insulation method thermographic NDT. Experimental studies were made on the applicability to the detection of artificial delamination defects in concrete blocks. Concrete blocks were periodically heated by quartz lamps combined with the light dimmer controller. The controller was operated by the same reference signal for the lock-in thermography. It was found that the delamination defects were detected by the localized contrast change in the phase delay images. It was also found that the location and size of the delamination defects can be estimated by the area of contrast change in the phase delay images which was clearly observed compared with conventional thermography techniques. The relationship between the values of phase delay and heating period was examined for several defect depths and several heating periods. It was found that the phase delay curve for certain defect depth shows the peak of the contrast in phase delay image at certain heating period and the depth of the delamination defects can be estimated from this relationship. Finally, the proposed lock-in thermographic NDT technique was applied for the quantitative measurement of the actual delamination defects found under railway bridge. It was fund that the depths of the delamination defects can be estimated using the master curve of the relationship between the values of phase delay and heating period made by the experiments for artificial delamination defects.