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Ageing steel bridges suffer fatigue cracks thereby necessitating immediate inspection, structural integrity evaluation or repair in the life cycle of steel bridges. We propose nondestructive evaluation techniques employing infrared thermography enabling us to remotely inspect the fatigue cracks in steel bridges and evaluate the structural integrity on the basis of thermoelastic stress measurement. This study presents a structural integrity assessment of steel bridges using remote measurement of the stress field around the fatigue cracks. We focus upon experimental results confirming reduction in the severity of the stress distribution around the fatigue cracks after the repair or reinforcement of members in steel bridges.

In our previous studies, a new surface modification method by combination of ceramic coating and heat treatment, named “substrate quenching after coating” was developed. The thrust type rolling contact fatigue tests were carried out for TiN coated steels and CrAlN coated steels processed by substrate quenching after coating, and the effects of the type of ceramic coating (TiN or CrAlN) and the quenching methods (by furnace quenching or by laser quenching) on the flaking initiation life were investigated. For the specimens quenched by furnace, the flaking life of CrAlN coated specimen was longer than that of TiN coated specimen. This reason could be explained by the difference of the oxidization of CrAlN and TiN in their furnace quenching process. For CrAlN coated specimens, the flaking life of the specimens quenched by laser was longer than that of the specimens quenched by furnace. This reason could be explained by the difference of the process time of the furnace quenching and the laser quenching. It is considered that laser quenching after coating could be an effective way to improve the flaking initiation life under rolling contact fatigue.

Introduction It is difficult to make a definite diagnosis of a cracked tooth solely based on an inspection within the root canal, especially in case of microcracks. At present, there seems to be no established method to detect dentinal microcracks in roots; therefore, the current detection techniques need to be improved. Vibrothermography (VibroIR) helps to detect microcracks by the friction heat generated from ultrasonic vibration. The purpose of this study was to establish a novel method using VibroIR to detect dentinal microcracks. Methods The root canals of 20 roots with cracks and control roots were prepared after removing the tooth crowns. A tapered indenter was inserted into the root canal and pressed until a microcrack was created under an optical microscope. Using VibroIR, the detection trials for dentinal microcracks were performed with an ultrasonic vibration power ranging from 0.43 to 1.48 W at an angle of 0°, 30°, 45°, 60°, and 90° between the ultrasonic vibration point and the microcrack line. After the detection test, the microcrack width was measured with an optical microscope. Results Frictional heat was detected in the microcracks with thermography at 0.89 to 1.48 W and at an ultrasonic vibration point angle less than 60° from the crack line for 10 seconds. Microcracks with a width of 4 to 35.5 μm were detected with this method. Conclusions VibroIR may be an effective method for the diagnosis of root dentinal microcracks.

A new remote nondestructive inspection technique that is based on thermoelastic temperature measurement by infrared thermography was developed for the evaluation of fatigue cracks propagated from welded joints in steel bridges. Fatigue cracks were detected from localized high thermoelastic temperature changes observed at crack tips induced by stress singularity under variable loading resulting from traffic on the bridge. A self-reference lock-in data processing technique was developed for improving the signal/noise ratio of the thermal images recorded in the crack detection process. In this study, remote and nondestructive detection of fatigue cracks in an actual steel bridge in service was performed by the self-reference lock-in thermography method. The accuracy of this method was improved with a motion compensation technique.

A new remote nondestructive evaluation technique based on thermoelastic temperature measurement by infrared thermography was developed for the evaluation of fatigue cracks propagating from welded joints in steel bridges. Fatigue cracks were detected from localized thermoelastic temperature changes at crack tips due to stress singularities generated by wheel loading from traffic on a bridge. A self-reference lock-in data-processing technique was developed to improve the signal-to-noise ratio of the thermal images obtained in the crack detection process. Thermoelastic stress analyses in the vicinity of crack tips were carried out after the crack detection process by self-reference lock-in thermography. The stress distribution under wheel loading by traffic was measured by infrared thermography. Stress intensity factors were directly evaluated from the measured stress distribution. It was found that these fracture mechanics parameters can be evaluated with reasonable accuracy by the proposed technique, enabl...