No abstract

A high power ultrasonic burst excitation technique was used to study the potential of the ultrasound excited thermography (vibro-or sono-thermography) to detect and to estimate the depth position of cracks in ceramics as well as voids in glass fibre reinforced polymer. By verification using high-frequency (50 MHz) ultrasound techniques it was proven that sono-thermography can detect relatively deep defects in ceramics. The depth of the defect can be estimated using a theoretical model based on analytical calculations dealing with friction heat sources equally distributed over the crack flanks stimulated by the ultrasonic vibrations of the specimen. In carbon fibre reinforced polymers, the thermographic indications of defects were correlated with 3D X-ray cross sections of the specimen.

Single planar fuel cell elements consisting of metallic interconnectors that are bonded and sealed by a thin glass solder layer form the core of a solid oxide fuel cell. For reliable operation, the bonding layer has to adhere well and must be without voids or foreign material inclusions, which might cause gas leakage, electrical shorts or mechanical weakening and structural failure. Nondestructive testing (NDT) by the high-frequency ultrasound in immersion technique and by air-coupled ultrasound was optimized to find such defects. Another technique was flash light excited thermography for detection of voids. The NDT techniques used are complementary to each other, as they are based on different physical principles. Voids and small steel platelets of different sizes were prepared in the glass solder layer before the high-temperature bonding process and then monitored by the NDT techniques through the interconnector plates. Two selected NDT techniques were then validated in a probability of detection (POD) study. The study resulted in detection limits for the two main types of defects. As a step towards production testing, a demonstrator was built combining testing by air-coupled ultrasound and that by flash thermography. During the testing steps, the cell elements were handled by a collaborative robot.