Christian Homma

Study of the Heat Generation Mechanism in Acoustic Thermography

In this paper we investigate the heat generation mechanisms that occurs during excitation of a specimen with high‐power ultrasound (20 kHz and above). In order to obtain stable and easy to interpret results we use a set‐up with a tunable piezo instead of an ultrasound welding system commonly used and excite the specimens at their resonance frequencies. We will report the results of recent investigations which reveal several different mechanisms contributing to the overall thermal signal. Besides frictional effects at crack faces also thermoplastic heating may occur at crack tips. In materials with high sound attenuation the heating of the bulk material itself can be measured. In this case the detected infrared signal corresponds to local stress fields of the induced vibration.

The iterative involvement of internal Experts into the technology scouting process

This paper describes technology scouting from the Scouts perspective: He or she needs to rapidly develop a working understanding of the search field, start leading a meaningful search for cooperation partners and be able to identify hitherto unknown and relevant research avenues - all while maintaining a delicate balance between providing value to, and taking time from the assigned Experts in the field. The time of the Experts is usually the most limited resource. The methodology and selection of tools that have evolved to satisfy these needs are modular and highly situational. At their center lies a solid core procedure which is used to establish a framework for Expert/Scout understanding, which we call Solution Space. The paper presents the methodology and a selection of tools that have evolved as a best practice approach in several scouting processes.

Acoustic Thermography Testing of Industrial Gas Turbine Blades in the Service Stage

Gas turbine blades are operating under very demanding conditions. In modern industrial gas turbines, the rotating blades and the guide vanes of the first stages are hollow to allow internal cooling. This means that there is a possibility of having crack initiation on the internal surface of the components. Due to the complex casting geometry, this type of defects is very difficult to detect with conventional nondestructive testing techniques such as ultrasonic and radiographic testing. Siemens has developed a new non-destructive testing technique based on acoustic thermography, SIEMAT. The test object is energized by an ultrasonic excitation device. Due to the vibrations, a very slight heating will develop at cracks in the test object. The local increase of temperature is captured by a highly sensitive IR camera. The SIEMAT technique is capable of detecting both surface-breaking and internal cracks, including cracks under coatings. The testing time is very short, and the IR sequences are recorded for subsequent analysis. A major advantage for service applications is that the technique is mostly sensitive to closed defects such as cracks, since open defects where no contact between the faces is present, for example pores and scratch marks, will not cause any heat generation. Siemens is currently implementing the SIEMAT technique for assessment of service-exposed turbine blades from medium size gas turbines, which are due for reconditioning. By being able to verify that no internal cracks are present, the reliability of the reconditioned blades will be increased. This paper describes the SIEMAT testing technique, and the results obtained when applied on service-exposed industrial gas turbine blades.Copyright