Max Rothenfusser

MECHANISMS AND MODELS FOR CRACK DETECTION WITH INDUCTION THERMOGRAPHY

Induction thermography is a non‐contacting, non‐destructive evaluation method with a wide range of applications. A deeper understanding of the detectability of cracks requires fundamental knowledge about the induced current density distribution in the component under test. A calculation of the current distribution provides information how much current is flowing at which location of the component, how a crack disturbs the current density, how much heat is produced at which location of the component, and how the heat diffuses to the surface. The heating process depends on the type of crack. On the one hand there are cracks which can be detected mainly by direct observation of the heating process due to an increased current density, and on the other hand there are cracks which can be detected mainly because of a modification of the heat diffusion. This paper presents an analytical model for the calculation of the current distribution, including the back‐flow current along with finite‐element calculations. F...

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.

INDUCTION AND CONDUCTION THERMOGRAPHY: OPTIMIZING THE ELECTROMAGNETIC EXCITATION TOWARDS APPLICATION

Active thermography, using electromagnetic excitation, allows detecting defects like cracks which distort the flow of current in the component under examination. Like other thermography techniques it is rapid and reliably utilizing infrared imaging. Electric current can be used in two ways for thermography: In induction thermography a current is coupled to the component by passing an AC current through a coil which is in close proximity to the component inspected, while in conduction thermography the current is coupled directly into the component. In this paper, the specific advantages of both coupling methods are discussed, including the efficiency of the coupling and optimization strategies for testing and also the necessary algorithms required to analyze the data. Taking these considerations into account a number of different systems for laboratory and practical application were developed.

The method of non-linear ultrasound as a tool for the non-destructive inspection of structural epoxy-metal bonds—a résumé

Abstract The method of non-linear ultrasound (NLUS) is frequently discussed as a promising non-destructive inspection tool to detect the deteriorating effect of operational loading on the performance of structural adhesive metal bonds. The non-linear component in the ultrasound pulse propagating across the joint arises in the weakest region of the adhesive. As the weakest region limits the mechanical strength of the bond a correlation between the ultrasonic-mechanical deformation behaviour and the bond strength is expected. This paper reports on efforts to verify such a correlation for epoxy–aluminium bonds undergoing practice relevant hydrothermal or mechanical loading. In conclusion, no corresponding NLUS signal is measured although the joints are aged significantly or strained up to fracture.

AUTOMATED INDUCTION THERMOGRAPHY OF GENERATOR COMPONENTS

Using Active Thermography defects such as cracks can be detected fast and reliably. Choosing from a wide range of excitation techniques the method can be adapted to a number of tasks in non‐destructive evaluation. Induction thermography is ideally suited for testing metallic components for cracks at or close to the surface. In power generation a number of components are subjected to high loads and stresses—therefore defect detection is crucial for a safe operation of the engines. Apart from combustion turbines this also applies to generators: At regular inspection intervals even small cracks have to be detected to avoid crack growth and consequently failure of the component. As an imaging technique thermography allows for a fast 100% testing of the complete surface of all relevant parts. An automated setup increases the cost effectiveness of induction thermography significantly. Time needed to test a single part is reduced, the number of tested parts per shift is increased, and cost for testing is reduced significantly. In addition, automation guarantees a reliable testing procedure which detects all critical defects. We present how non‐destructive testing can be automated using as an example an industrial application at the Siemens sector Energy, and a new induction thermography setup for generator components.