Hubert Mooshofer

A resonant CMUT sensor for fluid applications

Fluid waves at the interface of CMUTs (capacitive micromachined ultrasound transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results showing the sensitivity of the sensor to the viscosity of different silicone oil samples and will discuss the results.

Ultrasonic Defect Characterization in Heavy Rotor Forgings by Means of the Synthetic Aperture Focusing Technique and Optimization Methods

Ultrasonic nondestructive testing of steel forgings aims at the detection and classification of material inhomogeneities to ensure the components fitness for use. Due to the high price and safety critical nature of large forgings for turbomachinery, there is great interest in the application of imaging algorithms to inspection data. However, small flaw indications that cannot be sufficiently resolved have to be characterized using amplitude-based quantification. One such method is the distance gain size method, which converts the maximum echo amplitudes into the diameters of penny-shaped equivalent size reflectors. The approach presented in this contribution combines the synthetic aperture focusing technique (SAFT) with an iterative inversion scheme to locate and quantify small flaws in a more reliable way. Ultrasonic inspection data obtained in a pulse-echo configuration are reconstructed by means of an Synthetic Focusing Technique (SAFT). From the reconstructed data, the amount and approximate location of small flaws are extracted. These predetermined positions, along with the constrained defect model of a penny-shaped crack, provide the initial parametrization for an elastodynamic simulation based on the Kirchhoff approximation. The identification of the optimal parameter set is achieved through an iteratively regularized Gauss-Newton method. By testing the characterization method on a series of flat-bottom holes under laboratory conditions, we demonstrate that the procedure is applicable over a wide range of defect sizes. To show suitability for large forging inspection, we additionally evaluate the inspection data of a large generator shaft forging of 0.6-m diameter.

Sensing physical fluid properties with CMUT arrays

Fluid waves at the interface of CMUTs (Capacitive Micromachined Ultrasound Transducers) to the surrounding fluid are an often discussed and unwanted effect for medical imaging applications, as they cause ringing artifacts. A new approach for a surface wave sensor is presented which uses these dispersive surface waves for sensing fluid properties like mass density and viscosity. After a short introduction to the theory and our FEM model we will present first results. A mixture of water and glycerin was used to investigate the sensitivity of the sensor to dynamic viscosity.

Investigation of the Synthetic Aperture Focusing Technique resolution for heavy rotor forging ultrasonic inspection

This contribution determines the resolution achievable by Synthetic Aperture Focusing Technique (SAFT) processing ultrasonic inspection data acquired along circular scan paths. If the inspection is conducted using a transducer with a narrow divergence angle, the lateral resolution after SAFT reconstruction becomes highly depth dependent. In Nondestructive Testing (NDT), such a configuration is typical for the ultrasonic testing of heavy rotor steel forging blanks for energy industry components, such as rotor shafts and turbine discs. There, the inspection is performed in contact mode over specimens mantle surface with longitudinal waves. For defect classification and quantification, knowledge of the resolution is essential. The radial dependency of the resolution is determined by SAFT processing simulated far field point spread functions at different depths. The simulation results are discussed and compared with actual inspection data of turbine disc forgings containing reference reflectors and natural indications.

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.

Simulative Ultraschall-Untersuchung von Pitch-Catch-Messanordnungen für große zylindrische Stahl-Prüflinge und gradientenbasierte Bildgebung

Zusammenfassung Große zylindrische Stahlprüflinge werden mittels der Methode der finiten Differenzen im Zeitbereich (engl. finite differences in time domain, FDTD) simulativ untersucht. Dabei werden Pitch-Catch-Messanordnungen verwendet. Es werden zwei Bildgebungsansätze vorgestellt: ersterer basiert auf dem Imaging Principle nach Claerbout, letzterer basiert auf gradientenbasierter Optimierung eines Zielfunktionals.