Detektion und Evaluation von Inhomogenitäten im Volumen und an inneren Grenzflächen in polymeren Isoliersystemen mittels Ultraschall

  • Detection and Evaluation of Inhomogeneities in the Volume and at Inner Interfaces in Polymeric Insulation Systems by means of Ultrasound

Walter, Philipp James Ludwig; Schnettler, Armin (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2006, 2007)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2006


In this thesis, the appliance of ultrasonic diagnostics for the condition assessment of polymeric insulation systems is investigated. Especially, singular flaws in the insulation volume and inhomogeneities at inner material interfaces are regarded including a relevant risk potential concerning the isolation capability. The detection of such inhomogeneities and the evaluation of parameters that affect the risk potential like sizing of a singular flaw or determination of a delamination at an inner boundary are not well investigated yet. This thesis presents methods for the detection and evaluation of the inhomogeneities described above. An ultrasonic measurement system is used that allows automated measurements applying the immersion technique with water as acoustic coupling material. The data processing and interpretation of the measurement results are done with the developed software DISCUS. The reference block method is suited for sizing a detected and localised flaw. By evaluating the spherical and discoidal reference reflector size, the minimal and the expected dimension of an gas-filled void with a size in the range of 0,2 mm to 3 mm can be specified independent of the depth position. Furthermore, influencing parameters on the sizing result like kind of the regarded silicone rubber or temperature are analysed. Conclusions on the flaw material can be made by evaluating the phasing of the reflected ultrasonic impulses. Information of the flaw geometry and orientation are gained by setup the transducers in different angles. As an evaluation method, the dynamic of the reflectance over the intromission angle is presented. It can be shown that a differentiation between planelike and ball-shaped flaws is possible if the boundary specific effects are considered like dependence of intromission angle on the echo transmissibility or the refraction at the test piece surface. Regarding the diagnostic of inner material interfaces, an automated method is developed basing on a correlation algorithm. The method evaluates the phase jump resulting at a delamination by calculating the similarity measure of echo impulses. As a result the method provides a colour scaled correlation map (K-Scan) of the investigated material interface. Thereby, open delaminations with a diameter of 1 mm can be detected automatically at interfaces such as silicone - metal, silicone - epoxy resin and epoxy resin - metal, in dependence of the layer thickness above the interface. In addition, singular flaws at interfaces with a size of 0,4 mm can be detected. The capability analysis of the presented methods shows that a multitude of relevant influences on the risk potential of an inhomogeneity in polymeric insulation systems can be investigated and evaluated.