Development of a Method to Determine the Dashpot Coefficient of an Isolator Material in Vibrothermography

• Main Author: Rau, Austin
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2017
• Subjects: Aerospace engineering
• ISBN: 1392751608; 9781392751602

Vibrothermography is a nondestructive evaluation technique which is used to find cracks, delaminations, and other flaws. High amplitude sonic or ultrasonic vibrations are used to excite a specimen; when the flaws in the material are subjected to these vibrations, they generate heat due to friction. This heat can be viewed on an infrared camera to locate the defect. Vibrothermography requires the specimen to be mounted with compliant isolators at the contact points between the specimen and the test frame in order to prevent metal on metal contact, eliminate joint resonances, and make the specimen vibration more repeatable. The specimen is excited using a transducer or ultrasonic horn with a layer of material as a couplant to assist in energy transfer. Half-cylindrical clamps are used as contact points to provide a predictable and consistent mechanical interface. The isolators act as absorptive springs and end up absorbing some of the energy put into the specimen. Paper cardstock is one of the more common materials used as isolators; however, cardstock’s properties are highly susceptible to humidity, and under high loads the cardstock deforms and changes its properties. These factors of cardstock reduce repeatability in vibrothermography testing which motivates experimentation with other materials. To predict vibrothermography performance we need to be able to predict the damping, which is dominated by absorption in the isolators in the mounting setup. The damping controls resonance bandwidth and amplitude, and the amplitude determines testing performance. In this thesis, we propose the use of cylindrical mounts for consistent and repeatable contact and develop a method to determine the dashpot coefficient of a material that is used as an isolator at the contact points for mounting a specimen and, given the excitation amplitude, the test’s sensitivity to cracks. This method does not require a highly specialized setup, relying on basic electronic and material testing instruments and is meant to be performed by any person who wants to evaluate a material for use as an isolator for vibrothermographic tests.