Henry Gerhard

Two new techniques to improve interferometric deformation-measurement: Lockin and Ultrasound excited Speckle-Interferometry

Two new techniques have been presented which improve interferometric defect detection. Optical lockin interferometry extracts depth-resolved weak structures from a strong background of overall deformation. The mechanism involved in this interferometric tomography is frequency dependence of thermal wave depth range. Phase images have the advantage of more depth range and insensitivity to variation of surface absorption and scattering. The second technique-ultrasound activated interferometry-responds selectively to the mechanical losses in defects because ultrasound is converted into heat. This is an alternative method to reduce the influence of background deformation and to enhance the probability of detection (POD).

Thermal wave imaging using lockin-interferometric methods

We report about a technique where we transferred the Lockin-principle from Lockin-thermography to interferometry to perform thermal wave lockin-interferometry. This technique is based on speckle-interferometric imaging of periodical height changes going along with the temperature modulation in a thermal wave. We used both electronic speckle pattern interferometry and shearography setups and operated them with low frequency periodical heat deposition while a stack of interferometric fringe patterns was recorded. After unwrapping, each pixel of the stack was Fourier-analysed at the Lockin-frequency, giving an amplitude image and phase image of low frequency thermal deformation. Though this is very much like Lockin-thermography, the image generating mechanism is substantially different: The thermal wave generates periodical thermal expansion correlated with an overall deformation where the depth integral of the thermal wave is involved. At such a low frequency (below 1 Hz), deformation occurs simultaneously everywhere except in areas where thermal wave propagation is modified e.g. by boundaries, which affect the phase of deformation. Depth range is adjusted via modulation frequency as in lockin thermography.

Active Thermography for Defect Detection in Carbon Fiber Reinforced Composite Materials

Thermography methods are well suited for applications in non‐destructive evaluation. In active thermography a thermal wave is induced in the inspected specimen by modulated heat deposition or by a burst. Thermal wave reflections due to subsurface defects (like delaminations) in the sample affect amplitude and phase of the temperature modulation on the surface. By Fourier transformation of a recorded image sequence one obtains a phase and an amplitude image containing information on the thermal structure and hidden defects.

Lockin-speckle-interferometry for non-destructive testing

Interferometrical methods like Shearography or Electronic-Speckle-Pattern-Interferometry (ESPI) are being used for remote deformation measurements. For non-destructive testing, usually not the deformation of the whole inspected object is of interest, but only the changes in the deformation field that are caused by hidden defects. By applying the lockin technique, small local discontinuities can be monitored even on a large background deformation. Dynamic excitation is performed by modulation of absorbed light intensity while object deformation is continuously recorded to give a stack of fringe images. Instead of using only the information contained in the image with the best contrast, our technique evaluates the whole image stack with respect to the local response to the coded input. The periodical component of the deformation is extracted by Fourier transformation for the time dependent signal at each pixel. This way the relevant information contained in the image stack is compressed to an amplitude- and a phase angle image. As only defects contribute to a signal change in the phase image, the method is defect selective. Furthermore, the phase change depends on depth where the defect is located since thermal waves are involved. One more advantage is the substantial improvement of the signal-to-noise ratio.

Lock-in-Shearografie: Prinzip und Anwendung (Lock-in Shearography: Principle and Application)

Die Shearografie ist ein etabliertes optisches Verfahren zur Erfassung verborgener Defekte in Bauteilen. Es eignet sich für die kontaktfreie flächige zerstörungsfreie Prüfung, wobei nur mechanisch relevante Defekte erfasst werden. Die Verwendung einer periodischen Bauteilbelastung und einer speziellen Auswertung (Lock-in-Technik) bietet im Vergleich zur konventionellen Shearografie erhebliche Vorteile, wie z.B. Defektselektivität, verbessertes Signal-Rausch-Verhältnis und Tiefenauflösung. Die Leistungsfähigkeit dieser Lock-in-Shearografie wird im Folgenden anhand von Modellproben und Realbauteilen vorgestellt. Shearography is an optical method commonly used to detect hidden defects in materials. It is suited for remote imaging and non-destructive testing of components to reveal defects of mechanical relevance. Modulated object excitation in combination with special data processing (lock-in technique) provides substantial advantages compared to conventional shearography, e. g. defect selectivity, enhanced signal-to-noise ratio, and depth resolution. This article demonstrates the performance of lock-in shearography on model samples and industrial components.

Combined Application of NDE Techniques for Characterization of Short Fiber‐Reinforced Thermoplastics in Intact and Damaged State

NDE methods differ in the way they display the specimen under test. This is caused by the specific interaction with the material and its physical properties. Combining electromagnetic, optical, thermal, and elastic wave techniques, surface or integral information is achieved. Fiber orientation of injection molded shortfiber reinforced thermoplastics can be measured, which determines the mechanical and thermomechanical features of these materials. NDE techniques show a broad range of lateral resolution. It will be shown how they can reveal different types of damages and monitor damage processes. Some of the investigations can be performed in‐situ during tensile tests.