Jürgen Gruber

Fachhochschule Oberösterreich | fh-ooe

Fiber waviness in composite materials, also referred to as wrinkles, can be considered as one of the most significant manufacturing effects. The severe knockdown of mechanical properties, such as stiffness, strength and fatigue leads to a dramatically reduced load carrying capacity of the material. Wavy plies can appear in arbitrary shapes and loca...

Photothermal radiometry with an infrared camera allows the contactless temperature measurement of multiple surface pixels simultaneously. A short light pulse heats the sample. The heat propagates through the sample by diffusion and the corresponding temperature evolution is measured at the surface of the sample by an infrared camera. The main drawb...

Carbon fiber reinforced polymer is a material widely used nowadays in different application areas. All sectors, especially the aeronautics industry, demand components of high mechanical performance, but also with high durability and reliability, hence the need for advanced health assessing methods and to monitor the integrity of the components thro...

Kurzfassung Die thermografische Rekonstruktion von Messsignalen einer Infrarot-Kamera hat große Vorteile gegenüber einer Ultraschall-Prüfung: Es wird kein Koppelmedium benötigt, und die Temperatur kann an vielen Pixeln der Kamera parallel gemessen werden. Der Hauptnachteil im Vergleich zu Ultraschall ist die sich stark verschlechternde Auflösung mi...

Using an infrared camera for thermographic imaging allows the contactless temperature measurement of many surface pixels simultaneously. From the measured surface data, the structure below the surface, embedded inside a sample or tissue, can be reconstructed and imaged, if heated by an excitation light pulse. The main drawback in active thermograph...

In this work, it is shown that image reconstruction methods from ultrasonic imaging can be employed for thermographic signals. Before using these imaging methods, a virtual signal is calculated by applying a local transformation to the temperature evolution measured on a sample surface. The introduced transformation describes all the irreversibilit...

This work presents the application of methods for the determination of the thermal diffusivity well suited for flat bodies adapted to cylindrical bodies. Green’s functions were used to get the temperature time history for small and large times, for the approach of intersecting these two straight lines. To verify the theoretical considerations noise...

Reconstruction of subsurface features from ultrasound signals measured on the surface is widely used in medicine and non-destructive testing. In this work, we introduce a concept how to use image reconstruction methods known from ultrasonic imaging for thermographic signals, i.e. on the measured temperature evolution on a sample surface. Before usi...

Porosity in carbon fibre reinforced polymers (CFRP) degrades the engineering performance, especially the interlaminar shear strength. In this work the porosity of CFRP was successfully determined by means of pulsed thermography. Additionally cone beam X-ray computed tomography (XCT) measurements were carried out. The observation time and the result...

This work shows the prospects of pulse phase thermography (PPT) compared to ultrasonic testing when applied to carbon fiber reinforced polymer (CFRP) sandwich parts with aluminum honeycomb cores. Measurements were carried out on full-scale components with flaws like disbonds, septum disbonds, staggers and displaced cores, where the last two are not...

This work shows the prospects and limitations of the non-destructive testing methods Digital Shearography and Active Thermography when applied to CFRP sandwich parts with honeycomb cores. Two specimens with different core materials (aluminum, NOMEX) and artificial flaws such as delaminations, disbonds and inclusions of foreign material, are tested...

In this paper we show a detailed verification of an analytical thermal diffusivity model using finite element simulations. The real pore morphology for the simulation models are obtained by computed tomography measurements. The heat transfer by conduction is simulated in transient analyses. The thermal diffusivity values are calculated from the tem...