KEYWORDS: Diffusion, Thermography, Signal to noise ratio, Aerospace engineering, Infrared cameras, Ultrasonics, Manufacturing, Testing and analysis, Time metrology, Wave propagation
High strength and light weight, justify the frequent use of carbon fibre reinforced plastics for aeronautical applications. The manufacturing process of such material systems is a multi-stage process and susceptible to the formation of air-filled voids. This porosity weakens the epoxy matrix and causes noticeable degradation of mechanical properties. Active thermography with optical-excitation is an advantageous photothermal method because due to the infrared camera it is a non-contacting, fast testing method for the estimation of material properties or for defect detection. We use the Virtual Wave Concept, which allows ultrasonic testing methods for photothermal measurement data. Based on this ability, we apply the through-transmission method to determine the Time-of-Flight of virtual waves, which is directly related to the porosity dependent diffusion time. A signalto-noise ratio dependent approach is used for the temporal truncation of measurement data to get the optimum evaluation time. This ensures to evaluate only time-ranges which contain information of the heat diffusion inside the sample. In addition, undesired effects of heat losses due to convection and radiation are reduced. After the evaluation procedure is shown for simulated data, we demonstrate the experimental pixel-wise estimation of the porosity affected thermal diffusion times on a real aerospace part in transmission configuration. The results are validated by X-ray computed tomography reference measurements, where a good match can be achieved with active thermography results.
In active thermography, the structure below the surface can be reconstructed from measured surface temperature signals. The main drawback in active thermographic is the degradation of the spatial resolution with imaging depth. Recently, we used a mathematical compensation method to transform each measured surface temperature signal into a virtual acoustic wave, which is the solution of the wave equation and therefore ultrasound image reconstruction methods can be used. This allows a 3D thermo-tomography, which combines the advantages of thermographic and ultrasonic imaging, but the degradation of spatial resolution for deeper lying structures is still significant. A possibility to overcome this degradation is to incorporate prior information such as positivity and sparsity in the reconstruction process. Based on pulsed thermography data we show that the thermographic detection limit is extendible by a factor of four.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.