Virtual wave concept for thermal non-destructive evaluation of delamination in composites

Speaker:
Mayr, Günther; University of Applied Sciences; Austria

Authors:
Mayr, G.; University of Applied Sciences Upper Austria; Austria
Burgholzer, P.; RECENDT - Research Center for Non-Destructive Testing GmbH; Austria
Stockner, G.; University of Applied Sciences Upper Austria; Austria
Hendorfer, G.; University of Applied Sciences Upper Austria; Austria

ID: ECNDT-0129-2018
Download: PDF
Session: Thermography and Thermosonics 2
Room: H1
Date: 2018-06-14
Time: 10:50 - 11:10

In comparison to ultrasonic testing, thermographic imaging with a focal plane array IR detector has a main advantage: it allows a rapid and non-intrusive inspection of large and complex-shaped structures. The main disadvantage of active thermography is the degrading spatial resolution with increasing depth. The quantitative extraction of information about buried defects from measured temperature data is therefore still on research level. Most of the thermographic reconstruction techniques deal with 1D heat conduction. The aim is to apply inverse analysis technique to multi-dimensional reconstruction problems.
In a recent work (P. Burgholzer, J. Appl. Phys. 121, 105102 (2017)), we have shown that image reconstruction methods from ultrasonic imaging can be employed for thermographic signals. Through a new virtual wave concept, well-known reconstruction methods of ultrasonic testing, e.g. time reversal, SAFT and F-SAFT, are accessible for thermal NDE. Before using these imaging methods, a virtual wave signal is calculated by applying a local transformation on the temperature evolution measured on the sample surface. To apply this virtual wave concept also for NDE of composites, some modifications must be made in respect to opaque materials and non-adiabatic boundary conditions.
Heat conduction simulations with the Finite-Element-Method (FEM) and experimental investigations based on pulsed thermography measurements are used to validate the modified virtual wave concept. In this first step, we carried out FEM simulations with two-dimensional geometries and an assumed isotropic heat conduction and short excitation pulses (idealized as Delta-functions in time). The thermographic image reconstruction allows a good estimation of the size and depth of the artificial defects. The experimental results of the thermographic image reconstruction of a delamination in a composite component show a good agreement to 3D X-ray computed tomography data.