Description
In aerospace, the trade-off among performance, cost and safety poses an extreme engineering challenge to structural integrity. While composite materials have become a staple in aerospace industries due to their high strength-to-weight ratio and adaptability, they can develop complex 3D damage mechanisms undetectable on the surface. Non-destructive evaluation (NDE) methods detect interior damage non-invasively. This thesis focuses on the application of thermographic signal reconstruction (TSR) to pulsed infrared thermography for the quantitative assessment of impact damage in composite aerospace structures. Specifically, temporal deviations from a uniform 1D rate of cooling are emphasized and quantified by specific features in the temperature time history of pulsed thermography data on stiffened carbon fiber reinforced polymer (CFRP) panels. Past studies utilizing TSR have investigated the validity of the method itself, however this thesis aims to break down the principles of TSR and incorporate methods to detect and quantify the depth and characterization of impact damage. Specific features extracted from the derivatives are studied closely, referred to as augmented TSR, is adaptable to any data set utilizing TSR. The method is applied on skin-to-stringer CFRP panels previously impacted with various energies on the skin side, where the skin meets the stringer flange. Conventional ultrasonic testing data will constitute a reliable comparison to assess the performance of pulsed thermography with TSR. The results were able to detect all the main damage features, however still presented issues unique to thermography and the experimental setup, such as image blurriness. This meant that only the detection of larger delaminations was possible. A large advantage to TSR, however, is the ability to observe damage evolution with respect to depth by observing the TSR time derivatives progression in time. Identifying damage severity and mode non-destructively is crucial towards understanding failure mechanics in composites (specifically CFRP) laminates and identifying failure patterns for future structural prognostics. This can aid the development of models to predict and prevent failure and assist the decision-making process of structure maintenance. Pulsed thermography with TSR can prove to be an effective engineering solution, also capable to inspect large areas at once, to be utilized in the aerospace industry.
