Structural Health Monitoring Of Aircraft Composite Structures Using Ultrasonic Guided Wave Propagation

"Despite enhancements in terms of specific strength and stiffness by using composite in aircraft structures, their susceptibility to hide damage is still a major point of concern. The objective of this work is to investigate guided wave propagation in composite structures to detect delaminations, disbond and impact damage.The majority of the work focuses on assessment of composite joints. Primarily, a simple composite structure configuration was chosen to evaluate the effect of artificial and real damage on guided wave behaviour. The results show that non-mid-plane artificial delamination can accurately represent real impact, particularly barely visible impact damage (BVID). Next, a composite skin-stringer assembly and a composite scarf repair were chosen in order to represent typical aerospace structural joint features. The reflection, transmission and scattering behaviour of the plane guided waves are studied as a function of mode, frequency, excitation angle and the quality of the joint. For the composite skin-stringer, two inspection strategies are applied. From the first strategy, the within-the-bond, it is concluded that the antisymmetric mode (A0) transmission is highly sensitive to the damage for frequencies below 350 kHz, while the symmetric mode (S0) reflection around 200 kHz could be employed for monitoring an echo induced by the disbond. For imaging the disbond based on the scattering of the waves, the S0 mode appears as the best candidate below 350 kHz, by inducing an increase of 60 % of the scattered field in the presence of a disbond. The results from the second strategy, the across-the-bond, indicate that the A0 mode behaves more directionally while S0 is more refracted, specifically at low frequencies. For damage imaging, the S0 mode appears to be sensitive enough to disbonds (an increase of 30 % of the scattered wave) at around 150 kHz. Comparison of the pristine and damaged repair joint indicates reflection at the tip of each layer in the scarf (the reflections from the steps' edges), which can be an indication for evaluation of the quality of the joint. The anti-symmetric mode in the pulse-echo configuration seems to be an efficient mode and strategy for disbond detection in composite repairs. " --