Ultrasonic guided wave inspection of adhesive bonds between composite laminates

Nondestructive evaluation (NDE) of adhesively bonded joints requires a technique that is sensitive to adhesive defects without direct access to the bonded region. The intrinsic advantages of ultrasonic guided waves, an ability to inspect for different types of defects and travel through a structure having nonuniform cross section, make them a powerful tool for NDE of adhesive joints. An adhesive defect that exists at the interface between the skin and adhesive layer of a laminated composite skin–stringer structure is considered. Guided wave modes of interest have relatively large in-plane displacement at the interface indicating high shear stress there. However, the two geometry transitions in a skin–stringer configuration result in mode conversion and make it more challenging to generate sensitive modes for inspection. A wave structure based correlation coefficient is defined to predict the mode conversion at the geometry transition. An effectiveness index is calculated as a product of in-plane displacement at the interface and the correlation coefficients at the transitions. Based on the effectiveness index, two high-index incident modes are selected for simulation and experiments; mode 6 at 0.75 MHz and mode 7 at 0.95 MHz. Finite element analysis shows that both modes have relatively large in-plane displacement at the interface and that the shear stress is near a local maximum there as well. The experimental results show that both modes are sensitive to adhesive defects by either frequency content or amplitude ratio.