Which NDT method would not be suitable for detecting cracks in a composite structure?

Dye penetrant testing is not suitable for detecting cracks in composite structures primarily due to the nature of composite materials and the application method of dye penetrant. Composite structures often have a layered arrangement and may include materials such as fibers and resin matrices that can absorb or entrap the penetrant, leading to false indications or missed defects. Additionally, the surface of composites can be uneven or porous, preventing effective penetration of the dye and subsequent observation of cracks.

In contrast, ultrasonic testing is effective for composite materials because it can differentiate between layers and detect delaminations or internal cracks using high-frequency sound waves. The tap test is also practical for composites, as it relies on sound waves transmitted through the material, which can reveal defects based on the acoustics of the structure. X-ray testing can provide detailed images of internal structures, including cracks, through various densities of the composite material. Thus, the interaction of the dye penetrant with the composite's unique properties renders it the least suitable option for crack detection in this context.