MILLIMETER WAVE HOLOGRAPHICAL INSPECTION OF HONEYCOMB COMPOSITES

Multi‐layered composite structures manufactured with honeycomb, foam, or balsa wood cores are finding increasing utility in a variety of aerospace, transportation, and infrastructure applications. Due to the low conductivity and inhomogeneity associated with these composites, standard nondestructive testing (NDT) methods are not always capable of inspecting their interior for various defects caused during the manufacturing process or as a result of in‐service loading. On the contrary, microwave and millimeter wave NDT methods are well‐suited for inspecting these structures since signals at these frequencies readily penetrate through these structures and reflect from different interior boundaries revealing the presence of a wide range of defects such as isband, delamination, moisture and oil intrusion, impact damage, etc. Millimeter wave frequency spectrum spans 30 GHz–300 GHz with corresponding wavelengths of 10−1 mm. Due to the inherent short wavelengths at these frequencies, one can produce high spatial...