Dayal V.

Metallic inserts are embedded into honeycomb sandwiches as hard points. These inserts are normally circular in shape. The disbond between the honeycomb face sheets and the puck is of concern. Air-coupled ultrasound has been used for detecting disbond between insert and facesheet. It was discovered in such inspection that surprisingly large amplitude could be transmitted through thick metallic inserts (e.g., 3/4' thick, 1.5' diameter titanium), whereas a thin plate of titanium can only transmit a much weaker signal. This paper reports a theoretical, analytic, and experimental study of the geometrical effects of such inserts. Modal analysis of cylindrical inserts was made closed form solutions and using finite element ANSYS codes. This work discusses the limitations of various models of the vibrations of rods and discs. The transmission efficiency of air-coupled ultrasound correlated well with the vibration modes of the cylinder, i.e., axial versus torsional. The energy transfer through the insert has been related to the resonant phenomenon. Of all the resonances observed in such a structure, the axial vibration mode transmits the largest amount of energy and the bending and torsional modes do not. An understanding of the resonant transmission through metallic inserts is important for the inspection of such structures with narrow-band or broadband air-coupled ultrasonic systems. This paper presents the theoretical basis and experimental evidence of energy transfer through the inserts in honeycomb sandwich panel.