The Detection of Subsurface Defects in Composite Structures by the Application of a Remote Acoustic Doppler Technique

摘要

The non-contacting Acoustic/Doppler system discussed in this paper is unique and employs a proprietary design acoustic transducer which produces an air coupled shock or pressure wave, similar to that produced from a small explosion. This is achieved by discharging a high voltage electrical capacitor within a period of <5 μsec. The discharge is contained within a small ceramic chamber with an annular design anode providing an exit for the hot gasses which are a result of the discharge. The result of this configuration is that a single brief, but extremely high velocity shock wave of broadly unidirectional characteristics is launched into the air and used to impact and excite the object undergoing testing. The objective of the brief impact of white noise is to excite natural relaxation frequencies in the object and avoid any "blanketing" effect that would be present if continuos wave white noise was applied. Surface relaxation frequencies for any given material are dependent upon the underlying substructure of the object. In many respects this technique is analogous to tapping a surface with the edge of a coin and if there is a subsurface defect such as a debond then the resulting relaxation frequencies make a hollow sound. Audible responses are very difficult to quantify; furthermore physical contact is not always an option. However, the properly applied study of these relaxation frequencies is a very good nondestructive testing technique. In the case of this technology we are recording and analyzing the resulting relaxation frequencies excited by our acoustic pressure wave up to 50 kHz.