DIFFRACTION AND SHADOWING ERRORS IN -6 DB DEFECT SIZING OF DELAMINATIONS IN COMPOSITES

The 6 dB drop method is widely used in the sizing of delamination defe cts in fibre-reinforced plastic materials. However there are certain s ystematic uncertainties involved in this measurement. Previous investi gations have looked at the effect of defect size relative to the ultra sonic beam width us well as the effect of curved defect edges and diff raction effects in the beam. That work concentrated on the use of the plane-wave region of an unfocused transducer. This paper considers fur ther uncertainties in the 6 dB drop method caused by diffraction of ul trasound at the defect edges and shadowing of the return signal by the defect when carrying out double through-transmission and back-wall ec ho-gated pulse-echo scans. The effects are considered using focused an d unfocused transducers. Methods are proposed for determining the syst ematic uncertainties in the 6 dB drop method for several types of C-sc an.