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 defects in fibre-reinforced plastic materials. However there are certain systematic uncertainties involved in this measurement. Previous investigations have looked at the effect of defect size relative to the ultrasonic beam width as well as the effect of curved defect edges and diffraction effects in the beam. That work concentrated on the use of the plane-wave region of an unfocused transducer. This paper considers further uncertainties in the 6 dB drop method caused by diffraction of ultrasound at the defect edges and shadowing of the return signal by the defect when carrying out double through-transmission and back-wall echo-gated pulse-echo scans. The effects are considered using focused and unfocused transducers. Methods are proposed for determining the systematic uncertainties in the 6 dB drop method for several types of C-scan.