The received time-domain waveform signals at 100 kHz frequency in the second
elbow: (a) crack in extrados of space-Z pipe, (b) crack in intrados of space-Z pipe, (c) crack in
extrados of plane-Z pipe, (d) crack in intrados of plane-Z pipe, (e) crack in extrados of U pipe,
and (f) crack in intrados of U pipe