[jaden]

firstly, the reason that ASTM EE3044 does not require the mode-converted time interval is because the shear mode is about 35dB lower than the P-mode so its amplitude will be insignificant.
Both the preparation of the fusion surface and the material content could be sources of the indications. The contact surfaces of the pipe should be clean and dry. But it is also possible that there are inclusions in the HDPE pipe. A common source is unmixed carbon-black. Perhaps you could examine the chip under a microscope to see if you ca identify any impurities in the void.