to inspect an internal zone of a tubular cavity having an open end , for example , a zone of possible damage in a heat - exchanger tube , an elastically hardenable casting plastic is transported to the zone and cast against the surface embraced by the internal zone , and after hardening to an elastic casting , is pulled from the zone for inspection outside of the open end of the cavity . being elastic , the casting is not permanently deformed when pulled from the zone , the casting providing an undistorted physical image of the surface within the zone , permitting a determination of possible damage including pitting , cracks and the like .