Internal defects that are detected by radiography may also be detected by sound. Sound waves have been used by fisherman to locate hot fishing spots and depth of water and by the U.S. Navy to identify approaching objects. In casting inspection, ultrasonic testing uses high frequency acoustic energy that is transmitted into a casting. Because ultrasonic testing allows investigation of the cross-sectional area of a casting, it is considered to be a volumetric inspection method.
The high frequency acoustic energy travels through the casting until it hits the opposite surface or an interface or defect. The interface or defect reflects portions of the energy, which are collected in a receiving unit and displayed for the analyst to view. The pattern of the energy deflection can indicate the location and size of an internal defect, as well as wall thickness and the nodule count of ductile iron.
Ultrasonic testing requires a high does of knowledge and experience for an accurate interpretation of the results, which will affect the cost added to the part for the inspection.