Fig. 8 Fracture surface of broken blade
Fig. 9 Result of dye penetrant inspection on face side near the fracture position
5. Judgment Criteria in Ultrasonic Inspection
5.1 Information Obtained by Ultrasonic Inspection
The information obtained by the ultrasonic inspection is the echo of reflected wave from the discontinuous area inside a propeller casting.
The shape of the defect itself cannot be identified. It merely provides the size and depth of the defect by a comparison between the actual defect and a dummy defect of the calibration specimen manufactured from the same material as the propeller material. In the ultrasonic inspection, flat bottom defects of 2.0 mm in diameter are formed in the calibration specimen beforehand, a DAC (Distance Amplitude Correction) curve is prepared using these dummy defects of which depths are different from each other, and the defect is detected as a defect having the size equivalent to the flat bottom disks which are the same as the dummy defects. Namely, the information obtained actually is not the actual shape of the defect, but merely the information indicating that, if a flat bottom circular defect is present, an echo is reflected from the defect of such a size. As shown in Fig.10 and Fig.11, an example indicates that a defect of 10 mm in diameter detected as the defect equivalent to the flat bottom defect was actually two minute defects spaced at an interval of approx. 10 mm (defect formed, for example, by pushing it with the tip of a pencil). In addition, when the size of the actual defect in the casting is detected by the comparison with the calibration specimen, care must be taken because anultrasonic wave attenuation rate is different between the specimen and the actual casting. As a result, the specimen is desirable to have the same heat history (cooling rate) as well as the same chemical components as the actual casting.
Fig. 11 Actually internal defect
5.2 Judgment Criteria of Internal Defects
Fig.11 shows the judgment criteria line for determining the acceptability of the internal defects of a propeller.
Fig.11 is used to judge, by the calculation of propagation of cracking using the fracture mechanics, whether or not a fatigue defect develops and grows from a defect of any size and leads to a fracture during the service life of the propeller. In Fig. 11, the depth of defects is plotted on the abscissa and the size of defects is plotted on the ordinate, and the level of variable stress acting on the defective part of the propeller blade is shown as a parameter, Accordingly, the deeper a defect is. the larger the allowable defect is, In addition, the smaller the acting variable stress is, the larger the allowable defect size is.
