Specifically, when all propeller castings have been finished, the dye penetrant inspection is performed on the pressure surface of the blade in the area from the blade root fillet section to a radius of 0.4. The areas of 15% from the leading edge in blade lateral direction and the area of 20% from the trailing edge can be omitted to be inspected. The same rule is also specified in the inspection rule of the classification society. As a result, when the inspection is not specifically requested for a skewed propeller, basically the lateral center section of the blade is inspected mainly. In the present classification rules, the judgment whether the propeller is a skewed propeller or not is performed merely by the magnitude of its skew angle, and it is not considered that the feature of stress distribution of blade stress. The stress distribution on the trailing edge of the blade is depended on relation between the aspect ratio of the blade and the skew angle. Because, for a skewed propeller having a large aspect ratio, a stress is remarkably increased on the trailing edge even if the skew angle is relatively small, the inspection area should be determined for each propeller.
It was pointed out before that a skewed propeller must be designed considering that the maximum blade stress is located at the center of the blade. For a propeller having a stress distribution proper to a skewed propeller, the purpose of this is, except for the inspection performed at the time of new production, to prevent such an incorrect inspection to be performed in periodic inspections after launching that the dye penetrant inspection is not performed on those areas where stress is truly high. In particular, many of the defects ranging from the surface to the inside of the blade, which may be the start point of breakage of the blade, are found to be open to the surface by the polishing of surface of the blade carried out for each periodic inspection at a dock after launching. In this respect, the breakage of the blade starts, in many cases, from the defects not found in the periodic inspection.
4.2 Ultrasonic Inspection Procedure
The ultrasonic inspection uses a single probe or a double-divided probe. The inspection of a propeller casting uses the double-divided probe. The advantage of this probe is that it is suitable for detecting those defects located just directly under the surface of the blade at positions shallow from the surface. However, the area, which is 3 mm just below the surface of the blade, is an undetectable area called the dead zone which cannot be detected even using this probe. Fig.7 shows the frequency of occurrence of defective echoes among the detected results arranged according to the depth of detection.
From Fig.7, it can be found that the defects nearest the surface are present in this area at a position approx. 3 mm deep from the surface.
Fig. 7 Frequency of occurrence of detective echoes to the depth of detection
The presence of this dead zone becomes a problem for the ultrasonic inspection. Recently, therefore, an idea is proposed for the execution of the ultrasonic inspection that the inspection is performed on the thicken blade of the dead zone before final finishing.
In this paper, however, it is pointed out that, the problem on the dead zone can be solved by the ultrasonic inspection combined with the dye penetrant inspection and its reliability can be assured in practical use. Namely, the defects which become the start point of the fatigue cracking of a propeller blade as mentioned before are characterized in that, as shown in Fig.6, they are produced not only in the casting but also exposed partly to the surface of the blade. Therefore, if a defect is present in the dead zone, a defect may also present on the surface of the blade which can be detected by the dye penetrant inspection. In addition, shrinkage cracks resulted from the ununiform distribution of cooling temperature and foreign matter mixed into the molten metal are not present independently, but tend to be scattered even to the peripheral areas, if they are present in the casting. For a propeller casting having defects in the dead zone just below the surface of the blade, much exudation is found in the dye penetrant inspection. Fig.8 and Fig.9 show the appearances of the material specimens cut out from the fracture surface and residual section of the propeller of which blade is broken. From these figures, it is also found that a plurality of defects produced dispersedly and exudation is produced by the dye penetrant inspection.
