Fig. 12 Judgment criteria for determining the acceptability of the internal defects
5.3 Safety Factor for Judgment of Defect
As mentioned before, the information on the ultrasonic inspection is the echo of a defect converted into the diameter of a flat bottom disk. In actual, however, even if a flat bottom defect having a surface in parallel with the surface of the propeller blade is present just below the surface of it, considering the acting direction of stress, a fatigue crack does not propagate during the service life.
Therefore, a defect is first converted from a detected flat bottom-converted diameter to the diameter of a spherical defect which provides a reflected echo equivalent to the flat bottom-converted diameter, and the diameter of the spherical defect is evaluated as a disk in parallel with the cross-section of the blade at the stage of crack advancement calculation by the fracture mechanics.
Namely, because the actual image of a defect cannot be known by the judgment of the ultrasonic inspection results, the defect is evaluated larger (first safety factor) in its conversion to a spherical diameter and, in the process of application of the fracture mechanics, the defect is analyzed using a disk shape having a large stress concentration factor (second safety factor) in order to assure the reliability, In addition, this process covers a part of the uncertainty such as the affection of attenuation factor in the use of the calibration specimen.
Fig. 13 Correlation between a variable stress and an average stress at 0.25R of blade
5.4 Estimation of Stress Acting on Blade
As already described, variable stress acting on the propeller blade is used for the evaluation of an internal defect. Here the method of estimating the variable stress is described. First average stress (averaged value of stresses which acts on the blade during one rotation of the propeller and corresponds to an average thrust developed by the propeller in steady forward movement) acting on the surface of a propeller blade (pressure surface) is obtained by the FEM calculation. The load conditions then are the blade surface pressure distribution and centrifugal force obtained by the theory of propeller blade. Also, though the shape of the propeller blade is considered as those elements obtained by dividing the geometric shape of the propeller blade in radial and lateral directions, an increase in thickness of the blade by the fillet at the root of the blade is not taken into account (third safety factor).
On the other hand, a variable stress causing a fatigue crack in the propeller blade is produced because the propeller is operated in the wake at the stern, In general, it reaches the maximum when the blade passes just its highest position and reaches the minimum when it passes both board sides. In order to obtain these maximum and minimum stresses, first a variation in thrust of the blade during one rotation of the propeller must be calculated using the theory of unsteady propeller, and then each stress must be analyzed by the FEM based on the blade surface pressure distribution at the maximum and minimum thrusts. In this case, the data on the stern wake distribution is naturally required for each ship.
Because the wake distribution data itself is obtained for actual ships by some modification of the measured results of a model ship, some problems on the size of the model ship and the validity of the modification calculation are left unsolved. In the actual operation, therefore, as shown in Fig.13 [1], it is practicable to estimate a variable stress from the average stress obtained before using the correlation between the average stress and the variable stresses of the propellers used for various types of ships. For example, it can be said from Fig.13 that, for the propeller of a full body ship, the variable stress will be sufficient if it is approx. half the average stress.
