PSI - Issue 61

Rachid Azzi et al. / Procedia Structural Integrity 61 (2024) 241–251 Rachid Azzi and Farid Asma / Structural Integrity Procedia 00 (2023) 000 – 000

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service but also the new propellers of 4-5 months in operation (Pantazopoulos et al. (2011)). Investigative failure analysis methods such as visual inspection, optical microscopy, fractographic evaluation ... etc. applied to the blades after damage to reveal the source of the rupture; the results showed that the failure was caused by fatigue (Chang-Sup (2002)). The occurrence of cracks on the propeller blades is due to several factors such as material defect, cavitation erosion, and collision with foreign objects in the water (Blednova Zh.M et al. (2016)). The damage to the leading edge of the propeller blade is caused by the shocks ’ accidental, negligence during the transport of the propellers without protecting them and also caused by contact with cables or chains of a mooring buoy (Carlton (2018)). In most cases, impact damage involves only minor damage to the leading edge region, which consists of small fragments of matter; these fragments constitute a privileged site for the occurrence of fatigue cracks propagating under cyclic loads. These matter fragments on the leading edge if they are detected can be treated and repaired by grinding or by welding according to the recommendations of the manufacturer (Carlton (2018)). This study focuses on the detection of damage to the leading edge by comparing the vibrations of the damaged blade and those of the healthy blade. The leading edge damage takes varied geometrical shapes and occurs at different positions all along the leading edge. In the literature, we refer to this defect as an "edge notch" and was modeled by various geometrical shapes. The presence of a defect in the blade presents a weakening of the rigidity of the blade and consequently a change in modal characteristics of the propeller blade and its dynamic response. The propeller blade is embedded in the hub and during service it turns around the propeller axis, the rotational movement generates an inertia force, which increases the Eigen frequencies of the blade and the width of the defect (Saito (2009)). In this study, the modal and dynamic behaviors of one damaged propeller blade are compared with those of a healthy blade. The damage has two shapes, and varied lengths and takes several positions along the length of the leading edge. A modal analysis of both damaged and healthy propeller blades has been performed in rotating and non-rotating conditions. Then, a dynamic analysis is also carried out for healthy and damaged blades. Finally, the obtained natural frequencies, mode shapes, and dynamic response have been compared and discussed. 2. Geometric model of propeller blade The propeller blade was modeled by SolidWorks. Firstly, we represented the 21 sections of the blade, sections which are piled up the ones on the others, to radial positions of the axis of the propeller, and the seven curves guides that join the various sections. Open prop software (Epps et al. (2013)) is used to obtain the three-dimensional curves. The geometrical and hydrodynamic characteristics of the propeller are given in Table 1. Then, the sections of the blade represented are combined with the ones on the others by the function of Loft to obtain a 3D model. The geometrical model of the propeller blade is shown in Fig. 1.

Table 1. Geometric properties of the blade. Rotor Diameter 2R (m)

0,25

Number of blades

3

Rotation speed (RPM) Hub diameter 2r (m)

120

0,08 m

Meanline type Thickness type

NACA a=0.8 (modified) NACA 65A010 (modified)

The modeling of the defect was carried out by the drawing of profile "V" or "I" on the face of the blade at a position of the section of the blade. The width of the defect is constant and equal to 0.5 mm and its length is "L". Then this profile is extruded on all the thicknesses of the blade with the removal of matter. Thus, we obtain a model of a blade whose leading edge is damaged with a defect of length "L" and at radial position "S x ", with x=2, 5, 8, 12, and 17. During this parametric study, the damage position takes the following values S2, S5, S8, S12, and S17 which correspond to radial positions r = 0.2632R, 0.4475R, 0.6182R, 0.8084R and 0.9609R, respectively. The damage length