– dropped objects 9% – corrosion 6%
As can be seen, fatigue is the main cause of damage, followed by groups that can be designated as accidental damage. Corrosion damage is ranked as ranked as the fourth most frequent cause of damage. Figure 2.1 shows a fatigue failure of a propeller shaft in a shuttle tanker. The fracture occurred in the intermediate part of the shaft. The crack started from the surface of the shaft due to a weld arch strike. The fatigue surface is characterized by its smooth appearance with almost no plastic strain. At several stages during crack propagation, marks which are due to low stress variations are left as traces on the fatigue surface. These so-called beach marks correspond to changes in the fatigue loading; the crack front will make a mark during the time of slow growth due to smaller stress cycles. These marks are analogous to the dark winter rings found in the cross section of a tree. As can be seen, the beach marks have a typical semi-elliptical shape indicating the position of the crack front at various stages during the crack propagation. When the fatigue crack has reached the size of about three-quarters of the shaft diameter (D = 360 mm), the final fracture has occurred due to lack of the remaining ligament of the shaft cross section. It is a ductile fracture governed by the maximum occurring shear stress. The 45-degree share planes are easy to recognize for the final fracture. In the example in Figure 2.1, the fatigue failure leads to a severe leakage in the stern bearing tube and the blackout of the main engine. Nobody in the crew was injured.
– dropped objects 9% – corrosion 6%
As can be seen, fatigue is the main cause of damage, followed by groups that can be designated as accidental damage. Corrosion damage is ranked as ranked as the fourth most frequent cause of damage. Figure 2.1 shows a fatigue failure of a propeller shaft in a shuttle tanker. The fracture occurred in the intermediate part of the shaft. The crack started from the surface of the shaft due to a weld arch strike. The fatigue surface is characterized by its smooth appearance with almost no plastic strain. At several stages during crack propagation, marks which are due to low stress variations are left as traces on the fatigue surface. These so-called beach marks correspond to changes in the fatigue loading; the crack front will make a mark during the time of slow growth due to smaller stress cycles. These marks are analogous to the dark winter rings found in the cross section of a tree. As can be seen, the beach marks have a typical semi-elliptical shape indicating the position of the crack front at various stages during the crack propagation. When the fatigue crack has reached the size of about three-quarters of the shaft diameter (D = 360 mm), the final fracture has occurred due to lack of the remaining ligament of the shaft cross section. It is a ductile fracture governed by the maximum occurring shear stress. The 45-degree share planes are easy to recognize for the final fracture. In the example in Figure 2.1, the fatigue failure leads to a severe leakage in the stern bearing tube and the blackout of the main engine. Nobody in the crew was injured.
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