中文版
The fatigue fracture of ship machinery fittings is the damage gradually formed in a long time. Under the action of alternating load, a micro crack appears at the surface defect of the ship engine parts first, and then the crack expands and stagnates sometimes, and finally breaks. This fracture process is confirmed by three zones of fracture characteristics of fatigue fracture.
The formation of fatigue cracks
The source of fatigue crack is located in the surface of ship machinery parts where the maximum stress, that is, the place with stress concentration or the material near the surface of ship machinery parts, that is, there are serious metallurgical defects or tissue defects in the material. The crack sources on the surface of ship machinery parts are mostly micro-cracks formed by alternating stress concentration on the surface, such as oil holes, transition fillet corners, steps and coarse knife marks. Miniature grain in the near surface material of ship machinery parts due to the defects of smelting, cold and hot processing, crystal slip and grain boundary defects under the action of alternating stress.
② Fatigue crack propagation
Once the fatigue microcracks appear on the surface or near the surface of ship machinery parts, they will expand under the action of alternating stress. Fatigue crack propagation is divided into two stages. First, the crack at the micro-crack has been formed, that is, the crack at the fatigue source extends along the direction of the maximum shear stress (approximately 45° in the direction of the normal stress) to the interior of the ship machinery parts. The depth of expansion is shallow, and the rate of expansion is small. This is the first stage of crack propagation. Then the direction of crack propagation changes, and the crack propagates along the direction perpendicular to the normal stress, which is the second stage of crack propagation. At this time, the normal stress plays an important role in the crack propagation, which makes the crack propagate deeper and faster than the first stage. Under the action of alternating normal stress, the crack sometimes expands and sometimes stops. The two sides of the ship machinery parts are sometimes closed and sometimes separated, so as to form a "shellfish pattern" on the two sections. The crack propagation near the crack source is slow, and the two fracture surfaces rub and grind each other for a long time, which makes the fracture surface smooth and has sparse shellfish stripes. The farther crack propagates faster, and the grinding time between the two fracture surfaces is shorter, which makes the fracture surface rough and the shellfish grain fine.
③ Fatigue fracture
When the fatigue crack expands to a certain depth, the actual bearing area of the ship engine parts decreases. When the remaining area cannot bear the load, sudden fracture occurs, and the final fracture zone appears on the section.
