What are the Causes of Fatigue Cracks in Flexible Metal Conduits?

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Fatigue crack propagation area of flexible metal conduit:

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In general, the micro morphology of the fatigue crack propagation zone has the characteristics of cleavage or quasi-cleavage fracture. However, when the material grain boundary is significantly weakened, the fatigue fracture can also be characterized as intergranular fracture. The morphological features of the fatigue crack growth zone also include macro fatigue fringes (conchoidal patterns) and radial fringes. Micro fatigue streaks are traces left by the crack propagation process under the action of each stress cycle. The micro fatigue streaks are not continuous due to the microscopic defects of the material (inclusions, second phases, grain boundaries, etc.) or the difference in crystal phase , But distributed on small faces of different heights and shapes. The micro fatigue streaks are slightly curved arcs, usually, the arc center points to the beginning of the crack. When the material strength is very high, because it is very brittle, the micro fatigue cracks are not obvious enough on the cleavage surface of the fatigue fracture. At this time, the fatigue properties of the component fracture can be determined according to the macro fatigue streak or the fatigue streak termination line. The radial pattern is another microscopic feature of the fatigue crack propagation zone. It is the fringe formed by tearing due to the height difference between different propagation paths during the fatigue crack propagation process.

According to the above analysis:

The main failure mode of weld cracks in flexible metal conduits is fatigue cracking, which exhibits the characteristics of rational fracture. However, it is a general situation that the fatigue lines of austenitic materials with better toughness mentioned in the textbooks have obvious distribution characteristics. When the grain boundaries of austenitic materials are weakened, grain boundary cracks may also appear. Is the fatigue pattern of stainless steel material itself not particularly obvious? Not necessarily, the state of force, the direction of force, temperature, environment, etc. will all affect the morphology of fatigue cracks, but as long as there are macroscopic features of fatigue shell patterns, it must be fatigue cracking (except for corrosion patterns). Only the microscopic stripes of fatigue fracture of high-strength materials are not obvious.

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