Aero
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And just as a fan fact, A cracked in a engine blade or the axel will lead to a complete re-build of the engine. Fragments breaking off a blade is known to cause catestrophic failures. At those rpms edges of the blades extend a few mm longer and have surface temp of hundreds of degree centigrade. They are under a huge stress. Hence single crystal blades perform better than anything.
That's what i was saying, crack starts at grain boundaries but in jet engine blades grains are oriented in one direction to avoid separation at boundaries.
Grain Boundaries & Different Grain orientation
Grains oriented in one direction & no boundaries in between