Fatigue crack initiation at inclusion

Fatigue crack initiation at inclusion

SAMPLE: Solid steel hexagonal rod

ROOT CAUSE: Near surface inclusion from manufacture causing stress concentration

“Presented here is a fractured section of a solid hexagonal high strength torsion bar subjected to torsional loading in service. Visual inspection of the fracture surface showed evidence of fatigue cracking through the presence of concentric crack arrest markings. Cracking has initiated in one location near the surface of the rod, propagating only a short distance into the cross-section before the final overload fracture has occurred. Stereographic and metallographic inspection of the crack initiation point revealed a surface-breaking inclusion providing a geometric stress concentrator to initiate fatigue cracking. The helical orientation of the fracture surface is characteristic of torsional overload failure.”

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