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Fractography
The examination of fractured surfaces initially under high powered stereoscopic microscope followed by more detailed examination under the scanning electron microscope, can provide valuable information regarding failure mechanisms.
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Fatigue
Fatigue failure reveals itself by the presence of fine striations on a fracture surface due to the cyclic advance of the fatigue crack. Numerous fine striations indicate high cycle, low stress mechanism, whereas coarse striations often indicate low cycle high stress failure.
Figure 1
Fatigue Fracture of 316 SS (each striation caused by progress of fatigue crack during one stress cycle
Failure Analysis
Tensile overload
As a result of uniform overload under tension, a series of equiaxed voids is observed and this is termed microvoid-coalescence. In the case of mechanical overload in shear, the voids become elongated.
Figures 2 & 3
Ductile Overload in Shear Oval Dimples
Hydrogen embrittlement
The fracture face is almost always intergranular in nature with little evidence of oxidation or corrosion.
Hydrogen Embrittlement of Hastelloy X Seal Ring (clean, non oxidized grain boundaries)
Overheating of 6061 T6 Aluminum Tube (partial melting of grain boundaries)
Overheating
In this case instead of the grains of metal being sharply defined, incipient melting cozes the surfaces to become smooth with a times fine nodules due to solidification.
Brittle fracture
Failures, due to the impact loading of materials with low ductility will show fractures that are generally free of intergranular features.
Brittle fracture on Alumina Grit
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