Why we use v-notch in charpy impact test ?
It's a ductility [or toughness] test. A simple hardness test is not representative of structural integrity as it only tests the surface. For heat treated metals, a v-notch test is more representative.
He asked "Why"..
The notch is the start of the failure. Stress crack propagation results from the notch being cut into the sample, and the hammer hitting the sample right at the notch.
Predictable, repeatable fractures happen at the consistently weakest point on the bar.
For a good demonstration see:
That too, good point Russ.
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