Answer Posted / venkat chilla

1.Due to high compressive stress on the surface and very
hard surface the neck or crack propagation on the surface is
delayed during deformation. In general the crack will be
initiated at the core of the material in normal tensile
loading at the region of internal defects and inter metallic
particles. on further increasing the load the crack will
propagate further towards out side of the surface. As the
surface is hard the crack propagation will be stoped by this
hard case and causes to delayed failure .
2. While in fatigue loading the crack will initiate at the
surface and propagate later in to core. As the hardened
surface is not allowed to initiate a crack and so delayed
failure.

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