surface associated with ductile fracture is shown in the photograph for
a low carbon steel tested at high temperature. The top photograph shows the
fracture surface at the center region of the fracture surface and the lower
macroscopic pictures show the "cup" section of the ductile cup-cone
fracture surface. In this case, voids formed at the center of the sample
when neck formation starts at the ultimate tensile stress of the material join
together to form a central failure surface with a mean orientation normal to
the tensile axis. As this feature grows, the shear stress on the material
closer to the sample surface increases and the final failure surface in the polycrystalline
sample is at approximately 450 to the tensile axis,
the direction of maximum shear stress.
The microscopic picture shows
the textured surface left by the joining together of voids and the local microscopic
plastic deformation of the sample.