Notch characteristics in static strength of sintered steel was investigated experimentally and theoretically.
Circumferentially notched bars and edge notched plate specimens with various root radii were examined at room and liquid nitrogen temperatures. The fracture criterion was discussed based on the stress distributions calculated by the finite element method considering the non-elasticity of the material.
According to the experiments, sintered steel showed very low notch sensitivity at room temperature. Strength decrease by a notch was within 20% when the form factor was up to 3. At liquid nitrogen temperature it showed higher sensitivity, the strength decrease reached to about 50 for the same notch shape. These notch characteristics are similar to those of cast iron.
From the calculated stress distribution, it was revealed that the low notch sensitivity was caused by the following two factors, same as in cast iron: the non-elasticity of the material that reduces the maximum working stress to a much smaller value than the elastic estimation, and the existence of an over stressed region at the notch root where the stress is higher than the tensile strength of the material. The over stressed depth δ at fracture was 1-3mm at room temperature and 0.2-0.5mm at liquid nitrogen temperature, taking a smaller value in small notch radius.
The smallest value of δ was several times the particle size of raw powder, that is the minimum dimension below which the porous sintered steel can not be treated as a continuum solid. The larger value of δ at room temperature was inferred to be the propagating condition of an initiated cracks.