Effect of surface defects on fatigue properties of powder forged ferrous materials

In powder-forged ferrous materials, surface defects such as decarburized layers, surface roughness, and non-sintered areas (NSA) are observed. These NSA, peculiar to this material, play the most important role in determining the mechanical properties. In order to understand the behavior of NSA in dynamic conditions, fractography and measurement of the NSA distribution on the fatigue origin were investigated. The behavior of crack initiation and propagation was observed during fatigue tests by using ACPD (the alternative current potential drop method) measurement and the thermoelastic stress analysis system. The effects of NSA on fatigue properties were discussed on the basis of fracture mechanics. As a result, the mixture of static fracture with fatigue in fatigue origin indicates that process of fatigue fracture consist of 3 stages as follows: 1st stage; effective crack initiation (combination of NSAs by static and fatigue fracture) 2nd stage; propagation of effective crack 3rd stage; final rupture Observations of fatigue crack initiation and propagation with the crack growth monitoring system show that the fatigue fracture process consist of 3 stages mentioned above and effective crack initiation occurred at the beginning of less than 10% of fatigue life. From the measurement of NSA distribution of fatigue origin, the fatigue behavior of this material can be treated with fracture mechanics based on the effective crack length equal to the depth of NSA distribution of which NSA ratio is 14%.