In this study, fatigue properties of PM (Powder Metallurgy) soft magnetic materials were investigated by observation of the small fatigue crack propagation. The materials to which binder resin were added (Material BA) and three kinds of materials with different density (Material LD, MD and HD) were used in this study. Reversed plane bending fatigue tests were conducted at room temperature and small fatigue crack growths were studied in detail by means of replication technique. The fatigue strength became higher in the order, material HD, MD, LD, BA. In all specimens, cracks initiated at multi sites and each crack propagated with frequent deflection. Furthermore, cracks were coalesced repeatedly and specimen reached final fracture. Moreover, the difference between the materials was not recognized clearly in the da/dN-Kmax relationship. The maximum defect size, √area max, which was estimated by the statistics of extreme value became higher in the order, material BA, LD, MD, HD. As a result, the addition of the binder resin to PM alloy was not effective in improvement of fatigue strength. Furthermore, densification by increasing compacting pressure was effective in improvement of fatigue strength because the maximum defect size in each material became smaller. The ΔK0-Nf/√area max was estimated to consider whether the evaluation by the statistics of extreme value had propriety to the fatigue strength. Then, their curves were not corresponded. The disagreement was originated in the difference of coalescence frequency until the specimen reached to final fracture.
