Abstract
The fatigue crack growth tests near the stage I region were carried out on three spheroidal graphite cast irons (SGI) with matrix of ferrite, bull's-eye and mainly pearlite, having tensile strength of 380, 500 and 700MPa, respectively. The crack growth rate da/dN of the ferrite matrix SGI decreased continuously with a decrease in stress intensity factor range ΔK. On the other hand, the bulls-eye and pearlite matrix SGI showed discontinuous ranges on the da/dN vs. ΔK diagrams. The results were compared with local ΔK calculated at the crack tip by fracture mechanics. The ΔK at the lower end of the discontinuous range agreed well with the value calculated on the assumption that a single graphite particle is included in the tension-compression region at the crack tip. The ΔK at the higher end agreed well with the calculated value for including no graphite particles in this region. In the ferrite matrix SGI, however, the calculated value was smaller than ΔKth; the cyclic stress region is presumed to include plural graphite particles. The fatigue fracture mechanism of SGI is believed to be significantly affected by the interaction of graphite particle spacing, the size of plastic region at the crack tip, and the stress concentration around the graphite particle.
