Characteristics of fatigue crack growth in DP (with dual-phase, ferritic-martensitic microstructure) steel sheets have been investigated. SPHC (hot-rolled mild steel) sheets also have been examined for the purpose of comparison. For the both materials, the variation in fatigue crack growth rate (da/dN) with stress intensity factor range (ΔK) was measured by using CCT specimens, as well as the threshold values of ΔK by using CT specimens.
From the results of these measurements, m in the formula of Paris, da/dN=C(ΔK)m, was determined as 3.5 for DP steel as well as for SPHC steel. The threshold values (ΔKth) at R=0.1 were shown as 6.5MPa·m1/2 and 5.7MPa·m1/2 for DP and SPHC steels, respectively.
It is concluded that the characteristics of fatigue crack growth in DP steel sheet with a microstructure where a continuous ferrite matrix encapsulates islands of martensite are little different from those in conventional mild steel sheets.