Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10^-8 to 10^-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK₀ values reported to date, and certainly the highest combination of strength and ΔK₀ for steels (i.e., ΔK₀ values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations. Formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California