Materials Science and Engineering A 468–470 (2007) 114–119 Stress shielding and fatigue crack growth resistance in ferritic–pearlitic steel Y. Mutoh a,∗ , Akhmad A. Korda a , Y. Miyashita b , T. Sadasue c a Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan b Department of Mechanical Engineering, Nagaoka College of Technology, 888 Nishi-Katagai, Nagaoka, Niigata 940-8532, Japan c Steel Research Lab., JFE Steel Corporation, 1, Kawasaki-cho, Chuo-ku, Chiba, Japan Received 18 May 2006; received in revised form 20 May 2006; accepted 20 July 2006 Abstract The effect of pearlite morphology on stage IIb (Paris regime) fatigue crack growth behavior in ferritic–pearlitic steel was investigated. Networked and distributed pearlite structures were prepared. Constant-K fatigue crack growth tests were performed in situ in a scanning electron microscope. The results revealed that a distributed pearlite structure had better fatigue crack growth resistance than a networked pearlite structure. From the in situ observations, the distributed pearlite structure indicated a tortuous crack path, which induced crack interlocking as well as crack closure. For the networked pearlite structure, some crack branching was found on the crack path. The crack growth curves for the two microstructures, plotted using the effective stress intensity factor range K eff , where crack closure behavior is taken into consideration, did not coincide. The crack growth curves plotted using the crack tip effective stress intensity factor range K eff,tip , where crack tip shielding behavior as well as crack closure are taken into consideration, successfully coincided on one line. © 2007 Elsevier B.V. All rights reserved. Keywords: Fatigue crack growth resistance; Crack tip stress shielding; Pearlite morphology; In situ observation; Constant-K fatigue test; Crack closure 1. Introduction The process of steel plate production known as the thermo- mechanical control process has increasingly played a greater role in steel mass production. This process gives microstructures that are highly refined compared to those of conventionally processed steels, resulting in a significant improvement in strength and toughness [1]. It is known that failure problems of structures and machines are reported to be dominantly caused by fatigue [2,3]. Therefore, the improvement of fatigue strength and fatigue crack growth resistance of structural materials, as well as development of safety design, is strongly required for controlling fracture and assuring safety of structures. Fatigue crack growth life is domi- nant in the total life of components with notches and in welded joints, both of which are unavoidable in most structures and machines. ∗ Corresponding author. Tel.: +81 258 47 9735; fax: +81 258 47 9770. E-mail address: mutoh@mech.nagaokaut.ac.jp (Y. Mutoh). It is well known that in the near-threshold region, fatigue crack growth behavior is influenced by microstructural factors, whereas in the Paris regime, the microstructure has less influ- ence [4]. The effect of microstructure on fatigue crack growth behavior in steels has been widely investigated [5–10]. However, most of these studies are related to behavior in the near thresh- old region. Only limited information is available on the detailed influence of microstructure on fatigue crack growth behavior in the Paris regime [6–10]. The presence of a hard second phase in the soft ferrite matrix was found to influence fatigue crack growth behavior in duplex steels. The hard second phase, for example martensite, con- tributes to a superior fatigue strength and higher threshold value [11–14]. It is also reported that a hard second phase plays an important role in deflecting the crack path and then retarding crack growth [15]. It is well-known that crack closure phenomena enhance fatigue crack growth resistance and that the effective stress inten- sity factor K eff is the driving force for fatigue crack growth [16]. The crack tip stress shielding phenomena is also known to enhance crack growth resistance in ceramics, intermetallics, and composites, where the crack tip stress intensity factor K tip 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.07.171