Introduction

Understanding how a crack propagates in a given material is fundamental to all forms of theoretical postulations, modelling, analytical and numerical analyses. If the premise of crack growth path is wrong, no matter how elegant the mathematical or numerical expression may appear, the result will not represent reality. The knowledge of corrosion fatigue and its mechanism is of immense value in preventing failure in marine environment. Understanding the influence of microstructure on fatigue mechanism is fundamental because it supports: fatigue life prediction of structures, design of fatigue resistant materials and realistic fatigue modelling attempts. Fatigue is a complex problem and there have been several numbers of publications on the problem of fatigue, each presenting similar or different ideas or trying to modify existing theories – using from simple to complex analytical and numerical approaches. Generally, fatigue crack growth (FCG ) in metallic materials is divided into three regions. Each region has been reported to exhibit different mechanisms and characteristics. These regions are the threshold region (or early stages of fatigue crack development), the Paris Region (or the linear and steady crack growth stage) and region of final failure (or unstable/accelerated crack growth stage). The Paris Region is of interest in this study because it is the part commonly used and recommended in ASTM E647-15 [1] and BS EN ISO 11782-2:2008 [2] for engineering design.
In many reports [3][4][5][6][7][8], microstructure, mean stress, mechanical properties and initial crack length were said to have large influence on the threshold and final failure regions. In addition, the accelerated region is said to be influenced by sample thickness. These factors were reported to have little or negligible effects when the propagating crack has grown to a considerable length or beyond few grains, usually in the Paris Region [5][9][10][11][12][13][14]. In the threshold region, non-continuum or single shear mechanism is said to operate and the nature of the fractured surface is seen to be faceted. The crack tip was under both tensile and shear forces. The crack closure phenomenon was reported to be high and the plastic zone size is equal or less than the microstructural grain diameter. In the final or accelerated failure region, fatigue and additional static loading modes are said to operate. And, microvoid coalescence, intergranular or additional cleavage failure mechanism have been found on the fractured surface and the crack tip was under tensile loading. The plastic zone size in this final region was reported to be very much bigger than the grain diameter.
Many researchers have reported that fatigue failure in the Paris Region is generally by transgranular ductile striation mechanism [10][15][16]. The crack path is generally taken to be across the grains (or transcrystalline), although they may also propagate along the grain boundaries or intergranular depending on the material properties, loading and environmental conditions [17]. In this linear crack growth region, the crack tip is under tensile loading and the plastic zone size is reported to be greater than a grain diameter. The crack closure is low and mechanism of growth is by striation – alternating or simultaneous shear on two slip systems [16]. The conclusions [3][5][6][18] that microstructure has little or no influence on the fatigue crack growth rate (FCGR ) of metallic materials is often based on the frequent observation of ductile striation mechanism in the Paris Region in fractographs. One thing that must be noted is that most of the fatigue theories existing today – including that of crack extension [16], crack path [15] and crack closure [19][20] were propounded from experiments performed on non-ferrous materials. For example, two other phenomena which are associated with crack retardation across metallic materials in the literature are crack closure and interlocking. It is pertinent to note that crack closure effect may be more important for non-ferrous ductile metals. It is a common knowledge that the concept of crack closure and the use of effective stress intensity factor range (SIFR ) was proposed by Elber [19][20]. This theory is based on the fact that plastically deformed surface wake is left behind as the crack propagates. He argued that there is a premature contact of the crack faces during unloading from tension in a fatigue test. This effect reduces the effective stress at the crack tip. Elber proposed the use of effective SIFR instead of the conventional SIFR in plotting fatigue curve. The mechanisms such as the plasticity-induced and the roughness-induced crack closure can cause retardation of theFCGR . However, it is pertinent to note that Elber’s theory is based on his study of 2024-T3 aerospace aluminium alloy of a moderate ductility. The validity of this theory to steel will vary since; (a) steel can have high ductility in annealed or fine-grained condition to ‘near-brittle’ ductility in martensitic or very low temperature condition, and (b) the thickness effect in steel material. As sample thickness of steel is increased, the plain stress region at the outer surface tends to be eliminated leaving only plain strain condition where plastic zone becomes little or almost negligible. Hence, the concept of crack closure may not be rigorously applied to steel at all conditions, coupled with the fatigue test condition under plain strain condition – i.e. with little or negligible plastic zone at the crack tip. In other words, when the plastic zone is little, the effect of crack closure can then be ignored, or its consequences will be insignificant.
None of these studies in practical terms showed vividly how the crack propagated through the phases in the microstructure of the materials studied. There are many variables, e.g. alloying elements and their concentrations, forming process and the mode of deformation, heat treatment - involving temperature range, heating time, cooling time, etc. which can be combined in so many ways to obtain varieties of steel properties, e.g., ranging from high ductility to almost completely brittle steel. The primary goal of this paper, therefore, is to present some observed microstructural influence on the FCG phenomenon in the Paris Region of ferrite-pearlite (α-P) steels produced by Normalized-rolled (NR ) and Thermo-mechanical control process (TMCP ) in air and seawater (SW ).