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 ).