PSI - Issue 60

Vivek Srivastava et al. / Procedia Structural Integrity 60 (2024) 233–244 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 3(c): Detail view of ICCP setup with electrical connections

3. Results and Discussion 3.1 Corrosion fatigue crack growth behaviour

In fracture mechanics, stress intensity range (∆K) parameter is used to define the stress field around the crack tip and acts as the driving force for crack growth. CFCGR acts as the response variable, which is the crack growth per cycle (da/dN). ∆K versus da/dN plot (on a log-log scale) characterizes the cyclic crack growth behaviour of metals. This plot typically consists of three distinct regions as shown in Fig. 4. Region-I is called the long crack threshold region. Threshold stress intensity range (∆KT H) defines the sub-critical fatigue crack propagation resistance of the material in this threshold region, below which insignificant crack growth occurs (typically 1E-7 mm/cycle). Stepped-loading procedure was applied for threshold region measurements. Fur ther, ∆K was corrected to the effective ∆KEFF accounting for crack closure expected in the threshold region using Forth and Neuman model (Forth, Newman and Forman, 2002) in terms of low- value load ratio (R=0.1). Finally, ∆KTH_EFF was estimated at the limiting CFCGR value of 1E-7 mm/cycle. Region-II is the stable linear region, also called Paris region, and exhibits a power- law relationship between ∆K and da/dN. Most of th e fatigue crack growth life of a damage-tolerant structure is consumed in this region. Region-III is the region of unstable fast critical crack growth until final fracture(Anderson, 2005).

Fig. 4. Typical fatigue crack growth behaviour in metals (Anderson, 2005)