PSI - Issue 14

A. Poonguzhali et al. / Procedia Structural Integrity 14 (2019) 705–711 Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 000–000 in the form of Goodman/Basquin and data presented in fig 2a. A semi log plot of the mean stress versus the number of fatigue cycles resulted in a linear relationship over a range of stress amplitudes. Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 00 –000 in the form of Goodman/Basquin and data presented in fig 2a. A semi log plot of the mean stress versus the number of fatigue cycles resulted in a linear relationship over a range of stress amplitudes. Poonguzhali et al. / Structural Integrity Procedia 00 (2018) 000–000 in the form of Goodman/Basquin and data presented in fig 2a. se i log plot of the mean stress versus the number of fatigue cycles resulted in a linear relationship over a range of stress a plitudes. ' f  : fatigue strength coefficient, N f : Number of cycles to failure and b : fatigue ' f  : fatigue strength coefficient, N f : Number of cycles to failure and b : fatigue ' f  : fatigue strength coefficient, f : u ber of cycles to failure and b : fatigue strength exponent. Fatigue life increases linearly with an increase in cold work (CW) at high mean stress regimes as compared to lower mean stress regimes. Cold work increases the yield strength of material thereby fatigue initiation life. The linear part of the S-N curve was fitted to the Basquin type relation and the fatigue strength coefficient and exponents were evaluated to study the variation in the damage mechanisms. The fatigue life is estimated using the above relations in the stage I regime where the applied mean stress is linearly related to the number of cycles to failure as seen in fig.2b. The role of environment on the fatigue behavior is pronounced at lower mean stress σ mean = 375 MPa. At lower stress levels, the environmental effect and material interaction time are relatively higher. Hence, the crack initiation for 20% CW 316LN SS found to be higher. Since CF process involves the accumulation of damage through different stages, namely passive film breakdown, pit formation, pit-to-crack transition and crack growth is given as N N N N ptc N pit cf pf cfcg     (2) where � �� is the corrosion fatigue life, � p� , number of cycles to passive film break down, � �it , the number of cycles for pit formation, � ��� , the number of cycles to pit-to-crack transition � ��cg , the number of cycles for corrosion fatigue crack growth a b strength exponent. Fatigue life increases linearly with an increase in cold work (CW) at high mean stress regimes as compared to lower mean stress regimes. Cold work increases the yield strength of material thereby fatigue initiation life. The linear part of the S-N curve was fitted to the Basquin type relation and the fatigue strength coefficient and exponents were evaluated to study the variation in the damage mechanisms. The fatigue life is estimated using the above relations in the stage I regime where the applied mean stress is linearly related to the number of cycles to failure as seen in fig.2b. The role of environment on the fatigue behavior is pronounced at lower mean stress σ mean = 375 MPa. At lower stress levels, the environmental effect and material interaction time are relatively higher. Hence, the crack initiation for 20% CW 316LN SS found to be higher. Since CF process involves the accumulation of damage through different stages, namely passive film breakdown, pit formation, pit-to-crack transition and crack growth is given as N N N N ptc N pit cf pf cfcg     (2) where � �� is the corrosion fatigue life, � p� , number of cycles to passive film break down, � �it , the number of cycles for pit formation, � ��� , the number of cycles to pit-to-crack transition � ��cg , the number of cycles for corrosion fatigue crack growth a b strength exponent. Fatigue life increases linearly ith an increase in cold ork ( ) at high ean stress regi es as co pared to lo er ean stress regi es. old ork increases the yield strength of aterial thereby fatigue initiation life. The linear part of the S-N curve was fitted to the asquin type relation and the fatigue strength coefficient and exponents were evaluated to study the variation in the da age echanis s. he fatigue life is estimated using the above relations in the stage I regi e here the applied ean stress is linearly related to the number of cycles to failure as seen in fig.2b. The role of environment on the fatigue behavior is pronounced at lower ean stress σ mean = 375 Pa. t lo er stress levels, the environ ental effect and material interaction ti e are relatively higher. ence, the crack initiation for 20 316 SS found to be higher. Since F process involves the accu ulation of da age through different stages, na ely passive fil breakdo n, pit for ation, pit-to-crack transition and crack growth is given as N ptc N pit cf pf cfcg     (2) where �� is the corrosion fatigue life, p� , nu ber of cycles to passive film break do n, �it , the nu ber of cycles for pit for ation, ��� , the number of cycles to pit-to-crack transition � ��cg , the number of cycles for corrosion fatigue crack gro th a b   ' b f N f 2   ' b f N f 2   ' b f N f 2             2 2 (1) (1) (1) where,  : Stress range, where,  : Stress range, where,  : Stress range,

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Fig. 2. (a) S-N curve as a function of CW; (b) Fatigue strength coefficient vs. fatigue exponent for (σ mean  375 MPa) The formation of stable chromium-rich passive film help protect the substrate whenever crack opens up and the corrosion deposits formed near the microscopic crack delays the CF crack growth and thereby improves the fatigue life at lower stress levels. Open circuit potential (OCP) is monitored throughout and till the failure of the specimen. Fig.3 shows the variation of OCP of the metal-environment system with respect to time for 20% CW SS at a σ mean = 475 MPa. OCP fluctuated initially in the negative direction from -390 mV to – 404 mV during the start of the test and subsequently remained constant till 75% of the total life indicating rupture of the passive film and generation of stable pits from the metastable pit. Pit to crack transition is observed by a significant jump in OCP to a value of -250 mV at 75% N f till the end of the test with marginal fluctuations. Laser Raman spectroscopic analysis was carried out on the fractured surface after fatigue test at σ mean = 375 MPa to analyze the corrosion products responsible for the failure. Raman spectra of cold worked specimens as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as maghemite (γ-Fe 2 O 3 ) and goethite (α-Fe 2 O 3 ) were prominent for all steels (Oblonsky et al., 1995). Formation of Cr(III) oxides/oxyhydroxides inhibits anodic dissolution reaction by controlling oxygen reduction and electron transfer reactions and the presence of MoO 4 2- inhibits pitting corrosion of SS were observed in the LRS spectra (Walter J. Tobler, 2004). Fig. 2. (a) S-N curve as a function of CW; (b) Fatigue strength coefficient vs. fatigue exponent for (σ mean  375 MPa) The formation of stable chromium-rich passive film help protect the substrate whenever crack opens up and the corrosion deposits formed near the microscopic crack delays the CF crack growth and thereby improves the fatigue life at lower stress levels. Open circuit potential (OCP) is monitored throughout and till the failure of the specimen. Fig.3 shows the variation of OCP of the metal-environment system with respect to time for 20% CW SS at a σ mean = 475 MPa. OCP fluctuated initially in the negative direction from -390 mV to – 404 mV during the start of the test and subsequently remained constant till 75% of the total life indicating rupture of the passive film and generation of stable pits from the metastable pit. Pit to crack transition is observed by a significant jump in OCP to a value of -250 mV at 75% N f till the end of the test with marginal fluctuations. Laser Raman spectroscopic analysis was carried out on the fractured surface after fatigue test at σ mean = 375 MPa to analyze the corrosion products responsible for the failure. Raman spectra of cold worked specimens as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as maghemite (γ-Fe 2 O 3 ) and goethite (α-Fe 2 O 3 ) were prominent for all steels (Oblonsky et al., 1995). Formation of Cr(III) oxides/oxyhydroxides inhibits anodic dissolution reaction by controlling oxygen reduction and electron transfer reactions and the presence of MoO 4 2- inhibits pitting corrosion of SS were observed in the LRS spectra (Walter J. Tobler, 2004). Fig. 2. (a) S-N curve as a function of CW; (b) Fatigue strength coefficient vs. fatigue exponent for (σ mean  375 Pa) he for ation of stable chromium-rich passive film help protect the substrate whenever crack opens up and the corrosion deposits for ed near the icroscopic crack delays the F crack gro th and thereby i proves the fatigue life at lo er stress levels. pen circuit potential ( P) is onitored throughout and till the failure of the specimen. Fig.3 sho s the variation of P of the etal-environ ent syste ith respect to ti e for 20 CW SS at a σ mean = 475 MPa. OCP fluctuated initially in the negative direction from -390 mV to – 404 during the start of the test and subsequently remained constant till 75% of the total life indicating rupture of the passive film and generation of stable pits from the metastable pit. Pit to crack transition is observed by a significant jump in OCP to a value of -250 at 75 f till the end of the test with marginal fluctuations. aser a an spectroscopic analysis as carried out on the fractured surface after fatigue test at σ mean = 375 Pa to analyze the corrosion products responsible for the failure. a an spectra of cold orked speci ens as seen in fig. 4 showed mainly iron oxides and mixed chromium (III) oxides. Major phases of iron oxides such as maghemite (γ-Fe 2 O 3 ) and goethite (α-Fe 2 O 3 ) were prominent for all steels (Oblonsky et al., 1995). For ation of Cr(III) oxides/oxyhydroxides inhibits anodic dissolution reaction by controlling oxygen reduction and electron transfer reactions and the presence of MoO 4 2- inhibits pitting corrosion of SS ere observed in the S spectra ( alter J. obler, 2004).