Issue 48

S.C.S.P. Kumar Krovvidi et alii, Frattura ed Integrità Strutturale, 48 (2019) 577-584; DOI: 10.3221/IGF-ESIS.48.56 581 where,   t , N f , E,  f  ,  f  , b , c are total strain range, fatigue life, Young’s modulus, fatigue strength coefficient, fatigue ductility coefficient, fatigue strength exponent and fatigue ductility exponent respectively. The elastic and plastic strain amplitudes were obtained from the stabilized hysteresis loops at respective total strain amplitudes. The values for the coefficients for 316Ti SS determined from least square fit are given in Tab. 3. Hardening characteristics exhibited by the material are reflected in the cyclic stress-strain behavior as shown in Fig. 7. From the locus of the stress-strain maxima of the stable hysteresis loops of different strain amplitudes, cyclic stress-strain curve can be represented by a power law equation as follows:   / 2 2 n p K        (2) where,   ,   p , K  , n  are stress range, plastic strain range, cyclic strain hardening coefficient, cyclic strain hardening exponent, respectively. The value of K  and n  obtained by least square method is given in Tab. 3. Cyclic stress strain curve coefficients Basquin relation coefficients Coffin-Manson relation coefficients K  n   f  b  f  c 1326 MPa 0.259 1052 MPa -0.147 0.13 -0.51 Table 3 : Values of coefficients of strain-life relation for 316Ti SS Figure 6 : Strain- life plots for SS 316Ti at 823 K. Figure 7: The cyclic stress-strain curve of SS 316Ti at 823 K. Fractographic studies conducted on failed fatigue tested samples revealed the crack initiation and propagation as transgranular, Figs. 8(a) and (b). Crack initiation occurred at surface connected slip bands and crack propagation is reflected by fatigue striations on fracture surface, Figs. 8(a) and (b), respectively. Transgranular crack propagation was observed for 1 minute tensile hold test also. Since the LCF damage is mainly due to the accumulation of plastic strain, the hysteresis loop energy approach can provide accurate estimation of fatigue life of the material [20,21]. The relationship between fatigue life and hysteresis loop energy follows linear relationship on log-log plot and can be represented as power law relation   2 b t f W A N   (3) where, t W  is the strain energy density at half life, 2N f is the number of reversals to failure and A and b are material constants. The energy was calculated from the integration of hysteresis loop area at stabilization. The plot is shown in Fig. 9. The value of constants A and b were found to be 454.78 and -0.689 respectively.