PSI - Issue 14

Pankaj Kumar et al. / Procedia Structural Integrity 14 (2019) 96–103 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

101

6

Table 2 presents the experimental results for CR and ACR alloys. Test results depicts that the difference in fatigue life increases with increase in strain amplitude. Highest fatigue life difference is obtained at 0.6% strain amplitude. This signifies that ACR alloys perform well in low cycle fatigue regime.

Table 2. Fatigue life ( N f ) at different strain amplitudes during strain controlled LCF test. Strain amplitude 0.6 ( % ) 0.5 ( % ) 0.4 ( % ) Cryorolled 238 691 1319 Annealed cryorolled 420 963 1772

It has been observed that the tensile peak stress achieved during each cycle elapsed is lower as compared with ACR alloys. Thus, ACR alloys have more capability to sustain for long duration at each of the strain amplitudes. The possible reason is the re-alignment of elongated grain structure and dislocations due to recovery process. Hindrance in dynamic recovery process is decreased due to heat treatment and thus results in equiaxed and dislocation free grain structure due to which ACR alloys exhibit higher fatigue life as compared with CR alloys. This coincides with the reported results by Singh et al. (2014) for 5083 Al alloy.

Fig. 4. Stabilized hysteresis loops at different strain amplitudes for (a) cryorolled; (b) annealed cryorolled AA 5754 alloy

The stabilized hysteresis loops presented in Fig. 4 for both conditions of alloy shows that the tensile peak stress achieved is lower for ACR alloys at each of the corresponding strain amplitudes. However, the compressive peak stresses are higher for ACR alloys. PAHT results in dynamic recovery and growth of dislocation free recrystallized grains and thus absorbs more amount of energy before fracture and thus enhances the fatigue life of ACR alloys.

4.2. Numerical results

The initial cycle of hysteresis loops are numerically modeled by Chaboche kinematic hardening model. The axisymmetric model is displaced by fixed strain amplitude. This strain amplitude is same as that of what was achieved during experiment. Table 3 specifies the material coefficients determined from the experimental stabilized hysteresis loop at 0.6% strain amplitude for CR and ACR alloys.

Table 3. Chaboche kinematic hardening coefficients evaluated at stabilized loop of 0.6% strain amplitude. Parameters σ o ( MPa ) C 1 ( MPa ) C 2 ( MPa ) C 3 ( MPa ) γ 1 γ 2 γ 3 Q ( MPa ) b Cryorolled 215 45500 42500 7200 3000 2100 10 30 6.5 Annealed cryorolled 164 58500 57500 7200 1800 2300 0 105 7.5