PSI - Issue 14

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

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nitrogen prior to rolling operation doesn’t affect the strength of alloy. Thus, initial immersion time of 15 minutes is chosen and for subsequent passes the sample is dipped for 10 minute to achieve the liquid nitrogen temperature.

Fig. 1. (a) Cryorolling steup; (b) LCF specimen

The 5% thickness reduction is specified in each passes to avoid side cracking in the samples. The plate thickness is reduced from 23 mm to 12.5 mm. It has been reported by Singh et al. (2013) that post heat treatment significantly affects the material properties of cryorolled alloys. Thus, effect of post heat treatment on cryorolled alloys are also studied in the present work. Some of the cryorolled samples are subsequently annealed for 2 hours at 250 o C and cooled in furnace itself.

2.2. Strain controlled low cycle fatigue (LCF) test

The low cycle fatigue (LCF) test is performed at three different strain amplitudes from 0.3% to 0.6%. Specimens for strain controlled LCF tests are prepared according to ASTM E606 standard as shown in Fig. 1(b). Mirror finished polished specimens are prepared through proper grinding and polishing operations. The tests are conducted on a fully automated, closed loop, servo-hydraulic dynamic test machine (Model Instron 8802). The fatigue test is executed under fully reversed or symmetric tension compression loading (R E =ε min /ε max =-1). The strain rate of 0.5×10 -3 sec -1 is kept constant for all fatigue tests. All tests were performed under triangular waveform having zero mean strain. The corresponding testing frequency is evaluated from the relation governed by Borrego et al. (2004)

d

dt

  

f



(1)

2



where f is applied test frequency, d ɛ /dt is the strain rate and Δ ɛ⁄ 2 is the strain amplitude. Cyclic stress – strain curve or hysteresis loop is obtained at each cycle. Majority of materials exhibit cyclic hardening/softening or both when subjected to cyclic plastic deformation. The fatigue life is obtained at all strain amplitudes for CR and ACR alloys at same test parameters.

3. Numerical procedure

3.1. Chaboche kinematic hardening model

Finite element simulations are performed to characterize the LCF behaviour of CR and ACR alloys by using Abaqus 6.14 commercial software package. The cyclic plasticity model ―Chaboche kinematic‖ is used to simulate the hysteresis loop obtained from the strain controlled LCF test of the material. Axisymmetric finite element model is created to reduce the computational cost and time of the simulation. Chaboche kinematic hardening model is