PSI - Issue 14

8

S.C.S.P. Kumar Krovvidi et al. / Procedia Structural Integrity 14 (2019) 855–863 S.C.S.P. Kumar Krovvidi/ Structural Integrity Procedia 00 (2018) 000–000

862

Table 5: Comparison between the life of the bellows by analysis and by EJMA Based on linear elastic analysis Using cyclic stress strain diagram

using isotropic and kinematic hardening model

Life as per EJMA

Cycle life of bellows computed by analysis as per RCC-MR

3500

7800

19200

48000

The life of the bellows computed as based on linear elastic analysis is more conservative compared to that estimated based on elasto-plastic analysis based on cyclic stress stain curve. The detailed inelastic analysis predicts more life. Similar comparison for two other loading conditions was made and presented in Table 6.

Table 6: Life predicted by EJMA and by analysis for various loading conditions Life as per EJMA Based on Linear elastic FEA (N 1 )

Using cyclic stress strain diagram (N 2 )

Based on detailed inelastic analysis (N3)

Case 1: Pressure = 3bar and axial deflection = 20 mm. Case 2: Pressure = 3bar and axial deflection = 25 mm Case 2: Pressure = 3bar and axial deflection = 30 mm

48000

3800

7800

19200

16600

1300

4800

7000

7360

600

1800

3000

The life given by detailed inelastic analysis using LCF constants is considered as the life as per RCC-MR. The comparison of the fatigue life of the bellows estimated as per standards of EJMA and as per RCC-MR (based on the analysis) is given in Figure 10.

Figure 10: Comparison of the fatigue life of the bellows estimated as per EJMA and as per RCC-MR based on analysis.

3. Discussion The elasto-plastic strain range predicted by simple linier elastic analysis (as per the procedure given in RCC MR) is based on Neuber’s rule and the value predicted in this method is higher and hence the cycle life estimated is highly conservative when compared with the life estimated based on the inelastic analysis. Two different methods