PSI - Issue 28

12

Author name / Structural Integrity Procedia 00 (2019) 000–000

B.W. Williams et al. / Procedia Structural Integrity 28 (2020) 1024–1038

1035

Table 5: MMC Damage Model Parameters for TC128B at 100 s -1

� � �� �� � � � 0.150 53.61 0.969 1.0 2.6

Temperature (°C)

A (MPa)

n

m

24

96.70

0.180 0.188 0.196 0.204 0.212 0.220

0.01 0.03 0.05 0.05 0.10 0.10

0

101.68

0.156 0.162 0.168 0.174 0.180

56.64 48.00 37.33 34.56 30.87

0.969 0.969 0.969 0.969 0.969

1.0 1.0 1.0 1.0 1.0

2.3 2.2 2.0 1.8

-20 -40 -60 -80

85.75 66.35 61.13 54.33

1.01

decreasing temperature

Figure 11: Comparison of damage evolution failure curves for TC128B at various temperatures; plotted for normalized Lode angle of unity

5.3. Coupled Ductile-Brittle Fracture Response To capture the sudden load drop associated with cleavage fracture when preceded by ductile fracture, the coupled ductile-brittle model was used. The ductile fracture parameters were not modified from those detailed in the above section. The maximum principle stress at brittle fracture was adjusted in the Charpy simulations until reasonable agreement with the measured load drop was obtained. The predicted versus measured load displacement responses are compared for each temperature in Figure 12. The ductile-brittle load response is well captured by the coupled model. The discrepancy at 0 °C after maximum load is attributed to the MMC damage model parameters at this temperature which would need slight improvement. In all Charpy simulations, the maximum principle stress was normal to the crack plane. It was necessary to use a different value for the brittle fracture stress at each temperature. These values varied linearly as shown in Figure 13, ranging from 1800 MPa at -80 °C to 2600 MPa at 0 °C. Brittle fracture was not observed at 24 °C. A probabilistic distribution can be applied to the brittle fracture stress following the common Weibull form given report by Lei et al. (1998), where  u is the critical failure stress and  mp is the maximum principle stress. At -40 °C with  u = 2400 MPa, the 95% confidence intervals given brittle fracture stresses of 1989 MPa and 2280 MPa when using a large value for the Weibull modulus of m = 40. A high value was required to give a realistic scatter for the sudden load drop in the Charpy � � � � ��� �� � �� � � � � (5)