PSI - Issue 42

4

Gauri Mahalle et al. / Procedia Structural Integrity 42 (2022) 570–577 Mahalle et al./ Structural Integrity Procedia 00 (2019) 000 – 000

573

Fig. 2: Illustrations of the load v/s displacement data (a) at 400 (b)550ºC at two different displacement levels (in mm/min).

At 550 ºC, serrated yielding was observed indicating the presence of Dynamic strain ageing (DSA). the serrated flow curves were accompanied by increased ductility and loss of ultimate strengths. The yield and ultimate tensile strength are evaluated using Eqs. 1-2 with key points measured in SPT are listed in Table 2. It indicates that material properties of T91 are considerably dependent on strain rate and temperature.

Table 2. Average values of key points measured in the SPT

(KN) (KN) (MPa) (MPa)

Displacement rate (mm/min)

Test temperature (ºC)

fracture energy- J f

displacement to fracture d f (mm)

Fracture surface

400

0.003

0.23 0.26 0.18 0.21

1.31 1.33 0.79 1.01

17.25 120.95 2.01 ± 0.18

1.25 1.34 1.67 1.74

Ductile Ductile Ductile Ductile

0.3

19.5 13.5

122.81 1.81 ± 0.20 72.94 1.09 ± 0.21 93.25 1.44 ± 0.20

550

0.003

0.3

15.75

3.2. Constitutive analysis

It is crucial to identify the appropriate constitutive equations that relate flow stress of materials with strain rate at various test temperatures. These equations play an important role in predicting hot flow stress at various strain rates or in the selection of appropriate strain rates for material deformation to attain a particular flow stress level. One of the popular models is the Arrhenius model where flow stress is expressed as a function of Zener-Hollomon parameter: = 1 ln { ( ) 1 + [( ) 2 + 1]} (3) The Zener – Hollomon parameter ( Z ) describes merged effects of temperature and strain rate on the deformation behaviour. It is mathematically expressed as,