PSI - Issue 13

Hugo Wärner et al. / Procedia Structural Integrity 13 (2018) 843–848 Hugo Wärner et al./ Structural Integrity Procedia 00 (2018) 000 – 000

847

5

Table 1. Parameters for the cyclic stress-strain and Coffin-Mason curves.

c

K’ (MPa)

n’

ε' f

Test

TMF, t d = 0 s, Petráš et al. (2016) TMF, t d = 300 s ,Wärner et al. (2017) TMF Aged, t d = 300 s, Wärner et al. (2017) CF, t d = 600 s, Wärner et al. (2018) CF, ε a = 0.25 %, Wärner et al. (2018)

0.625 0.205 0.014 0.022 0.353 0.259 0.057

-0.953 -0.696 -0.309 -0.422 -0.799 -0.705 -0.481

840.8 541.8 1193 802.1 196.2 3734 14050

0.117 0.108 0.247 0.151 -0.065 0.3824 0.6241

CF Aged 650 °C, ε a = 0.25 % CF Aged 700 °C, ε a = 0.25 %

2.2. Fracture and damage behavior From the work of Petráš et al. (201 6) it is evident that, for that TMF condition, oxidation assisted surface initiation cracking and propagation along grain boundaries due to oxides formation during the tensile part and then cracking during low temperature compressive part or at high temperature tensile loading part, is the main fracture and damage behavior. Fig. 5 and Fig. 7 show similar fracture and damage mode for TMF with 5-minute dwell time and increased temperature range. Longer time in tension from the added dwell time and therefore a more continuous formation of oxides explains the lower fatigue life. From Fig. 7b it can also be observed that not only intergranular cracking orthogonal to the loading direction occurs during fracture for this testing condition, also transgranular cracking occurs. From Fig 6a it is seen that voids due to creep damage during relaxation form in grain boundaries and propagate during loading. This should lower the overall fatigue life.

Fig. 5. Oxidation at surface at grain boundaries and cracks; (a) TMF t d =0 s, ε a =0.4 %, ΔT=100 -800 °C virgin material; (b) CF t d =600 s, ε a =0.5 % (Horizontal loading direction for both micrographs).

Fig. 6. Damage by void formation at grain boundaries; (a) TMF t d =0 s, ε a =0.3 %, ΔT=100 -800 °C virgin material; CF t d =300 s, ε a =0.25 % (Horizontal loading direction for both micrographs).

The CF testing condition images, Fig. 5b, Fig. 6b and Fig 7c,d show similar damage and fracture initiation and propagation in the TMF condition. The main differences in the micrographs is that cracks propagate by alternating between intergranular and transgranular crack propagation, this can be seen in Fig 7c and 7d. The relationship between initiation and propagation mode compared with the amount of fatigue and creep condition has been discussed elsewhere, see Wärner et al. (2018), but further investigation and analysis are needed to fully understand this mechanism. The difference in fatigue life between the pre-aging of TMF and the CF testing conditions relates to a more creep damage governed cycling in the CF test condition that should be impeded by precipitation from aging. For the more extensive fatigue and temperature range governed TMF test condition, the precipitation at grain boundaries make it susceptible to brittle intergranular cracking. Although also in this domain, more analyses and investigations are needed to establish a full understanding of difference in fatigue lives.