Issue 42

A. Strafella et alii, Frattura ed Integrità Strutturale, 42 (2017) 352-365; DOI: 10.3221/IGF-ESIS.42.36

15-15Ti, as 560MPa (theoretical 620MPa ≤ Rp0.002 ≤ 840MPa, after 20%CW), all the three phases of creep can be recognized. These curves are plotted in Fig. 5 (until 1000h, current time value of test at 500MPa), providing important results on the creep behaviour because the secondary creep stage was reached and the duration was greater than of 1000h.

2,5

Creep Strain % - [560MPa] Creep Strain % - [300 MPa] Creep Strain % - [400MPa] Creep Strain % - [500 MPa]

2,0

T= 550°C Air

1,5

1,0

Creep Strain [%]

0,5

0,0

0

200 400 600 800 1000 1200 1400 1600

Time [h]

Figure 4 : Comparison of 15-15Ti(Si) creep curves in air at 300, 400, 500 and 560 [MPa].

0,20

Creep Strain % - [300 MPa] Creep Strain % - [400MPa] Creep Strain % - [500 MPa]

0,18

0,16

0,14

0,12

T= 550°C Air

0,10

0,08

0,06 Creep Strain [%]

0,04

0,02

0,00

0

200

400

600

800

1000

time [h]

Figure 5: Comparison between creep curves in air at 300, 400 and 500 MPa, until 1000h. All tests enabled to calculate the creep rate of secondary stage, also defined steady strain creep rate (sscr) being the minimum and constant creep rate.

The sscr values are obtained and compared with two methods, as illustrated in Fig. 6: - Using the first derivative on curve strain-time to determinate the minimum; - Plotting Fit linear of strain-time curve in secondary creep stage.

356