PSI - Issue 3

A. Strafella et al. / Procedia Structural Integrity 3 (2017) 484–497 A. Strafella, A. Coglitore, E. Salernitano / Structural Integrity Procedia 00 (2017) 000–000

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5

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

Fig. 3. Comparison of 15-15Ti(Si) creep curves in air at 300, 400, 560 MPa. 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 T= 550°C Air Creep Strain [%] Time [h]

The creep strain at the same time value increased with the applied stress and the shape of creep curve is consistent with the expectations: at 300 and 400 MPa, after 1200 h, the strain curves didn’t reach the tertiary creep stage, still remaining in the secondary creep phase. At stress values near the yield strength of 15-15Ti, as 560MPa (theoretical 620MPa ≤ R p0.002 ≤ 840MPa , after 20%CW), all the three phases of creep can be recognized. During the test at 560MPa, the rupture was reached at 1165h, unlike the tests at 300 and 400MPa still in progress, whose curves are plotted in Figure 4. These tests provide important results on the creep behaviour because the secondary creep stage was reached and the duration was greater than of 1200h.

0,10

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

T= 550°C Air

0,08

0,06

0,04

Creep Strain [%]

0,02

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 0,00

Tme [h]

Fig. 4. Comparison between creep curves in air at 300 and 400 MPa. 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 Figure 5:

• Plotting Fit linear of strain-time curve in secondary creep stage • Using the first derivative on curve strain-time to determinate the minimum