PSI - Issue 68

Amy Milne et al. / Procedia Structural Integrity 68 (2025) 666–673 Milne et. al. / Structural Integrity Procedia 00 (2025) 000–000

669

4

Figure 1: Build plate showing C(T) samples with inset (right) illustrating X, Y, Z sample orientations

The C(T) samples nominal dimensions were width, W = 25 mm, thickness, B = 12.5 mm. The samples were fatigue pre-cracked from a chevron notch to generate a sharp crack front, Davies (2023) at an R ratio of 0.1 and a ΔK of approximately 12.5 MPa√m. An optical microscope was then used to determine the exact size of the pre-crack and record the a/W parameter which was in the range of a/W = 0.45 – 0.48. The samples were tested in the as-built condition, i.e. with no post-process heat treatment. 3.2. CCG Test Procedure Since there was no prior CCG data on this material, initially the loads applied to the samples were estimated using the reference stress of the samples and uniaxial creep rupture data, Williams et al. (2020a). These estimates led to a range of test durations, with the average test lasting approximately 2 months. All tests were performed at 650 °C and the temperature across the sample was monitored using three K type thermocouples. The temperature remained at 650 ± 2 °C for the test duration across all three thermocouples. During the tests, the load line displacement (LLD) was recorded using linear variable differential transducers (LVDTs) external to the furnace via extensometery. The direct current potential drop technique (DCPD) was also employed to monitor crack growth. Where possible, tests were stopped when crack growth acceleration (displayed as a sharp increase in the gradient of the LVDT traces) was noticed. However, the fast crack growth rate in some test samples means that the tests could not be interrupted before full failure. Post test, some samples were sectioned in half using electro-discharge machining (EDM) to analyse the crack path and observe the microstructure. One half of the sample was ground, polished and etched and examined under an optical microscope, while the other half was fatigued open to analyse the fracture surface and quantify the creep crack growth area and hence average crack length through the samples thickness. These physical crack length measurements were then used to calibrate the PD measurements ASTM (2023). Table 1 shows a list of the CCG tests completed with details of the sample orientation, dimensions, applied K and test duration.

Table 1: Table showing CCG tests completed

Test ID

Orientation Horizontal Horizontal Horizontal

K (MPa√m)

Test Duration (h)

B (mm)

B net (mm)

W (mm)

1AY 2MY 2IY 1NX 1PX 2LZ 1LZ 1OZ

20.7 12.0 12.0 17.2 15.2

33

12.3 12.3 12.2 12.3 12.5 12.5 12.2 12.3

9.8 9.9 9.7 9.9

31.2 25.0 25.0 25.0 25.1 25.0 24.8 24.6

983 373

Vertical Vertical

48 88

10.0 10.0

Horizontal Horizontal Horizontal

9.2 8.2

4513 4700

9.8 9.8

11.9

59