PSI - Issue 38

638 Sara Eliasson et al. / Procedia Structural Integrity 38 (2022) 631–639 Author name / Structural Integrity Procedia 00 (2021) 000 – 000 The behaviour of the specimens could be sorted in two distinct regions, a clear increase in strain and a region of almost no increase in strain until final failure. The two regions are separated by a breaking point, ( 3 , 3 ), illustrated in Fig. 9. The breaking point was determined with a piecewise linear function, Eq. 1, and was extracted from the stiffness degradation data (Fig. 8) and average strain data (Fig. 10). The breaking point occurred between 14 149 cycles all the way up to 419 548 cycles (Table 3). 8

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Table 3. Breaking points (x3, y3) for the piecewise linear fit of stiffness degradation (*, Fig. 8) and average strain in y-direction (**, Fig. 10).

Specimen AT0_3 AT0_4 AT0_5 AT0_6 AT0_7 AT0_10

x 3

y 3 (*)

y 3 (**) 0.01043 0.00948 0.01101 0.00795 0.01007 0.01066

Cycles 785 750 500 370 412 530

330 321 19 499 14 149 419 548 148 967 31 332

0.9533 0.9895 0.98546 0.94891 0.94677 0.84529

> 5 000 000

1 386 450

78 370

5. Discussion The scatter of the tension-tension fatigue tests makes it challenging to fit a Wöhler curve to the S-N data. The scatter for different load levels is significant and at the load level of 90 % of UTS there is a specimen failing after two million cycles and a specimen failing before 1 000 cycles. The spread of the data at this high load fatigue regime points towards the issues of transferring the load to these anisotropic materials with high specific strength and stiffness. The fatigue limit seems not to be far off from the 80 % of UTS load level, where we see several runout specimens. Looking at the stiffness degradation, the first two stages from Fig. 1b are captured, but not the final stage with the rapid stiffness reduction before final failure. The last stage of the stiffness degradation might occur so rapidly that the frequency of the imaging is not high enough to capture the event. The limiting factors are the amount of data produced if the HSC is to be triggered more frequently and when the failure occurs, since there is a large scatter. Focusing on the stiffness degradation, the piecewise linear fit (Table 3) indicates that the first region (Fig. 9) of higher stiffness degradation is ongoing until 14 149 – 419 548 cycles. The highest value of 419 548 cycles corresponds to the runout specimen, AT0_6. All four specimens in Fig. 8 have different failure modes. The specimens AT0_4 and AT0_7 have lateral failures inside the tab (Fig. 10a and c), and they have an incipient leveling out of the stiffness degradation, meaning that the fatigue life of these specimens could have been underestimated. Looking at the strain measurements (Fig. 10) for the three specimens, AT0_4, AT0_7, and AT0_10 the first region could be split in two or three substages. The first substage is where there is an initial strain increase and then one jump (or two jumps) to a continued strain increase, before leveling out in region two. These jumps could be indications of defects in the specimens inducing considerable damage before the system reached a more stable state. The flaws could have arrested additional damage growth or led to an early failure. These substages are evident for the specimen AT0_10, and the strain jumps are large and resulting in a very high stiffness degradation. This high stiffness degradation could explain the early failure. The specimen, AT0_6, does not show these substages,