PSI - Issue 42

Rafael Magalhães de Melo Freire et al. / Procedia Structural Integrity 42 (2022) 672–679 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

675

4

Table 2 - Pre-strain pattern Mark Sample

Test type

Pre-strain pattern

P1 P2 P3 P4 P5 P6 P7 P8 P9

0% → +1% → -1% 0% → - 1% → +1%

thin

Experimental tests

0% → +1% → - 1% → +2% → - 2% → +3% → -3% 0% → - 1% → +1% → - 2% → +2% → - 3% → +3%

0% → +1% → -1% 0% → - 1% → +1%

side-groove

Experimental tests

0% → +1% → - 1% → +2% → - 2% → +3% → -3% 0% → - 1% → +1% → - 2% → +2% → - 3% → +3%

As rolled

Experimental tests

No pre-strain 0% → + 1% 0% → -1%

P11 P12 P13 P14 P15 P16 P17 P18

thin

Simulation

0% → + 1% → -1% → + 2% → -2% 0% → -1% → + 1% → -2% → + 2%

0% → + 1% 0% → -1%

side-groove

Simulation

0% → + 1% → -1% → + 2% → -2% 0% → -1% → + 1% → -2% → + 2%

3. Results and Discussion Figure 3 (a) shows examples of CTOD test results. They were used with a two-parameter Weibull distribution equation to obtain the cumulative fracture probability versus critical CTOD, shown in Figure 3 (b). It is clear that the pre- strained specimen’s curves were shifted to the left, which means the pre-strained material has lower critical CTOD in comparison to the P9 specimens without pre-strain. In Figure 3 (c), a summary of the MOTE of critical CTOD results for pre-strain cycles is shown in a graphic with the damage degree as abscissa, which can be calculated as the Miner law, explained in Minner (1945). For this calculation, the Manson-Coffin equation, Coffin (1954), Manson (1965), is applied to approximate the number of cycles to failure, and the fatigue ductility index and the material constant were adopted to equal 0.575 and 0.380, respectively, according to the previous study done by Tateishi et al (2004). According to Figure 3 (c), the fracture toughness for the specimens decreases as the damage degree increases. However, this relation is more evident for thin specimens than side-groove specimens (thick plate). In general, the lower critical CTOD was obtained when the last strain was pre-compression (plots with bar).

Figure 3 – a) P vs Vg curve from the CTOD tests; b) Weibull distribution for each pre-strain pattern; c) Mote critical CTOD vs. Damage degree

Since the last pre-strain cycles that were executed by compression load seem to be more harmful to fracture toughness, Weibull stress, which is believed to be a more robust parameter for fracture initiation, was calculated and assessed considering the specimens with these pre-strain patterns. Beremin (1983), Bordet (2005), and Bordet (2005)