PSI - Issue 13

Tsubasa Kumamoto et al. / Procedia Structural Integrity 13 (2018) 710–715 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

712

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Table 1. Chemical composition of the steel (wt.%).

C

Si

Mn

P

S

Al

N

Fe

0.108

0.498

1.55

0.009

0.0023

0.032

0.0004

Bal.

Fig. 1. A set of microstructure and strain distributions used for the damage quantification. (a) Optical micrograph of the intercritically annealed and quenched specimen before tensile testing. (b) DIC strain map just before the fracture with an initial strain rate of 10 − 2 s − 1 without hydrogen charging. (c) Local strain profile of (b).

3. Results and discussion 3.1. Tensile test

The results of the tensile tests are listed in Table 2. The 0.2 proof stress, tensile strength, and uniform elongation did not change significantly under any test conditions, while the elongation to fracture deteriorated under the test condition with an initial strain rate of 10 − 4 s − 1 with hydrogen pre-charging, as grey-highlighted in Table 2. Specifically, lowering the strain rate with hydrogen pre-charging does not affect the uniform elongation but reduces the local elongation. Hence, we conducted a more detailed inspection focused on the damage evolution in the necking regime of the fractured specimens. Table 2. Tensile properties with and without hydrogen pre-charging. Strain rate [s − 1 ] 0.2% proof stress  0.2 [MPa] Tensile strength  B [MPa] Uniform elongation  U. El [%] Total elongation  El [%] Relative local elongation,  RLE *[%] Without H 10 − 4 637 960 11.1 16.8 - 10 − 2 666 992 11.7 19.5 - With H 10 − 4 687 985 9.37 13.7 75.8 10 − 2 690 989 10.1 17.3 92.3 The damage evolution behaviors with and without hydrogen pre-charging were investigated quantitatively using the SEM-based technique. Fig. 2 shows the evolution of (a) damage number density, (b) damage area fraction, and (c) average damage size as a function of corresponding local strain. As can be seen in the test condition at an initial strain rate of 10 − 4 s − 1 without hydrogen pre-charging, the number of damage incidents increased with increasing the local strain (Fig. 2a). In contrast, the average damage size was roughly constant at about 0.5 µm 2 in the damage arrest regime (Fig. 2c). In particular, all the damage that occurred were arrested once to grow in this regime. Finally, damage growth occurs at a local strain of 67% (Fig. 2c). These three regimes of damage incubation, damage arrest, and damage growths are also observed under other test conditions in the present study. *  REL =  Local elongation, with H /  Local elongation, without H 3.2. Quantitative analysis of damage evolution