PSI - Issue 47

Ezio Cadoni et al. / Procedia Structural Integrity 47 (2023) 630–635

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Author name / Structural Integrity Procedia 00 (2023) 000–000

a)

b)

Fig. 3. Stress versus strain curves of the three gauge lengths with 18.7 kN of preload.

a) b) Fig. 4. Comparison between tensile stress and strain-rate versus time (a) and strain (b) curves having di ff erent gauge length and preload.

a)

b)

c)

Fig. 5. Specimens having di ff erent gauge length; a) 5 mm; b) 10 mm; c) 15 mm.

the preloading level in the pretensioned bar from 18.7 kN to higher values. The specimen with the gauge length of 10 and 15 mm have been tested with 27.9 and 37.1 kN, respectively. Fig. 4a shows how the strain-rate obtained is exactly the same (558 s − 1 ) for the two lengths. In Fig. 4b it can be noted how the stress versus strain curves are identical up to the necking phenomenon. The photos of broken specimens are shown in Fig. 5. In specimens with 5 mm and 10 mm gauge lengths, the neck develops in the middle, whereas in specimens with longer gauge lengths, the neck develops near the end at 20-30% of the gauge length as found by Osovski et al. (2013). The specimen profile has been measured before and after the test. Fig. 6 shows the case of 5 and 15 mm gauge length. Thus, the plastic strain is derived from the natural logarithm of the ratio between the initial and final sample cross-sections (see Fig. 7). The true failure plastic strain obtained for specimen having same plastic strain rate (e.g. 18, 27.9 and 37.1 kN, for 5, 10, and 15 mm, respectively) is 1.19 as well as is constant the dimension of the necking zone (see Figs. 5 and 6 ). The true failure stress, obtained by using the Bridgman correction, is 975 MPa.

4. Numerical simulation

A numerical simulation of dynamic tensile tests was conducted using the commercial finite element software Ls Dyna / Explicit to analyse the distribution and evolution of plastic strain within specimens of 5, 10, and 15 mm gauge length, as well as the dynamics of neck formation. To model the SHTB test structure, eight-node hexahedral solid elements were used, including the input bar, specimen, and output bar. Input bar interfaces were loaded with experi mentally recorded stress waves, improving simulation realism and reducing calculation times. Input / output gauge sig-