PSI - Issue 36

Valeriy Kharchenko et al. / Procedia Structural Integrity 36 (2022) 145–152 Valeriy Kharchenko, Eugene Kondryakov, Andriy Kravchuk et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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Under quasi-static conditions, the material in the shear zone is deformed uniformly until a fracture occurs due to voids coalescence. In this case, the nucleation and uniform growth of voids occur throughout the entire shear zone. The surrounding voids coalesce during growth that forms a rough fracture surface within the shear zone. With an increase of the strain rate, plastic deformation of the material occurs easier due to the adiabatic rise of temperature. When thermal softening of the material exceeds the hardening effect of strain and strain rate, the fracture will begin in the weakened areas where microcracks or microvoids exist. Thus, inhomogeneous plastic deformation occurs in a narrow region of the shear zone, where voids of a larger size are formed than under quasi-static conditions. Therefore, due to the coalescence of voids in this narrow deformation region, a relatively flat fracture surface is formed. At the same time, at such strain rates for specimens from steel 20, no signs of material melting were found in the fracture surfaces. On the macroscopic fracture surfaces of specimens from steel Armox 500T as compared with steel 20, there are signs of metal melting due to adiabatic rise of temperature (Fig. 9(f)). It implies the significantly lower values of shear strains at the time of fracture initiation for steel Armox 500T. In the specimens of steel 20 at strain rate 2600 s -1 (Fig. 9(b)), both zones of inhomogeneous fracture with the formation of “shear lips” and a homogeneous zone with microvoids are observed. This fact indicates a mixed type of fracture and transition from quasi-static to dynamic loading conditions. Macroscopic fracture surfaces of specimens from steel Hardox 450 (Fig. 9(e)) show the zones of dimples and microvoids, but the fracture surface predominantly has a homogeneous structure.

Fig. 9. Macroscopic fracture surfaces of the specimens of steel 20 for L = 4 mm (a), L = 2 mm (b), L = 1 mm (c – transverse direction, d – longitudinal direction), Hardox 450 (e) and ARMOX 500T (f) steels. The results demonstrate that the selected loading scheme allows one to practically exclude wave effects that arise under dynamic loads (Fig. 10). Figure 10 illustrates the influence of the strain rate effect on the calculation results. An increase in the values of the C parameter of the Johnson-Cook material model leads to an increase in the level of shear stresses. There is a good agreement between the experimental and calculation results at C = 0.01.

Fig. 10. Comparison of numerical simulation with experimental data (various values of parameter C of the Johnson-Cook material model)