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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3. Experimental Technique 3.1. Specimens, Material and Experimental Equipment

Based on the results of numerical simulation, a batch of specimens with different lengths L = 1, 2 and 4 mm, as well as with different radii of chamfers R = 0.25 and 0.5 mm were prepared. Several specimens had side grooves. The specimens are made of the following steels: structural steel 20, Hardox 400 high strength steel, and ARMOX 500T armored steel. The specimens are cut in two directions: longitudinal and transverse. The tests are performed employing the instrumented drop-weight impact testing machine with a high-speed registration system. From the test results, the loading diagrams are in the force-time coordinates P (t). All the tests are made at a loading velocity of 5.2 m/s. The mass of the loading part is m = 23.5 kg. The first tests were performed on the specimens of steel 20. Figure 4 illustrates the loading diagrams in coordinates force-time F(t) (Fig. 4(a)) and shear stress - shear strain τ(γ) (Fig. 4 (b)) for the specimens of steel 20 with different lengths of the shear zone L = 1, 2 and 4 mm. The strain rates for such specimens at the same loading rate V = 5.2 m/s are 5200, 2600, and 1300 s -1 , respectively. It implies that the character of fracture is the same for all specimens. The process of deformation and fracture can be divided into three stages. The first stage is elastic, where the force increases sharply. An increase in the deformation rate leads to an increase in the level of forces and stresses. After the onset of yielding, the material demonstrates an obvious work-hardening effect for all the strain rates, and the stress increases to the maximum values. This is the stage of uniform plastic deformation of the material. At the third stage, when the maximum values of the forces and stresses are reached, a crack forms at the shear zone and the specimen undergoes fracture within these zones resulting in an abrupt drop of the force-time curve. Thus, the developed experimental-computational technique allows one to evaluate the effect of the strain rate within the range of 1300 - 5200 s -1 on the resistance of materials deformation under dynamic shear conditions. 3.2. Experimental Results and Discussion

Fig. 4. Loading diagrams in coordinates force - time F(t) (a) and shear stress - shear strain τ(γ) (b ) for the specimens of steel 20 with different lengths of the shear zone L = 1, 2 and 4 mm. The effect of the strain rate is also seen in Fig. 5(a), where the yield stress and the fracture stress are given as functions of the strain rate. The yield stress increases with an increase in the strain rate. The fracture stress is generally larger than the yield stress for each strain rate, however, they can decrease at certain values of strain rate. Fig. 5(b) shows the fracture initiation plastic strains plotted against the shear strain rate. An increase in the strain rate from 1300 s -1 to 2600 s -1 results in the fracture initiation strain increase. However, the fracture initiation strain shows a slight decline with the increase of the strain rate from 2600 s -1 to 5200 s -1 . It implies that the fracture processes may be controlled by die rent mechanisms under quasi-static and dynamic conditions.