PSI - Issue 13

Ivan Smirnov et al. / Procedia Structural Integrity 13 (2018) 1336–1341 Ivan Smirnov et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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2.2. Compression tests Compression tests were carried out on cylindrical specimens with a diameter of 8 mm and a length of 4 mm. The specimens were cut by an electro erosion machine, so that the axis of the cylinder and the rod coincided. Quasi-static tests were performed using a standard test machine with a compression plate speed of 1 mm/min (strain rate of 0.004 1/s). Specimen deformation was measured by a traverse displacement. Dynamic compression tests were performed on a setup with a split-Hopkinson pressure bar by the Kolsky method (Kolsky, 1963). A set of steel measuring bars with a diameter of 20 mm was used. The system was loaded by a steel hammer with a length of 300 mm and without the use of a pulse shaper. The tests were performed for two strain rates of ~1200 and 3800 1/s. 2.3. Bending tests Bending tests were carried out on beam-shaped specimens with dimensions of 55×9.2×9.2 mm 3 for the UFG states and 55×8.5×8.5 mm 3 for the CG states. The specimens of UFG copper states had a square cross-section after ECAP processing. The square shape of the cross-section of the CG specimens was obtained by grinding, but the cross-section had rounded corners. Beams had a V-shaped notch of 2 mm with a tip curvature radius of 0.25 mm. Quasi-static tests on three point bending were performed using a standard test machine with a load point speed of 1 mm/min. Specimen deformation was measured by a DIC method. The dynamic bending tests were carried out by the Charpy method according to ASTM E23 using a drop weight impact test machine (Instron 9350). The impact velocity and the impact energy were 5 m/s and ~150 J, respectively. Additional experiments were performed for the test temperature of – 50 and 100 ° С, as well as for the impact velocity and the impact energy 10 m/s and ~256 J, respectively. The samples were held before testing at a reduced temperature for 30 min and at an elevated temperature for 60 min. Load diagrams were determined by using a strain gage imbedded in the impactor. Deflection curves were calculated by integration of the force time curve according to Newton's second law.

3. Results and discussion 3.1. Compression tests

Figure 2 shows stress and strain diagrams at the dynamic compression of copper samples with different treatments. The material before and after ECAP treatment demonstrates the same elastic response to the impact load (inclination of the tangent to the elastic loading region). The materials showed almost the same degree of deformation for each of the impact speeds. Nevertheless, there is a significant difference in yield strengths. The four-pass and eight-pass specimens showed a considerable jump in yield strength over the initial specimen.

a)

b)

750

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

DS

4 ECAP

8ECAP

DS

4 ECAP

8ECAP

500

250

True strain

~1200 1/s

True stress (MPa)

~3800 1/s

0

0

40

80

120

160

0

40

80

120

160

Time (  s)

Time (  s)

Fig. 2. Dynamic compression diagrams of copper: a) true stress; b) true strain.