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

1339

4

The offset yield strength (at a strain of 0.2%) for different loading rates is shown in Fig. 3. The initial material (DS) showed the greatest sensitivity of the yield stress to the loading rate. However, the material with the different ECAP treatments demonstrated essentially different stress rate sensitivities of yield strength. The copper after four ECAP passes has a slope coefficient of linear approximation that is four times smaller than the material after eight passes and six times smaller than the initial material. Thus, if we assume that the dependence of yield stress on loading rate is linear in a wide range of loading rates, then at higher load rates, the curves will intersect. Then, the yield stress of the CG material will be higher than the yield strength of the UFG material.

450

 8ECAP 0.2

=363+0.0008 

400

350

 4ECAP 0.2

=349+0.0002 

300

 DS 0.2 =256+0.0012 

250

0 1x10 4 2x10 4 3x10 4 4x10 4 5x10 4 6x10 4 200 Yield strength 0.2% (MPa)

Stress rate (GPa/s)

Fig. 3. Stress rate dependence of offset yield strength for copper with different treatments.

According to the study of microstructural evolution in copper subjected to severe plastic deformation, Mishra et al. (2007), differences in the behaviour of copper with different treatments can be related to the ratio of low angle and large angle grains, and also the density and mobility of dislocations. The initial structure has a large grain size and high density of high angle grain boundaries. After a few ECAP passes, the fraction of low angle grain boundaries increases significantly. For 4 passes, there is a balanced distribution of low angle and high angle grain boundaries. After 8 passes, the material structure has a small grain size with a high density of high angle grain boundaries. 3.2. Bending tests Typical test diagrams are shown in Fig. 4. All specimens for any test conditions were fractured in a ductile manner that follows from the slowly falling tail of the loading diagram. Exceptions are the specimens of the annealed initial material (A), which showed only intense plastic deformation without crack propagation. Therefore, the test results of such samples cannot be considered as impact toughness KCV. In this connection, the test results are further considered relative to the comparison of the ability of the structural states of the material to carry a load under predetermined test conditions.

10

A DS

8

4ECAP 8ECAP RT, 5 m/s, 156 J

6

4

Force (kN)

2

0

0

2

4

6

8

Time (ms)

Fig. 4. Force diagrams in dynamic three-point bending tests of copper specimens with different processing treatments.