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

1340

5

A comparative analysis of the loss of load carrying capacity of specimens with different processing treatments can be carried out by estimating the consumed energy on deformation or fracture. The consumed energy can be represented as energy up to the load peak E * (fracture initiation energy), energy after the load peak (fracture energy) and total energy E full of deformation and fracture of a specimen. The total energy is the sum of the other two. Figure 5a shows the dependence of the energy consumed at deformation and micro-fracture before the force peak on the loading rate. The loading rate was determined by the angle of tangent inclination to the most extended rectilinear section of the elastic region on the force diagram. A linear approximation was constructed for points of quasi-static and dynamic tests. The consumed energy prior to the beginning of the macro-fracture of a specimen is substantially higher for the materials after the ECAP processing than for the material in the delivery state. The material after eight ECAP passes showed a rate sensitivity of more than double the material in the delivery state.

b)

2000

800

a)

8ECAP A

8ECAP

4ECAP DS

A

E full

500 Consumed energy (kJ/m 2 ) 1000 1500

600

400 E* (kJ/m 2 )

E*

200

0

0

-50

0

50

100

0

25

50

75

100

Temperature ( o C)

Force rate (MN/s)

Fig. 5. Consumed energy on deformation and fracture at bending tests of copper specimens after various processing: a) loading rate dependence of energy E * consumed before the load peak; b) dependence of consumed energy on temperature of dynamic tests. Metals with an fcc crystal lattice do not exhibit a state of cold brittleness and brittle fracture. However, the dynamic behaviour of UFG fcc metals at low and high temperatures is of practical interest. Figure 5b shows a comparison of the consumed energy at dynamic deformation of the annealed initial copper specimens and dynamic fracture of copper specimens after eight ECAP passes for different test temperatures. The total energy and energy before the beginning of macro-fracture of the UFG material have pronounced temperature dependence. Their values decrease with increasing temperature. Despite the lower consumed energy before the load peak E *, the total consumed energy E full for the UFG material is higher than for the CG material, which was deformed plastically without crack propagation. The fracture surface of the specimens consisted mainly of dimples. A detailed analysis of the grain structure of UFG pure copper (99.99%) after the impact toughness test was reported by Liang et al. (2017). EBSD mapping revealed that the UFG specimens had micro cracks at the grain boundaries and triple junctions due to limited plasticity and dislocation activity. Recrystallised refined grains in the specimens after two ECAP passes and large grown grains in the specimens after 16 ECAP passes were found along the macro crack. The aim of this work was to obtain data on the response of UFG copper at dynamic loads. The UFG structure was obtained by four or eight ECAP passes. The specimens were subjected to two different tests: split Hopkinson pressure bar compression of cylindrical specimens and three-point bending of a beam with a V-notch The main results are as follows. 1. The UFG copper dynamic yield strength under compression tests is significantly higher than the dynamic yield strength of the CG material. However, the UFG copper sensitivity to the compression loading rate is significantly 4. Conclusions