PSI - Issue 13

Alexey Evstifeev et al. / Procedia Structural Integrity 13 (2018) 886–889 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

888

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micro level. It can be taken into account within the confines of incubation time criterion in the form (Petrov et al. (2015), Petrov et al. (2013)):



− 

( )     , s ds

(1)

st

1



where  is the incubation time, σ st is the tensile strength for quasi-static loading, ( ) t  is the time dependence of the tension stress. In experimental scheme tension stress is linear function of time and deformation. It allows solve equation (1) and find threshold stress for different strain rate.

50 100 150 200 250 300 350 400 450 500 550 600 Ultimate strength, MPa

0,1

1

10

100

1000

Strain rate, 1/s Fig. 2. Tension strength versus strain rate for aluminum alloys: circle – 1230, square – 5556, triangle - 2024. The theoretical line was modelled using the concept of incubation time with material parameters: for 1230 alloy τ = 1.2 µs, σ st = 80 MPa; for 5556 alloy τ = 0.9 µs, σ st = 321 MPa; for 2024 alloy τ = 0.95 µs, σ st = 437 MPa. Experimental results and theoretical lines of ultimate strength for different aluminum alloys are shown on fig.2. Experimental points show a strain rate effect on the dynamic tensile strength. Curves are plotted by formula (1) using the material parameters: for 1230 alloy τ = 1.2 µs, σ st = 80 MPa; for 5556 alloy τ = 0.9 µs, σ st = 321 MPa; for 2024 alloy τ = 0.95 µs, σ st = 437 MPa. Aluminum alloys 5556 and 2024 have a close dynamic strength in term of incubation time parameters, that just a little less than have pure aluminum alloy. This is seen on the curve of the ultimate strength versus the strain rate. Non linear part of curve for alloy 1230 starts earlier than for another alloys. The obtained results also have good correlation with the microstructure investigations. The fracture surface of specimens after tension test for different loading rate was studied on an Axio-Observer Z1-M optical microscope in the dark field. The viscous fracture surface is characterized by a dim grey appearance with characterist ic “fibers”. The brittle fracture surface is crystalline without visible signs of plastic deformation on the fracture surface. The percentage of the viscous fracture component S (shear area) (in %) was determined according to the ASTM E 436 03. The results of measuring the percentage of fibers S on the fracture surface are given in fig.3. As can be seen from the presented data, the destruction of the 2024 and 5556 alloys after tension becomes more brittle than for the initial alloy. In addition, Fig. 3 shows the features of the fracture mechanisms of aluminum alloys at a high strain rate. The nonlinear change in the strength of the materials, observed in Fig. 2, correlates with the mechanism of destruction. In the case of quasi static loading more part of deformation have plastic nature. After increasing the strain rate, the mechanism of fracture changes to a more fragile.