Enhanced assessment of technological factors for Ti-6Al-4V and Al-Cu-Mg strength properties

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 “Step 3” contained 2.640 harmonic loading cycles at a frequency of 4 Hz for a total test duration of 660 seconds. holding at “Step 2” is necessary to establish thermodynamic equilibrium in the sample after its loading at “Step 1”. The magnitude of the load amplitude increment for each step σ a step was calculated by the formula: σ a step = σ a max / N cycle , where σ a_ max is the maximum stress amplitude in “Step 3”, N cycle is the number of cycles in “Step 3”. During the tests, the load, axial and transverse deformations, radiation temperature of the working surface of the sample were measured simultaneously. The tests were carried out at room temperature. After completing the test program, the measurement data were analyzed and the extreme strain and temperature values were determined for each extreme stress in the test program. The data obtained made it possible to isolate the components associated with irreversible deformation from the total deformations of the sample, as well as the parts associated with thermoelastic and dissipative heating of the sample from the temperature change, and determine the critical stress of the sample above which the process becomes irreversible. Results and discussion Stress-strain properties of the samples (alloys VT6 and D16) with and without a stress concentrator under cyclic loading For samples made of VT6 titanium alloy as delivered, Fig. 4 shows a comparison of the dependences of the increments of the average temperature values ( ∆ T m ) with the dependences of the stepping strain ( ) formula , (1) p xm  on the stress amplitude ( σ a ) (Fig. 4, a ). Fig. 4, b shows a comparison of the dependences of the increments of the average temperature values ( ∆ T m ) with the amplitudes of irreversible longitudinal strain ( ) formula , (2) p xm  on the stress amplitude ( σ a ). Here: 0 p xm xm xm    = - , (1) 0 p a xa xa d E    = - , (2) where max min ; 2 x x xm    + = max min 2 x x xa    - = – average and amplitude values of total longitudi- nal strain, ε x max , ε x min – extreme values of longitudinal strain, ε xm 0 total longitudinal strain after completion of “Step 2” of the loading program, σ m ; 0 a d xa E   = – secant dynamic modulus of elasticity, is calculated at the beginning of “Step 3”, where inelastic deformations are insigni fi cant. Fig. 4 shows the dependences of the average temperature and the components of plastic strain on the stress amplitude. The numbers “1” and “2” denote the dependencies for samples with a hole and without a hole, respectively. The average stress was set to σ m = 476 MPa and the maximum stress amplitude σ a max = 529 MPa. If the experiments are performed for other average stresses, then it is possible to estimate the in fl uence of the average stress in the loading cycle on the magnitude of the stress amplitude at which dissipative heating begins and the process of accumulation of inelastic strain is activated. The presence of a concentrator in the sample in the form of a hole during periodic deformation with a constant average stress in a cycle decreases the value of the stress amplitude ( σ a ) at which the process of plastic strain of the material is activated (Fig. 4, a ); a nonlinear change in the average plastic axial strain and an increase in the average temperature of heating the sample are observed. The presented diagram makes it possible to estimate the limit of cyclic elasticity of the VT6 material (Fig. 4, a ). Average values of irreversible plastic strain ( ) p xm  for samples with a hole and an increase in