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

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 of axial plastic strain remained constant p xm  » 1 %. Whereas for the D16 sample with a weld, the average temperature value decreased during cycling (from ∆ T m = 0.8 °C to ∆ T m = 0.55 °C) at a constant stress amplitude σ a = 167 MPa. In this case, the value of axial plastic strain remained constant – more than p xm  » ≈ 2,2 %. The average values of durability at the studied loading level of D16 alloy are 2.9×10 3 and 15.7×10 3 cycles, respectively, for samples with and without a weld. Thus, a decrease in the characteristics of low- cycle fatigue for samples with a weld is obtained by more than 5 times. This shows the sensitivity of the method to the stress concentrator. It can be assumed that during periodic asymmetric deformation of the sample beyond the elastic limit, two deformation effects are realized in the material. The fi rst effect is accompanied by an increase in the conventional yield stress (hardening) at which the accumulation of irreversible average strain is detected (the step of the plastic hysteresis loop). This effect is similar to material creep. The second effect is accompanied by a simultaneous increase in the amplitude of irreversible strain and an increase in the temperature of dissipative heating, which is accompanied by deterioration in strength properties. These effects require further research. An increase in temperature and accumulation of average plastic strain has the same tendency: these processes begin at lower stress amplitudes for samples with a stress concentrator. The results of experimental studies show the fundamental possibility of describing degradation processes under cyclic loading. The method will also allow assessing the in fl uence of technological in fl uences to which the sample of the material was previously subjected. The method can be used as an alternative to the destructive methods of testing structures for strength and assessing the characteristics of resistance to fatigue fracture of metallic materials. Comparison of experimental data and results of mathematical simulation The results of fi nite element modeling ( FEM ) of cyclic stretching of a strip of VT6 with a hole and a strip without a hole, as well as a strip of D16 with a weld and a strip without a weld are compared. The models were a quarter of a sample with a measurement base of L 0 = 12.5 mm. Symmetry conditions were set on surfaces coinciding with the coordinate planes O xz and Oyz . A load was applied to the upper surface of the samples, the history of which corresponded to that in the experiment. The values of the elasticity and plasticity parameters of the material were assumed to correspond to the VT6 and D16 alloys, respectively. The weld material for the D16 sample was assumed to be the same as for the base material, but with a reduced yield stress and strength equal to 0.85 of σ 0.2 and σ u . The limiting deformation of the weld material was assumed to be equal to 1.15 of the limit for the base material D16 (see Fig.2). The calculated deformed con fi gurations and the values of the intensity of total strain for samples from the alloys under consideration are shown in (Fig. 7). Fig. 8 shows the diagrams of the dependence of the average component of the strain of the sample measurement base in the cycle on the stress amplitude in the cycle of a fl at sample with a hole from VT6 (Fig. 8, a ) and for a fl at sample with a weld after laser welding (Fig. 8, b ). Lines 1 correspond to the data of simulation the cyclic deformation of fl at samples with stress concentrators (a hole for a VT6 samples and a weld for D16 samples), lines 2 denote data for samples of the corresponding dimensions, for each alloy, without stress concentrators. It can be noted that the voltage amplitudes correspond satisfactorily between the experimental values and the results of mathematical simulation. At these stress amplitudes, irreversible accumulation of strain of the sample occurs on the measurements base of 25 mm. Data on sample sizes and stress amplitudes are given in Table.