PSI - Issue 79

Oleg Plekhov et al. / Procedia Structural Integrity 79 (2026) 168–175

171

3. Stress wave and residual stress measurements The geometry of the samples made from Ti-6Al-4V utilized for pressure impulse measurement is shown in figure 3a. To investigate the shock wave decay a range of specimen thicknesses was employed in the experiments 0.8, 1.0, and 1.4 mm. The fatigue test specimens (Fig. 3 b,c) are manufactured on a CNC lathe and subsequently polished to minimize the effect of surface roughness during the experiments. No heat treatment is applied to the material before testing. After polishing, the part of the specimens is surface treated by the LSP method, and the rest part is the reference group. Using the PDV device, the velocity of the free surface of the specimens is measured when it is exposed to a laser pulse. The shape of the laser beam spot on the specimen surface is a circle with a diameter of 2 mm. The pulse duration is 10 ns. The measurements are carried out with the values of energies of 2, 3, 4, and 6 J, which corresponded to the energy power densities of 7, 10, 13, and 19 GW/cm². The laser i s focused at the centre of the front surface of the specimen. The PDV measures the velocity in the centre of the back surface of the specimen. The characteristic results of the free surface velocity profiles are shown in figure 4a. The pressure (figure 4b) can be calculated based on the results presented in figure 4a as follows: = 1 2 (1) where P - pressure; V – the free surface velocity; c – the velocity of the longitudinal acoustic wave; ρ – the specimen density.

Table 1. Chemical composition. Ti Al V

Fe

C

N

H

O

Bal

6.48

3.94

0.20

0.018 0.007 0.002 0.17

The pressure impulse amplitude depends non-linearly on the laser power density. The pressure amplitude increases with increasing laser power density. However, the rise in the pressure amplitude from 12 GW/cm² to 19 GW/cm² is insignificant. Additionally, it can be noted that the power de nsity 6 GW/cm² is relatively low, resulting in minimal plastic deformation of titanium. The duration of stress wavefront at a depth of 0.8 mm was 20 ns. Due to the plasma extension process limited by the resistance of the water layer, the stress equal to half the height of the pulse remained for more than 50 ns.

a

b

c

Fig. 3. (a) The geometry of the sample for pressure impulse measurement; (b) the specimen for high cyclic fatigue test; (c) the specimen for fatigue test in gigacyclic regime.