PSI - Issue 72

Mariia Bartolomei et al. / Procedia Structural Integrity 72 (2025) 135–140

136

and White et al. (1963) which consists in the ability to generate shock waves and plastic deformation in metallic materials by means of high-energy laser short-pulse impact. The application of this technology in various industries allows one to increase the cyclic durability of products, improve the strength characteristics and corrosion resistance of structural parts, so the interest in the LSP process has been maintained for several decades, Peyre et al. (1995, 1998), Clauer et al. (2009, 2019). During laser shock treatment, the beam hitting the absorbing layer on the sample surface forms a plasma, which, expanding, forms a high-pressure elastoplastic wave. The wave, propagating in the material, causes plastic deformations due to residual stresses are formed. The studies devoted to fatigue curve changes before and after laser shock treatment of samples from various titanium alloys are discussed in Pan et al. (2019). In Ouyang et al. (2022), numerical evaluation of the change in fatigue characteristics before and after LSP is studied and compared with experimental data. The magnitude and depth of the generated residual stresses are influenced not only by the parameters with which LSP is carried out, Cao et al. (2012), but also by the sequence and direction of treatment, Kallien et al. (2019), and pulse duration Sun et al. (2022). Not all variants of LSP application can provide improvement of structural integrity Achintha et al. (2014) and fatigue life. In this work we studied the influence of LSP patterns on the fatigue life of notched specimens from TC4. The main goal of the work is to determine the range of possible increase in external cyclic load within which the compressive residual stress field generated by LSP remains effective. 2. Material and experimental conditions Fatigue tests were performed on flat specimens made by electroerosion cutting from TC4 titanium alloy sheet with thickness of 3 mm. The geometry of the specimens is shown in Fig. 1. The specimens were weakened by a lateral circular notch of radius 8 mm to localize the crack propagation zone.

Fig. 1. Geometry of TC4 samples.

To estimate fatigue strength fatigue tests were carried out on servohydraulic testing machine Biss Bi-00-100 with constant stress amplitude, the stress ratio R=0.1 and loading frequency of 10 Hz. The number of cycles to failure was recorded based on the fatigue test results. Three LSP patterns different from each other by their location relative to the notch are used to form the residual stress field in the notched area. The first pattern includes treatment of the sides; the second pattern suggests treatment of the notch; the third pattern is a combination of the previous two and represents the treatment of all surfaces (Fig. 2). LSP of the samples was carried out on an installation that includes a Nd:YAG solid-state laser Beamtech SGR Extra-10 and a robotic six-axis manipulator STEP SR50. The laser wavelength is 1064 nm, the maximum laser pulse frequency is 5 Hz, the theoretical maximum energy in a pulse is 10 J (the energy loss in radiation is 10%, so the maximum realized pulse energy is 9 J), and the pulse duration is 10 ns. The laser spot had the shape of a square with a side of 1 mm, and the laser shots were performed without overlapping. Then the fatigue life of treated and untreated specimens was determined during fatigue tests. Fig. 3 shows the number of cycles that the specimens were subjected to before they cracked and failed at a load of 10 kN under the different LSP patterns. According to the results of fatigue tests, it can be seen that LSP pattern №1 gave practically