PSI - Issue 79

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

169

1. Introduction Despite decades of research of metals fatigue, this phenomenon remains catastrophic and unpredictable. From a practical standpoint, controlling fatigue crack initiation and propagation processes is of paramount importance. One particularly effective control method involves the generation of significant compressive residual stresses (CRS). Laser shock peening (LSP) can induce CRS exceeding 1 mm in depth. During LSP processing, the metal surface is subjected to nanosecond-scale, high-energy laser pulses that generate shock waves. The propagation of these waves through the material results in plastic deformation of the surface layer, producing CRS with amplitudes reaching -1 GPa and depth more than 1 mm. Since the processing is very promising, for example, from the point of view of the aerospace industry, both experimental [1 – 3] and numerical [4,5] researches in this area continue. The enhancement of fatigue properties of materials under LSP is already well known in both ultrasonic [6,7] and conventional testing [8 – 12]. Ref [9] demonstrates that the fatigue resistance of LSP-processed specimens tested at low stress ratios exhibits a significant increase compared to the unprocessed ones. LSP led to a pronounced increase in fully reversed fatigue lives and fatigue strengths in Ti6Al4V in the paper devoted to the comparison of different surface treatment methods [8]. In [10], it was shown that the enhancement in the fatigue life of Ti-6Al-4V was the result of the high-amplitude compressive residual stress on the surface and in depth induced by LSP. A substantial decline in the rate of fatigue crack growth within the laser peened area was documented in [12]. In [10], the fatigue life of the Ti-6Al-4V alloy increased by at least 1.28 times after laser shock peening due to the introduction of deep compressive residual stresses with minimal change in surface topography. Finally, [11] presents a comprehensive review of LSP applications in aircraft engineering detailing the positive effects and challenges associated with this technique. Various methods of surface treatment, including LSP, do not always have a positive effect. Despite extensive research, predicting residual stresses based on LSP parameters remains challenging [13]. In [14], the fatigue strength decreases significantly when the laser pulse power density reaches a certain limit, which is attributed to the formation of internal cracks. The lower fatigue limit observed in [15] is explained by an excessive grain refinement. In the context of LSP, the coating and the overall stability of absorbing and confinement overlays are of paramount importance [16]. Consequently, LSP is a multifaceted process with numerous parameters that must be considered for effective implementation. LSP includes several coupled physical-mechanical phenomena: material ablation, high rate plastic deformation of material caused by propagation of nanosecond-duration elastic-plastic waves, generation and redistribution of self equilibrated CRS. To investigate LSP, an experimental setup was developed enabling real-time measurement of pressure pulse profiles and subsequent reconstruction of through-thickness CRS distributions. To evaluate the effectiveness of surface treatments (depth and amplitude of the induced CRS) a range of experimental methods for residual stress measurement are used [17 – 19]. The most widely adopted among these are the hole-drilling method (ASTM E837 [20]) and X-ray diffraction (ASTM E915 [21]). The hole-drilling method involves measuring strains around a small hole drilled into the material using strain gauges. Stresses are calculated by analyzing material relaxation. Key advantages of this method include equipment accessibility and applicability to large-scale components. However, its primary limitations include: localized surface damage, shallow analysis depth (1 – 2 mm), accuracy dependency on the correctness of mathematical models used for data interpretation, inapplicability to curved surfaces. In contrast, X-ray diffraction (XRD) is a non-destructive method based on analyzing shifts in interplanar spacings within the crystal lattice under stress. XRD offers high spatial resolution and is suitable for measuring surface stresses (depth range: 10 – 20 μm). To study stress gradients with d epth, XRD is often combined with layer removal by etching: sequential removal of thin material layers followed by measurement enables the reconstruction of residual stress distribution profiles in subsurface zones. In this study, the hole-drilling method was used to evaluate residual stresses in flat specimens with stress concentrators, while X-ray diffraction was employed to assess residual stresses on the surface of cylindrical specimens. The effectiveness of LSP application can only be illustrated by fatigue experiments with standard samples or elements of real structures. Two fatigue tests were carried out to demonstrate LSP's effectiveness while highlighting the need for careful application: one involving stress-concentrated plate samples (high-cycle fatigue) and another using cylindrical samples (gigacycle fatigue). For stress-concentrated samples, LSP increased fatigue life by a factor