Acoustic-Emission Features of Fatigue Crack Propagation in TC4 Titanium Alloy after Laser Shock Peening

The study aims at describing characteristic features of acoustic emission (AE) signals during fatigue crack development in the residual stress field pre-created by laser shock peening (LSP). This method consists in the formation of the residual stress field by high-power short-pulse laser impact and generation of the elastic-plastic wave in the material. As a result of such impact, the material structure in the surface layer undergoes changes that affect not only the possibilities of fatigue crack initiation and development, but also the AE characteristics in the process of cyclic deformation. In the course of work, a series of samples from titanium alloy TC4 were subjected to LSP in the stress concentrator zone in order to retard fatigue crack initiation and development. Further, the treated specimens were tested under cyclic deformation conditions to demonstrate the effectiveness of the selected LSP mode. The crack length was measured by the electric potential drop method. AE signals were recorded on the investigated specimens during testing. On the basis of the experimental data obtained, cluster analysis of AE signals was carried out. The AE data were clearly divided into two clusters, which may indicate the presence of two dominant sources of AE signals, qualitatively reflecting the two predominant fracture mechanisms. It has been shown that the value of the cumulative energy of the AE signals in each of the clusters is significantly higher in the post-LSP samples compared to the base samples before the moment of crack growth. Thus, as a result of LSP, significant changes in the evolution of AE signals during cyclic deformation of titanium alloy TC4 specimens with a lateral semicircular notch are observed. This indirectly indicates that LSP affects the structural characteristics of the material, creating a residual stress field and extending the fatigue life of the specimens.