Relaxation of residual stresses in a surface-hardened prismatic specimen with a stress concentrator of a semicircular profile under conditions of high-temperature creep

The study presents a numerical method for analyzing the relaxation kinetics of residual stresses in a prismatic sample with semi-circular notches after prior vibro-impact ultrasonic surface plastic hardening by shot peening under high-temperature creep conditions, based on the finite element method. The method includes the stage of reconstructing (recovering) residual stresses in a smooth hardened specimen based on known experimental information, solving problems of stress redistribution after notch application, and calculating the kinetics of residual stresses under creep conditions. To illustrate the method, problems are solved for a prismatic sample of 100×10×10 mm with notch radii of 0.1 to 0.5 mm from EP742 alloy at a temperature of 650°C under thermal exposure for 300 hours. The compliance of the calculated data using the developed method with experimental data and grid method calculations was verified in the special case of a smooth specimen. The kinetics of residual stress distribution due to creep from the depth of the notch root into the sample at different time intervals over 300 hours was analyzed. Based on the calculation data, it was shown that the highest relaxation rate is observed in stress concentrators with ρ = 0.1 mm and ρ = 0.2 mm, but in all cases, after 300 hours of creep, compressive residual stresses are maintained in the area adjacent to the notch as well as away from the concentrator. It is shown that the notches practically do not affect the geometry of the samples compared to the hardened smooth samples, while a reduction in the deflection of the hardened samples is observed during the creep process.