Design and experiment of simulated specimen for cold expansion strengthening of bolt holes in turbine disk based on stress gradient matching

To address the fatigue life issue of bolt holes in aero-engine turbine disks under high-temperature and high-stress service conditions, a stress gradient matching design method for simulated specimens used in bolt hole cold expansion strengthening is proposed based on the principle of stress field equivalence around the hole. A thermo-mechanically coupled finite element model is established to systematically investigate the effect rules of four key geometric parameters-notch depth, parallel section width, notch radius, and thickness-on the stress field distribution of the simulated specimen, and a single-variable sequential optimization strategy based on parameter sensitivity analysis is developed. The results show that increasing the notch depth and parallel section width can reduce the stress gradient error, while the cold expansion strengthening stiffness and the test load range should be taken into account simultaneously; the selection of the notch radius should reserve sufficient allowance based on the bolt hole radius to avoid unexpected fracture at the notch caused by stress concentration competition; the thickness has no significant effect on stress distribution or cold expansion strengthening effectiveness. Modified by the competition mechanism analysis of notch versus bolt hole stress concentration, a reasonable simulated specimen configuration was optimized, achieving a maximum stress gradient error of merely 7.37%, which can reproduce the stress state of the actual turbine disk bolt hole with high fidelity. High-temperature low-cycle fatigue tests at 550 ℃ demonstrate that fractures of the simulated specimens all occur in the expected assessment zone at the inner wall of the bolt hole, and the average fatigue life after cold expansion strengthening increases by 203.8% compared with that of the unstrengthened specimens. The measured residual stresses are in good agreement with the simulation results, with a relative error of 2.4% for the surface residual compressive stress on the hole wall. The proposed specimen design method addresses the effectiveness at three levels, namely stress field equivalence accuracy, fatigue failure mode reproduction, and strengthening process characterization, which can provide experimental basis for fatigue life evaluation and cold expansion strengthening process optimization of bolt hole structures in turbine disks.