A double-side symmetric cold expansion method to improve fatigue performance of holed structures: fatigue failure mechanism and life prediction

• DSSCE improves fatigue life by ∼3 times over non-strengthened specimens. • DSSCE offers ∼55% longer life than conventional cold expansion (CE). • Fatigue life prediction model shows <35% average error with experiments. • DSSCE induces ∼20% higher compressive stress than CE at hole entrance. • DSSCE induces ∼0.9 μ m finer grains than CE at hole entrance. In this study, a double-side symmetric cold expansion (DSSCE) method is proposed to address the critical issue of fatigue failure in open-hole structures, which are ubiquitous yet vulnerable components in aerospace and mechanical assemblies. Experimental investigations were conducted on AA6061-T6 specimens to compare the fatigue performance of DSSCE with conventional cold expansion (CE) and non-strengthened (NS) workpieces. The results demonstrate that the DSSCE process significantly enhances fatigue life: achieving an average life of 103370 cycles, which is approximately 3 times that of NS specimens (38288 cycles) and 1.5 times that of CE specimens (66517 cycles). This improvement is mechanistically attributed to DSSCE generating ∼20% higher compressive residual stress at the critical mandrel entrance side and inducing a finer average grain size (13.9 µm vs. 14.8 µm for CE), thereby promoting more uniform stress distribution and superior crack resistance. Furthermore, a novel fatigue life prediction methodology integrating residual stress effects with crack initiation and propagation analysis was developed. The model exhibits strong predictive accuracy, with an average error of less than 35% against experimental data and excellent stability (error variance ≈ 0.1). This work not only introduces an effective and industrially practical surface enhancement technique but also provides a validated predictive framework for the fatigue life assessment of cold-expanded structures.