DUNCHEVA, G.V., et al. EFFECTIVENESS OF DIFFERENT METHODS FOR FATIGUE LIFE ENHANCEMENT OF FASTENER HOLES IN D16AT ALUMINUM ALLOY. Journal of the Technical University of Gabrovo. 2019, 58 5-15. ISSN 1310-6686.
An evaluation of the effectiveness of three different methods for enhancement of fatigue life of fastener holes in D16AT aircraft Al-alloy has been made. Objects of comparative analysis are the friction stir hole expansion (FSHE), solid mandrel cold working and symmetric cold expansion (SCE) methods. The results are generalized on the basis of fatigue tests, S-N curves obtained, X-ray diffraction and micro-structural analyses. Under the high-cycle fatigue performance, the SCE provides more than 66 times longer fatigue life as compared to solid mandrel method and more than 82 times greater fatigue life in comparison with FSHE method. Through X-ray diffraction analysis it has been found out that the higher efficiency of the SCE method is due to the symmetric distribution (with respect to the plate middle plane) of the introduced residual hoop stresses around the hole. On the other hand, the solid mandrel cold working method causes a significant gradient of the residual stress distribution in the thickness plate direction, which is a precondition for nucleation and propagation of corner fatigue cracks. It has been established that the FSHE method efficiency depends primarily on the heat generated and the equivalent plastic strain size. The combination of these factors determines the beneficial micro-effect of the microstructure modifying immediately around the hole and the useful macro-effect due to the introduced compressive residual stresses. It has been concluded that SCE method should be used for pre-stressing of fastener holes in the most loaded components in the D16AT aircraft structures - wings and fuselage, while FSHE method can be applied for processing of fastener holes in less loaded aircraft components.
