Interference fit effect on improving fatigue life of a holed single plate

This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates made of a medium-carbon forging steel (35KB2), heat treated by quenching followed by tempering, up to a hardness of about 350BH, obtaining a sorbitic microstructure. Tensile and impact tests showed an ultimate tensile strength of about 1100MPa, a yield strength of about 1000MPa, an elongation to failure of 15% and an impact toughness KV of 43J at room temperature. Axial fatigue tests were performed on holed specimens with or without a pin, made of the same material, press fitted and still left into their central hole. The tension-tension fatigue tests have been performed with a stress ratio R=0.1. The effect on fatigue strength was investigated both experimentally and numerically. Three different conditions were investigated by using open hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S-N) curves pointed out an increased fatigue life of the interference fit specimens, compared with the open hole ones. The numerical investigation was performed in order to analyse the stress field by applying an elastic plastic 2D simulation, with commercial finite element software. The stress history and distribution around the interference-fitted hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) because a residual and compressive stress field is generated by the pin insertion.