The aim of this study is to maximize the fatigue life of a high strength steel. For this purpose, two different kinds of double surface treatments were studied: shot peening plus vibratory finishing and shot peening plus grit blasting. First, a high intensity peening treatment was applied, with the aim of inducing a deep region submitted to high compressive residual stresses, and a second surface treatment followed, to reduce the roughness induced in the first treatment and mitigate the damage produced on the specimen surface. The use of such double treatments was demonstrated to be a good choice to achieve high surface hardening and deep compressive residual stress profiles and to minimize surface defects. The fatigue life of the steel was greatly enhanced following the application of both double surface treatments. A vibratory treatment applied for 24 hours after the application of a previous shot peening treatment was able to increase the average fatigue life of the single shot peened samples almost sevenfold, but a much greater fatigue enhancement was produced using grit blasting for only 60 seconds. Anyway, both second treatments are able to remove the damaged surface layer produced in the first high intensity shot peening treatment. The obtained results also revealed that an appropriate combination of surface treatments (shot peening+secondary treatment) avoided the initiation of cracks at the surface of the specimens under cyclic bending loads. Instead, crack initiation took place beneath the compressive residual stress field, induced in alumina inclusions that acted as stress concentrators.
The structure and crystallographic texture of zinc strips (Zn–Cu–Ti alloy) produced by the continuous horizontal twin roll strip casting method has been characterized. In longitudinal sections normal to the transverse direction, the strips display an approximately symmetrical chevron patterned structure of columnar grains inclined about 30° from the rolling direction. In association with such structure, the macroscopic texture is mainly a [Formula: see text] ‘normal’ (not cyclic) fibre texture tilted approximately ±30° around the transverse direction plus a similarly tilted weak <0001> fibre texture. A thin layer of small equiaxed grains with a strong (0001) basal texture is present at the free surfaces. The observed structure/texture combination agrees quite well with the expected macrostructure of solidification of the alloy in the twin roll casting process.
The objective of this paper is to present a preliminary study of the effects of ball burnishing with vibration assistance on cycle fatigue endurance of cylindrical specimens subjected to bending stress and obtain the best input parameters. The specimens were burnished through 6 different combinations of preload force, number of passes and vibration assistance, finding that burnishing while applying a 90-N preload and 5 passes derives in the longest lifespan, being the vibration assistance not significant with this combination. In terms of roughness, it has been proved that the vibration assistance improves the finishing significatively, allowing to reduce the number of passes compared to NVABB and, as optimal ball-burnishing parameters, the 90-N preload and 5 passes combination derivates in a 93% improvement in terms of mean roughness of the surface. It is also been proved the hardening effect provoked by the ball-burnishing, being an improvement estimated around the 51% in the best case.
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