Effects of Artificial Small Defect and Corrosive Environment on Torsional Fatigue Strength of High Strength Spring Steel

Fatigue strength of spring steel with small scratches

“…At the surfaces of both specimens, a shear‐mode crack at fracture origin and the subsequent mode I crack propagation normal to the principal stress direction were observed. The similar crack pattern has been reported for the torsional fatigue failure of smooth spring steel specimens . Especially, Kanazawa and Abe observed a nonpropagating crack at the torsional fatigue limit defined at N = 10 8 cycles and concluded that the fatigue limit was determined by the nonpropagation condition of a crack initiated on the maximum shear stress plane.…”

“…Furthermore, to improve the fatigue strength of spring steel, the manufacturing process sometimes involves a peeling process for removal of defects in the surface layer, an eddy‐current test process for detection of defects, a nitriding treatment process for enhancement of surface hardness, or a shot‐peening process for the introduction of compressive residual stress. However, the effects of these processes on fatigue strength have yet to be well understood, and essentially, the knowledge about the influence of small defects on the VHCF of spring steels under torsional loading is still limited . In practice, the ability to access the effects of small defects, inclusions, and inhomogeneities on the uniaxial fatigue strength has grown rapidly over the last decades .…”

“…In the literature, several studies with an emphasis on the effects of small artificial defects on the torsional fatigue strength of spring steels are available. Wakita et al examined the fatigue limit defined at N = 10 7 cycles for JIS SUP7 spring steel specimens containing semispherical pits of 300 μm in depth by performing torsional fatigue tests at f = 30 Hz in air and under a corrosive environment of saltwater mist. Takahashi et al investigated the effects of small drilled holes and semicircular slits with the size ranging from 100 to 400 μm on the fatigue strength of shot‐peened and unpeened JIS SUP9A spring steel specimens tested at a stress ratio, R , of 0 at f = 20 Hz in the HCF regime up to N = 10 7 cycles.…”

“…However, the effects of these processes on fatigue strength have yet to be well understood, and essentially, the knowledge about the influence of small defects on the VHCF of spring steels under torsional loading is still limited. 3,4 In practice, the ability to access the Nomenclature: f, test frequency; HV, Vickers hardness; N, number of cycles; N f , number of cycles to failure; R, stress ratio; ffiffiffiffiffiffiffiffiffi area p , square root of the defect projected onto the plane perpendicular to the major principal stress direction; t, scratch depth; ϕ*, mismatch angle between the scratch direction and the principal stress plane; σ a , normal stress amplitude (, (σ max − σ min ) / 2); σ w , uniaxial fatigue limit for notched specimen; σ w0 , uniaxial fatigue limit for smooth specimen; σ 1 , major principal stress; σ 2 , minor principal stress; τ a , shear stress amplitude (, (τ max − τ min ) / 2); τ w , torsional fatigue limit for notched specimen; τ w0 , torsional fatigue limit for smooth specimen ρ, notch root radius; FEA, finite element analysis; HCF, high cycle fatigue; SEM, scanning electron microscope; SIF, stress intensity factor; VHCF, very high cycle fatigue. effects of small defects, inclusions, and inhomogeneities on the uniaxial fatigue strength has grown rapidly over the last decades.…”

Estimating the Effects of Microscopic Stress Concentrations on the Fatigue Endurance of Thin-walled High Strength Steel

“…[13][14][15][16][17][18][19][20][21][22] Frost and Dugdale 13) systematically investigated the effects of the nominal stress, s nominal , and the macroscopic stress concentration factor, a a , on the fatigue endurance and the generation of nonpropagating cracks. They found that there are three cases depending on the combination of s nominal and a a : (1) fractured, (2) not fractured with nonpropagating cracks, and (3) not fractured without nonpropagating cracks.…”