Influence of inclusion size on S‐N curve characteristics of high‐strength steels in the giga‐cycle fatigue regime

Abstract

A B S T R A C T Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10 6 cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusioninduced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.Keywords fatigue crack growth rate; giga-cycle fatigue; high-strength steels; nonmetallic inclusion; subsurface crack. N O M E N C L A T U R E da/ dN= crack propagation rate d inc = depth of non-metallic inclusion from the specimen surface N f = total fatigue life (number of cycles to failure)A IG = size of internal inclusion with GBF K = stress-intensity factor range K GBF = stress-intensity factor range at GBF K S = stress-intensity factor range at surface inclusion K I = stress-intensity factor range at internal inclusion without GBF K IG = stress-intensity factor range at internal inclusion with GBF K th = threshold stress-intensity factor range for surface crack growth σ a = stress amplitude σ at = true stress amplitude considering the stress gradient

Notice of Retraction: Load frequency effect on gigacycle fatigue properties of superalloy inconel 718

“…The area (II) of the region outside the initial site is larger than in the higher cycle failure case with N f =5.366x10 8 . The radial streaks for area (III) were more clear for N f =5.366x10 8 than for N f =3.089x10 5 , and the roughness of area (III) for N f =3.089x10 5 is larger than for N f =5.366x10 8 . This corresponds to faster crack propagation for lower cycle case with high stress levers.…”

“…Gigacycle fatigue failures for high strength materials have become a serious industrial matter [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The conventional fatigue experiment with a load frequency of 10 Hz costs more than one thousand days to reach 10 9 cycles.…”

Fatigue crack initiation behaviors throughout friction stir welded joints in AA7075-T6 in ultrasonic fatigue

“…Moreover, the Mg 2 Si particles in the NZ have a larger size than the Fe-rich particles, because the Fe-rich particles exhibited multiple cracks, which is much more serious than that of Mg 2 Si particles [25]. Generally, the defect size is the crucial factor to control the fatigue strength in the VHCF range for high strength steel with non-metallic inclusion, and the fatigue crack initiated from the inclusion with maximum diameter [31,32]. In this test, the intergranular particles in the NZ are supposed to play similar roles as the inclusions in high strength steel for the similar failure mechanisms as presented in Fig.…”

A review about the effects of structural and operational factors on the gigacycle fatigue of steels

“…It was established that the major part of fatigue life is spent during crack propagation, whereas the fraction of fatigue life spent during initiation is usually much smaller. In the 30 recent years, the industrial technology development has led to some machine components failures beyond 10 7 cycles, and consequently, the asymptote might not exist in some cases of S‐N curves of steels . Bathias et al suggested to ban the fatigue limit concept, and to replace it by fatigue strength for a considered number of cycles …”