Effects of inclusion size and location on very-high-cycle fatigue behavior for high strength steels

Abstract

a b s t r a c tFatigue tests with rotary bending (52.5 Hz) and ultrasonic cycling (20 kHz) methods were performed, respectively on two high strength steels so as to investigate the effects of inclusion size and location on the fatigue behavior from low cycle fatigue to very-high-cycle fatigue (VHCF) regimes. Under rotary bending test, four fracture patterns were observed with respect to different locations of inclusion on fracture surface. The mean crack growth rate within FGA (fine granular area of crack origin) was estimated to give the value with the magnitude of 10 -13 m/cycle for the specimens with the fracture pattern of ''FGA þ surface fisheye''. Under ultrasonic cycling test, the size effect of inclusions on VHCF was investigated with two groups of specimens. One is with the inclusion size between 20 and 60 mm and the other is with the inclusion size between 10 and 30 mm. The results show that large inclusions lowered the proportion of net FGA at given stress levels and led to the shortening of VHCF life. The degradation of VHCF strength caused by the increase of inclusion size is ascribed to the decrease of the critical stress of FGA formation for large inclusions.

Effects of loading condition on very-high-cycle fatigue behaviour and dominant variable analysis

“…For rotating bending, most of interior defects are the types of non-inclusions; only one specimen fractures from interior inclusion for RB150. With the increase of tempering temperature, inclusion initiation becomes dominant for RB150 and RB180 as are often reported in the literature [5,15,22,[27][28][29]. ΔK Inc and ΔK FGA are different from each other for the three heat treatment procedures.…”

“…The location of dominated inclusion either near the surface or far from the surface is the key factor to the failure mode transition with or without FGA, and leads to the large scatter of S-N curves [16,22]. A number of studies on fatigue crack propagation also show that fatigue crack growth behaviour near the threshold differs with respect to the change of tempering temperature [23][24][25].…”

Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti–6Al–4V alloy

“…For the case of crack initiation from the interior of the specimen, the morphology of rough area containing flat facets was observed at the crack initiation region for titanium alloys, which is similar to the FGA with rough surface at the interior initiation region of high-strength steels in the VHCF regime [31][32][33]. Shiozawa et al [34] analyzed the effects of stress ratio on the value of the stress intensity factor range (SIF), which is calculated from the size of facets for a beta-type titanium alloy Ti-15V-3Cr-3Sn-3Al.…”

The formation mechanism of characteristic region at crack initiation for very-high-cycle fatigue of high-strength steels

“…4a (also in Fig. 5d) show that the nonmetallic inclusion consists of Al 2 O 3 , CaO, MgO and SiO 2 [28]. An FIB sample was cut from the FGA region (marked rectangle in Fig.…”