13th International Conference on Fracture June 16–21, 2013, Beijing, China torsion, they noted that at amplitudes above the SN knee, slip markings appeared on a specimen after a few cycles. At amplitude below the knee they appeared after a million or so cycles. Wei and Baker found wall developed arrays of dislocation loops in iron specimen after only 10 cycles in push-pull fatigue [12]. Such observation at small number of cycles is difficult at high frequency in VHCF. However, fatigue deformation features of iron at very high cycle fatigue regime are limited. PSBs formation at subsurface and at initiation area was not well discussed on literature. Current work intends to investigating the fatigue initiation and localized irreversibility of b.c.c. metals ARMCO iron (α-Fe) at very high cycle fatigue regime. In order to study this phenomenon a new flat specimen, easy for surface observation, was designed to be tested at 20 kHz up to 1010 cycles. Observations were performed on specimen surface during testing and fractographic after failure. 2. Experimental The studied material is a polycrystalline iron whose chemical composition is given in Table 1 and mechanical properties of corrected true stress-strain is given in Fig 1. The microstructure is ferrite with equiaxe grains. The ferrite grain size is included in 10 to 40 μm. No specifically orientation was observed by EBSD. Table 1. Chemical compositions (wt.%) C P Si Mn S Cr Ni Mo Cu Sn Fe 0.008 0.007 0.005 0.048 0.003 0.015 0.014 0.009 0.001 0.002 Balance Fig 1. Stress-strain curve Fig 2. Sketch of flat ultrasonic specimen For the reason of surface online observation condition by IR camera and optic microscope, a new design of 1 mm flat specimen (Fig. 2) was used to carry out fatigue tests. Specimen, special attachment and piezoelectric fatigue machine constituted the resonance system working at 20 kHz. The cyclic loading is tension-compression. The stress ratio is then R = -1. Before testing, both sides surface of the flat specimen were polished. One surface of the flat specimen was electrolytic etched.
RkJQdWJsaXNoZXIy MjM0NDE=