Issue34

M. Goto et alii, Frattura ed Integrità Strutturale, 34 (2015) 427-436; DOI: 10.3221/IGF-ESIS.34.48 428 envisaged structural applications of ultrafine grained (UFG) metals, attention has been paid to fatigue performance. Studies of fatigue on UFG materials processed by ECAP has been focused mainly on cyclic deformation, S-N plots, formation of shear bands (SBs) and underlying microstructural mechanisms [5-11]. Since the fatigue life of machine components and structures is mainly controlled by the growth life of a fatigue crack, the crack growth behavior should be clarified for the design of safe machine components and structures. Recently, in the high-cycle fatigue (HCF) tests, the growth behaviors of long (millimeter-range) cracks in UFG metals [12-16] were studied for compact-tension, single edge-notched, single edge bend and center-cracked tensile specimens. With regard to the growth characteristics of long fatigue cracks in UFG materials, the higher growth rates in low and medium values of the stress intensity factor (SIF) range and the lower growth thresholds have been reported. These phenomena appear to be attributed to the weakened roughness-induced crack closure caused by the much smoother fracture surface and lower deflection of the crack path. The smoother crack-path/fracture-surface is caused by decreased grain sizes and limited crack tip plasticity. It has been shown that the crack growth life from an initial size to 1 mm accounted for about 70% of the fatigue life of plain specimens of many conventional grain-sized metals [17, 18]. Therefore, the growth behavior of small cracks must be clarified to estimate the fatigue life of smooth members. Regarding the crack growth in strain-controlled low-cycle fatigue (LCF) tests and stress-controlled fatigue tests at high stress amplitudes corresponding to LCF regime, study on fatigue crack growth mechanism has been relatively rare, and only a few reports can be found [19, 20]. Meanwhile, for ECAPed samples, the yz -, zx -, and xy -planes are defined by three mutually orthogonal sectioning planes that are perpendicular to the longitudinal axis of the pressed sample, parallel to the sample side, and parallel to the sample top faces at the point of exit from the die, respectively. In strain-controlled LCF tests, SBs in the zx -plane were oriented at 45° to the loading axis parallel to the longitudinal axis of the pressed samples, while those in the xy -plane were nearly perpendicular to the loading axis [21]. The SBs appear on the zx -plane at 45° to the loading direction mainly because it is the plane of maximum resolved shear stress [22]. Fatigue cracks were initiated in and propagated along these SBs. Consequently, the LCF crack on the zx -plane grew along the direction inclined at 45° to the loading axis. Up to now, LCF crack growth behavior of UFG materials have been mainly discussed from the viewpoints of microstructure and morphological features of surface damage. On the other hand, the discussion from the mechanical viewpoints should be done for a better understanding of the fatigue damage of UFG materials. However, such studies are few and certain questions remain unanswered. The objective of this study is to investigate the physical background of the formation mechanism of crack growth paths in the HCF and LCF regimes in terms of the mixed-mode deformation at the crack tip. E XPERIMENTAL PROCEDURES ure oxygen-free copper (99.99 wt% Cu) was used in the experiment. Prior to ECAP processing, the samples were annealed at 500°C for 1 h (average grain size: 100  m). The ECAP die had a 90° angle between intersecting channels. The angles at the inner and outer corners of the channel intersection in the die were 90° and 45°, respectively. Repetitive ECAP was accomplished according to the Bc route (after each pressing, the billet bar was rotated 90° around its longitudinal axis). Eight extrusion passes resulted in an equivalent shear strain of approximately 7.8. The microstructure of an ECAP rod obtained using a transmission electron microscope showed fine equiaxed grains of approximately 300-nm diameter and large elongated grains. Fatigue specimens 5 mm in diameter were machined from their respective processed bars. Although the specimens had shallow circumferential notches (20-mm notch radius and 0.25-mm notch depth), the fatigue strength reduction factor for this geometry was close to 1, meaning that they could be considered plain. The fatigue specimens were electrolytically polished (approximately ≈25  m from the surface layer) prior to mechanical testing to remove any preparation-affected surface layer. Polishing was carried out at 25°C using an electrolyte consisting of 600 mL of phosphoric acid, 300 mL of distilled water, and 100 mL of sulfuric acid. Prior to testing, a small blind hole (both diameter and depth of 0.1 mm) was drilled as a crack starter on the middle surfaces of the plain specimens. Fig. 1 shows the location of the drilling hole. A hole was drilled on the surface where an intersection between the shear plane of the final pressing and the specimen surface makes an angle of 45° ( zx -plane) or 90° ( xy -plane) with respect to the loading axis. All fatigue tests were performed at room temperature using a rotating-bending fatigue machine (constant bending-moment type) operating at 50 Hz. The fatigue damage on the specimen surface was observed using an optical microscope (OM) and a scanning electron microscope (SEM). The crack length, l , was measured along the circumferential direction of the surface. The P