Statistical fatigue properties and small fatigue crack propagation in bimodal harmonic structured Ti-6Al-4V alloy under four-point bending

Abstract

Small fatigue crack propagation in bimodal harmonic structured titanium alloy (Ti-6Al-4V) with high strength and ductility was examined under four-point bending at a stress ratio of 0.1 in the ambient laboratory atmosphere. The crack profiles were observed using optical microscopy and scanning electron microscopy, and analyzed using an electron backscattered diffraction to examine the mechanism of small fatigue crack propagation. Fatigue crack paths were not influenced by the bimodal harmonic structure, and the crack growth rates, da/dN, in the harmonic structured Ti-6Al-4V were almost the same as those in a material with coarse acicular microstructure for comparable values of stress intensity range, K. In contrast, the harmonic structured Ti-6Al-4V had a higher resistance of fatigue crack initiation due to the grain refinement induced by mechanical milling, which resulted in an increase of the fatigue life and fatigue limit. Furthermore, the statistical fatigue properties of Ti-6Al-4V alloy were analyzed using the stress dependence of Weibull parameters to quantitatively examine the effects of the bimodal harmonic structure on its fatigue life.

Mechanism of Fatigue Crack Initiation and Propagation in Commercially Pure Titanium and Titanium Alloy with Bimodal Harmonic Structure

“…Relationship between crack growth rate and stress intensity range for untreated and MM90 series explaining small crack growth rates do not depend on the harmonic structure (Ti-6Al-4V)27) . Typical fracture surface features of the MM90 series failed at long fatigue life (σ max = 722 MPa, N f = 3.4 × 10 6…”

“…Results of four-point bending fatigue tests, showing stress amplitude as a function of cycles to failure explaining that MM90 series has the higher fatigue limit and fatigue life (Ti-6Al-4V)27) .…”

“…2 [25][26][27][28][29] 29 -32) Ti-6Al-4V 24) Plasma rotating electrode process: PREP 33) Ti-6Al-4V SUJ2 SUJ2 3.3 ) (Ti-6Al-4V) 26) . Fig.…”

Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

“…Hardness value of the MM series is higher than that of the untreated series (125.2±3.6 HV). The sintered materials were sliced into disks approximately 1.5 mm thick and machined into a blunt-notched specimen for four-point bending fatigue tests [26,27]. After machining, the specimen surface was polished with emery paper (#240 to #4000) to a thickness of 1 mm and polished in a SiO 2 suspension to obtain a mirror finish.…”

“…Our research group has designed a harmonic structure using powder metallurgy to sinter mechanically milled stainless steel powders [18][19][20][21][22][23], which improve both their strength and ductility by suppressing necking during tensile deformation [20]. In particular, we have focused on the fatigue properties under fourpoint bending [24][25][26][27] and near-threshold fatigue propagation of long cracks [28][29][30] in titanium-based materials with a bimodal harmonic structure. To achieve sufficient performance with the newly developed harmonic structured stainless steels, their fatigue properties and fatigue crack propagation behavior need to be examined.…”