PSI - Issue 23

A. Eremin et al. / Procedia Structural Integrity 23 (2019) 233–238 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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In situ DIC monitoring provides qualitative (visual patterns in terms of strain fields) and quantitative (strain values at each local region point on the specimen surface) data on the deformation behavior. Fig. 3 illustrates the strain fields for CG (a) and UFG titanium (c) at the moments prior to fracture, and SEM images of fracture surfaces for both material states. DIC images witness that plastic deformations localize in the neck area, but the high strains in CG Ti widely distributed along the gauge length and thus involves more material in deformation reducing stresses. In the cases of UFG Ti the almost all specimen surface experience low elastic strain, while the high deformations observed in the neck area. It was also revealed the difference in the fracture process development: the CG Ti exhibits the normal tearing, but the UFG is mainly fractured by the shear stresses being oriented at the angle of 45° to the loading axis.

0.8

(a)

(b)

0.4

0

ε=33.4%

0.8

(c)

(d)

0.4

0

ε=17.77%

Fig 3. Patterns of the major strain fields (ε 1 ) measured by the DIC technique prior to fracture for CG (a) and UFG (c) titanium specimens; SEM panoramic images of fracture surfaces for CG (b) and UFG (d) titanium.

The theses are also proved by SEM images of fracture surfaces. The texture of the fracture surface for the CG Ti is uniform and represents the normal tearing with significant ductility. Fracture surface pattern for the UFG Ti consists of three different zones: i) central region, ii) two regions with step-like ductile fracture textures on the right and left side from the central part that develops through the specimen cross-section, and iii) the region with smooth surfaces on the specimen periphery resulted from the action of shear stresses (oriented at the angle of 45° to the loading axis). Three types of fracture textures are attributed to the variation in deformation mode. The middle region was formed due to normal tearing under the plane strain conditions (in presence of constraints caused by surrounding material). The smooth surface regions on the periphery were under plane stress state and exhibit a shear fracture pattern. The regions with the step-like ductile texture are transient zones between various fracture modes.

4.2. Fatigue tests results

Fatigue tests show that the average fatigue durability of UFG Ti was reduced compared to CG one from 210000±28000 cycles to 180000±38000 cycles. The crack growth process was divided into two stages: (1) crack nucleation and (2) crack propagation. The duration of the first stage for CG was 186000 cycles (88% of the lifetime) and 135000 cycles (77% of the lifetime) for UFG Ti, whilst the second stage – 25000 cycles (12%) and 41000 cycles (23%) correspondingly. It should be noted that the crack appearance in UFG titanium occurs earlier than in CG Ti,