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Processing of commercially pure copper tubes by hydrostatic tube cyclic extrusion–compression (HTCEC) as a new SPD method

Eftekhari M., Faraji G., Bahrami M.

Archives of Civil and Mechanical Engineering

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2021

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Vol. 21, no. 3

595--606

EN

In this study, severe plastic deformation (SPD) process of hydrostatic tube cyclic extrusion–compression (HTCEC) was performed through two passes on the commercially pure copper tubes with the purpose of fabricating relatively long ultrafine-grained (UFG) tubes. In HTCEC process, the presence of pressurized hydraulic fluid around the piece plays a key role in the reduction of the friction load and, consequently, in the reduction of required pressing load. In principle, this facilitates the production of long and large tubes. After processing by HTCEC, the mechanical characteristics and microstructure evolution were examined. Microstructure analysis revealed that after the first pass of HTCEC process, an ultrafine cell microstructure with an average size of ~ 993 nm was attained. After two passes of HTCEC, the average size of cells/subgrains was reduced to ~ 340 nm. This was while the average grain size of the annealed sample was 41 μm. Also, after two passes of HTCEC process, a remarkable increase in the yield strength from 154 to 336 MPa, and the ultimate strength from 223 to 414 MPa was observed. Furthermore, the mean value of microhardness increased from 74 to 149 HV, and a more uniform distribution of microhardness along the thickness was seen, compared to the first pass of HTCEC. Meanwhile, unlike most conventional SPD methods, the value of elongation to failure was slightly lessened from 59.5 to 41.6%. SEM fractography analysis denoted that mostly ductile fracture occurred in the HTCEC-processed samples. In general, two main advantages of HTCEC process can be the production of relatively long ultrafine-grained tubes and the significant increase in the strength and hardness besides a low loss of ductility.

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Determination of crack initiation and propagation in two disc shaped specimens using the improved maximum tangential stress criterion

Eftekhari M., Baghbanan A., Mohtarami E., Hashemolhosseini H.

Journal of Theoretical and Applied Mechanics

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2017

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Vol. 55 nr 2

469--480

EN

The crack initiation angle and propagation path for two different disc shaped test specimens (i.e., SCB and CBD specimen) are investigated experimentally and theoretically. The Maximum Tangential Stress (MTS) criterion does not calculate the crack initiation angle in SCB and CBD specimens correctly. Moreover, at the angles after occurrence of pure mode II, where the stress intensity factor of mode I becomes negative, this criterion is not applicable. Therefore, in this research work, Improved MTS (IMTS) criterion which has been implemented in the extended finite element method and is applicable under tensile and compressive loading conditions to examine the crack propagation path in the aforementioned disc shaped specimens. Furthermore, an experimental study on a cracked Brazilian disc specimen has been conducted at different angles. Results of IMTS criterion in these specimens show that the crack propagation path and the crack initiation angle can be predicted theoretically by using IMTS criterion.

3

Vibration analysis of single-walled carbon nanotubes conveying nanoflow embedded in a viscoelastic medium using modified nonlocal beam model

Hosseini M., Sadeghi-Goughari M., Atashipour S. A., Eftekhari M.

Archives of Mechanics

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2014

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Vol. 66, nr 4

217--244

EN

In this study, the vibration and stability analysis of a single-walled carbon nanotube (SWCNT) coveying nanoflow embedded in biological soft tissue are performed. The effects of nano-size of both fluid flow and nanotube are considered, simultaneously. Nonlocal beam model is used to investigate flow-induced vibration of the SWCNT while the small-size effects on the flow field are formulated through a Knudsen number (Kn), as a discriminant parameter. Pursuant to the viscoelastic behavior of biological soft tissues, the SWCNT is assumed to be embedded in a Kelvin–Voigt foundation. Hamilton’s principle is applied to the energy expressions to obtain the higher-order governing differential equations of motion and the corresponding higher-order boundary conditions. The differential transformation method (DTM) is employed to solve the differential equations of motion. The effects of main parameters including Kn, nonlocal parameter and mechanical behaviors of the surrounding biological medium on the vibrational properties of the SWCNT are examined.