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1

Cyclic loading–unloading creep behavior of composite layered specimens

100%

Hu B. , Yang S. Q. , Xu P. , Cheng J. L.

Acta Geophysica

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2019

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tom Vol. 67, no. 2

449--464

EN

Soft and hard interbedded rocks show obvious time-dependent deformation after deep tunnel excavations, and it is therefore necessary to research the mechanical behavior of the layered rock. However, it is hard to obtain ideal transversely isotropic rocks in fields, so rock-like specimens were poured by using artificial materials. Cyclic loading–unloading creep experiments were performed on the artificial layered cemented specimens with various layer angles (0°, 30°, 60° and 90°) at a 20 MPa confinement. Time-independent deformations and time-dependent deformations of the rock-like specimens were distinguished to investigate the visco-elasto-plastic deformation characteristics. Instantaneous elastic strain and instantaneous plastic strain had linear correlations with stress ratio, whereas creep strain, including visco-elastic strain and visco-plastic strain, increased nonlinearly with an increasing stress ratio. The specimens with a small layer angle had more noticeable time-independent and time-dependent deformations and larger steady-state creep rates than those of the specimens with a large layer angle. Attenuation creep and secondary creep could be observed at relative low stress levels, whereas accelerating creep until failure occurred at the creep failure stress level. The time for creep failure can be predicated according to the axial steady-state creep rate or volumetric creep curve. Damage in the rock-like specimens showed linear correlation with the stress ratio. Dip angle has a significant effect on the creep failure mode under cyclic loading–unloading conditions.

2

A review on the deformation mechanism and formability enhancement strategies in incremental sheet forming

88%

Ullah S. , Li Y. , Li X. , Xu P. , Li D.

Archives of Civil and Mechanical Engineering

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2023

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tom Vol. 23, no. 1

art. no. e55, 2023

EN

The process formability of incremental sheet forming (ISF) is better than the conventional forming processes. Stretching, through-thickness-shear, bending-under-tension (BUT), and compressive forces are the proposed deformation mechanisms for improved formability; however, researchers have not corroborated (on consensus) the relative significance of any one among these. Similarly, researchers observed abrupt fractures (brittle fracture) and fractures preceded by necking (ductile fracture) for different case studies, which initiated a new debate and is still unanswered. Besides, researchers have extended the ISF to energy-assisted ISF to improve the process formability further for materials having a high strength-to-weight ratio. Three prominent energy-assisted ISF are (a) Electric-assisted ISF (E-ISF) works on the principle of lowering the yield stress by raising the temperature and has shown promise for Magnesium and Titanium alloy. (b) The ultrasonic vibration-assisted (UV-ISF) process works on the principle of acoustoplastic softening effect and thus far improved the room temperature material formability while reducing the forming forces. (c) Electromagnetic-assisted ISF (EM-ISF) is a non-contact, high-speed process that utilizes the pulsed magnetic field to apply inertial force, which improves formability by dislocation slips. The EM-ISF and UV-ISF have shown promise to counter the challenges during aluminum alloy forming; however, the work in this regard is still in the initial phase and has not explored its full potential. This study updates the potential research on the current status of the energy-assisted ISF. Different customized testing equipment is discussed that help understand the process mechanism. Microstructural changes in the material occur at normal ISF and with energy-assisted ISF are discussed in detail. Discussion and future work are presented based on the insight from various articles at the end.

3

Development and verification of a high-precision laser measurement system for straightness and parallelism measurement

75%

Xu P. , Li R. J. , Zhao W. K. , Chang Z. X. , Ma S. H. , Fan K. C.

Metrology and Measurement Systems

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2021

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tom Vol. 28, nr 3

479--495

EN

A laser measurement system for measuring straightness and parallelism error using a semiconductor laser was proposed. The designing principle of the developed system was analyzed. Addressing at the question of the divergence angle of the semiconductor laser being quite large and the reduction of measurement accuracy caused by the diffraction effect of the light spot at the long working distance, the optical structure of the system was optimized through a series of simulations and experiments. A plano-convex lens was used to collimate the laser beam and concentrate the energy distribution of the diffraction effect. The working distance of the system was increased from 2.6 m to 4.6 m after the optical optimization, and the repeatability of the displacement measurement is kept within 2.2 m in the total measurement range. The performance of the developed system was verified by measuring the straightness of a machine tool through the comparison tests with two commercial multi-degree-of-freedom measurement systems. Two different measurement methods were used to verify the measurement accuracy. The comparison results show that during the straightness measurement of the machine tool, the laser head should be fixed in front of the moving axis, and the sensing part should move with the moving table of the machine tool. Results also show that the measurement error of the straightness measurement is less than 3 m compared with the commercial systems. The developed laser measurement system has the advantages of high precision, long working distance, low cost, and suitability for straightness and parallelism error measurement.

4

Free vibration of composite cylindrical shells with orthogonal stiffeners

75%

Wu Y. , Zhao C. , Liang H. , Yao S. , Xue J. , Xu P.

Journal of Theoretical and Applied Mechanics

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2022

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tom Vol. 60 nr 2

239--252

EN

This paper proposes theoretical and numerical approaches to scrutinize the free vibration of orthogonal stiffened cylindrical shells. According to K´arman-Donnell shell theory, the total energy of the stiffened cylindrical shells is derived. Based on the principle of minimum potential energy, the eigenfunction related to the frequency is established and solved by developing a Matlab program. Analytical solutions of the natural frequency for free vibraion of the stiffened cylindrical shells are calculated and are verified against the finite element results from ABAQUS software. On account of the observations from the parametric study, an optimization scheme of the stiffeners is proposed.