Issue 3

JTAM, Sofia, vol. 43 Issue 3 (2013)

Propagation of Cylindrical Rayleigh Waves in a Transversly Isotropic Thermoelastic Diffusive Solid Half-Space

Rajneesh Kumar1, Tarun Kansal2
1Department of Mathematics, Kurukshetra University, Kurukshetra-136119, India
2Department of Mathematics, M. N. College Shahabad (M.), 136135 India

The propagation of cylindrical Rayleigh waves in a transversely isotropic thermoelastic diffusive solid half-space subjected to stress free, isothermal/insulated and impermeable or isoconcentrated boundary conditions is investigated in the framework of different theories of thermoelastic diffusion. The dispersion equation of cylindrical Rayleigh waves has been derived. The phase velocity and attenuation coefficients have been computed from the dispersion equation by using Muller’s method. Some special cases of dispersion equation are also deduced.

JTAM, Sofia, vol. 43 Issue 3 pp. 03-20 (2013), [Full Article]

A Numerical Approach for Estimating the Effects of Multiple Earthquakes to Seismic Response of Structures Strengthened by Cable-Elements

Asterios Liolios1, Angelos Liolios1, George Hatzigeorgiou2
1Division of Structural Engineering, Dept. of Civil Engineering, Democritus-University of Thrace, Xanthi, Greece
2Lab. of Ecological Mechanics and Technology, Dept. of Environmental Engineering, Democritus-University of Thrace, Xanthi, Greece

The behaviour of reinforced concrete (RC) frames, which have been strengthened by cable elements and are subjected to multiple earthquakes, is numerically investigated. The purpose is to estimate damage indices in order to compare the seismic response of the structures before and after the retrofit by cable element strengthening and to select the optimum strengthening version.

JTAM, Sofia, vol. 43 Issue 3 pp. 21-32 (2013), [Full Article]

Stress of Make-Up Torque on the Gas Storage Well Bore and Coupling Connecting Thread

Peiqi Liu1, Liming Zhang1, Siyuan Xu1, Zhixiang Duan2, Zuzhi Chen2, Wenjing Zhao2
1Dalian University of Technology, Dalian 116012, China
2China Special Equipment Inspection and Research Institute, Beijing 100013, China

The structure of wellbore and coupling is connected by thread, it is the position where fatigue failure accidents are happened in gas storage well, and the make-up torque of thread is the key factor that influences the mechanical properties of this structure. In this paper, a finite element method was established to discuss the calculation method of make-up torque and the influence law on the stress of coupling in the gas storage well. Results show that the make-up torque cannot be simply ignored due to its great impact on internal stress distribution of gas storage well. Experimental results showed the correctness of the model. Under the action of the make-up torque, the overall stress level inside the wellbore is higher than the external coupling. However the maximum stress exists in the external coupling. The overall stress gradually increases with the increase of interference rotation number (abbr. RN) between coupling and wellbore, but the overall distribution trend is almost the same. The first thread on the left of the connection part is the area where fatigue failure is most likely to occur. The make-up torque between threads increases linearly with the increase of RN between coupling and wellbore.

JTAM, Sofia, vol. 43 Issue 3 pp. 33-42 (2013), [Full Article]

Numerical Procedure for Identification of Constitutive Equations Based on Experimental Data

Anguel Baltov, Ana Yanakieva
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

In this research experimental data obtained by well-known methods of displacement measurement are considered, whereas a thin cell meshwork has been fixed to the surface of the observed loaded object (thin metal sheet, for instance) and a set of point displacements has been obtained. Based on such experimental evidence, an eight point numerical procedure is proposed in the paper to identify the constitutive relations for small time increments. The presented approach is also applicable in case of micro deformations and facilitates the derivation of nonlinear constitutive equations in stress/strain increments. Approximate error estimation of the procedure is additionally performed, and a test example is given.

JTAM, Sofia, vol. 43 Issue 3 pp. 43-50 (2013), [Full Article]

Orthogonal Dynamic Model of Viscoelastic Deformation under Variable Concentration and Loading

Svetla Lekova1, Aleksander Aleksandrov1, Milena Milenova1, Tzolo Tzolov2
1Dept. Applied Mechanics, UCTM – 1756 Sofia, Bulgaria
2Dept. Polymer Engineering, UCTM – 1756 Sofia, Bulgaria

In the present work, a generalized mathematical model is considered for describing the process of viscoelastic deformation of polymer materials. The model is applied for describing the creep of polyethylene in the conditions of diffusion in liquid environment interacting chemically with the polymer and variable loading. The orthogonal expansions method is used and is extended for a process with integrating properties (without self-settling). In this case, it is necessary to separate the integral component, which is also presented as parameter-dependent on loading and saturation. The output experiments are executed with samples which possess reached beforehand different levels of saturation with cyclohexan and have been afterwards submitted to mechanical testing with brief creep in pure torsion at various constant levels of loading.

JTAM, Sofia, vol. 43 Issue 3 pp. 51-58 (2013), [Full Article]

Long-Term Strength of Polymer Blends from Recycled Materials

R. K. Krastev, S. Djoumaliisky, I. Borovanska
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

This report presents data on the long-term strength of five composites made of plastic waste. They contain low density polyethylene, high density polyethylene, polypropylene and polystyrene (LDPE, HDPE, PP and PS). Long-term strength is determined experimentally by tensile creep to fracture. The experimentally determined long-term strength is compared to predictions for its probabilistic boundaries. The calculation method of these predictions uses data from short-term experiments. The calculated predictions are true for four compositions which exhibit ductile fracture. The composite containing 50 wt.% PS has the greatest strength (of the tested specimens) and has brittle fracture. Its calculated estimate of long-term strength is not consistent with the experimental one.

JTAM, Sofia, vol. 43 Issue 3 pp. 59-66 (2013), [Full Article]

Applied Study on Mechanics of Nanocomposites with Carbon Nanofillers

I. Petrova, E. Ivanov, R. Kotsilkova, Y. Tsekov, V. Angelov
Open Laboratory of Experimental Mechanics (OLEM), Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

The incorporation of small amount of hard and soft carbon nanofillers, such as nanoscale diamond, carbon ash and multiwall carbon nanotubes (MWCNT) have different effect on the surface properties of the neat polyester resin and polypropylene. The soft nanofiller, carbon ash did not influence the wear properties of the polyester resin up to 5 wt% filler content. In contrast, 0.5–5 wt% hard nanodiamond strongly decreased the wear mass loss of the polyester resin. The scratch resistance and the coefficient of friction of the polypropylene is improved significantly while there is 0.1–3 wt% MWCNT. Small amount of MWCNTs change significantly the molecular structure of the polypropylene, producing a nucleation effect. This was found to assist in the changes of surface properties by increasing the hardness, friction and wear of polypropylene.

JTAM, Sofia, vol. 43 Issue 3 pp. 67-76 (2013), [Full Article]

Strength-Deformation Properties of Fine-Grained Fibre-Reinforced Concretes

M. Mironova, V. Naidenov
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

The main strength-deformation properties of fine-grained fiber-reinforced concretes with different type and quantity of fibres, used as repair overlays, are discussed. The results of mechanical properties of experimental compositions are obtained and generalized for two basis ages and standard laboratory environment. The experimental results are mathematical processed using MATLAB procedure. The basic characteristics – residual strength, toughness indexes and residual strength factors are obtained as a function of type as well as quantity of the hybrid fibre reinforcement.

JTAM, Sofia, vol. 43 Issue 3 pp. 77-86 (2013), [Full Article]

Strain Energy Release Rate Determination in the Case of Mode II Crack in Overhanging Bilayered Composite Beam

Victor I. Rizov, Angel S. Mladensky
University of Architecture, Civil Engineering and Geodesy, 1, Chr. Smirnensky Blvd, 1046 Sofia, Bulgaria

Mode II crack in overhanging bilayered composite beam is investigated. The beam has rectangular cross-section and is made by two unidirectional fiber reinforced composites. The formula for strain energy release rate, G, is obtained by linear elastic fracture mechanics compliance technique. The validity of the expression derived is established by comparison with solution for G in which the internal forces in front and behind the crack tip are used. The influence of the two layers moduli of elasticity ratio on the strain energy release rate is investigated. The dependence among the strain energy release rate and the ratio of the lengths of the overhang beam part and the beam span is also analyzed.

JTAM, Sofia, vol. 43 Issue 3 pp. 87-96 (2013), [Full Article]