Mechanics of Advanced Materials and Modern Processes最新文献

Fiber–reinforced polymer composite materials are widely used in different branches of industry due to their distinctive features such as high specific strength and stiffness and due to as considerable opportunity to formulate materials with controllable variation of properties in response to the action of external factors (smart-materials). A distinguishing feature of products made of composite materials is that the processes of product and material fabrication are inseparable. Therefore the estimation of composite properties based on the composite architecture and properties of the reinforcing fibers and matrix is a very actual task.

The model of polymer behavior at glass transition recently developed by the authors was generalized to the case of fiber-reinforced polymer matrix composites using two approaches: one is base on the concept of free specific energy, the other – on the growth of matrix stiffness. For homogeneous materials these two approaches are of equal worth, whereas for composite materials they give different results under deformation in the transverse direction. The stiffness growth approach is more accurate, but is very expensive computationally and, is highly sensitive to the experimental data errors.

Using the finite element method and averaging technique the thermoelastic constants of composites containing different types of fibers in the glassy and high-elastic states were calculated based on the fiber and matrix properties. Softening of the matrix has an insignificant effect on the longitudinal modulus of a composite but leads to a considerable decrease of the transverse and shear moduli. The coefficient of thermal expansion in the transverse direction is much higher than the coefficient of thermal expansion in the longitudinal direction, especially when the composite is in the high-elastic state.

The model of polymer behavior at glass transition recently developed by the authors can be generalized to the case of fiber-reinforced polymer matrix composites. The thermoelastic constants of composites containing different types of fibers can be calculated from the fiber and matrix properties using the finite element method and averaging technique.

Significant amount of research, both experimental and numerical, has been conducted to study the mechanical behaviour of biodegradable polymer PL(L)A due to its wide range of applications. However, mechanical brittleness or poor elongation of PL(L)A has limited its applications considerably, particularly in the biomedical field. This study aims to study the potential in improving the ductility of PLA by blending with PBS in varied weight ratios.

The preparation of PLA and PBS blends, with various weight ratios, was achieved by melting and mixing technique at high temperature using HAAKE? Rheomix OS Mixer. Differential Scanning Calorimetry (DSC) was applied to investigate the melting behaviour, crystallization and miscibility of the blends. Small dog-bone specimens, produced by compression moulding, were used to test mechanical properties under uniaxial tension. Moreover, an advanced viscoplastic model with nonlinear hardening variables was applied to simulate rate-dependent plastic deformation of PLA/PBS blends, with model parameters calibrated simultaneously against the tensile test data.

Optical Microscopy showed that PBS composition aid with the crystallization of PLA. The elongation of PLA/PBS blends increased with the increase of PBS content, but with a compromise of tensile modulus and strength. An increase of strain rate led to enhanced stress response, demonstrating the time-dependent deformation nature of the material. Model simulations of time-dependent plastic deformation for PLA/PBS blends compared well with experimental results.

The crystallinity of PLA/PBS blends increased with the addition of PBS content. The brittleness of pure PLA can be improved by blending with ductile PBS using mechanical mixing technique, but with a loss of stiffness and strength. The tensile tests at different strain rates confirmed the time-dependent plastic deformation nature of the blends, i.e., viscoplasticity, which can be simulated by the Chaboche viscoplastic model with nonlinear hardening variables.

Nickel-based superalloys are usually exposed to high static or cyclic loads in non-ambient environment, so a reliable prediction of their mechanical properties, especially plastic deformation, at elevated temperature is essential for improved damage-tolerance assessment of components.

In this paper, plastic deformation in a single-crystal nickel-based superalloy CMSX4 at elevated temperature was modelled using discrete dislocation dynamics (DDD). The DDD approach was implemented using a representative volume element with explicitly-introduced precipitate and periodic boundary condition. The DDD model was calibrated using stress–strain response predicted by a crystal plasticity model, validated against tensile and cyclic tests at 850?°C for <001?>?and <111?>?crystallographic orientations, at a strain rate of 1/s.

The DDD model was capable to capture the global stress–strain response of the material under both monotonic and cyclic loading conditions. Considerably higher dislocation density was obtained for the <111?>?orientation, indicating more plastic deformation and much lower flow stress in the material, when compared to that for <001?>?orientation. Dislocation lines looped around the precipitate, and most dislocations were deposited on the surface of precipitate, forming a network of dislocation lines. Simple unloading resulted in a reduction of dislocation density.

Plastic deformation in metallic materials is closely related to dynamics of dislocations, and the DDD approach can provide a more fundamental understanding of crystal plasticity and the evolution of heterogeneous dislocation networks, which is useful when considering such issues as the onset of damage in the material during plastic deformation.

The machining of conductive alumina ceramic was successful by the electric discharge grinding (EDG). Therefore, the aim of the present work is to increase the material removal rate (MRR) during EDG of conductive alumina ceramic by addition of ceramic powder with dielectric.

To achieve the objective through experimental investigation is carried out and the influence of input process parameters (powder concentration, duty ratio, pulse on time, table speed and wheel speed) on surface roughness (SR), MRR and surface integrity has been studied. The fine grade silicon carbide powder of #1000?mesh sizes was mixed in dielectric medium with varying concentration to understand the influence of the powder concentration and its interaction with other process parameters during powder mixed electric discharge grinding (PMEDG). The central composite rotatable design (CCRD) has been used to plan the experiments. Optimization of the obtained statistical models of MRR and SR has been done to obtain highest MRR and lowest SR.

It was observed that the MRR achieved by PMEDG was 3 – 10 times higher than EDG. It was found that all the process factors and interactions show significant contribution on SR. The SR obtained by PMEDG was 2 – 4 times higher than EDG.

It has been established that the PMEDG process is a better option for processing of Al₂O₃–SiC_w–TiC ceramic material as preliminary operation before EDG to achieve high MRR. In the present work the surface and subsurface damages were also assessed and characterized by the scanning electron microscope (SEM).

In an aerospace industry, the efficient use of thermally sprayed coatings for high-temperature applications is achieved by improving the thermal characteristics (TC) such as thermal drop/barrier (TD) and thermal fatigue cycles (TFC). The characterization of ceramic coatings demands a better understanding of TC and their performance.

In this paper, an attempt has been made to use hybrid Taguchi design method based grey relation analysis (GRA) for optimizing the control factors such as the thickness of coating, type of coating, bond coating and exposed temperature. The necessary experiments were carried out using Taguchi L16 factorial design of experiments for analysis based on the larger the better signal-to-noise (S/N) ratio. The multi-response/output optimization and grading of control factors were successfully carried out by GRA.

The significance of each factor as regards TD and TFC were investigated. The ANOVA results showed that most important parameters at 95?% confidence level and were validated with a confirmation test using optimum process factors. It shows an improvement in the TC of thermal barrier coatings.

This work revealed that the hybrid GRA with Taguchi technique had improved the durability and efficient usage of TBC for high-temperature applications.

ZnO/CNT nanocomposites were prepared using Zinc acetate source materials and with the assistance of copper plate, glycine and sugar solution. The combined behavior between these two materials may give rise to the production of advanced materials with a wide range of applications in electronics and optoelectronics.

The ZnO-CNT nanostructures are successfully prepared by simple perfume spray pyrolysis method on copper substrate. The possible growth mechanism of ZnO-CNT nanocrystals formation by this method has been tried to explore the sensor and optical properties has been demonstrated.

The as-synthesized ZnO-CNT nanostructures were characterized using the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) pattern measured with Cu Kα radiation. Studies of the morphologies of the ZnO-coated CNTs revealed no significant change in the internal structures single walled graphite sheets and the diameters of the CNTs, but the ZnO appeared to form a layer of thinfilm single crystalline particles attaching to the surface of the nanotubes. The photoluminescence (PL) measurements excited by the 380?nm were done at room temperature. CNTs are easy to be entangled and agglomerate due to their long length and low diffusive mobility in base fluids.

The lower mobility was found to occur for the ZnO/CNT composite where a linear sensitivity behavior was measured and it reaches high at the temperature of 200 °C. The samples luminescence is dominated by well-structured ultraviolet band emission and almost no deep level emission was observed, revealing a high optical quality of the produced structures.

In this paper the mechanical properties of armco-iron are studied experimentally. The ultrasonic testing machine USF-2000 was used to carry out fatigue tests involving 10⁶–10¹⁰ loading cycles. The frequency of loading is 19.5?±?0.5?kHz. Based on the experimental data, the W?hler curve was obtained.

For the analysis of physical condition, two new sensors were designed, one of which measured the electric resistance of the sample and the other measured the magnetic permeability. Second measuring method can be applied to ferromagnetic materials only, though it may be useful for studying the mechanical properties of metals in general.

It was shown that the significant changes in physical processes accompanying the evolution of structural defects in the material were observed in the final stages of the experiment.

The applied measurement techniques allowed us to exactly determine the time of fatigue crack initiation below the surface the material, which cannot be monitored by the standard optic methods.

Glass and rubbery transitions under cooling and heating of polymeric materials underlie a shape memory effect, that is a material ability to save temporarily the deformed shape and restore the original one under the external influence. The present work aims to model the shape memory effect for an axially compressed polymeric rod in its post-buckled equilibrium state, which is the generalization of Euler’s elastica for a glassy material case.

For modeling, we use a new type of constitutive relations describing the thermomechanical behavior of amorphous polymers over a wide temperature range. To define the model parameters for lightly-linked epoxy resin a series of experiments was conducted using the Dynamic Mechanical Analyzer.

Post-buckled states of an epoxy rod equilibrium during the temperature change have been found from numerical simulation. The obtained results illustrate the shape memory effect in case of axially compressed rod buckling.

The thermomechanical shape-memory cycle includes the stages of deformation development and preservation and the subsequent recovery of the initial shape. According to the obtained results, maximum deflection corresponds to the first loading step at the rubbery material state, because the elastic modulus is very low. During cooling under a constant load the deformation remains constant. After unloading in glassy state the deflection decreases by a small value, because the glassy elastic modulus significantly exceeds the rubbery one. During subsequent heating the rod recovers its initial undeformed shape.

Understanding of viscoelastic behaviour of a periodontal membrane under physiological conditions is important for many orthodontic problems. A new analytic model of a nearly incompressible viscoelastic periodontal ligament is suggested, employing symmetrical paraboloids to describe its internal and external surfaces.

In the model, a tooth root is assumed to be a rigid body, with perfect bonding between its external surface and an internal surface of the ligament. An assumption of almost incompressible material is used to formulate kinematic relationships for a periodontal ligament; a viscoelastic constitutive equation with a fractional exponential kernel is suggested for its description.

Translational and rotational equations of motion are derived for ligament’s points and special cases of translational displacements of the tooth root are analysed. Material parameters of the fractional viscoelastic function are assessed on the basis of experimental data for response of the periodontal ligament to tooth translation. A character of distribution of hydrostatic stresses in the ligament caused by vertical and horizontal translations of the tooth root is defined.

The proposed model allows generalization of the known analytical models of the viscoelastic periodontal ligament by introduction of instantaneous and relaxed elastic moduli, as well as the fractional parameter. The latter makes it possible to take into account different behaviours of the periodontal tissue under short- and long-term loads. The obtained results can be used to determine loads required for orthodontic tooth movements corresponding to optimal stresses, as well as to simulate bone remodelling on the basis of changes in stresses and strains in the periodontal ligament caused by such movements.

Absorption of laser energy into the worksheet surface during laser bending process is an important and critical factor for accurate computation of the bend angle. This paper presents an integrated FEM-ANN approach to compute accurate value of bend angle during laser bending process.

Initially, a finite element method (FEM) based three-dimensional nonlinear transient thermo-mechanical numerical model is developed using ABAQUS package. Using FEM model and data obtained in actual experiments, the proper values of absorptivity for various sets of process conditions are computed by inverse analysis technique. Based on the proper values of absorptivity, an artificial neural network (ANN) model is developed for accurate and quick prediction of absorptivity for given input process conditions. The predicted absorptivity is then employed in the FEM model for accurate computation of bend angle.

The performance of the integrated approach is verified by conducting experiments.

The verification results showed that the proposed approach is able to compute the bend angle with a very good accuracy (average prediction error of 4.14?%). The proposed approach can also be suitable for the numerical simulations of other laser based manufacturing processes.