Mechanics of Random Discontinuous Long-Fiber Thermoplastics—Part I: Generation and Characterization of Initial Geometry

2013 ◽  
Vol 80 (5) ◽  
Author(s):  
Ahmed I. Abd El-Rahman ◽  
Charles L. Tucker
Keyword(s):  
Aspect Ratio ◽  
Spatial Correlation ◽  
Distribution Functions ◽  
Random Structure ◽  
Quasi Equilibrium ◽  
Fiber Aspect Ratio ◽  
Metropolis Monte Carlo ◽  
Long Fiber Thermoplastics ◽  
Deformation Mechanics ◽  
Orientational Correlation

The deformation mechanics of dry networks of large-aspect-ratio fibers with random orientation controls the processing of long-fiber thermoplastics (LFTs) and greatly affects the mechanical properties of the final composites. Here, we generate initial geometries of fiber networks in a cubic unit cell with a fiber aspect ratio of l/d = 100 and fully periodic boundary conditions for later numerical simulation. The irreversible random sequential adsorption (RSA) process is first used to generate a quasi-random structure due to the excluded-volume requirements. In order to investigate the nonequilibrium character of the RSA, a second method, which is similar to the mechanical contraction method (MCM) (Williams and Philipse, 2003, “Random Packings of Spheres and Spherocylinders Simulated by Mechanical Contraction,” Phys. Rev. E, 67, pp. 1–9) and based on a simplified Metropolis Monte Carlo (MC) simulation is then developed to produce quasi-equilibrium fiber geometries. The RSA packing results (ϕ ≈ 4.423% when using a fiber aspect ratio of 100) are in good agreement with the maximum unforced random packing limits (Evans and Gibson, 1986, “Prediction of the Maximum Packing Fraction Achievable in Randomly Oriented Short-Fibre Composites,” Compos. Sci. Technol., 25, pp. 149–162). The fiber structures were characterized by several distribution functions, including pair-spatial and pair-orientation distributions, based on either the center-to-center distance or the shortest distance between the particles. The results show that the structures generated by the RSA have an easily-detectable long-range spatial correlation but very little orientational correlation. In contrast, the quasi-equilibrium structures have reduced spatial correlation but increased short-range orientational correlation.

Disorientations in dislocation structures: Formation and spatial correlation

2002 ◽  
Vol 17 (9) ◽  
pp. 2433-2441 ◽  
Author(s):  
Wolfgang Pantleon
Keyword(s):  
Plastic Deformation ◽  
Spatial Correlation ◽  
Distribution Functions ◽  
Scaling Behavior ◽  
Dislocation Dynamics ◽  
Slip Systems ◽  
Experimental Findings ◽  
Good Agreement

During plastic deformation, dislocation boundaries are formed and orientation differences across them arise. Two different causes lead to the formation of two kinds of deformation-induced boundaries: a statistical trapping of dislocations in incidental dislocation boundaries and a difference in the activation of slip systems on both sides of geometrically necessary boundaries. On the basis of these mechanisms, the occurrence of disorientations across both types of dislocation boundaries is modeled by dislocation dynamics. The resulting evolution of the disorientation angles with strain is in good agreement with experimental observations. The theoretically obtained distribution functions for the disorientation angles describe the experimental findings well and explain their scaling behavior. The model also predicts correlations between disorientations in neighboring boundaries, and evidence for their existence is presented.

Modeling the Large Deformation and Microstructure Evolution of Nonwoven Polymer Fiber Networks

2018 ◽  
Vol 86 (1) ◽  
Author(s):  
Mang Zhang ◽  
Yuli Chen ◽  
Fu-pen Chiang ◽  
Pelagia Irene Gouma ◽  
Lifeng Wang
Keyword(s):  
Mechanical Properties ◽  
Aspect Ratio ◽  
Mechanical Behavior ◽  
Large Deformation ◽  
Biological Tissues ◽  
High Porosity ◽  
Polymer Fiber ◽  
Fiber Networks ◽  
Fiber Aspect Ratio ◽  
Network Microstructure

The electrospinning process enables the fabrication of randomly distributed nonwoven polymer fiber networks with high surface area and high porosity, making them ideal candidates for multifunctional materials. The mechanics of nonwoven networks has been well established for elastic deformations. However, the mechanical properties of the polymer fibrous networks with large deformation are largely unexplored, while understanding their elastic and plastic mechanical properties at different fiber volume fractions, fiber aspect ratio, and constituent material properties is essential in the design of various polymer fibrous networks. In this paper, a representative volume element (RVE) based finite element model with long fibers is developed to emulate the randomly distributed nonwoven fibrous network microstructure, enabling us to systematically investigate the mechanics and large deformation behavior of random nonwoven networks. The results show that the network volume fraction, the fiber aspect ratio, and the fiber curliness have significant influences on the effective stiffness, effective yield strength, and the postyield behavior of the resulting fiber mats under both tension and shear loads. This study reveals the relation between the macroscopic mechanical behavior and the local randomly distributed network microstructure deformation mechanism of the nonwoven fiber network. The model presented here can also be applied to capture the mechanical behavior of other complex nonwoven network systems, like carbon nanotube networks, biological tissues, and artificial engineering networks.

Mechanical Properties of Electrospun Carbon Nano Fiber (ECNF)/Epoxy Nanocomposites

2007 ◽  
Author(s):  
Darunee Aussawasathien ◽  
Erol Sancaktar
Keyword(s):  
Mechanical Properties ◽  
Aspect Ratio ◽  
Epoxy Resin ◽  
Carbon Nanofiber ◽  
Mechanical Analysis ◽  
Short Fiber ◽  
Neat Epoxy ◽  
Epoxy Nanocomposites ◽  
Cure Reaction ◽  
Fiber Aspect Ratio

Electrospun polyacrylonitrile (PAN) fiber precursor based Carbon Nanofiber (CNF) mats were produced and impregnated with epoxy resin. The mechanical properties of as-prepared nanofibers in the mat and short fiber filled epoxy nanocomposite forms were determined to demonstrate the effect of fiber aspect ratio and interconnecting network on those properties. Our experimental results reveal that epoxy nanocomposites containing Electrospun Carbon Nano Fibers (ECNF) with high fiber aspect ratio and high interconnecting network in the non-woven mat form yield better mechanical properties than those filled with short ECNFs. The ECNF mat in epoxy nanocomposites provides better homogeneity, more interlocking network, and easier preparation than short ECNFs. Mechanical properties of ECNF mat-epoxy nanocomposites, which we obtained using tensile and flexural tests, such as stiffness and modulus increased, while toughness and flexural strength decreased, compared to the neat epoxy resin. Dynamic Mechanical Analysis (DMA) results showed, higher modulus for ECNF mat-epoxy nanocomposites, compared to those for neat epoxy resin and short ECNF-epoxy nanocomposites. The epoxy nanocomposites had high modulus, even though the glass transition temperature, Tg values dropped at some extents of ECNF mat contents when compared with the neat epoxy resin. The cure reaction was retarded since the amount of epoxy and hardener decreased at high ECNF contents together with the hindering effect of the ECNF mat to the diffusion of epoxy resin and curing agent, leading to low crosslinking efficiency.

Theoretical Assessment of Stress Analysis in Short Fiber Composites

2004 ◽  
Vol 261-263 ◽  
pp. 1421-1426
Author(s):  
Hong Gun Kim ◽  
Sung Mo Yang ◽  
Hong Gil Noh ◽  
Dong Joo Lee
Keyword(s):  
Stress Concentration ◽  
Aspect Ratio ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
Stress Transfer ◽  
Shear Lag ◽  
Modulus Ratio ◽  
Fiber Aspect Ratio ◽  
Shear Lag Model ◽  
Lag Model

An investigation of composite mechanics to investigate stress transfer mechanism accurately, a modification of the conventional shear lag model was attempted by taking fiber end effects into account in discontinuous composite materials. It was found that the major shortcoming of conventional shear lag theory is not being able to provide sufficiently accurate strengthening predictions in elastic regime when the fiber aspect ratio is very small. The reason is due to its neglect of stress transfer across the fiber ends and the stress concentrations that exist in the matrix regions near the fiber ends. To overcome this shortcoming, a more simplified shear lag model introducing the stress concentration factor which is a function of several variables, such as the modulus ratio, the fiber volume fraction, the fiber aspect ratio, is proposed. It is found that the modulus ratio is the most essential parameter among them. Thus, the stress concentration factor is expressed as a function of modulus ratio in the derivation. It is also found that the proposed model gives a good agreement with finite element results and has the capability to correctly predict the variations of the internal quanitities.

REGULAR VIRTUAL COORDINATE SYSTEM OF COORDINATES OVER THE FIELD OF MOVING VISUAL LANDMARKS

2020 ◽  
pp. 11-17
Author(s):  
T. A. Tikhomirova ◽  
G. T. Fedorenko ◽  
K. M. Nazarenko ◽  
E. S. Nazarenko
Keyword(s):  
Coordinate System ◽  
Random Structure ◽  
Quasi Equilibrium ◽  
Global Instability ◽  
Visual Landmarks ◽  
T Matrix ◽  
Grid Nodes ◽  
Special Case ◽  
Over Time ◽  
Virtual Coordinate

The problem objects tracing in respect to large sample of visual landmarks whose position, shape, and optical properties change over time is considered. A Regular Virtual Coordinate System (RVCS) based on visual landmarks triangulation is proposed. RVCS is created using a linear iterative algorithm to relax initial triangulation to the quasi equilibrium state – shagreen, in which the grid nodes are located close to centers of their surroundings. It is shown that shagreen has a dense boundary with strongly elongated cells and the central core with almost regular triangles. We proposed to use the shagreen core as a virtual barycentric mesh because its geometry is regular and resistant to random displacements of visual landmarks, while correctly mirror affinity transformations. The use of RVCS allows to minimize the amount of information for representing the object trajectory while maintaining the accuracy of its localization close to the maximum achievable when using all available landmarks. It is shown that due to its global instability of the shagreen with random structure the triangulations with less then 100 vertices should be relaxed by no more then T = 10 – 20 simple iterations. This process can be represented by single linear operator which in special case T = 2t is created by t matrix multiplications. In addition, shagreen indicates the smooth trend for rough landscape, which increases the reliability of the estimation of local normals.

Electrical Conductivity of Barium-Ferrite-Vulcanized Polychloroprene Filled with Short Carbon Fiber

1992 ◽  
Vol 65 (1) ◽  
pp. 7-23 ◽  
Author(s):  
P. B. Jana ◽  
S. K. De ◽  
S. Chaudhuri ◽  
A. K. Pal
Keyword(s):  
Carbon Fiber ◽  
Aspect Ratio ◽  
Carrier Concentration ◽  
Barium Ferrite ◽  
Volume Resistivity ◽  
Positive Temperature ◽  
Short Carbon Fiber ◽  
Fiber Aspect Ratio ◽  
Mixing Method ◽  
Short Carbon

Abstract Short-carbon-fiber-filled polychloroprene composites, prepared by mill-mixing and cement-mixing methods were vulcanized by barium ferrite. The effects of concentration and aspect ratio (L/D) of short carbon fiber on volume resistivity have been studied. Composites prepared by the cement mixing method having high fiber aspect ratio (L/D 100) show higher hardness and modulus accompanied by lower elongation and volume resistivity than the composites prepared by the mill mixing method, which give low fiber aspect ratio (L/D 25). Experimental values of volume resistivity agreed reasonably well with the calculated values from the theory based on probability of formation of conductive network. The temperature dependence of volume resistivity of the composite shows a positive temperature coefficient. The shape of the hysteresis loop, observed during the heating and cooling cycle in the variation of volume resistivity with temperature, depends on the concentration and aspect ratio of short carbon fiber. Hall-effect studies indicate that increase in fiber concentration and aspect ratio in the composites causes increase in carrier concentration and decrease in mobility of the carriers. With increase in temperature, both conductivity and carrier concentration decrease and mobility of the carriers increases.

Reynolds number scaling in cryogenic turbulent Rayleigh–Bénard convection in a cylindrical aspect ratio one cell

2017 ◽  
Vol 832 ◽  
pp. 721-744 ◽  
Author(s):  
Věra Musilová ◽  
Tomáš Králík ◽  
Marco La Mantia ◽  
Michal Macek ◽  
Pavel Urban ◽  
...  
Keyword(s):  
Aspect Ratio ◽  
Large Scale ◽  
Temperature Fluctuations ◽  
Reynolds Numbers ◽  
Distribution Functions ◽  
Bénard Convection ◽  
Benard Convection ◽  
Helium Gas ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

We perform an experimental study of turbulent Rayleigh–Bénard convection up to very high Rayleigh number, $10^{8}<Ra<10^{14}$, in a cylindrical aspect ratio one cell, 30 cm in height, filled with cryogenic helium gas. We monitor temperature fluctuations in the convective flow with four small (0.2 mm) sensors positioned in pairs 1.5 cm from the sidewalls and 2.5 cm vertically apart and symmetrically around the mid-height of the cell. Based on one-point and two-point correlations of the temperature fluctuations, we determine different types of Reynolds numbers, $\mathit{Re}$, associated with the large-scale circulation (LSC). We observe a transition between two types of $\mathit{Re}(\mathit{Ra})$ scaling around $\mathit{Ra}=10^{10}{-}10^{11}$, which is accompanied by a scaling change of the skewness of the probability distribution functions (PDFs) of the temperature fluctuations. The $\mathit{Re}(\mathit{Ra})$ dependencies measured near the sidewall at Prandtl number $\mathit{Pr}\sim 1$ are consistent with the $\mathit{Ra}^{4/9}\mathit{Pr}^{-2/3}$ scaling above the transition, while for $\mathit{Ra}<10^{10}$, the $\mathit{Re}(\mathit{Ra})$ dependencies are steeper. It seems likely that this change in $\mathit{Re}(\mathit{Ra})$ scaling is linked to the previously reported change in the Nusselt number $\mathit{Nu}(\mathit{Ra})$ scaling. This behaviour is in agreement with independent cryogenic laboratory experiments with $\mathit{Pr}\sim 1$, but markedly different from the $\mathit{Re}$ scaling obtained in water experiments ($\mathit{Pr}\sim 3.3{-}5.6$). We discuss the results in comparison with different versions of the Grossmann–Lohse theory.

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