A multiplanar model for the pore radius distribution in isotropic near-planar stochastic fibre networks

A model is presented for the pore radius distribution in isotropic near-planar stochastic fibre networks. At a given areal density, the mean pore radius of two-dimensional random networks is shown to decrease with increasing fibre width and to increase with increasing fibre linear density.For structures with a structural component in the third dimension the standard deviation of pore radii is shown to be proportional to the mean for changes in areal density and porosity in agreement with data reported in the literature. At a given porosity, near-planar networks exhibit an increase in mean pore radius with increasing fibre width and linear density.     

Computer simulation of effects of the pore size distribution on the kinetics of pressure-assisted final-stage densification

Most theoretical treatments of pressure-assisted densification of porous solids assume a single size for all pores. We remove this assumption and consider a distribution of pore sizes. Dissolution of intragranular pores by volume diffusion and dissolution of intergranular pores by grain-boundary diffusion are both treated. The evolution with time of pore size distributions is calculated for distributions that are initially described by log-normal and Weibull functions, and differences in predicted behaviours are discussed. The pore size distribution is then related to two important quantities: porosity and number of pores per unit volume. The assumption of a distribution of pore sizes is found to avoid certain unrealistic predictions obtained from models with a single pore size, such as abrupt disappearance of all pores and rapid approach to full density.     

Comments on the pore radius distribution in near-planar stochastic fibre networks

The pore radius distribution in near-planar stochastic fibre networks is known to be influenced by changes in the mean number of fibres per unit area and their distribution in the plane. Experimental data is presented that confirms the established result that the standard deviation of pore radii is proportional to the mean. The data shows also that this proportionality is the same for changes in the number of fibres per unit area and for changes in the uniformity of their in-plane distribution. Data from the literature suggests that processes that increase the mean pore radius, increase also the coefficient of variation of pore radii. Theoretical considerations and experimental data are presented that show that the coefficient of variation of pore radii is in fact constant for near-random and non-random stochastic fibre networks.     

Pore size characteristics of nonwoven structures under uniaxial tensile loading

Nonwovens are highly porous structures consisting pores of complex shapes and sizes which are responsible for desired functional characteristics. In general, a nonwoven is often subjected to uniaxial tensile loading in various applications and it is of paramount importance to account for changes in structural characteristics including pore sizes during the loading conditions. In this research work, the pore size of thermally bonded nonwoven structures under uniaxial tensile loading at various levels of strains has been investigated. A theoretical model has been proposed that accounted for fibre reorientation and changes in the fibre volume fraction during the application of tensile strain. A comparison has been made between theoretical and experimental pore size distributions of thermally bonded nonwoven structures at defined levels of strains. Moreover, an attempt has been made to rationalise some of the contradictory literature results of pore size distributions of nonwoven structures under uniaxial tensile loading.     

Electrospun polyimide/titanium dioxide composite nanofibrous membrane by electrospinning and electrospraying

Fibrous membrane with a fibre diameter of 229 +/- 35 nm was fabricated from polyimide solution by electrospinning. Nanofibrous membrane with a fibre diameter of 251 +/- 37 nm was fabricated by combined electrospinning and electrospraying for polyimide/TiO2. Among the different solvents studied, ethanol was the effective solvent for dispersing the TiO2 nanoparticles in the nanofibrous matrix during electrospraying. The average pore size of polyimide membrane was obtained in the range 0.79-0.89 microm whereas the average pore size of polyimide/TiO2 membrane was found to be in the range 1.23 microm. The tensile stress of polyimide nanofibrous membrane and also polyimide/TiO2 composite fibrous membrane determined to be 0.36 MPa and 0.65 MPa respectively. Nanofibrous membrane containing TiO2 nanoparticles on the surface of the polyimide nanofibres improved the mechanical stability of the membrane.     

A model for fibre contact in planar random fibre networks

A model is presented for the expected degree of contact between fibres in isotropic near-planar random fibre networks. The statistics of fibre contact in two-dimensional fibre networks are considered and the expressions derived are developed to allow the fractional contact area in structures formed by the superposition of two-dimensional structures to be derived. These expressions allow the fractional contact area to be expressed in terms of the network porosity only. For thin networks, the fractional contact area may be expressed in terms of network porosity and the expected coverage of the network. Theory permitting the determination of the expected area of one contact and hence the expected number of contacts per fibre is presented also. Good agreement is found between the expressions derived for the fractional contact area and data from the literature.     

Characterization of Nicalon fibres with varying diameters: Part II Modified Weibull distribution

Diameters vary significantly in a tow of commercial NicalonTM fibres, which is one of the most attractive ceramic reinforcements for structural composites. It was found that the strength distribution of Nicalon fibres could not be adequately characterized using either single- or bi-modal Weibull distribution. A recently proposed modified Weibull distribution can account for the effect of varying diameter in the characterization of fibre strength. To verify the validity of the modified Weibull distribution, comprehensive mechanical testing and fractographic studies have been conducted on Nicalon SiC fibres with diameters varying from 8 to 22 m. The experimental results have been reported in Part I. Part II of this paper further modifies the derivation of the modified Weibull distribution to yield a relationship which is similar in form, but soundly based on experimental findings. Factors considered in the modified Weibull distribution include the dependence of fracture toughness and flaw density on fibre diameter, both of which may vary with fibre diameter, as reported in Part I. Comparison with experimental data shows that the current modified Weibull distribution works very well, while both single-modal and bi-modal Weibull distributions are inadequate for describing Nicalon fibres with varying diameters. © 1998 Chapman & Hall     

3D mapping of water in oolithic limestone at atmospheric and vacuum saturation using X-ray micro-CT differential imaging

Determining the distribution of fluids in porous sedimentary rocks is of great importance in many geological fields. However, this is not straightforward, especially in the case of complex sedimentary rocks like limestone, where a multidisciplinary approach is often needed to capture its broad, multimodal pore size distribution and complex pore geometries. This paper focuses on the porosity and fluid distribution in two varieties of Massangis limestone, a widely used natural building stone from the southeast part of the Paris basin (France). The Massangis limestone shows locally varying post-depositional alterations, resulting in different types of pore networks and very different water distributions within the limestone. Traditional techniques for characterizing the porosity and pore size distribution are compared with state-of-the-art neutron radiography and X-ray computed microtomography to visualize the distribution of water inside the limestone at different imbibition conditions. X-ray computed microtomography images have the great advantage to non-destructively visualize and analyze the pore space inside of a rock, but are often limited to the larger macropores in the rock due to resolution limitations. In this paper, differential imaging is successfully applied to the X-ray computed microtomography images to obtain sub-resolution information about fluid occupancy and to map the fluid distribution in three dimensions inside the scanned limestone samples. The detailed study of the pore space with differential imaging allows understanding the difference in the water uptake behavior of the limestone, a primary factor that affects the weathering of the rock.     

Electrospun nanofibrous membranes for temperature regulation of microfluidic seed growth chips

This paper reports a simple and effective approach to control statistical pore size distributions within electrospun nanofibrous membranes (ENMs), by choosing appropriate spinning times of electro-spinning deposition. Mean pore diameter of ENMs decreases exponentially with increasing spinning time. This pore-size control method is demonstrated to regulate amount of heat energy reaching microfluidic seed growth chips (SGC) and thus growth temperature of seeds on the chips, without using sophisticated semiconductor manufacturing techniques or additional on-chip electronic circuits. Decreasing mean pore diameter of ENMs causes to decrease the on-chip temperature, following a second-order polynomial trend. Phenotypic study based on real-time observation of root architecture is conducted on multiple SGCs under various temperature conditions obtained by using ENMs with different pore size distributions.     

Modeling the compression-induced morphological behavior of nonwoven materials

Nonwovens have the most complex morphologies in textile materials and they are often being compressed in a range of applications. The morphology of a typical nonwoven is defined in terms of fiber orientation, fiber volume fraction, number of fiber-to-fiber contacts, distance between the contacts, porosity, and pore size distribution. In this study, an attempt has been made to predict the morphological characteristics of nonwoven materials under the state of compression. A concept of compression ratio has been introduced in predicting the fiber volume fraction at a defined level of compression strain that has significantly influenced the other morphological characteristics of nonwoven materials. A mechanistic model of pore size distribution of nonwoven has been proposed by updating the structural and morphological parameters under predefined compressive stresses. A comparison has been made between theoretical and experimental pore size distributions of compressed nonwoven fibrous materials. In addition, the out-of-plane fiber orientation distribution was experimentally obtained by analyzing the two-dimensional (2D) cross-sections of fibers in the thickness direction.     

The fibre pull-out energy of misaligned short fibre composites

A theoretical study on the fibre pull-out energy has been carried out for short fibre-reinforced composites. Two probability density functions were introduced for modelling the fibre-length distribution and the fibre-orientation distribution. By taking into account the effect of snubbing friction between fibres and matrix at the fibre exit point during fibre pull-out, and that of the fracture stress of fibres obliquely crossing the fracture plane (i.e. the inclined strength of fibres), the fibre pull-out energy of composites has been derived as a function of fibre-length distribution and fibre-orientation distribution, as well as interfacial properties. The previously existing fibre pull-out energy theories can be deduced from the present model. The effects of fibre-length distribution, fibre-orientation distribution, interfacial properties, snubbing-friction coefficient and parameter A for determining the inclined strength of fibres on the fibre pull-out energy, have been studied in detail. The present study provides the necessary information as to which fibre-length distribution, fibre-orientation distribution and interfacial property are required to achieve a desired fibre pull-out energy and hence a desired composite toughness. High-strength fibres, a large fibre-volume fraction and a large fibre diameter for a comparatively large mean fibre length, are shown to be favourable for achieving a high fibre pull-out energy. This revised version was published online in November 2006 with corrections to the Cover Date.     

Relationship between mechanical performance and microstructure in composites fabricated with flow-enhancing fabrics

The resin transfer moulding process involves the long-range flow of resin into a closed mould which is filled with dry reinforcement. High-performance composites require a high volume fraction of fibres, which results in low porosity of the fibre pack and therefore slow rates of mould filling. Commercial reinforcement fabrics are becoming available which promote faster resin flow than conventional fabrics, by engineering regions of large pore space into the reinforcement stack. However, theoretical models of the property-microstructure relationships have indicated that resin-rich areas (corresponding to filled large pore space) and fibre clustering will lead to degradation of the mechanical performance of the laminate. This report describes a series of compression and interlaminar shear tests on a range of twill-weave fabrics having ‘flow-enhancing’ tows substituted in the warp direction. The results provide some experimental support for existing theoretical models.     

Diameter dependence of the apparent tensile modulus of hemp fibres: A morphological,structural or ultrastructural effect?

The aim of this paper is to investigate the origin of the diameter-dependence of Young’s modulus in hemp fibres. In view of the considerable experimental difficulties encountered when determining the 3D morphology of elementary fibres, the influence of the fibre morphology and size on the E-modulus is studied using a mathematical model. An approach based on the 3D elastic theory is used to construct a model of the fibre structure, and to predict its mechanical properties. We clearly show that the modulus is dependent on the size of the lumen and on the outer fibre diameter. This structural effect, induced by the cylindrical geometry, the multi-layered organisation, and the orientation of the cellulose microfibrils only partly explains the large, experimentally determined dispersion of apparent E-modulus, as a function of fibre diameter. Ultrastructural parameters, such as cellulose crystallinity and microfibril angles, are identified to be the main factors involved in this dependence.     

Determination of the fibre orientation distribution of a mineral wool network and prediction of its transverse stiffness using X-ray tomography

A method to determine the orientation and diameter distributions of mineral wool fibre networks using X-ray tomography and image analysis is presented. The method is applied to two different types of mineral wool: glass wool and stone wool. The orientation information is obtained from the computation of the structure tensor, and the diameter is estimated by applying a greyscale granulometry. The results of the image analysis indicate the two types of fibres are distributed in a 2D planar arrangement with the glass wool fibres showing a higher degree of planarity than the stone wool fibres. The orientation information is included in an analytical model based on a Euler–Bernoulli beam approximation. The model enables prediction of the transverse stiffness. It is indicated that the glass wool transverse stiffness is lower than the stone wool transverse stiffness. Comparison with experimental results confirms the assumption that the underlying deformation mechanism of mineral wool is the bending of fibre segments between bonds.     

A novel method for the measurement of elastic moduli of fibres

The elastic modulus of fibres used in composite materials is a parameter of prime importance in the determination of overall mechanical behaviour. However, evaluation of Young’s modulus, E, of a fibre is a delicate operation given the small dimensions (diameter typically a few tens of microns), and therefore low forces involved in tensile testing. This article treats a novel method of modulus assessment involving the bending of fibres, clamped at one extremity, by forced vibrations. The fibre behaves as a beam, and when the forced oscillations approach (one of) the resonant frequency(ies) of the fibre, the bending amplitude increases. Classical beam theory allows evaluation of Young’s modulus from knowledge of resonant frequency, and fibre dimensions and density. Preliminary application of the technique using fibres of E-glass, having well known elastic characteristics, has given good results and shown its inherent potential. Subsequently, the technique developed was used on recycled fibres in order to obtain their Young’s modulus and to assess their loss of mechanical properties when compared to virgin fibres.     

Analysis of the hygroexpansion of a lignocellulosic fibrous material by digital correlation of images obtained by X-ray synchrotron microtomography: application to a folding box board

This study provides original experimental data on the microstructural mechanisms of the hygroexpansion of a material made up of lignocellulosic fibres. A paperboard made up of several layers was chosen and subjected to relative humidity variations during X-ray microtomography scanning. The 3D images of the evolving media were analysed using a digital image correlation technique to measure the displacement field within the studied material. This technique allowed the hygroexpansion of the studied material and of each layer of this latter to be analysed in the in-plane and out-of-plane directions. Results show that the hygroexpansion is highly anisotropic. The microstructural hygroexpansive mechanisms for the pore and fibre phases could also be revealed. They have been shown to depend strongly on the fibre content of the fibrous layers. This analysis provides also useful information concerning the size of the Representative Elementary Volume (REV) for the hygroexpansion phenomenon of dense lignocellulosic fibrous networks. In view of the obtained results, the relevancy of common theoretical models used to predict the hygroexpansion of materials such as papers and boards is also discussed.     

煤基炭泡沫孔结构调控

以肥煤镜质组富集物为前驱体, 采用高压渗氮法制备煤基炭泡沫, 研究了发泡温度、发泡压力和发泡时间对炭泡沫孔结构的影响。利用SEM观察炭泡沫的孔胞形貌, 同时利用Nano Measurer分析软件统计SEM照片孔胞直径分布和孔喉直径分布以及平均孔径。结果表明: 微孔塑料成核理论可以定性解释炭泡沫的孔结构变化趋势。发泡温度的升高导致成核密度增加, 同时导致气体在胶质体的溶解度降低, 不利于孔胞长大。发泡压力的增大导致炭泡沫的孔胞密度增加, 临界成核半径降低, 同时加剧了热聚合反应, 导致胶质体的粘度增大, 不利于孔胞长大。发泡时间的延长会使热聚合更加充分, 影响胶质体粘度, 进而影响孔结构。     

The size effects on the mechanical behaviour of fibres

The size of a fibre affects its mechanical properties and thus is of theoretical and practical importance for studies of the rupturing process during loading of a fibrous structure. This paper investigates the overall effects of length on the mechanical behaviour of single fibres. Four types of fibres, ranging from brittle to highly extensible, were tested for their tensile properties at several different gauge lengths. Different from most previous studies where the focus has been on the gauge length effects on a single property such as fibre strength or breaking strain, this paper look comprehensively into the effects of length on all three of the most commonly studied mechanical properties, namely strength, breaking strain and initial modulus. Particular emphasis is placed on initial modulus and on the interactions between all three parameters. Influences of strain rate and fibre type on the size effects are also investigated. The effect of potential fibre slippage on experimental error is examined. An image analysis method is used to measure the real fibre elongation in comparison to the same fibre elongation obtained directly from an Instron tester. Finally, a statistical analysis is carried out using the experimental data to test the fitness of the Weibull theory to polymeric fibres. This was done as the Weibull model has been extensively utilized in examining fibre strength and breaking strain, although it is supposed to be valid only for the so-called classic fibres to which more extensible polymeric fibres do not belong. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fabrication and in vivo osteogenesis of biomimetic poly(propylene carbonate) scaffold with nanofibrous chitosan network in macropores for bone tissue engineering

A biomimetic poly(propylene carbonate) (PPC) porous scaffold with nanofibrous chitosan network within macropores (PPC/CSNFs) for bone tissue engineering was fabricated by a dual solid–liquid phase separation technique. PPC scaffold with interconnected solid pore wall structure was prepared by the first phase separation, which showed a high porosity of 91.9% and a good compressive modulus of 14.2 ± 0.56 MPa, respectively. By the second phase separation, nanofibrous chitosan of 50–500 nm in diameter was formed in the macropores with little influence on the pore structure and the mechanical properties of PPC scaffold. The nanofibrous chitosan content was calculated to be 9.78% by elemental analysis. After incubation in SBF for 14 days, more apatite crystals were deposited on the pore surface as well as the nanofibrous chitosan surface of PPC/CSNFs scaffold compared with PPC scaffold. The in vitro culture of bone mesenchymal stem cells showed that PPC/CSNFs scaffold exhibited a better cell viability than PPC scaffold. After implantation in rabbits for 16 weeks, the defect was entirely repaired by PPC/CSNFs scaffold, as opposed to the incomplete healing for PPC scaffold. It indicated that PPC/CSNFs scaffold showed a faster in vivo osteogenesis rate than PPC scaffold. Hereby, PPC/CSNFs scaffold will be a potential candidate for bone tissue engineering.