Reinforcement permeability

For random mat non-woven reinforcements, permeability is isotropic in-plane while, for other textile structures, the permeability will be different in different directions, depending upon the nature of the textile structure (number and size of tows, warp and weft densities etc.). This differential permeability will result in complex flow patterns in the tool, making flow prediction even more important, although the use of D Arcy s equation then becomes an over-simplification. 

The determination of reinforcement permeability is a requirement for any LCM simulation. Fibrous reinforcements typically have different permeability in different directions, due to the arrangement and orientation of the fibres and the presence of stitching. For most fibrous reinforcement, three principal permeability coefficients are defined and 33 for the three principal... 

Buoyancy in some form is employed in nearly all categories of underwater and surface systems to support them above the ocean bottom or to minimize their submerged weight. The buoyant material can assume many different structural forms utilizing a wide variety of densities. The choice of materials is severely restricted by operational requirements, since different environmental conditions exist. For example, lighter, buoyant liquids can be more volatile than heavier liquids. This factor can have a deleterious effect on a steel structure by accelerating stress corrosion or increasing permeability in reinforced plastics. 

Exfoliating layered particles such as the clays, mica, or graphite is being used to provide very effective reinforcement of elastomers at loading levels much smaller than in the case of solid particles such as carbon black and silica [228-231]. Other properties can also be substantially improved, including increased resistance to solvents, and reduced permeability and flammability. 

Two principal types of fabric are adaptable to filter use woven fabrics, which are used in shaker and reverse-flow filters and felts, which are used in reverse-pulse filters. The felts made from synthetic fibers are needle felts (i.e., felted on a needle loom) and are normally reinforced with a woven insert. The physical properties and air permeabilities of some typical woven and felt filter fabrics are presented in Tables 17-6 and 17-7. The air permeability of a filter fabric is defined as the flow rate of air in cubic feet per minute (at 70°F, 1 atm) that will pass through 1 ft2 of clean fabric under an applied differential pressure of Vt in water. The resistance coefficient KF of the clean fabric is defined by the equation in Table 17-6, which may be used to calculate the value of KF from the air permeability. If Ap, is taken as 0.5 in water, t as 0.0181 cP (the viscosity of air at 70°F and 1 atm), and Vj as the air permeability, then //, = 27.8/air permeability. 

Ionic polymers other than Nation have also been included in ionic/non-ionic PEM blends. Poly(ether sulfone) (PES) has been used to strengthen SPEEK as well as sulfonated poly(ether sulfone) (SPES) with contents ranging from 20 to 60 wt%. The conductivity of the SPEEK component was relatively the same as unmodified SPEEK up to about 40 wt%. A similar effect was seen for PES/SPES blends, although the drop in MeOH permeability was more dramatic for PES/SPES from unmodified SPES than for PES/SPEEK from unmodified SPEEK. PVDF has also been used as a blending material to reinforce SPEEK. s The strength of the PEM was increased over unmodified SPEEK. Although conductivity levels decreased as a function of increasing PVDF content, the selectivity (ratio of proton conductivity to MeOH permeability) of the blended PEMs was increased over that of unmodified SPEEK and Nation. 

The protection of steel reinforcements. Concrete produces a layer of passivity at the steel/concrete interface and any breakdown of this can increase the chance of reinforcement corrosion. In addition, it is important that concrete be maintained in a state of low permeability to minimize the passage of moisture and air to the steel. 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

Under conditions of internal restraint, the expansion produced is proportional to the ratio of steel to concrete and the dosage of the admixture. Special care should be taken to ensure that reinforcement is located in its proper position during placement and consolidation so that adequate restraint and good bond to steel is obtained. Restrained expansion increases the density of the matrix and produces concrete or mortar with a lower coefficient of permeability than that of corresponding Portland cement concretes and mortar. 

To improve the quality of hardened concrete such as increased early and long-term strengths and modulus of elasticity, decrease permeability (and hence inhibit corrosion of embedded steel reinforcement) and absorption, increase abrasion resistance and increase bond with reinforcement. 

A typical automobile tyre consists of several parts (Fig. 5). The tread and sidewall make up the outside of the tyre. The tread is the portion of the tyre that comes into contact with the road, whereas the sidewall, or wall, is the side of the tyre that connects the tread and the bead. The tread is reinforced from underneath by the belt, which comprises a set of fabrics and/or wires. The plies, found in association with the belt, are layers of rubber-coated cords, which are typically made of fabric, polymer, fibreglass, or steel. The set of belts and plies is often collectively referred to as the cord, or cord body, of the tyre. The cords of the tyre wrap around the bead, the part of the tyre that rests on the tyre rim. The bead consists of both mbber bead filler and a series of steel bead wires. The last major part of the tyre, the innerliner, is a low-permeability... 

The coefficient of proportionality K is called the permeability of the reinforcement. According to theory [5] K is only dependent on the geometry between the fibers in the reinforcement (the pore space ). Several models for the dependence of K on the fiber volume fraction Vf has been proposed. The most-cited model is the so-called Kozeny-Carman model [16,17], which predicts a quadratic dependence on the fiber radius R in addition to the dependence on Vf... 

The best way to use the Kozeny-Carman model and other permeability models (e.g. the anisotropic model by Gebart) [18], is to use them as interpolation formulas for intermediate volume fractions between known values. Extrapolation should be done with extreme caution because the models are developed for idealized reinforcements. Typical values for the permeability of different types of reinforcement are given in Table 12.1. 

The permeability can be determined experimentally in several different ways (e.g., in a radial flow [19] or unidirectional flow experiment [20]). The experiments can also be done with either an advancing flow front (wetting flow) or a fully saturated reinforcement under steady-state conditions. There is some debate in the scientific community whether the... 

Table 12.1 Typical Permeability Data for Some Reinforcement Materials...

The flow instability can best be understood by looking at a case with unidirectional flow (see Fig. 12.7). There will always be some nonuniformity between the sides. This will result in the flow front moving faster on one side of the core than it does on the other. The net force on the core from the resin will be higher on the side where the flow front has moved the farthest and as a result the core will be pushed away from this side. The displacement of the core will increase the permeability more and the flow front will move even farther ahead on the fast side, and so on. The process will reach an equilibrium when the reaction force from the reinforcement becomes large enough to balance the fluid pressure on the other side of the core. 

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