Stiffness of fabrics

ASTM D1388-96, Standard Test Method for Stiffness of Fabrics, American Society for Testing Materials, West Conshohocken, Pennsylvania, USA, 1996. 

Sondheim, W.S., 1941. The influence of structure on the stiffness of fabrics. Ph.D. Thesis, The University of Manchester, Manchester, UK. 

Transverse Dimensions or Fineness. Historically, the quantity used to describe the fineness or coarseness of a fiber was the diameter. Eor fibers that have irregular cross-sections or that taper along their lengths, the term diameter has no useful meaning. Eor cylindrical fibers, however, diameter is an accurate measurement of the transverse dimension. Though textile fibers can be purchased in a variety of cross-sectional shapes, diameter is stiU a useful descriptor of the transverse dimension. Eiber diameter is important in determining not only the ease with which fibers can be twisted in converting them to yams, but also fiber stiffness, ie, fabric stiffness, and, alternatively, fabric softness and drapeabiHty. 

Years of development have led to a standardized system for objective evaluation of fabric hand (129). This, the Kawabata evaluation system (KES), consists of four basic testing machines a tensile and shear tester, a bending tester, a compression tester, and a surface tester for measuring friction and surface roughness. To complete the evaluation, fabric weight and thickness are determined. The measurements result in 16 different hand parameters or characteristic values, which have been correlated to appraisals of fabric hand by panels of experts (121). Translation formulas have also been developed based on required levels of each hand property for specific end uses (129). The properties include stiffness, smoothness, and fullness levels as well as the total hand value. In more recent years, abundant research has been documented concerning hand assessment (130—133). 

The solution that has been adopted by makers of composite soundboards is to fabricate a sandwich structure where a layer of high-quality cardboard is glued between two identical layers of CFRP (Fig. 28.22). The philosophy of this design modification is to replace some CFRP by a much lighter material in those regions that contribute least to the overall stiffness of the section. 

The first type of bonded design for this application was the beaded doubler panel (Fig. 28). This design was fairly successful at addressing the problems with simple riveted structure but had two primary drawbacks. The area under the beads remained a single thickness sheet and was still prone to fatigue. Reducing the unbonded areas under the beads was not a solution because it reduced the overall stiffness of the panel. Secondly, tooling for these panels was complex and not very robust. Autoclave pressure applied to the beaded areas of the doubler would cause them to collapse, so thick frames were fabricated with cutouts for the beads to protect them. A rubber layer bonded to the surface of the frames... 

A regular antisymmetric cross-ply laminate is delined to have laminae all of equal thickness and is common because of simplicity of fabrication. As the number of layers increases, the bending-extension coupling stiffness B.,., can be shown to approach zero. 

In the case of plastics, emphasis is on the way plastics can be used in these structures and why they are preferred over other materials. In many cases plastics can lend themselves to a particular field of application only in the form of a sophisticated lightweight stiff structure and the requirements are such that the structure must be of plastics. In other instances, the economics of fabrication and erection of a plastics lightweight structure and the intrinsic appearance and other desirable properties make it preferable to other materials. 

The treatment should not lead to harshness or stiffness of the fabric, thus obviating the need for a softener. Indeed, if the polymer itself provides a degree of softness, this is an added bonus. 

Solid PET polymer is relatively hard and brittle. It must be formed into very fine fibers in order to exhibit a bending stiffness that is low enough for textile materials. Most commercial PET fibers are produced in a diameter range of about 10-50 pm, considerably smaller than a human hair. Within this range lie large differences in the softness, drape and feel of fabrics formed from the fibers, since the bending stiffness of a cylindrical fiber depends on the 4th power of its diameter. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Wool is a natural protein fiber characterized by the scaly structure of its external surface-cuticle (Fig. 1). This structure, i.e. the stiffness of cuticle and smoothness of the epicuticle as well as the ability of wool to contract, causes the shrinking of wool fabric during mechanical washing processes. 

Fabricated silicone rubber parts are traditionally made from high consistency silicone gum stock. Because of the stiffness of the gum stock, the material must be worked with rubber masticating equipment and preformed before fabrication. A new fabricating process using a low consistency liquid silicone rubber was introduced by Dow Corning Corporation recently (1, 3). This process is called liquid polymer system (LPS). 

The stiffness of a fabric can be objectively determined as the average of the flexural rigidities (in warp and weft direction). These depend on the shear modulus and the coefficient of friction both are influenced by swelling and, therefore by humidity. 

Fillers offer a variety of benefits increased strength and stiffness, reduced cost, shrinkage reduction, exothermic heat reduction, thermal expansion coefficient reduction, improved heat resistance, slightly improved heat conductivity, improved surface appearance, reduced porosity, improved wet strength, reduced crazing, improved fabrication mobility, increased viscosity, improved abrasion resistance, and/or impact strength. Fillers also can have disadvantages. They may limit the method of fabrication, inhibit cure of certain resins, and shorten pot life of the resin. 

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