Textile performance, mechanics mechanical properties

The use of Kevlar has been confined to specialised applications 98), where high mechanical performance and lightweight properties are essential, because of its present relatively high cost compared with conventional textile materials. These applications can be conveniently divided into two main categories, one where the fibres alone form the final product such as in cables and fabrics and the other where they act as reinforcing elements for the production of composite structures. 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

High performance polymer fibers (HPPF) have excellent mechanical properties compared to traditional textile fibers such as nylon. The typical HPPFs are aramid and polyethylene fibers (6). Aramid is a generic name for a class of aromatic polyamide fibers, most of which are varieties of poly(p-pheny-lene terephthalamide). Kevlar is the trade name of the varieties of aramid polymers introduced conunercially by Dupont. The molecules in the fibers of these materials are oriented in the axial direction. Poly(p-phenylene terephthalamide) is a rigid molecule with the following structure ... 

The mechanical properties of textile fibers, yarns, and fabrics may be more fully determined by subjecting the substrate to small forces in directions other than along the fiber axis. Tear, bending, and shear strengths, as well as recovery from bending and abrasion resistance, etc., also influence the wear properties of textiles. Finally, time-dependent extension and recovery, termed creep, or creep recovery, respectively, is of importance in determining the performance of fibers in industrial applications. A discussion on the measurement of these parameters, however, is beyond the scope of this article. 

Source Reprinted with permission from Kawabata S, Measurement of the transverse mechanical properties of high performance fibres, J Text Inst, 81(4), 432-447, 1990. Copyright 1990, The Textile Institute. 

Because textile materials are lightweight, flexible and strong polymers and biological tissues are themselves fibrous polymers, with very similar dimensional, physical and mechanical properties, they have found numerous applications as bioimplants. From their use as sutures and ligatures many thousands of years ago, to hernia repair meshes and vascular grafts in the present century, textiles continue to be explored for use in newer and better performing medical products. The currently available implants can be categorized as one-, two- or three-dimensional structures. 

For textiles to perform life-support and life-saving functions, they must remain functional in the environment of the body over a predicted period of time. The material selected should be chemically stable and physiologically acceptable and it should have optimum physical and mechanical properties. It must be defect-free, uniform in size and predictable in tensile and other properties. There are, therefore, both general requirements for a textile material before it can be selected for construction of an implant, as well as specific requirements, such as optimum strength, flexibility, abrasion resistance and long-term patency, which may vary from product to product. The last major section in the chapter discusses the general requirements for implant success and the key properties needed to optimize physical performance. 

During the initial stages of biotextile product development many in vitro and in vivo tests are performed to assess the key parameters of the implantable device, such as the chemical composition of the material, the level of surface contamination, the design of the textile structure, the initial mechanical properties, the thrombogenicity (the rate of blood clot formation) and... 

Different types of protective clothing need to withstand attack from specific environmental hazards and thus require the constituent textile materials to have high performance in specific properties. For example, a body armour system requires the textile materials used to have superior mechanical properties firefighters clothing requires its component materials to have exceptional fire resistance properties and chemical protective clothing requires the materials to be resistant to the attacks of chemical agents. 

The mechanical properties of a conductive material play an important role when selection is performed for any particular textile application. Metallic filaments usually compromise good electroconductive characteristics with poorer mechanical properties. A very high stiffness and lower stretchability of metallic fibers not only makes the woven or knitting process difficult but also reduces their service life. On the other hand, polymeric fibers or yams exhibit good elongation and recovery properties. The combination of nonstretchable metallic fibers with stretchable polymeric yams creates a new class of metal-based electroconductive fibers, which is known as co-spun polymeric-metal yams (Fig. 28.3). 

In order to demonstrate the mechanical properties, especially the performance of the in situ joined zone, results of a dynamic crash test are shown. The test profile is a closed cap profile made of glass fiber textile and polypropylene matrix material (TwinTex - see Fig. 8.29). An impact energy of... 

There are at least four reasons that explain the impressive improvements in the mechanical performance (i) in the current SPCs, the reinforcing component dominates strongly, and has much better mechanical properties when compared with the isotropic matrix, (ii) excellent adhesion between matrix and reinforcement is achieved since they have the same chemical composition, (iii) better orientation of PET macromolecules is expected in the nanofibrils compared to that in microfibrils and textile filaments of the same PET, and (iv) in the test specimens of SPCs, the nanofibrils are urdaxiaUy aligned and the testing has been pierformed in the drawing direction only. 

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