Fiber macroscopic level

In general, heterogeneities in structural materials are often the source of mechanical failure, but specific types also provide ways to disperse energy without failure. For example, some silks, at a microscopic and macroscopic level, are able to form structures such as spherulite inclusions that will develop into elongated cavities in the solid fibers (Akai, 1998 Frische et al., 1998 Robson, 1999 Tanaka et al., 2001). Interestingly, Isobe et al. (2000), in a significant but largely overlooked paper, showed that synthetic A/ i 4o produced spherulites that had the essential features of Alzheimer s amyloid senile plaques (Kaminsky et al., 2006). 

Fig. li. The change in failure at the single-fiber level as shown by the embedded single-fiber test is mirrored by the failure behavior at the macroscopic level. The increase in adhesion also tracks the same between the ITS results and 0° flexure data. 

Both the molecular template and the self-assembly techniques presented above have limited control over the final shape of the solid, since this is generally obtained in the form of a powder, fibers, or thin films. It is possible, however, to control the shape and size of solids by combining the former techniques with techniques that restrict the volume in which the synthesis takes place. The final goal is to have control over the solids at the molecular as well as macroscopic level, in order to have in a single material properties emerging from several levels of scale. Such structures are referred to as hierarchical [2, 6]. 

The macroscopic level of consideration takes into account fiber length and differences in cell growth such as earlywood, latewood, reaction wood, sapwood, heartwood, mineral content, resin content, etc. Difierences in growth chemistry can cause significant differences in the strength of wood. 

At the macroscopic level, entire fibers actually distort in relation to one another, such that recovery of original position is now impossible. The wood cells or wood fibers are actually failing either by scission of the cell, in which the cell actually fails by tearing into two parts to give a brash type of failure, or by cell-to-cell withdrawal (middle lamella failure) where the cells actually pull away from one another to give a splintering type of failure. 

The failure of a ply depends on the loading and the strength of its constituents, the fibers, and matrix. At the macroscopic level, five different failure modes are recognized for a ply ... 

As mentioned above, heat and mass transfer in textile materials is a complex phenomenon that includes several mechanisms. Textile material properties significantly influence these mechanisms. Several researchers studied the effect of these properties on heat and mass transfers at three different levels (1) the microscopic level (chemical composition, morphological characteristics, fineness, cross-section, porosity, and water content of the component fibers), (2) the mesoscopic level (yam structure and properties), and (3) the macroscopic level (the fabric s physical and stmctural characteristics and finishing treatments) [3,22,23]. Thus, in the following section, heat transfer properties, such as thermal conductivity, thermal resistance, thermal absorptivity, and thermal emissivity and mass transfer properties, such as water vapor transmission and liquid water transmission, will be defined at fiber, yam, and fabric levels. 

Mechanical tests for advanced composite materials conform in many respects to the conventional test typology used for traditional isotropic materials. Despite the complication associated with the heterogeneity of composite systems, the interface between fiber and matrix, and the anisotropy at the micro- and macroscopic levels, the same characteristic property definitions generally used for conventional materials can be identified for these novel materials. In some cases additional constants are required and some differences in nomenclature are introduced especially when no isotropic counterpart exists. 

Measurement of mechanical properties of proteins, especially those of fibrous proteins, has been an important interdisciplinary concern in the history of protein science. In fact, the very early X-ray work by Astbury and his colleagues established the force dependent conformational transition of keratin fiber between a- and /3-forms [15]. A large body of work has since been accumulated on the measurement of mechanical parameters of fibrous structures made of keratin, collagen, dentin and other structural proteins [10, 14, 16, 17]. Measurement was done at the macroscopic level on higher order assemblies of fibrous proteins, applying established methods in materials science for the determination of, for example, static and/or dynamic elastic modulus [14],... 

To determine such a modulus, the measurement of the deformation is not made at the macroscopic level but rather at the molecular one using Raman spectroscopy and/or X-ray diffractometry. These experimental methods give access to the absolute modulus from the modifications induced by the applied stress in the Bragg refiection-—and therefore in the position of atoms—, one can indeed evaluate Young s modulus in the direction of stress—and even in the perpendicular direction—thanks to the Hooke law. The values of moduli obtained in this manner are remarkably high. Diamond, which is exclusively constituted of carbon-carbon bonds, has a tensile modulus equal to llbOGPa in the [110] direction for a cross-section of 0.049 nm. In comparison, polyethylene chains which also consist of C-C bonds substituted by hydrogen atoms and whose cross-section is 0.180 nm, should exhibit a tensile modulus of about 310 GPa [i.e 1160 (0.049/0.180)]. This value corresponds almost ideally to the absolute modulus of polyethylene fibers determined at the molecular level by X-ray diffractometry. In contrast, the tensile modulus obtained from a macroscopic measurement of the deformation represents... 

Absorbency can be broadly classified into two types, physical (macroscopic) and chemical (molecular). On the macroscopic level, the fluid first wets the surface of the absorbent material and is physically transported into and throughout a porous medium as a moving front of continuous liquid threads or columns. In this manner an absorbent batt of cellulose fluff will "physically" absorb about ten times its own weight of aqueous fluid. If fluid input ceases before the system reaches saturation, the liquid front will, for a time, continue to move into the capillary structure. The outermost pores will become depleted of free water, but retain adsorbed water and that which actually entered the walls of the fibers. 

The two main amphibole asbestos fibers are amosite and crocidoHte, and both are hydrated siHcates of iron, magnesium, and sodium. The appearance of these fibers and of the corresponding nonfibrous amphiboles is shown in Figure 1. Although the macroscopic visual aspect of clusters of various types of asbestos fibers is similar, significant differences between chrysotile and amphiboles appear at the microscopic level. Under the electron microscope, chrysotile fibers are seen as clusters of fibrils, often entangled, suggesting loosely bonded, flexible fibrils (Fig. 2a). Amphibole fibers, on the other hand, usually appear as individual needles with a crystalline aspect (Fig. 2b). 

Friction and Adhesion. The coefficient of friction p. is the constant of proportionality between the normal force P between two materials in contact and the perpendicular force F required to move one of the materials relative to the other. Macroscopic friction occurs from the contact of asperities on opposing surfaces as they sHde past each other. On the atomic level friction occurs from the formation of bonds between adjacent atoms as they sHde past one another. Friction coefficients are usually measured using a sliding pin on a disk arrangement. Friction coefficients for ceramic fibers in a matrix have been measured using fiber pushout tests (53). For various material combinations (43) ... 

The previous paragraph has made it clear that if there are elastic fibers and a constant macroscopic stress is applied, the longitudinal creep rate will eventually fall to zero. With constant transverse stresses applied as well, the process of transient creep will be much more complicated than that associated with Eqns. (27) and (28). However, it can be deduced that the longitudinal creep rate will still fall to zero eventually. Furthermore, any transverse steady creep rate must occur in a plane strain mode. During such steady creep, the fiber does not deform further because the stress in the fiber is constant. In addition, any debonding which might tend to occur would have achieved a steady level because the stresses are fixed. 

The specimen is composed of a mixture of matrix, unbroken fibers, and broken fibers. The volume fraction of intact fibers is given by Eqn. (57) with L = Ls, the specimen length. To the neglect of transients, the macroscopic stress supported by these intact fibers is given by Eqn. (60). The strain will now exceed the level of Eqn. (61) associated with the ultimate strength of the fiber bundle. Therefore, the stress supported by the intact fibers will be less than ac, which is the ultimate strength of the fiber bundle without matrix. The applied stress exceeds composite material to creep. 

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