Circumferential fibers

In the application of bidirectional patterns, the end domes can be formed by fibers that are laid down in polar winding patterns. The best geometrical shape of the dome is an oblated hemispheroid. Theoretically, the allowable stress level in the two perpendicular directions should be identical. However, the efficiency of the longitudinal fibers is less than that of the circumferential fibers. It is possible to estimate an optimum or length-to-diameter ratio of a cylindrical case for a given volume. 

VCFm Mean velocity of circumferential fiber shortening (circ/sec)... 

Figure 12. Left. Predicted free wall geometry in a human LV with an apical fibrous muscular aneurysm. Right. Variation of midwall circumferential fiber stretch with position. Note that adjacent to the aneurysm, elongation of the normal myocardium is severely limited. (Reproduced from Janz and Waldron Predicted effect of chronic apical aneurysms on the passive stiffness of the human left ventricle, Circ Res 42 255,1978 with permission from the American Heart Association.)...

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

When the operation of the hoUow-ftber membrane is to be reversed, and permeation from the bore to outer 2one is required, circumferential stress and pressure drop along the fiber capiUary (bore) must be considered in the design of the fiber unit. The circumferential stress, S is expressed as... 

The torsion-tube test described by Whitney, Pagano, and Pipes [2-14] involves a thin circular tube subjected to a torque, T, at the ends as in Figure 2-29. The tube is made of multiple laminae with their fiber directions aligned either all parallel to the tube axis or all circumferentially. Reasonable assurance of a constant stress state through the tube thickness exists if the tube is only a few laminae thick. However, then serious end-grip difficulties can arise because of the flimsy nature of the tube. Usually, the thickness of the tube ends must be built up by bonding on additional layers to introduce the load so that failure occurs in the central uniformly stressed portion of the tube (recall the test specimen criteria). Torsion tubes are expensive to fabricate and require relatively sophisticated instrumentation. If the shearing strain y 2 is measured under shear stress t.,2, then... 

The second special case is an orthotropic lamina loaded at angle a to the fiber direction. Such a situation is effectively an anisotropic lamina under load. Stress concentration factors for boron-epoxy were obtained by Greszczuk [6-11] in Figure 6-7. There, the circumferential stress around the edge of the circular hole is plotted versus angular position around the hole. The circumferential stress is normalized by a , the applied stress. The results for a = 0° are, of course, identical to those in Figure 6-6. As a approaches 90°, the peak stress concentration factor decreases and shifts location around the hole. However, as shown, the combined stress state at failure, upon application of a failure criterion, always occurs near 0 = 90°. Thus, the analysis of failure due to stress concentrations around holes in a lamina is quite involved. 

A candidate interlayer consisting of dual coatings of Cu and Nb has been identified successfully for the SiC-Ti3Al-I-Nb composite system. The predicted residual thermal stresses resulting from a stress free temperature to room temperature (with AT = —774°C) for the composites with and without the interlayers are illustrated in Fig. 7.23. The thermo-mechanical properties of the composite constituents used for the calculation are given in Table 7.5. A number of observations can be made about the benefits gained due to the presence of the interlayer. Reductions in both the radial, and circumferential, o-p, stress components within the fiber and matrix are significant, whereas a moderate increase in the axial stress component, chemical compatibility of Cu with the fiber and matrix materials has been closely examined by Misra (1991). 

Fig. 7.23. Thermal residual stress distributions in the (a) radial, (b) circumferential and (c) axial directions for a SCS-6 SiC fiber/TisAI + Nb matrix composite with and without Cu, Nb and Cu/Nb coatings. Total coating thickness t/a = 0,1. After Arnold and Wilt (1992).

An anatomic circumstance that sometimes creates exceptions to the above rules for differential nerve block is the location of the fibers within the peripheral nerve bundle. In large nerve trunks, fibers located circumferentially are the first to be exposed to the local anesthetic when it is administered into the tissue surrounding the nerve. In the extremities, proximal sensory fibers are located in the outer portion of the nerve trunk, whereas the distal sensory innervation is located in the central core of the nerve. Thus, during infiltration block of a large nerve, sensory analgesia first develops proximally and then spreads distally as the drug penetrates deeper into the core of the nerve. 

The optical micrographs, taken in polarized light, point out the existence of two different regions in the PAN-based carbon fiber [100] a central isotropic region and an outer circumferential layer. The existence of these two regions was also observed in a previous study by TEM [97], where it was observed that the raw PAN-based carbon fiber consists of an external 2 pm ring and a core of 3 pm diameter. 

The carbon fibers present a filamentary shape with a diameter of few tens of nanometers (nanofibers) to few tens of micrometers. The structure is that of short graphitic segments. The preferred orientation of the graphitic planes is parallel to the fiber axis. The morphology can be radial, cross-sectional, or circumferential [25]. The carbon fibers present excellent mechanical and thermal properties. The carbon fibers are produced by two ways [26] ... 

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