Unidirectional, laminar composite

Figure 5.117 Schematic illustration of unidirectional (a) and crossplied (b) laminar composites. Reprinted, by permission, from M. Schwartz, Composite Materials Handbook, 2nd ed., p. 3.71. Copyright 1992 by McGraw-HUl, Inc.

Fiber reinforced composites, depending on the properties needed, can be fabricated in three different ways. Very short fibers can be used as filler, short fibers can be organized with random orientation and long fibers can be laid in one direction to form unidirectional composites. Short staple fibers may also be twisted together to form continuous yams to fabricate unidirectional composite laminates similar to those made using long fibers. Several unidirectional laminates may be combined by layering in different directions to form laminar composites. Yarns may also be woven or knitted into fabrics to form similar laminar composites. 

Tranj-laminar fracture of composites with a certain amount of fibres in the throughthickness direction will lead to fibre breaking with significant effects on delamination resistance (see e.g. Refs [24,37] for details). Woven fibre mats, 3D fibre performs or additional 3D reinforcement (pins and stitching) of FRP composites with fibres ahgned in one plane (see e.g. Refs [84,85]) have been developed and can be tested for their fracture mechanics properties. The typical approach for testing these is to apply a standard test method developed for unidirectionally reinforced FRP composites and to assess the difference in delamination resistance compared to the standard laminate. So far, that approach has yielded (nominal) numbers, but their interpretation is not... 

The waU of a cellulose fiber in turn exhibits a complex, laminar structure, wherein each layer consists of smaller, unidirectional fibers, or microfibrils, in the range of 5 to 50 run wide, and anywhere from 100 run to several microns long, depending on the source [33]. Each layer of microfibrils varies with respect to fibril orientation (microfibril angle). Microfibrils in turn have a composite stracture, consisting of slender cellulose crystallites, or whiskers, with diameters on the order of 5 run, which are threaded together and embedded in the microfibrils between amorphous regions of cellulose and hemicellulose [34]. Hence, cellulose can be viewed as a composite material from the nanoscale perspective (whiskers in microfibrils), the microscopic perspective (microfibrils in fibers), and the macroscopic perspective (fibers in wood). 

C- and BN-interphase on SiC fibers for unidirectional SiCf/SiC composites was formed by EPD process, and their microstructure and mechanical properties were investigated. In the case of the C-SiCf/SiC composites, the laminar structures of carbon on SiC fibers were clearly observed, and the thickness of C-interphase on SiC fibers ranged from 50 to 400 nm. The C-SiCf/SiC composites showed a pseudo-ductile fracture behavior with large amount of fiber pullout. However, the BN-SiCf/SiC composites fractured in a brittle manner without fiber pullout in spite of sufficient thickness of BN interphase. It is inferred from the results of EDS that sintering additives would react with h-BN-interphase, and the interphase did not act effectively for toughening the SiCf/SiC composites. 

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