Curing temperatures for thermosetting resins are in

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Unformatted text preview: along the fiber-matrix interface while an airplane is on the ground, the moisture may freeze upon flight, causing further degradation of the interface and cracking. Thus, the nature of bonding at the interfaces is important. The fiber-matrix interfacial bond strength can be characterized by the shear stress required to cause sliding between the fiber and the matrix. The interface bonding can be mechanical, chemical, or both. Under certain conditions the differential thermal expansion described above can actually aid in the formation of a mechanical bond between the matrix and the fiber. Since most matrix materials have a higher coefficient of thermal expansion than the fibers, cooling from elevated processing temperatures results in compression, or frictional, bonding across the fiber-matrix interface. Curing temperatures for thermosetting resins are in the 100–200 C range, and the expansion coefficients of these materials are high compared with glass and carbon (see Tables 14.3–1 and 14.3–2), which are the most common reinforcing materials. Thus, this type of bonding can be easily accomplished in these composites. Mechanical bond strength can be increased by making the fiber surface rougher, which must be done carefully to avoid compromising fiber strength. However, in most instances, mechanical bonding alone is not sufficient; it is used to supplement strength derived from chemical interactions. Chemical bonding can be in the form of a wettability or secondary bond, which results from electron interactions between the fiber and matrix. This type of bonding occurs when, during fabrication, the molten matrix “wets” the fiber. As discussed in Section 8.2.4, the important variables are the surface energies of the matrix and fibers, which in turn determine the contact angle, the area of contact, and, ultimately, the bond strength. Because the electronic interactions associated with this type of bond occur over a distance of only 2–4 angstroms, it is important that the two bonding surfaces come into intimate contact with each other. Cleanliness of the fiber surface is necessary for such contact to occur. Layers of dust or oil can prevent these interactions, thereby reducing the bond strength. A second type of chemical bonding, referred to as reaction or primary bonding, can occur as a result of mass transfer through solid-state diffusion between the fiber surface and the surrounding matrix. This can result in the formation of a compound at the interface. In such cases, the interface becomes less sharply defined and is more appropriately | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC2 rps MP 589 pg590 [V] G2 7-27060 / IRWIN / Schaffer Part III Properties ;;;;; ;;;;; ;;; ;; Interface Solid solutions M F Intermetallics Metal F Concentration M Fiber Fiber Metal FIGURE 14.3–5 Schematic of the interface region in a metal-matrix composite showing the metal (M) and fiber (F) concentrations and the metal-fiber reaction compounds that may form in the interface region. (Source: K....
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.

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