tailieunhanh - Mechanics Analysis Composite Materials Episode 2

Tham khảo tài liệu 'mechanics analysis composite materials episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 1. Introduction 21 The next step in the development of composite materials that can be treated as matrix materials reinforced with fibers rather than fibers bonded with matrix which is the case for polymeric composites is associated with ceramic matrix composites possessing very high thermal resistance. The stiffnesses of the fibers which are usually metal steel tungsten molybdenum niobium carbon boron and ceramic Sic A12O3 and the ceramic matrices oxides carbides nitrides borides and silicides are not very different and the fibers do not carry the main fraction of the load in ceramic composites. The function of the fibers is to provide strength and mainly toughness resistance to cracks of the composite because non-reinforced ceramics is very brittle. Ceramic composites can operate under very high temperatures depending on the melting temperature of the matrix that varies from 1200 C to 3500 C. Naturally the higher is this temperature the more complicated is the manufacturing process. The main shortcoming of ceramic composites is associated with a low ultimate tensile elongation of the ceramic matrix resulting in cracks appearing in the matrix under relatively low tensile stress applied to the material. An outstanding combination of high mechanical characteristics and temperature resistance is demonstrated by carbon carbon composites in which both components - fibers and matrix are made from one and the same material but with different structure. Carbon matrix is formed as a result of carbonization of an organic resin phenolic and furfural resin or pitch with which carbon fibers are impregnated or of chemical vapor deposition of pyrolytic carbon from a hydrocarbon gas. In an inert atmosphere or in a vacuum carbon carbon composites can withstand very high temperatures more than 3000 C . Moreover their strength increases under heating up to 2200 C while modulus degrades under temperatures more than 1400 C. However in an oxygen atmosphere they oxidize and .