Unformatted text preview: on ﬁbers in a class of composites known as carbon-carbon
composites. The primary purposes of the matrix materials are to provide lateral support
to the ﬁbers and transfer loads. They also are a source of toughness in the composite, since
the majority of ﬁber materials are brittle. Cracks that have propagated through a brittle
ﬁber are stopped when their tips encounter relatively tougher matrix materials. An exception to ductile matrix materials is ceramic matrix materials, which are inherently brittle.
Composites using ceramic matrices, such as reinforced concrete, are used in compressive
load applications, or the brittle behavior is countered by carefully tailoring the interface
properties. This approach is discussed in Section 14.3.4. Typical matrix materials and
their properties are given in Table 14.3–2.
TABLE 14.3–2 Typical properties of matrix materials. Density
Note: CTE | v v 588 iq Elastic
10 6 18
10 6 3.6
10 6 3.2 400–440 310 4.8 10 6 c oefﬁcient of thermal expansion. | e-Text Main Menu | Textbook Table of Contents CTE
( C 1)
6 6 6 pg589 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 Composite Materials 14.3.4 Role of Interfaces | v v Interfaces play an important role in determining the properties of composites. Consider 1 cm3 of a unidirectional composite made from 25- m -diameter continuous ﬁbers
embedded in a matrix. If the ﬁbers are arranged in a square array and are on average
50 m apart, the volume fraction of ﬁbers is approximately 20%. The total ﬁber-matrix
interface area is approximately 314 cm2, compared with the 6 cm2 external surface area.
The typical ﬁber volume fraction in composites is 2 to 3 times the above amount, and
the interfacial area increases proportionally. The large interfacial area can signiﬁcantly
affects the properties of composites, in particular the crucial properties of toughness and
If the matrix and the ﬁbers have different coefﬁcients of thermal expansion, then cooling from a high fabrication temperature causes differential thermal contraction between
the ﬁber and the matrix and results in thermal stresses at the interfaces (see Section 13.5).
This problem can be minimized by matching the expansion coefﬁcients of the ﬁbers and
matrix. However, some differential expansion will always be present, and the ﬁber-matrix
interface must have sufﬁcient strength to survive temperature changes. Interfaces also
provide a preferential path for oxygen diffusion or moisture uptake that may facilitate
composite property degradation. For example, if moisture diffuses...
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