Continuous Fibre Composites
We shall now consider the case where the matrix is ductile and the elastic strain to fracture in the fibres is less than the elastic/plastic extension of the matrix as would occur in fibre reinforced metal matrix composites or thermoplastic matrix composites. At low volume fractions of fibres, the chain of events is analogous to the case where the matrix fails first in that the fibres will break and the load will transfer to the matrix which, having a reduced cross-section, will see a sudden jump in stress. Again, what happens next depends on the magnitude of the increase in the stress in the matrix - will it fracture or not?
[this site has been made possible by Lamiflex Composites]
The stress on the composite at the point of fibre fracture (ef) is
The force on the composite is just the product of the stress and the cross-sectional area, so the stress on the matrix after the fibres break is
So the stress on the matrix increases by
. If the rise in stress is not sufficient to fracture the matrix then it will
continue to support the applied load. Thus the fracture strength of the composite will be given by
where sm is the ultimate tensile strength of the matrix; i.e. the addition of fibres leads to a reduction in the strength of the composite to levels below that of the unreinforced matrix. Fortunately, as the fibre volume fraction increases, the fibres carry more of the applied load. When the fibres break, the load transferred to the matrix is large and the much reduced cross-sectional area of the matrix will be unable to support the load and the matrix too will fail. The strength of the composite, like the previous example, is determined by the strength of the fibres i.e.
materials for this section of the website has been in part drawn from the
class Composite Materials at MIT [for further information please point your browser to:
http://callisto.my.mtu.edu/MY472/ ]
 |
