  Composite materials are often stronger than their constituents. We
demonstrate this through a spring network model on a square lattice. Two
different types of sites (A and B) are distributed randomly on the lattice,
representing two different constituents. There are springs of three types
connecting them (A-A, B-B and A-B). We assign two spring parameters for each
type of spring. these are a spring constant and a breaking threshold. Here we
show that intermediate compositions may require higher energy to induce the
first sample-spanning break than either pure A or pure B. So the breaking
energy goes through a maximum as the concentration of one component varies from
0 to 100%. The position and height of the peak depend on the spring parameters.
Moreover, certain combinations of spring parameters can produce composites,
which do not break upto a specified strain limit. Thus a fracture 'percolation
threshold' may be defined.
