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The incorporation of two or more fibres within a single matrix is known as "hybridisation" and the resulting material is a hybrid composite, often abbreviated to just "hybrid".  Hybrid composites have been reviewed by the author [1, 2, 3] and a classification scheme for hybrid composites was proposed by Short and Summerscales [2] which divided hybrid composites into 6 groups:

1. fibre-by-fibre mixtures (also known as "intimate" hybrids [4,5])
2. tow-by-tow mixtures (also known as "discrete" or "zebra" hybrids [4, 5])
3. layer-by-layer mixtures
4. skin-core-skin structures (also known as sandwich structures)
5. internal ribs
6. external ribs

The Young's modulus of a hybrid composite can be predicted using a separate term for each fibre type:

where subscript A denotes the low elongation fibre and subscript B denotes the high elongation fibre.  For a unidirectional composite, and assuming that the critical situation is fibre fracture and that the contribution from the resin matrix is negligible, a lower bound strength can be predicted using the higher value from either:

or

However, the above assumes that only one dominant fibre is carrying the load.  The low elongation fibre can be assumed to be the critical failure case and if the high elongation fibre is also carrying load at the same strain, then:

The graphical method for the strength at constant strain is described in reference [6].

Summerscales and Short [7] presented an alternative model which assumed that the failure strain of the low elongation fibre could be increased to that of the high elongation fibre by isolating the individual critical fibre failures such that broken fibres were uniformly distributed through the composite.  For fibres with closely matched strains to failure, where the high-modulus fibre has the low strain to failure and vice versa:

which predicts a strength that exceeds the rule of mixtures.  This synergistic strengthening is known as the hybrid effect (but is as elusive as the Philosopher's Stone).  The above equation was used to model data from Gruber [8] for Kevlar 49/E-glass hybrids, albeit giving an under-estimation of the values reported.

References

1. J Summerscales, D Short, Carbonfibre and glassfibre hybrid reinforced plastics, Composites, July 1978, 9(3), pages 157-166.
2. D Short, J Summerscales, Hybrids - a review, part 1: techniques, design and construction, Composites, October 1979, 10(4), pages 215-221.
3. D Short, J Summerscales, Hybrids - a review, part 2: physical properties, Composites, January 1980, 11(1), pages 33-38.
4. SV Kulkarni, BW Rosen and HC Boehm, Cost performance evaluation of hybrid composite materials, Proceedings of the 31st Annual Technical Conference of the Reinforced Plastics/Composites Institute (SPI, Washington DC), February 1976, paper 17A.
5. SV Kulkarni and BW Rosen, Evaluation of the cost-effectiveness of hybrid composite laminates, Proceedings of the 17th Structures, Structural Dynamics and Materials Conference, (AIAA/ASME/SAE, King of Prussia PA), May 1976 and Journal of Aircraft, December 1977, 14(12), 1153-1154.
6. HE Edwards, NJ Parratt and KD Potter, Synthesis and application of aligned discontinuous composites, Proceedings of the 2nd International Conference on Composite Materials, TMS of AIME, Toronto, April 1978, 975-933 and 1300-1301.
7. J Summerscales, The mechanical properties of carbon fibre with glass fibre hybrid reinforced plastics, PhD thesis, CNAA/Plymouth Polytechnic, May 1983.  UOP Library
8. MB Gruber, mechanical properties of hybrid composites, Master of Mechanical and Aerospace Engineering thesis, University of Delaware, June 1981.