Short Fiber Composite Materials Andy Fox Eric Trelease

Three Groups of Materials Short Fibers are aligned parallel to each other Materials that contain in-plane randomly oriented long chopped fibers  CSM=chopped-strand mat (long fibers)  SMC=sheet molding compound (short fibers)  DMC=dough molding compound (short fibers) Injection molded products which have short fibers as a reinforcing element

Short Aligned Fibers For continuous fibers the stress parallel to the fiber alignment is: The critical length of the fibers is the minimum length of fiber required for the stress to build up to the fracture strength of the fiber and is given by:

Two Failure Conditions Fiber fracture occurs before matrix fracture  The average stress in the fiber is given by:  Substituting into the continuous fiber equation the composite strength is given by:

Second Failure Condition Matrix fracture occurs before fiber fracture  The average stress in the fiber is given by:  Assuming l < l c and substituting into the continuous fiber equation, the strength of the composite is given by:

In-Plane Random Fibers  CSM=chopped-strand mat (long fibers)  SMC=sheet molding compound (short fibers)  DMC=dough molding compound (short fibers)

In-Plane Random Fibers K 1 is the strengthening or reinforcing efficiency of short fibers K 2 is the orientation efficiency Tensile strength of the random material is given by:

CSM=chopped-strand mat (long fibers) Little fiber fracture occurs after the fibers have been chopped to a certain length

SMC=sheet molding compound (short fibers) Some fiber degradation and dispersion occurs Depends on the size of treatment given to the fibers When strongly bound, bundles act as reinforcement Therefore K 1, fracture characteristics & work of fracture will depend on the amount of dispersion

Injection Molded Short Fiber Materials Usually made with thermoplastic polymer matrices Fiber orientation effects greater than matrix effects Fiber orientation depends on melt flow Orientation distribution variation exists  Impossible to collect experimental data characteristic of the random fiber material

Stress Transfer in Composites Based on experimental data Critical fiber length for efficient stress transfer to the fiber is a unique function of the ratio between the Young elastic moduli of fiber and matrix. The effect of the bonding efficiency between the two components was studied and dramatic increases in the critical length are observed when adhesion falls below 30%.

References Hull, Derek An Introduction to Composite Materials, Cambridge University Press, Journal of Materials Science Publisher: Springer Science+Business Media B.V., Formerly Kluwer Academic Publishers B.V. ISSN: (Paper) (Online) DOI: /BF Issue: Volume 22, Number 2 Date: February 1987 Pages: Papers “Theoretical study of the stress transfer in single fibre composites