Short Fiber Composite Materials

1
Short Fiber Composite Materials

• Andy Fox
• Eric Trelease

2
Three Groups of Materials

• Short Fibers are aligned parallel to each other
• Materials that contain in-plane randomly oriented
  long chopped fibers
• CSMchopped-strand mat (long fibers)
• SMCsheet molding compound (short fibers)
• DMCdough molding compound (short fibers)
• Injection molded products which have short fibers
  as a reinforcing element

3
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

4
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

5
Second Failure Condition

• Matrix fracture occurs before fiber fracture
• The average stress in the fiber is given by

• Assuming l lt lc and substituting into the
  continuous fiber equation, the strength of the
  composite is given by

6
In-Plane Random Fibers

• CSMchopped-strand mat (long fibers)
• SMCsheet molding compound (short fibers)
• DMCdough molding compound (short fibers)

7
In-Plane Random Fibers

• K1 is the strengthening or reinforcing efficiency
  of short fibers
• K2 is the orientation efficiency
• Tensile strength of the random material is given
  by

8
CSMchopped-strand mat (long fibers)

• Little fiber fracture occurs after the fibers
  have been chopped to a certain length

9
SMCsheet 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 K1, fracture characteristics work of
  fracture will depend on the amount of dispersion

10
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

11
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.

12
References

• Hull, Derek An Introduction to Composite
  Materials, Cambridge University Press, 1981.
• Journal of Materials Science Publisher Springer
  ScienceBusiness Media B.V., Formerly Kluwer
  Academic Publishers B.V. ISSN 0022-2461 (Paper)
  1573-4803 (Online) DOI 10.1007/BF01160761
  Issue  Volume 22, Number 2 Date  February 1987
  Pages 504 - 508 Papers
• Theoretical study of the stress transfer in
  single fibre composites