In: Mechanical Engineering
Composite engineering
In a design assignment, you are asked to use continuous carbon
fibres to reinforce an epoxy matrix
to achieve Young's modulus of 250 GPa along the longitudinal
direction of the composite. The
epoxy matrix to use has a Young's modulus of 2 GPa and the
continuous carbon fibres 400 GPa. If
the allowed maximum loading of the C fibres is 60 vol%, is the
design goal achievable? You are
required to derive any equation needed in the calculation and state
any assumptions made.
In fibre reinforced composites, fibre material is so selected that it's Young's modulus is always more than the Young's modulus of matrix.
Ef >> Em
Ef = Young's modulus of fibre ( Continuous carbon fibre)= 400 GPa
Em = Young's modulus of matrix ( epoxy matrix) = 2 GPa
vf = Volume fraction of the fibres
vm = Volume fraction of the matrix
For longitudinal loading, the figure is as shown.
Load P, is applied parallel to the fibres. The amount of strain is equal for the matrix and fibres. However the load is ditributed among the fibres and the matrix.
Now the allowed maximum loading of C fibres is 60 vol% , i.e., vf = 0.6, vm = 0.4
Ec = 2 * 0.4 + 400* 0.6 = 240.8 GPa
This is the maximum value of Young's modulus of composite along the longitudinal direction, beacuse of the given limit of Carbon fibre volume%. Therefore, the design goal is not achievable.
It can be achieved if vf = 62.31 %