Question

The mechanical properties of composites can often be estimated based on a weighted average (by volume) of the constituent materials’ properties. If the case study part that was supposed to get 40% (by volume) resin was made of polyester and S-glass, compute its expected stiffness and the stiffness of the defective parts with 55% resin. Use 1,015,000 psi as the modulus of elasticity (stiffness) for polyester and 12,000,000 psi for the stiffness of S-glass fibers.

Answer 1

Volume fraction of resin matrix = V_m = 0.4

Volume fraction of fiber S--glass = 1- Volume fraction of resin matrix = 1-0.4= V_f = 0.6

Supposed stiffness=?

Stiffness of composite = E= E_f V_f + E_mV_m.

Stiffness of composite = E= 12000000*0.6+1015000*0.4 = 1126000 psi.

Now the defective part has 55 % resin.

So the volume fraction of resin matrix = V_m = 0.55

Volume fraction of fiber =V_f= 1- volume fraction of resin matrix = 1-0.55 = 0.44

Strength of defective composite = E = E= E_f V_f + E_mV_m

Stength of defective composite = E = 12000000*0.45+1015000*0.55 = 5958250 psi.