Composite beams and young's modulus

[ABoul]

Homework Statement

"A polymer, with Young’s modulus 2GPa and yield stress 50MPa, is reinforced by
uniaxially aligned glass fibres which have Young’s modulus 70GPa and fracture at
1400MPa. The volume fraction of fibres is 40%.


(a) Calculate Young’s modulus for the composite in the direction of the fibres.
(b) The composite is loaded parallel to the fibres. What would be the stress on the
matrix when the composite yields ?"

Homework Equations

The Attempt at a Solution

(a) E = xE_f + (1-x)E_m
E = 0.4(70e9) + 0.6(2e9) = 29.2 GPa

firstly, i don't know if the above is right. also, i have o clue how to do part (b).

 

[member 553083]

i tried using the E from part (a) in the equation for yield stress, but this didn't work. any help would be greatly appreciated.