Answer:
1. Ef > Em and 5. lf < lcritical
Explanation:
The critical length of fiber is given by
[tex]L_c=\frac {\sigma_f\times d}{2\tau_c}[/tex] where d is fiber diameter, [tex]\sigma_f[/tex] is ultimate strength of fiber, and [tex]\tau_c[/tex] is fiber matrix bond strength.
When fiber length increases above critical length, the reinforcement becomes more effective since maximum fiber load is achieved by most part of the fiber hence
[tex]L>L_c[/tex]
In matrix composite, matrix phase remains intact when fiber phase fails since the fracture strain in matrix is more than in fiber hence
[tex]\epsilon_m>\epsilon_f[/tex]
Moreover, strain is inversely proportional to elastic modulus hence
[tex]\frac {1}{E_m}>\frac {1}{E_f}[/tex] meaning [tex]E_m<E_f[/tex] or [tex]E_f>E_m[/tex]
