During Stage II, which phase experiences plastic deformation?

• A. Fiber Phase
• B. Both Phases
• C. No phase deforms plastically
• D. Matrix Phase

Answer: (D)

In Stage II creep of fiber-reinforced composites, the matrix does the plastic deformation. The fibers are very stiff and strong, so they stay largely elastic under the steady-state creep stresses, while the softer matrix yields and flows, producing the creep strain. This is why the ongoing plastic deformation is associated with the matrix phase rather than the fiber phase. Hence, the matrix phase is the phase that deforms plastically in Stage II.