What is the significance of Stage I in the failure of a composite?

• A. Plastic deformation occurs in the matrix only
• B. Plastic deformation occurs in the fiber only
• C. Both Phases yield
• D. No Plastic deformation in either phase

Answer: (D)

Stage I is the purely elastic response of a fiber-reinforced composite. In this stage, the load is below the yield strengths of both the matrix and the fiber, so both constituents deform elastically and follow a linear stress–strain relationship. There is no plastic flow in either phase, which is why this stage is defined by the absence of plastic deformation. Only when the stress reaches the yield point of one or both constituents does plastic deformation begin and Stage II of damage/failure progress. That’s why the statement about no plastic deformation in either phase best captures Stage I.