What are typical values of Mode I and Mode II interlaminar fracture toughness values G_IC and G_IIC, and what do they signify?

• A. G_IC about 0.5–1.5 kJ/m^2; G_IIC about 1–3 kJ/m^2; they quantify resistance to crack growth under opening (Mode I) and sliding (Mode II) interlaminar loading.
• B. G_IC ~ 2–3 kJ/m^2 and G_IIC ~ 5–6 kJ/m^2; they measure thermal resistance.
• C. G_IC ~ 0.1–0.2 kJ/m^2; G_IIC ~ 0.2–0.4 kJ/m^2; they measure strength.
• D. G_IC ~ 10–20 kJ/m^2; G_IIC ~ 20–40 kJ/m^2; they measure conductivity.

Answer: (A)

Interlaminar fracture toughness values G_IC and G_IIC are the energy required per unit area to drive a delamination crack along the laminate interface under opening (Mode I) and in-plane shear (Mode II) loading. They quantify how resistant the composite is to crack growth along the interlaminar region. In practice, these are expressed in kJ/m^2 and reflect the toughness of the interface: higher numbers mean the laminate can withstand more energy before the crack propagates.

For many common fiber-reinforced laminates, a reasonable ballpark is G_IC around 0.5–1.5 kJ/m^2 and G_IIC around 1–3 kJ/m^2, which matches the stated values in the correct choice. This conveys that Mode I opening requires energy on the order of fractions to a couple of kilojoules per square meter, while Mode II shear typically requires similar or slightly higher energy.

These values are determined from fracture tests (like DCB for Mode I and ENF for Mode II) and can vary with material system, layup, temperature, and moisture, but the concept remains: they are about resistance to delamination growth, not thermal properties, strength in the usual sense, or electrical/conductive properties.