Which of the following statements about Tg is consistent with typical thermoset carbon/epoxy systems?

• A. Tg is the glass transition temperature; above Tg, stiffness and strength generally decrease.
• B. Tg is the melting temperature; at Tg the polymer becomes fully crystalline.
• C. Tg is unrelated to mechanical properties.
• D. Tg is the temperature at which fibers fuse.

Answer: (A)

In thermoset carbon/epoxy systems, the glass transition temperature (Tg) is the temperature where the epoxy network goes from a rigid, glassy state to a softer, rubbery state as the polymer chains gain mobility. Below Tg, the material is stiff and strong because the crosslinked network restrains motion; the matrix and fibers work together to carry load, giving high stiffness. Above Tg, the increased segmental mobility of the resin reduces the storage modulus and often lowers the load-bearing capacity, so the composite becomes less stiff and may creep more under stress. This behavior meaningfully ties Tg to mechanical performance, especially in temperature ranges close to or above Tg.

The other statements don’t fit because Tg is not the melting temperature (thermosets don’t melt; Tg is about a glassy-to-rubbery transition, not crystallization), Tg is not unrelated to mechanical properties (it strongly influences stiffness, strength, and creep behavior), and Tg is not the temperature at which fibers fuse (that concept doesn’t apply to fiber mats in this context).