Final answer:
Crazing is the phenomenon preceding fracture in thermoplastics below their glass transition temperature, where microvoids form, leading to potential crack propagation and eventual fracture. The glass transition temperature influences the mechanical properties of a polymer, dictating whether it is in a brittle glassy state or a more flexible rubbery state.
Step-by-step explanation:
The phenomenon that can precede fracture (crack propagation) in thermoplastics below their glass transition temperature is called crazing. Crazing occurs when microvoids and small cracks form within the plastic due to stress, often appearing as a whitish area due to light scattering. These crazes can absorb energy and can be stable or grow under sustained load, eventually leading to fracture. The presence of crazes indicates that the material has exceeded its elastic limit and has begun to deform plastically, which in thermoplastics can be reversible if the material is heated above its Tg but irreversible in thermosets. The glass transition temperature is a critical point for thermoplastic materials, influencing their mechanical properties and suitability for certain applications.
Understanding the thermal and mechanical behavior of polymers is crucial in applications where materials may experience a range of temperatures that could lead to fracture. Polymers below their glass transition temperature are in a glassy state, which is brittle, while above the Tg they enter a rubbery state, which is more flexible and ductile.