Plastic materials

Definition

Plastic materials are synthetic substances made from polymers that can be molded into various shapes and forms when heated or subjected to pressure. These materials exhibit versatility in applications due to their ability to undergo significant deformation without breaking, which relates closely to stress and strain concepts in physics, as well as their mechanical properties defined by elastic moduli.

5 Must Know Facts For Your Next Test

1. Plastic materials can be classified into two main categories: thermoplastics, which can be reheated and reshaped, and thermosetting plastics, which harden permanently after being set once.
2. The mechanical behavior of plastic materials under stress is often nonlinear, especially at larger strains, contrasting with metals that typically show linear elasticity up to their yield point.
3. Different types of plastic materials have unique properties, such as tensile strength, ductility, and toughness, which determine their suitability for specific applications.
4. The temperature and rate of loading can significantly influence the mechanical properties of plastic materials, leading to phenomena like viscoelasticity, where time-dependent strain occurs.
5. Plastic deformation in materials refers to a permanent change in shape or size when subjected to stress beyond the elastic limit, which is crucial for understanding how plastics behave under various forces.

Review Questions

• How do plastic materials demonstrate both elastic and plastic behavior when subjected to stress?
  • Plastic materials initially exhibit elastic behavior when a small amount of stress is applied, allowing them to return to their original shape after the force is removed. However, once the stress exceeds a certain threshold known as the yield point, these materials undergo plastic deformation, resulting in a permanent change in shape. Understanding this transition helps in selecting plastics for applications requiring specific mechanical properties.
• Discuss the role of Young's Modulus in assessing the mechanical performance of plastic materials compared to metals.
  • Young's Modulus quantifies the stiffness of a material and is critical for comparing how plastics and metals respond under similar loads. Plastics generally have lower Young's Modulus values than metals, indicating they are less stiff and can deform more easily under stress. This property is essential when designing components that require flexibility versus those that need rigidity.
• Evaluate the implications of temperature and loading rate on the mechanical properties of plastic materials during application.
  • Temperature and loading rate have significant effects on the mechanical properties of plastic materials. As temperature increases, plastics may become softer and more ductile, allowing for easier shaping and forming. Conversely, rapid loading can lead to brittle behavior in plastics that would normally exhibit ductility under slower conditions. Understanding these factors is crucial for engineers when selecting plastics for applications that encounter varying environmental conditions and loading scenarios.

Related terms

Polymer: A large molecule composed of repeating structural units called monomers, which are bonded together, forming the basis of plastic materials.

Elasticity: The ability of a material to return to its original shape after being deformed by an applied stress, reflecting how materials respond to forces.

Young's Modulus:

A measure of the stiffness of a material, defined as the ratio of tensile stress to tensile strain in the linear elastic region of the stress-strain curve.