Questions: Elastic and Plastic Behavior of Materials

Question 1

A spring is compressed and then released, returning to its original length. This behavior is:

• A. Plastic
• B. Elastic
• C. Fatigue
• D. Fracture

Answer: (B)

The spring returned to its original shape when the force was removed. This is the definition of elastic behavior -- deformation is reversible.

Question 2

If you bend a metal wire past its yield point and release it, the wire will spring back to its original straight shape.

• T. True
• F. False

Answer: False

Beyond the yield point, deformation is plastic (permanent). The wire will spring back slightly due to elastic recovery, but it will retain a permanent bend. This is why it is called plastic deformation -- the shape change is irreversible.

Question 3

Why do engineers design structures to stay within the elastic range of their materials?

Model answer: In the elastic range, the structure returns to its original shape after every loading cycle, so it can be loaded and unloaded indefinitely without accumulating permanent damage. If the material enters the plastic range, permanent deformation occurs, which can change the structure's geometry and eventually lead to failure.

Elastic behavior ensures repeatable performance. A bridge beam that stays elastic will have the same strength and stiffness after carrying its millionth truck as it did carrying its first. Plastic deformation would cause progressive sagging, misalignment of connections, and eventual structural failure.