Question

The phenomenon where a metal part tends to partially return to its original shape after the forming tool is removed is known as

• A. Spring back
• B. Elastic recovery
• C. Strain hardening
• D. Plastic deformation

Hint:

To compensate for spring back in manufacturing operations: - Overbending: Design the punch angle to be sharper than the desired final target angle. - Bottoming Coining: Apply extreme compressive stress at the bottom of the stroke to set the material plastically.

Answer 1

The Correct Option is A

Concept: When a piece of sheet metal is bent or formed, the total deformation applied by the tooling consists of two distinct components: \[ \epsilon_{\text{total}} = \epsilon_{\text{plastic}} + \epsilon_{\text{elastic}} \] Where:
• \( \epsilon_{\text{plastic}} \) is the permanent, non-reversible deformation that shapes the part.
• \( \epsilon_{\text{elastic}} \) is the temporary, reversible deformation governed by Hooke's Law.

Step 1: Describe what happens during unloading. When the forming tools (punch and die) are retracted, the external bending forces drop to zero. The material releases its stored elastic strain energy. While the plastic portion of the deformation remains permanent, the elastic portion recovers. This causes the metal part to unbend slightly and try to return toward its original unformed flat shape.

Step 2: Identify the correct industry term. Although this structural phenomenon occurs due to elastic recovery, the specific manufacturing term given to this physical geometric dimensional change in sheet metal operations (like V-bending or U-bending) is Spring back. It results in an increase in the final bend angle and a shift in the bend radius compared to the dimensions of the tool.