Why is a DD rivet heat treated before use?

A DD rivet is heat treated before use primarily to harden and increase its strength. The heat treatment process enhances the mechanical properties of the material, ensuring that the rivet can withstand the forces applied during the aircraft's operation. This treatment improves factors like tensile strength and fatigue resistance, both of which are critical in aviation applications where safety and reliability are paramount.

The other options, while potentially related to materials and rivets in general, do not specifically address the purpose of heat treating DD rivets. For example, relieving internal stress is sometimes a consideration in materials processing but is not the primary reason for heat treating DD rivets. Softening the material to facilitate riveting could reduce strength, which is counterproductive in an aerospace context. Lastly, while preventing breaking is a consideration in the overall design and use of rivets, it is the increase in strength through hardening that directly relates to the heat treatment process.