How is preload best described?

Preload refers to the initial tension applied to a fastener, such as a bolt or screw, before any external loads are applied to the joint. When a fastener is tightened, it stretches slightly and thus creates a clamping force, which is known as preload. This clamping force is critical because it ensures that the parts being joined remain securely fastened under load.

By establishing a specific preload, the fastener can effectively withstand the forces that may try to loosen it when the assembly is under operational conditions. Therefore, referencing preload as causing a fastener to tighten or loosen in the joint correctly highlights its role in maintaining the integrity and functionality of the joint under various conditions.

Other options do not accurately represent the concept of preload. For instance, while preload can affect friction, it doesn’t primarily aim to reduce friction in the joint. Additionally, preload does not eliminate torque requirements; rather, it necessitates them to achieve the desired clamping force. Lastly, preload is very relevant to joint integrity, as insufficient or excessive preload can compromise the joint's performance.