Residual stress

Residual stress is like a kind of "tension" that gets stuck inside something, like a balloon. Just like a balloon can be filled with air and get really big and tight, materials like steel can be made really tight. No matter how tight the material gets, it still cannot be perfectly even like a balloon filled with air. There are still some bits that are a little bit tighter than the other bits. The tight parts are called residual stress.