system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Car crash specialists, mechanics, etc.: Is it true that after some types of collisions, a car will never be the same again? Is there really such a thing as damage that cannot be fixed?

You hear every now and then about cars that were involved in some colisions that would not be suitable for repair, or owners complaining that their cars were "never the same again".  Apart from the obvious economic reasons (fixing it would be more expensive than buying another car), why is it that some types of damages are beyond the possibility of repair?  Isn't anything fixable?
gpt
Yes. Imagine you break a stick and then put pieces back together with duct tape. Is repaired stick same as new? Of course not. It's the same with auto parts. They are prepared to deform in specific ways during collision but they only can deform once. During that process internal bonds between atoms inside part are broken,  rearranged, moved and twisted. And you can't put them back in place, just like you can't make crumpled paper look pristine again after straightening. Even if part is straightened and looks OK visually, it's broken inside and won't perform OK.

Edit: it's especially important for parts that are designed as crumple zones during collisions. Crumpling is way to dissipate energy during collision. But part that was crumpled and straightened won't dissipate energy at all-it will just get back to previous, broken shape. That means other parts of the car - like passengers - would have to get that energy on themselves...