Nut shells are hollow without the nuts inside, which makes them not at all tough to crack. Tubes and bamboos may not break that easily when you hit them from top to bottom, but will certainly crack or bend when you hit them in the middle. Car bumpers are hollow in some respects, which make them ineffective in totally negating destructive force that's thrown at your car. What's hidden underneath is a bumper reinforcement that does all the negation to protect your car and you during accidents.
The car bumper reinforcement is made from highly durable steel that can absorb force before it can be transmitted to your car's frame and then to your head. Basically, bumpers are just the decorative outer surface that matches your car's paint job. The force reduction and absorption is done by the steel construction you'll find when you strip your car of its bumpers. These reinforcements don't usually need replacements and are expected to last the lifetime of your car.
However, if you're not careful, you might find these reinforcements all rusty and muddy. The rear bumper reinforcement, in particular, can be stuck with a build-up of mud that comes from the rear wheels. This build-up becomes really tough to clean when left untouched for a long time and from there, rust will surely form. You can scrape them with all your might, but when the damage gets too extensive, you are left with no choice but to replace the rear reinforcement as a whole.
The front bumper reinforcement gets the most abuse from the most abusive of drivers. With you ramming at every parking meter or thrash bin, your front reinforcement will surely be all dented by now. That's certainly not a good habit, whichever way you want to look at it. In any case, damage to your front bumper could also mean that you have damaged the reinforcement as well. Minor dents can be left untouched, but you will surely not get any help from it when the assembly is bended badly. Sane and proper driving habits will help you keep your car parts and body parts together, so it is better if you keep note of that.
Your head isn't a tough nut to crack and a full force collision can definitely ruin your life. That's why car care is so important to make sure that you are safe inside your car. You don't need to give the bumper reinforcement any special attention. Just make sure you clean it every once in a while and, please, drive carefully. In case you were in an accident and need to rebuild your bumpers, Parts Train can help you gather all the necessary tools for the rebuilding. You can easily maneuver their online catalog, perfect if you don't have the time to scramble.