Another one of my weird ideas. Any engineers reading this?

I think a better example is a boxing glove. Punch a refrigerator with a boxing glove and both the fridge and your hand are negligibly affected. Put on a steel glove and swing with the safe force and you will not only damage the fridge but your hand as well.

Remember that F=M x A. When you increase mass you increase force which acts in both push and pull.

So a steel bumper would apply more push force on the fridge but also more pull force on the rest of the truck. In your case the wheel barrow would pull your body at greater force into the impact than if you had a pillow.

 

I believe the crumple zone that plays the biggest role in energy absorbing is in the frame. The aluminum crumple bumper is to reduce the possibility of frame damage if someone backs into you in a parking lot.

So it is a crumple zone for your crumple zone and likely would not be there if the front end was not plastic.