I am really surprised this didn't kick up

Its been a long time since I thought about the failure mechanics of steel or sheet metal, but as I remember it, all materials go through the same failure mechanism, (72bluNblu and others, feel free to correct me) material stretch and return to equilibrium with a 2% offset (commonly called relaxation), plastic deformation, some form of hysteresis, work hardening and ultimately failure. All those functions can be diagramed and explained in classic engineering stress strain curves. Some materials such as glass have properties with stress strain cures that are quite short, while materials such as carbon steel have properties that can be manipulated to make the steel ductile, tough, hard, etc.
The strength of the unibody is determined, in part, by the number, size, and placement of the spot welds. In todays cars some spot welds are being replaced by adhesives specifically designed for automotive use which are structural in nature.