It's not really how much it bends (that's just a function of the stiffness and the applied load), it's really all about whether you go beyond the elastic limit of the material. In the elastic range it is simply a function of load (stress) and movement (strain), double the load and the strain should also double..... (simple stuff really, actually it gets a bit more complicated if you start looking into fatigue, but that's the next grade....!)
Metals (and lots of other materials) have a degree of elasticity and you can stretch and bend them in a purely reversible manner up to a certain, point. Once you exceed the elastic limit that's when the trouble starts.... Then you start getting non-linear response and things start to fail, e.g. double the stress and, oops, the wing fell off!
Still that's what materials engineers get paid to calculate and, let's face it, most of the time they get it right.