MM. Once it is in a hardened state is it possible for that same material to return to a gel like state?
It sure does. As far as I know, it has the same name in both states, since it's not really a change of state, but a change in viscosity.
Corn starch is actually a great example of a shear thickening fluid. Mix up some corn starch in water until it gets kind of thick and goopy, and it will be a shear thickening fluid. If you pick it up and just hold it in your hand, it will ooze through your fingers and flow like a thick fluid.
However, if you roll it around in your hands, you will be subjecting it to a shear force, and it will act more like a solid. You can actually roll it up into a ball, and as long as you keep rolling it around, it will keep the ball shape. As soon as you stop shearing it, though, the viscosity will drop again, and it will become fluid-like and start to flow.
So the sort answer is yes, it's reversible. And the reversibility is as simple as stopping applying a force. So if you're wearing the armor, it hardens when you crash into the tree, and as soon as you bounce off the tree, the shear force goes away and it becomes soft again.
