It cures moderately hard, not brittle, at least at room temperature. If you're looking at using it on a porous substrate that has a little flex, it should be okay, especially if it's clamped during cure to keep the adhesive layer thin. If you need something that's very tough and flexible, there's always 3M 5200, but that's more expensive. Or a low durometer epoxy, which is both more expensive and requires skilled measuring and mixing. Or a Very High Bond double-side tape like 3M 300LSE, which works on flexible, non-porous substrates.