[StingRay]

The problem with washers is the sharp edge. Hardened is worse. In an impact the washer cuts and button holes through the steel. The softest most deformable washer would best if a flat washer is used. I would stack a smaller and larger diameter washer so that the outer dia. of the large diameter has the opportunity to deflect, deform and perhaps cut in less. The attached pic is the RV industry standard for a reinforcing washer in seat belt applications. Even it is not a guarantee. Best practice would put it near other structure or stamped shape and where there are multiple layers of material. The radius flange allows the washer to deform and stretch the floor without cutting. Poorly applied even they will pull through.