out” waiting to be hit, so Su is not nearly as important as KIc. Also, they do not lose functionality after yielding, as long as their permanent deformations do not interfere with the inner structure, drivetrain or weapon system, so Sy is not that important either. And the armor plates are already mounted either over stiff internal structural walls or over shock mounts (as pictured to the right, with shock mounts made out of curled steel cables), therefore they don’t need too much stiffness themselves. Top cover plates can be included in this category if they do not act as structural elements, if they are just plates used to protect the robot interior, without having to support any internal components such as the drivetrain system. So, stiffness and yield strength are not that important, as long as the robot has a few internal supports that will prevent the top cover from bending too much into the robot and smash some critical component. Therefore, non-structural top cover plates that are well supported can be modeled as traditional armor elements, so the following analysis also applies to them. Note that we’re assuming here that the fracture toughness KIc can also be a measure of impact toughness. This is true for most metals, unless they have notch sensitivity problems, explained later. In addition, the value of KIc depends on the loading rate (the impact speed). All KIc measurements included in the previous table were made under very slow tests, therefore they are also called static KIc. If the tests had been performed at very high load rates, to evaluate the effect of the impact speed, the resulting dynamic KIc values would probably be much lower. For instance, one of the Ti6Al-4V armor plates from our middleweight Touro shattered almost like glass due to a high speed impact from the bar spinner The Mortician, during RoboGames 2006. Very little plastic deformation can be seen in the remaining plate (pictured to the right), and the removed portion shattered into tiny pieces. At lower impact speeds, the very ductile Ti-6Al-4V would certainly absorb more energy, meaning that its static (low speed) KIc = 72MPa√m is probably much higher than its dynamic (high speed) KIc.
90