Yes, let me state that the toughness of Aluminum vs. Steel is extremely dependent on the alloy and temper. 1100 Aluminum, for example, will elongate forever. I believe Aluminum tank are made from a 6000 series alloy, and the temper state is one of the work-hardened tempers (2-3 digits). The specific measurement that you want is the Charpy or Izod toughness. (Minor differences between the two; Charpy is more likely the one you want). (And no, Izod test has nothing to do with alligators
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The tensile strength of a hardened aluminum alloy can be higher than that of a soft steel (I can quote you alloys), but since I have no idea what alloys are used in tanks, I'll shut up here...