If you can see the crack, it's not a fatigue regime you have to worry about. It's not going to take 10,000 or a 100,000 cycles to failure. We will be dicussing numbers in the low hundreds down to one. You enter a failure regime controlled by the KIC fracture toughness of which aluminium has a lower fraction of steel or titanium's value, than it does a comparison of their strength or elongation properties.
It's not even junk. As a general rule, five pounds of scrap won't just kill someone. It's a disaster zone waiting to happen.