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Aluminum and other metals undergo microcracking only when stretched beyond its elastic limit.  Beyond the elastic limit, permanent deformation will occur. The elastic limit is, therefore, the lowest stress point at which permanent deformation can be measured. So if you slam a carabiner point blank at a concrete surface irreversible deformation will occur superficially (a scratch or dent), but the material as a whole will have rebounded perfectly.  The elastic limit of aluminum is about 400 million pascals or about 58,000 pounds per square inch (far greater than any other point in the safety chain).  

If a carabiner has a cross section of a quarter square inch you would have to pull with about 32000 pounds (58000/4 x 2, (the factor of 2 is for the two sides of the biner) before irreversible deformation or load bearing would begin to be compromised. However long long before the ultimate load bearing capacity of the biner it would have to be undergo severe elongation.  That is why as a rule, if the gate still works, the biner is still safe to use.  If it has been stretched the half mm of clearance to interfere with the gate, it has been irreversibly deformed and should not be used for life or death situations unless there is no other option.  Better reserve said biners for stretching your hammock or dangling them from the back of your jeep.