[Slick61]

The devil's in the details... do you want it to bend, or do you want it to break. Either grade could work with a properly designed connection.

To a degree, you're splitting hairs here between grades 5 & 8. The nominal compositions of each could be identical/overlapping. Same for the mechanical properties. Hard to say what composition you'll actually GET if buying bolts at the hardware store, tho economics dictate that the manufacturer will use the cheapest route possible: either the lowest alloy content that's heat treated to its limit, or a higher alloy content with a more mild heat treat... whatever gets them the strength they need for the lowest cost.

Yes, the Grade 8 bolt is marginally stronger, but this strength comes at the expense of ductility (elongation)... so it will be less ductile/more brittle (~2% difference).

A given bolt material could be heat treated to either Grade 5 OR Grade 8 requirements.

Hardness, yield & tensile strength only tell you part of the story. For a static application with axial loading, grade 8 should provide the highest strength. For the application in question, I'd look at toughness... since dynamic/impact-type loads would be of primary concern. Specifically, I'd look at the Charpy V-notch values at room temp. Granted, Charpy values are primarily an indicator of notch sensitivity, but they also provide an indication of general toughness.

If you're looking at shear-type applications, I'd try to design the connection so that all threads closest to the load application are engaged, thus minimizing notch sensitivity in the threads.

There's several ways to skin this cat...