[Scoober]

In high vibration or thermal areas the harder the material(higher grade) the less elastic it is and the more prone it is to fracture in those situations. The softer materials are more stetchy(for lack of a better term) and can accomodate more vibration and thermal changes. Most people that work on cars have put too much torque on a fastener and have had it break at some point or another. This has stretched the bolt beyond its elastic limit, most often under the head or thread area(smallest diameter) and you will see it neck down a bit. In shear type of assemblies the part hold the loads and the bolts are used to create surface friction between the parts, never use a bolt as a pin in a shear assembly!! lugs are holding the wheel to the hub and create high friction rate between the two surfaces, lugs too loose and you will shear the lugs off under acceleration or braking. All fasteners stretch to create clamp load and there has to be the right amount of stretch to allow for expansion under loads or if there is contraction like in gaskets.