[blykins]

Being a Mechanical Engineer, I've had lots of courses on materials, machine design, and finite element analysis.

A material has a yield strength. That's the stress limit at which the material deforms....this could either happen plastically....or it could happen like Dan was explaining....in a "brittle" fashion.

In most cases, the ultimate shear strength is within a certain percentage of the ultimate yield and tensile strengths...if I remember correctly from school, it's in the upper 80% range. However the material's metallurgical properties can be altered by different methods of working the metal. This would backup Dan's argument that a G8 bolt would be less likely to plastically deform....but it would just fail at stresses equal to its tensile/shear strength.

A good example of this would be a high strength steel and a nodular iron. Both would have extremely high strengths, but a nodular iron is more of a "brittle" material....which would be less likely to plastically deform...

What you have to analyze though in this situation, is whether the strength difference is enough to warrant the grade difference.

30000 psi over a 1/2 bolt would allow around 6000 lbs more force in a tensile strength situation.

So even if the G5 bolt plastically deforms, it would still start deforming before the G8 bolt would....even though the G8 bolt would yield in a much more violent manner.

One thing I don't quite understand is the explanation on how the G8 bolts fail with different modes of loading. I'd like to hear more on that Dan, if you could sometime.

It seems to me, when you put a fastener in a high load situation such as the high clamping mode you were talking about in your post, that would be in an axial tensile strength situation.

However, you say that a G8 bolt is more likely to fail under a bending moment, side load, or impact.

A bending moment is a function of tensile strength. When you bend a piece of metal, you are essentially stressing it at the top and bottom of the metal. You're pulling it on the top, and compressing it on the bottom.

The side load or impact would be a complete shear load....not a function of tensile strength, but of shear strength.

I don't have a list in front of me of the fastener shear strengths.....only tensile strengths. This difference that you were referring to could be a function of shear strength only......not tensile strength or axial loading.