Re: D44 crossover problems *FYI*
Once again, I don't think the shear strength is the big deal here, it's the clamp load of the connection that holds it together. You should have a clamp load of about 18,000 pounds or more from each fastener holding the two machined surfaces together. This creates a lot of friction force and that's what holds it together.
The "brittle" nature of stronger fasteners is not always that big a deal, especially since metallurgy is always advancing and TOUGHNESS is what everyone is after. But regardless, (to use some numbers from a chart from one of our suppliers) if you tighten a grade 5 bolt up to 75% of it's proof strength (just under the yield point), you get a 13K# clamp load. The same situation with grade 8 gives you 18K# clamp load. Now take a fastener that's rated at 180-190K psi tensile strength compared to a grade 8 at 150K psi and you get even more clamp load out of it. Where "brittle" comes into this is the relationship of yield strength and ultimate tensile strength. When they get close together, we consider the bolt "brittle". For example a grade 5 bolt has a yield point of 92ksi and a yield of 120ksi so yield is 77% of the tensile. Grade 8 with yield of 130ksi and tensile of 150ksi gives a percentage of 87%. Stressproof has 100ksi and 115ksi so it's percentage is also about 87%. But is this "brittle"? Only if you have a fastener with poor thread roots, bad fillet radii or other messed up design features. Just because a low grade fastener bends farther before it breaks doesn't mean it's better. Weaker is weaker and after a fastener yields, it's ruined. Consider that a hard hit on a fastener that breaks a grade 5 equivalent stud with 115ksi ultimate strength won't even get close to the yield point of a grade 8 or stronger part.
There is a LOT to all of this especially when you start looking at all the private manufactures building higher strength fasteners and calling them whatever they want. There are no SAE specs over grade 8 so anything you want to use that's stronger, you have to check out the specs for yourself. There is nothing wrong with the higher (than grade 8) strength fasteners, there are lots of reputable companies building them and lots of demanding applications using them. If you want to know what they have, ask them what the yield and tensile strength are and you'll have the most pertinent infomation.
