Okay, this is yet another SAE Grade 5 vs Grade 8 fastener question -
are Grade 8s more brittle than Grade 5 and does it matter for
My main question in this post is "I thought that generally harder
materials were more brittle rather than tough?"
I've been going around in circles trying to definitively answer this
and just get conflicting answers. Yes it is. No it is not. Yes it
is but it doesn't matter because the Grade 5 will have already failed.
It is also my understanding that SAE does not specify notch toughness
or ductile/brittleness for Grade 5/8.
About the best explaination I've seen is at
is an automotive enthusiast site.
However, I disagree with what the author wrote in the second paragraph
I'm fully prepared to be shown wrong! :-)
It's been over 20 years since I've studied the stuff. :-(
****from roughly the middle of the weblink above ****
Again, you can see that the grade 8 will support over 1000 lbs more or
a 1/2-ton more. But there?s something more important to note. The
grade 5 fastener has already reached its ultimate load and FAILED
BEFORE the grade 8 starts to yield or stretch. Therefore, the argument
that you should not use grade 8?s because they are more brittle than
grade 5?s is not a true statement in most applications.
Toughness is an important feature of a fastener. It is the opposite of
brittleness and gives you an idea of how it will handle abuse without
being damaged and eventually weakening the fastener or can cause
fatigue to appear much earlier than normal. One way to ?measure?
toughness is by looking at the hardness rating of a fastener. The
higher the number (Brinell, Rockwell ?) the harder the material is and
the tougher it is to damage. According to Marks? Standard Handbook for
Mechanical Engineers, Grade 5?s typically have a core Rockwell
hardness of C25-C34 whereas a grade 8 typically has a core Rockwell
hardness of C33-C39. Based on this, grade 8?s are tougher than grade
- posted 18 years ago