David Billington wrote:

The 1st figure is 1/10 the tensile strength and 2nd figure is 1/10 the
ratio between the minimum tensile strength and the minimum yield stress
(or proof stress in the higher strength fasteners as this is), also IIRC
the units are in kgf/mm2. So the yield stress is 90 kgf/mm2 and the
tensile strength is 100kgf/mm2.

Grade 10.9 means:
tensile strength is 100 kp / mm^2 (there is no such thing as kgf)
Multiplying the first number with the second (10 * 9 = 900) gives the yield
strength times 10 (so 90 kp / mm ^2).

A 8.8 has 80 kp/mm^2 tensile and 64 kp/mm^2 yield strength.
The numbers aren't very precise.
That was the initial definition. Of course, the units now are in N and mm,
or in Pa.


Nick
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