The Proof Load is defined as the greatest load than can be applied to a
fastener that will not strain it beyond its elastic limit. In other
words, the fastener must not deform and it must remain with it's elastic
region when it is loaded up to its maximum proof load. Typically the
proof load is between 85 to 95% of the yield strength.
According to ISO 225 and ISO 965-1 the following definition applies:
Fp = Proof Load, measured in N (Newtons)
What is Yield Strength?
The Yield Strength is defined as the amount of stress at which a
predetermined amount of permanent deformation occurs.
What is Tensile Strength?
Tensile Strength is a measurement of the amount of force required to pull
something until it fractures. In other words, the tensile strength
of a fastener is the maximum amount of tensile strength it can handle
Rm = Tensile Strength, measured in N/mm2 or
Which nuts can come in higher tensile grades?
Fastener grade ranges are always given within the specification standard for particular nuts. We must ensure that a non-specified fastener is not quoted as a higher tensile one.
For example, it is common practice for customers to ask for Whitworth Grade 8 nuts. However, BSW/Whitworth nuts come in letter grades only. Or for customers to ask for lock nuts (thin) to be made in grade 8 - whereas these fasteners can not be torqued to grade 8 figures because there is not enough thread engagement.
See the following table for those standards covered under ISO 898-2 standard.