Do Higher Grade Bolts Have Lower Stretching Ability?

Myth: The higher the bolt grade, the lower the stretching ability of the bolt.

Truth: whatever their grade, bolts made from the same material have the same modulus of elasticity E:

• For carbon steel bolts (ISO 898-1): E=210 kN/mm2.
• For stainless steel bolts (ISO 3506-1): E=170 kN/mm2.

According to the law of Hooke, the stretching ability of a material depends on its modulus E while its stress σ and elongation ε are linked as follows: σ=E x ε. Therefore, the higher grade means the same elongation under the same force for the same bolt sizes, but a greater load can be supported before yielding or breaking.