Engineering Reference

Modulus vs strength

Modulus measures stiffness; strength measures the load at which a material fails.

Definition

The two are independent and are governed by different things. Modulus comes from atomic bonding and is essentially fixed for a material family — every steel is about 205 GPa. Strength comes from microstructure and is highly variable — steels range from 215 MPa to over 1,700 MPa yield.

The design consequence is direct: a deflection-limited part gains nothing from a stronger steel, and a yield-limited part gains nothing from a stiffer one. Deciding which limit governs is the first step in materials selection.

Common misuse

Switching to a higher-strength steel to fix a stiffness problem. The modulus is unchanged, so the deflection is unchanged.

Related