Glossary

Young’s modulus (also called the modulus of elasticity, E) is a material property that measures elastic stiffness—how much a material resists stretching or compressing within its elastic (fully recoverable) range. It is defined as the ratio of stress to strain in the straight-line portion of the stress–strain curve: E = σ / ε, and it’s expressed in units of pressure (e.g., psi or GPa).

In bolted joints, Young’s modulus is one of the key inputs that controls how much a fastener elongates for a given load and how the load is shared between the fastener and the clamped parts (joint stiffness). A higher E means the material is stiffer and will stretch less under the same stress; a lower E means it’s more compliant and will stretch more. This is why a steel bolt (E ≈ 200 GPa / ~29 Msi) generally elongates less than an aluminum fastener (E ≈ 69 GPa / ~10 Msi) for the same stress, and why modulus directly affects calculations for bolt stretch, preload retention, joint separation, vibration performance, and load distribution.

Young’s modulus is not a strength value. It does not tell you how strong the material is before it yields or breaks (that’s yield strength and tensile strength), and it’s not hardness. It only describes the slope of the elastic region—the “springiness” of the material—so two steels with very different strengths can still have nearly the same modulus.

It’s named for Thomas Young, the British scientist who introduced/defined an elastic modulus for material stiffness in 1807 (and then discussed it further in his Course of Lectures on Natural Philosophy and the Mechanical Arts). 

One nuance: the stress–strain proportionality idea that we now write as σ = Eε was discussed earlier by Leonhard Euler (1727), and Giordano Riccati (1782) is often credited with early experiments using the concept in a modern way—but the name “Young’s modulus” comes from Thomas Young’s 1807 treatment.

AKA: The Modulus of Elasticity