What is Strain?
Strain is a dimensionless measure of deformation representing the displacement between particles in the body relative to a reference length. It quantifies how much a material stretches or compresses under applied forces. Strain is one of the most fundamental concepts in materials science and structural engineering.
Engineering strain (also called Cauchy strain or nominal strain) is the simplest form, defined as the change in length divided by the original length. True strain (also called logarithmic strain or Hencky strain) accounts for the continuously changing length during deformation and is more accurate for large deformations.
Strain Formulas
Types of Strain
| Type | Formula | Use Case |
|---|---|---|
| Engineering (Normal) | ΔL / L₀ | Small deformations (<5%) |
| True (Logarithmic) | ln(L / L₀) | Large deformations, metals forming |
| Shear Strain | γ = tan(θ) | Angular deformation |
| Volumetric Strain | ΔV / V₀ | Hydrostatic compression |
Typical Strain Values for Materials
| Material | Yield Strain | Ultimate Strain |
|---|---|---|
| Steel (mild) | 0.001 - 0.002 | 0.10 - 0.25 |
| Aluminum | 0.002 - 0.005 | 0.08 - 0.20 |
| Concrete (compression) | 0.001 - 0.002 | 0.003 - 0.004 |
| Rubber | N/A | 5.0 - 8.0 |
Frequently Asked Questions
What is the difference between stress and strain?
Stress is the internal force per unit area within a material (measured in Pascals or psi), while strain is the dimensionless ratio of deformation to original dimensions. They are related through the material's modulus of elasticity: Stress = E x Strain (Hooke's Law).
When should I use true strain instead of engineering strain?
Use true strain when deformations exceed approximately 5% of the original length. True strain is additive for sequential deformations and provides more accurate results for metal forming, rubber stretching, and other large-deformation applications.
Can strain be negative?
Yes. Negative strain indicates compression (the material is getting shorter), while positive strain indicates tension (the material is getting longer). The sign convention helps distinguish between tensile and compressive loading.