Strain is the dimensionless measure of deformation in a material under stress. It is the change in length divided by the original length:
ε = ΔL / L₀
A strain of 0.01 (1%) means the material has stretched or compressed by 1% of its original dimension. Aerospace materials typically operate at strains below 0.5% in normal service — the deformations are real but invisible to the eye.
Elastic and plastic strain
Elastic strain is recoverable: remove the load and the material returns to its original shape. This is the regime aerospace structures are designed to operate in during normal flight. A wing that bends upward under lift and returns to its original shape after landing is deforming elastically. The relationship between stress and elastic strain is linear and governed by Young’s modulus:
σ = E × ε
Plastic strain is permanent: the material has yielded and will not return to its original shape. An aircraft wing that remains bent after a hard landing has experienced plastic strain. Conventional aerospace design requires no plastic strain at limit load (the maximum expected load). Expendable UAVs may accept some plastic strain under peak loads because the structure only needs to survive one mission.
The transition from elastic to plastic behavior occurs at the yield strength. Beyond yield, the stress-strain curve flattens — the material deforms much more for each additional increment of stress. Eventually, the material reaches its ultimate tensile strength and fractures.
Why strain matters for aerospace
Wing deflection. A wing in flight bends upward under aerodynamic load. The strain in the spar caps determines how much. Excessive deflection changes the effective angle of attack distribution along the span, degrading aerodynamic performance and potentially inducing flutter. A 747’s wingtips deflect ~3 meters in turbulence; the strains involved are still below 0.3%.
Fatigue. Every load cycle imposes a strain cycle. Over thousands of cycles, microscopic damage accumulates at stress concentrations even though the strain stays well within elastic limits. This is fatigue, and it is the primary structural life limiter for reusable aircraft.
Anisotropy in printed parts. 3D-printed FDM parts have different strain-to-failure along filament lines versus between layers. The inter-layer strain capacity can be 40–70% lower than the in-plane capacity, making print orientation a structural design variable.
Related terms
- Stress — the force per unit area that produces strain
- Young’s Modulus — the stiffness relating stress to elastic strain
- Yield Strength — the stress at which strain becomes permanent
- Structural Load — the external forces that produce stress and strain