Yield strength is the stress at which a material transitions from elastic (recoverable) to plastic (permanent) deformation. Below yield, remove the load and the material returns to its original shape. Above yield, the material retains a permanent set — it is deformed and will not spring back.
For metals, the yield point is usually defined as the 0.2% offset stress: the stress at which the material has accumulated 0.2% permanent strain. This is a practical convention because most metals do not have a sharp, unambiguous yield point.
Yield strength vs. ultimate strength
Yield strength — the onset of permanent deformation. This is the primary design limit for reusable aerospace structures because permanent deformation changes part geometry, introduces residual stresses, and can initiate fatigue cracks.
Ultimate tensile strength (UTS) — the maximum stress the material can sustain before fracturing. For ductile metals, UTS is 10–50% higher than yield strength. For brittle materials (composites, ceramics, some cast alloys), yield and ultimate strength may be nearly identical — the material fractures with little or no warning.
Aerospace structural design uses both:
- The structure must show no permanent deformation at limit load (1× maximum expected load) — this is a yield criterion.
- The structure must not rupture at ultimate load (1.5× limit load for crewed aircraft) — this is an ultimate criterion.
Values for aerospace materials
| Material | Yield strength (MPa) | Ultimate strength (MPa) | Density (kg/m³) | Specific strength (σ_y/ρ) |
|---|---|---|---|---|
| 7075-T6 aluminum | 503 | 572 | 2,810 | 179 |
| 4130 steel (normalized) | 435 | 670 | 7,850 | 55 |
| Ti-6Al-4V titanium | 880 | 950 | 4,430 | 199 |
| Carbon fiber composite (quasi-iso) | — | 500–800 | 1,550 | 320–520 |
| PLA (printed, along layers) | 50–60 | 50–65 | 1,240 | 40–48 |
| PLA (printed, between layers) | 20–35 | 25–40 | 1,240 | 16–28 |
| CF-nylon (printed, along layers) | 70–90 | 80–100 | 1,150 | 61–78 |
The last column — specific strength (yield strength divided by density) — governs weight-efficient design. Titanium and carbon fiber composite dominate because they combine high strength with low density. 3D-print filaments are dramatically weaker, especially between layers, which is why printed UAV structures rely on geometric efficiency (closed sections, infill) and supplementary materials (carbon rods as spar caps) to carry flight loads.
The PLA inter-layer values are notable: at 40–60% of the along-layer strength, the layer boundary is the weakest point in any printed airframe. Print orientation relative to the primary load path — discussed in Designing a Printed Wing — is the most consequential structural decision in FDM UAV construction.
Yield in expendable design
Expendable airframes can relax the yield criterion: accepting plastic deformation under peak loads (catapult launch, terminal maneuver) as long as the structure does not fracture. This allows lower safety factors (1.1–1.25 on ultimate) and lighter structures, because the designer sizes to ultimate strength rather than yield strength.
Related terms
- Stress — the quantity that yield strength bounds
- Strain — the deformation that becomes permanent beyond yield
- Young’s Modulus — the stiffness in the elastic regime below yield
- Safety Factor — the margin applied to yield and ultimate criteria
- Structural Load — the external loads that produce the stresses compared against yield