Specific Strength & Specific Stiffness: The Engineering Logic Behind “Plastic Replacing Steel”

Introduction: Rethinking Material Performance

In discussions about “replacing steel with plastic,” one critical concept is often overlooked: material performance should not be judged by absolute values alone.

The real question is not “which material is stronger,” but: under the same weight constraint, which material delivers higher load capacity and stiffness?

This is exactly where specific strength and specific stiffness become essential.

01 Why Strength Cannot Be Compared Directly

Absolute Strength vs. Lightweight Design

Steel typically reaches 400–1000 MPa, while engineering plastics are usually 100–200 MPa. At first glance, they seem incomparable.

However, when strength is normalized by density, the comparison changes completely.

• Plastic density: ~1.0–1.5 g/cm³
• Steel density: ~7.8 g/cm³

Why Density Changes Everything

• Larger section moments of inertia → higher stiffness
• Larger load-bearing areas → higher effective strength

02 Specific Strength & Specific Stiffness

Specific Strength (σ / ρ)

The maximum stress a material can withstand per unit weight. It determines load-bearing capacity under the same mass.

Specific Stiffness (E / ρ)

The stiffness a material provides per unit weight. It controls deformation under load, especially in brackets, housings, and frames.

04 A Simple, Intuitive Example

1 kg Structural Bracket

• Steel: Volume ≈ 0.13 L
• GF-PA6: Volume ≈ 0.74 L (same weight)

Nearly 6× volume expansion allows ribs, reinforcements, and higher moments of inertia.

Result: Comparable stiffness and strength with 60–70% weight reduction.

Conclusion

The essence of “plastic replacing steel” is not imitation, but achieving a better specific-performance balance under weight-constrained design.