In discussions about “replacing steel with plastic,” one concept that is often overlooked but crucial is this: material performance shouldn’t be judged by absolute values alone.

When we talk about replacing steel with plastic, we’re not really comparing “which is stronger or harder,” but rather — under the same weight, which material can carry more load and maintain higher stiffness.

This is exactly the significance of specific strength and specific stiffness.

01 Why Can't We Compare Strength Directly?

Steel often has a strength of 400–1000 MPa, while common engineering plastics only reach a few tens to around 100–200 MPa. At first glance, they seem incomparable. But if we convert this to stress per unit weight, the picture changes completely.

Plastics have a density of only 1.0–1.5 g/cm³, whereas steel is as high as 7.8 g/cm³. In lightweight design, what really constrains the design isn’t whether the material can “hold up,” but whether the structure’s weight can be reduced.

With the same mass of material, plastics allow for more volume, which means:

A larger section moment of inertia (beneficial for stiffness)

A larger load-bearing area (beneficial for strength)

Therefore, discussions about replacing steel with plastic must start with specific strength (σ / ρ) and specific stiffness (E / ρ). Ignoring density is equivalent to ignoring lightweight design itself. When density is taken into account, even a PP product can achieve performance comparable to—or better than—titanium.