How to Compare Engineering Materials: The Five Properties That Matter

EngiRef Guides · Updated July 2026 · ~8 minute read

"Which material is stronger?" is usually the wrong first question. Material selection is a trade study, and the materials that win real design competitions rarely win on any single raw number. Aluminum "loses" to steel on strength and stiffness, yet dominates aerospace structures. PEEK "loses" to almost every metal on every mechanical property, yet displaces metal in chemical and medical applications. The skill is knowing which properties govern your problem and comparing candidates on those — often as ratios, not raw values.

The five workhorse properties

1. Density (ρ)

Density multiplies through everything in a moving or mass-sensitive design: fuel burn, inertia, shipping cost, handling. Steels sit near 7.8 g/cm³, titanium alloys near 4.4, aluminum alloys near 2.7, and engineering plastics mostly between 0.9 and 1.5. When a design is weight-critical, density belongs in the denominator of every comparison you make — which is where specific properties (below) come from.

2. Elastic modulus (E)

Young's modulus measures stiffness — resistance to elastic deformation — and it is largely fixed for a metal family regardless of alloying or heat treatment. All steels sit near 200 GPa; all aluminum alloys near 69–72 GPa. Two consequences: you cannot buy stiffness by choosing a "stronger" grade of the same metal, and deflection-limited designs (see our beam deflection guide) are compared on E, not strength.

3. Yield and tensile strength

Yield strength marks the onset of permanent deformation; ultimate tensile strength marks the maximum stress before failure begins. Unlike modulus, strength varies enormously within a family — AISI 1018 mild steel and quench-and-tempered 4340 differ by several times in yield strength while sharing nearly identical density and stiffness. That's why "steel vs. aluminum" is underspecified: the honest comparison is grade vs. grade, temper vs. temper (6061-T6 and 7075-T6 are very different animals).

4. Thermal properties

Thermal conductivity decides whether a part spreads heat (copper, aluminum) or bottles it up (stainless steel, titanium, nearly all plastics). Thermal expansion decides whether an assembly of dissimilar materials fights itself as temperature swings. These properties routinely veto candidates that looked fine mechanically — a plastic bushing that creeps at 90 °C, a stainless heat sink that isn't one.

5. Ductility and toughness

Elongation at break and impact toughness describe how a material fails: gradually with warning, or suddenly. High-strength tempers usually buy their strength by sacrificing ductility. For anything safety-critical or subject to impact and overload, failure mode is a first-class selection criterion, not a footnote.

Compare ratios, not raw numbers

The most common beginner error is comparing raw strength when the design is weight-limited or stiffness-limited. The classic corrective is to compare specific properties:

The famous near-coincidence in that last column — steel, aluminum, and titanium all cluster around the same specific stiffness — explains why weight-limited, stiffness-driven designs are often decided by geometry, cost, corrosion, and manufacturing rather than by the metal itself: for a simple tension member, none of the big three buys you a fundamentally stiffer-per-kilogram structure. (In bending, thicker-walled light sections shift the math — geometry again.) Specific strength, by contrast, differs sharply between families and grades, which is where titanium and high-strength aluminum earn their keep.

The pitfalls that bite in practice

• Comparing families instead of grades. "Aluminum vs. steel" hides a 4× spread within each family. Always compare specific designations: 304 vs. 316 stainless differ in corrosion behavior; 6061-T6 vs. 7075-T6 differ hugely in strength and weldability.
• Ignoring temperature. Datasheet values are room-temperature values. Plastics lose stiffness rapidly with heat; some steels lose toughness in the cold. Check properties at your service temperature, not the lab's.
• Forgetting fatigue. A part loaded once and a part loaded a million times live in different worlds. Steels typically exhibit an endurance limit; aluminum alloys generally do not — meaning any cyclic stress eventually matters. Fatigue analysis, not static strength, governs rotating and vibrating hardware.
• Unit slips across property tables. Mixing MPa and ksi, or g/cm³ and kg/m³, quietly wrecks a comparison. This deserves its own discussion — see unit conversion errors in engineering.
• Optimizing one property to death. The best material is the one that satisfies all constraints — mechanical, thermal, chemical, manufacturing, cost — with margin, not the one that tops a single column.

A practical comparison workflow

1. Name the governing constraint. Is the part stiffness-limited, strength-limited, weight-limited, temperature-limited, or cost-limited? Most parts have one dominant constraint and two secondary ones.
2. Shortlist by elimination. Kill candidates that fail hard constraints (service temperature, chemical exposure, food/medical contact) before comparing numbers.
3. Compare side by side on the governing properties. Put two or three finalists next to each other — density, yield strength, modulus, thermal conductivity — and look at ratios relevant to your constraint.
4. Check the failure mode and the fatigue story. Then, and only then, let cost and availability break ties.