Material Properties
Strength, stiffness, density and thermal data for the metals and plastics engineers and makers use most, and what each number actually means.
Choosing a material is a trade-off between strength, stiffness, weight, heat, cost and how easily you can make the part. The numbers below are typical room-temperature values for common grades. Real values depend on the exact alloy, heat treatment, product form and, for plastics, the grade and how the part was made. For design work, use the minimum values in the material specification or the supplier’s certificate.
The key properties
| Property | Symbol and units | What it tells you |
|---|---|---|
| Density | ρ, g/cm³ (× 1000 = kg/m³) | How heavy a part will be for its size |
| Modulus of elasticity (Young’s modulus) | E, GPa | Stiffness: how much the material stretches or bends under load before it yields |
| Yield strength | σy, MPa | The stress at which it starts to bend permanently. The usual design limit for metals |
| Ultimate tensile strength | UTS, MPa | The highest stress it can take before it starts to neck and break |
| Elongation at break | % | Ductility: how far it stretches before breaking. Low values mean brittle |
| Hardness | HB, HRC, HV | Resistance to dents, scratches and wear. Roughly tracks strength in steels |
| Thermal expansion | α, µm per m per °C | How much it grows when heated |
| Thermal conductivity | k, W/m·K | How quickly heat flows through it |
| Melting point or glass transition | °C | Metals melt; most plastics soften well before they melt, at the glass transition temperature |
Reading a stress–strain curve
Most of these numbers come from a tensile test: a sample is pulled until it breaks while the force and stretch are recorded. Stress is the force divided by the original area; strain is the stretch divided by the original length.
- Elastic region: the straight first part. Remove the load and the part springs back. Its slope is E.
- Yield: the end of springy behavior. Beyond it, the part is permanently bent or stretched.
- Ultimate tensile strength: the top of the curve. After this the sample thins locally (necks) and the load it can carry drops.
- Fracture: the break. A long curve means a ductile material that bends and warns before failing; a short one means a brittle material that snaps.
Metals
| Material | Density (g/cm³) | E (GPa) | Yield (MPa) | Tensile (MPa) | Elongation (%) |
|---|---|---|---|---|---|
| Structural steel ASTM A36 | 7.85 | 200 | 250 (min) | 400–550 | 20+ |
| Mild steel 1018, cold drawn | 7.87 | 205 | 370 | 440 | 15 |
| Stainless steel 304, annealed | 7.9–8.0 | 193–200 | 205–290 | 515–620 | 40+ |
| Aluminum 6061-T6 | 2.70 | 69 | 276 | 310 | 12–17 |
| Aluminum 7075-T6 | 2.81 | 72 | 503 | 572 | 11 |
| Titanium Ti-6Al-4V (grade 5), annealed | 4.43 | 114 | 880 | 950 | 15 |
| Free-cutting brass C36000 | About 8.5 | 97–100 | 125–310 | 340–470 | Varies with temper |
| Copper C11000 | About 8.9 | 115–120 | 70 (soft) to 310 (hard) | 220–380 | Varies with temper |
| Gray cast iron class 30 | 7.2 | 90–110 | No clear yield (brittle) | About 210 (min) | Under 1 |
Ranges reflect different tempers and product forms. Specification minimums (for example ASTM A240 for 304 stainless: 205 MPa yield, 515 MPa tensile, 40% elongation) are what you should design with. Heat treatment changes strength a lot but barely changes stiffness: every steel has E close to 200 GPa.
Picking a metal
- Mild steel (A36, 1018): cheap, strong, stiff, easy to weld and machine. Rusts without paint or plating.
- 304 stainless: corrosion resistant and tough, but gummy to machine and work-hardens. 316 adds molybdenum for salt water and chemicals.
- 6061-T6 aluminum: the all-rounder for brackets, frames and plates. Light, machines well, welds (with some loss of strength near the weld) and anodizes.
- 7075-T6 aluminum: nearly twice as strong as 6061 for the same weight, but more expensive, less corrosion resistant and not generally weldable.
- Ti-6Al-4V titanium: very strong for its weight and corrosion resistant, but expensive and slow to machine.
- Brass C36000: the easiest metal to machine; good for fittings, bushings and small turned parts.
Plastics and 3D printing
| Material | Density (g/cm³) | Tensile strength (MPa) | E (GPa) | Heat limit | Good to know |
|---|---|---|---|---|---|
| PLA | 1.24 | 50–65 | 3.0–3.6 | Softens about 55–60 °C | Easiest to print; stiff but brittle; creeps and warps in a hot car |
| ABS | 1.04 | 35–45 | 2.0–2.4 | Glass transition about 105 °C | Tougher and more heat resistant; warps while printing; needs ventilation |
| PETG | 1.27 | 45–55 | 1.9–2.1 | Glass transition about 80 °C | Tough, good layer bonding, slightly flexible |
| Nylon 6/6 (dry) | 1.14 | 75–85 | 2.8–3.0 | Melts about 260 °C | Strong and wear resistant; absorbs moisture, which lowers stiffness |
| Polycarbonate | 1.20 | 60–70 | 2.3–2.4 | Glass transition about 145 °C | Very tough and impact resistant; clear grades available |
| Acetal (POM, Delrin) | 1.41 | 60–70 | 2.8–3.1 | Melts about 165–175 °C | Low friction, machines beautifully, good for gears and bushings |
| Acrylic (PMMA) | 1.18 | 65–75 | 3.0–3.3 | Glass transition about 105 °C | Clear and stiff but brittle; laser cuts cleanly |
| HDPE | 0.95 | 20–30 | 0.8–1.2 | Melts about 130 °C | Slippery, chemical resistant, floats |
| PTFE (Teflon) | 2.2 | 20–35 | 0.4–0.6 | Usable to about 260 °C | Lowest friction of any solid; soft and creeps under load |
Values are typical for molded or extruded material and vary widely between grades and brands. 3D-printed parts are usually weaker, often much weaker between layers than along them, and depend on infill, wall count, temperature and moisture. Check your filament maker’s datasheet, and see 3D Printing and Additive Manufacturing.
Strength and stiffness for the weight
When weight matters, compare properties divided by density. Specific strength (yield strength ÷ density) shows how much load a material carries for its mass:
Specific stiffness (E ÷ density) has a surprise: steel (200 ÷ 7.85 = 25.5), aluminum (69 ÷ 2.70 = 25.5) and titanium (114 ÷ 4.43 = 25.7) are almost identical. Pound for pound, none of them is stiffer in pure tension. Aluminum wins in bending only because, for the same weight, an aluminum part can be about 2.9 times thicker, and bending stiffness grows with thickness cubed: an aluminum plate is roughly 8.5 times stiffer than a steel plate of the same weight. See Beams and Profiles.
Worked example: what does 1 m of 20 mm round bar weigh?
Volume = π × 10² × 1000 = 314 159 mm³ = 314.2 cm³. Mass = volume × density:
- Steel (7.85 g/cm³): 2.47 kg
- Brass (8.5 g/cm³): 2.67 kg
- Titanium (4.43 g/cm³): 1.39 kg
- Aluminum (2.70 g/cm³): 0.85 kg
- PLA, if printed solid (1.24 g/cm³): 0.39 kg
Thermal properties
| Material | Thermal expansion (µm/m·°C) | Thermal conductivity (W/m·K) | Melting or softening (°C) |
|---|---|---|---|
| Carbon steel | 11–13 | About 50 | About 1450–1520 |
| Stainless steel 304 | 17.3 | 16 | 1400–1450 |
| Aluminum 6061 | 23.6 | 150–170 | 580–650 |
| Copper | 17 | About 390 | 1085 |
| Brass C36000 | 20.5 | About 115 | 885–900 |
| Titanium Ti-6Al-4V | 8.6 | 6.7 | 1600–1660 |
| Gray cast iron | 10–12 | 45–55 | About 1150–1300 |
| PLA | About 68 | 0.13 | Softens about 55–60 |
| ABS | 70–100 | 0.17 | Glass transition about 105 |
| Nylon 6/6 | 80–100 | 0.25 | Melts about 260 |
ΔL = α × L × ΔT
Worked example: aluminum bolted to steel
A 1 m aluminum rail is bolted to a steel frame, and the workshop warms up by 30 °C.
- Aluminum grows 23 × 10⁻⁶ × 1000 × 30 = 0.69 mm
- Steel grows 12 × 10⁻⁶ × 1000 × 30 = 0.36 mm
- The difference, 0.33 mm, has to go somewhere: into bending, bolt shear or a slotted hole. On precision machines, allow for it with slotted holes or a single fixed point.
- Conductivity: copper and aluminum spread heat fast (heat sinks, hot ends). Stainless and titanium hold heat, which is why stainless is hard to machine: the heat stays at the cut. Plastics are insulators.
- 3D printing: high expansion is why ABS and nylon warp as they cool. PLA expands less and is easier to print flat.
Hardness
Hardness is measured by pressing an indenter into the surface. The common scales:
- Brinell (HB): a hard ball, used for softer metals and castings. Mild steel 1018 (cold drawn) is about 126 HB; 6061-T6 aluminum about 95 HB; 7075-T6 about 150 HB.
- Rockwell C (HRC): a diamond cone, used for hardened steels. Ti-6Al-4V is about 36 HRC; hardened tool steel for cutters and dies is typically 58–64 HRC.
- Rockwell B (HRB): for softer steels, brass and aluminum.
- Vickers (HV): a diamond pyramid, usable across the whole range and for thin parts.
Conversions between scales are approximate and only valid for similar materials. For carbon and alloy steels, tensile strength in MPa is roughly 3.4 × HB, which is useful for a quick estimate but not for design.
Unit conversions
| Quantity | Conversion |
|---|---|
| Stress, strength, modulus | 1 MPa = 1 N/mm² = 145.04 psi. 1 ksi = 6.895 MPa. 1 GPa = 145.04 ksi |
| Density | 1 g/cm³ = 1000 kg/m³ = 0.0361 lb/in³ (steel: 0.284 lb/in³) |
| Thermal expansion | 1 µm/m·°C = 0.556 µin/in·°F |
| Thermal conductivity | 1 W/m·K = 0.578 BTU/(hr·ft·°F) |
| Temperature | °F = °C × 1.8 + 32 |
Related
- Beams and Profiles: use E and yield strength to size beams.
- Speeds and Feeds: how material choice changes cutting speeds.
- 3D Printing and Additive Manufacturing: choosing and printing filaments.
FAQ
What is the difference between yield strength and tensile strength?
Yield strength is where the material starts to deform permanently; tensile strength is the maximum it can take before it starts to break. Design normally keeps stresses well below yield, because a part that has yielded is already bent out of shape.
Should I use typical or minimum values?
Use typical values to compare materials and get a feel for a design. Use the specification minimum (or the mill certificate) for anything that must not fail, and add a safety factor on top.
Why is my 3D-printed part weaker than the datasheet?
Datasheet values usually come from molded test bars. Printed parts have layer lines, gaps and partial infill, and are weakest when pulled across the layers. Orient the part so the main load runs along the layers, add walls, and dry hygroscopic filaments such as nylon before printing.
Is stainless steel magnetic?
Common austenitic grades such as 304 and 316 are essentially non-magnetic when annealed but can become slightly magnetic after cold working, such as bending or machining. Ferritic and martensitic grades (for example 430 and 410) are magnetic.
6061 or 7075 aluminum?
Choose 6061-T6 for general parts, welded frames and anything that will be anodized or exposed outdoors. Choose 7075-T6 when you need the highest strength for the weight and the part will not be welded.
References
- ASM International, ASM Handbook, Volume 2 (aluminum, copper and titanium alloys) and Volume 1 (steels), and ASM aluminum datasheets for 6061-T6 and 7075-T6.
- ASTM A36 (structural steel) and ASTM A240 (stainless plate and sheet: minimum properties for 304).
- Ulbrich, Ti-6Al-4V (UNS R56400) datasheet.
- Copper Development Association, alloy data for C11000 and C36000.
- Filament and resin manufacturers’ technical datasheets for PLA, ABS, PETG, nylon and polycarbonate.
Last reviewed: October 2026. Values are typical at room temperature. Always confirm critical properties with the material specification or supplier certificate.