Material Selection · Strength

Yield. Tensile.
Fatigue.
Know what matters.

Choosing material for strength isn't picking the highest number on a spec sheet. It's matching material properties to load case — static vs fatigue, ductile vs brittle failure, weight constraints, cost. Here's the honest engineering guide.

Free DFM Review Material selection

01 · Strength types

Which "strength" matters?

Materials have multiple strength properties. Using the wrong one leads to under-designed parts or over-weight parts.

Yield strength

The important one

Stress at which material begins plastic deformation. Design limit for most structural applications — exceeding yield means permanent deformation.

Ultimate tensile

Pretty number

Maximum stress before fracture. Usually 20-50% higher than yield. Rarely the actual design limit — part has deformed by then.

Fatigue limit

For cyclic loads

Stress below which material survives infinite cycles. Typically 40-50% of yield for steel, varies for other materials. Critical for parts under repeated loading.

Specific strength

Strength per weight

Yield strength ÷ density. Critical for aerospace, racing, performance applications where weight matters. Titanium and carbon fiber dominate this metric.

02 · Comparison table

Common engineering materials.

Strength properties for standard engineering materials. Values are typical for indicated condition.

Material	Yield MPa	UTS MPa	Density g/cc	Specific strength
Aluminum 6061-T6	275	310	2.70	102
Aluminum 7075-T6	503	572	2.81	179
Mild Steel 1018	370	440	7.87	47
4140 Q&T	655	1000	7.85	83
304 Stainless	215	505	8.00	27
17-4 PH H1025	1000	1070	7.80	128
Ti-6Al-4V (Gr.5)	830	900	4.43	187
Inconel 718	1036	1241	8.22	126
PEEK unfilled	97	100	1.30	75
Carbon fiber (unidir.)	1600-2000	1600-2000	1.55	1100+

03 · By application

Material selection by load case.

Static load, stiff

• Mild steel (1018, A36) — cheapest, stiffest per cost
• 4140 Q&T — higher strength steel
• Aluminum 6061 — when weight matters
• Cast iron — for compression only
• Concrete/steel rebar — for very large structures

Fatigue load (cyclic)

• Steel 4140 QT for most cyclic applications
• 7075-T6 aluminum — aerospace fatigue-rated
• Titanium Ti Gr.5 — excellent fatigue life
• Stainless 17-4 PH — fatigue + corrosion
• Specify shot-peened surfaces for improved fatigue life

High strength/weight

• Ti-6Al-4V (Gr.5) — 187 specific strength
• Aluminum 7075-T6 — 179 specific strength
• Carbon fiber composite — 1100+ for unidirectional
• Beryllium alloys — specialty, 200+ specific strength
• Mg alloys — lighter but lower absolute strength

High temperature

• Inconel 718 — 650°C service
• Inconel 625 — corrosion + temperature
• Hastelloy X — 1000°C + oxidation
• Rene alloys — superalloys for turbine
• PEEK — 260°C continuous for plastics

04 · Common mistakes

How engineers get this wrong.

Specifying UTS instead of yield. UTS is the number on marketing brochures. Design for yield — that's where your part stops working. A 7075-T6 aluminum part designed to 400 MPa (below 503 MPa yield) is safe; designed to 500 MPa (below 572 MPa UTS) is not — you're already yielding at 503.

Ignoring fatigue for cyclic loads. Parts under repeated loading fail at much lower stress than static parts. 4140 Q&T with 655 MPa yield has fatigue limit ~300 MPa at 10^7 cycles. A bracket that never fails statically can break in 3 months of vibration. Always check fatigue limit for moving/vibrating parts.

Upgrading material instead of upsizing. Going from aluminum to titanium "for strength" often costs 10× material + 3× machining. Making the aluminum part 30% thicker solves the same problem for 50% more material cost. Material upgrade justified only when geometry is truly constrained.

Forgetting temperature knockdown. Strength decreases with temperature. Aluminum 6061 yields 275 MPa at 20°C but only 200 MPa at 150°C. For any application above room temperature, check derated properties. Specific attention for: aluminum above 100°C, steel above 250°C, plastics above 60°C.

Misreading composite specifications. Carbon fiber "2000 MPa tensile strength" is unidirectional fiber direction only. Cross-ply woven fabric: 1000 MPa. Quasi-isotropic layup: 400-600 MPa. Random fiber composite: 100-200 MPa. The marketing number is rarely the design number.

FAQ

How much safety factor should I use?

Typical safety factors: 1.5 for static load, known loading, ductile materials. 2.0 for uncertain loads or impact. 3.0 for brittle materials (cast iron, ceramics, glass). 4.0+ for safety-critical applications (pressure vessels, life-critical). Aerospace uses 1.5 because weight is critical and loads are well-characterized. Consumer products often use 2-3 to handle unexpected loading. Safety factor on yield, not UTS.

Ductile vs brittle failure?

Ductile materials (most metals): deform noticeably before failing — advance warning, usually safe. Brittle materials (cast iron, ceramic, hardened tool steel, carbon fiber parallel loading): fail suddenly with no warning. Design brittle materials with larger safety factor and avoid tensile loading. For ductile: yield is design limit. For brittle: UTS with higher safety factor is design limit.

Temperature effects on strength?

Generally strength decreases with temperature. Aluminum alloys: significant loss above 100°C. Standard steels: significant loss above 300°C. Stainless steels: retain strength to 500°C+. Superalloys (Inconel, Hastelloy): designed for high temperature service. Engineering plastics: wide variation, from 80°C for nylon to 260°C for PEEK. For any application outside room temperature, check derated properties for your specific temperature.

Strain rate sensitivity?

Materials behave differently under slow vs fast loading. Quasi-static (slow) loading: standard yield strength applies. Impact loading (high strain rate): apparent strength increases significantly. High strain rate: most materials show 20-50% higher strength. Fracture toughness: decreases with high strain rate (more brittle). Design impact-prone parts for low-temperature impact properties.

Anisotropy in rolled and forged materials?

Rolled plate is not isotropic. Strength in rolling direction (L) higher than transverse (T), which is higher than through-thickness (ST). For critical loading, specify material grain direction. Forged parts have more complex anisotropy following grain flow from forging. For safety-critical parts, loads should align with grain direction where possible.

What about creep for high-temperature applications?

Creep: slow permanent deformation under sustained load at elevated temperature. For service above 30% of melting temperature (absolute scale), creep is dominant design consideration. Aluminum above 100°C, steel above 400°C, superalloys above 600°C. Design against creep with: lower allowable stress (often 10-30% of yield), creep-resistant alloys, regular inspection. Not relevant for room-temperature applications.