Yield. Tensile. Fatigue. Know what matters.

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

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 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.

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.

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.

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.

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.