Material selection

Material selection chooses materials by matching their properties and processing routes to a component's requirements. First exclude candidates violating constraints such as operating temperature, corrosion resistance or minimum stiffness, then compare a performance objective such as mass or cost. A tension member carrying force $F$ over length $L$ needs area at least $F/{\sigma }_{\mathrm{allow}}$, giving mass at least $FL\rho /{\sigma }_{\mathrm{allow}}$. Thus specific strength is relevant for this strength-limited case. A stiffness-limited beam can instead have a different material performance index, so there is no universally best lightweight material independent of geometry and loading.

A plot comparing material property ranges, often on logarithmic axes. Constant material performance index values appear as straight guidelines when the index is a product of powers of the plotted properties. A family region is a screening aid, not a certified property value for every grade or loading direction.

A combination of material properties to maximize or minimize after eliminating free geometric variables from an engineering objective and constraints. Changing a geometric constraint can change the index: strength-limited rectangular cantilevers give ${\sigma }_f/\rho$ with fixed height, but $\sqrt{{\sigma }_f}/\rho$ with fixed width.

Specific strength is strength divided by density, with the strength convention stated explicitly. For a uniform tie carrying tensile force $F$, allowable stress ${\sigma }_f$ requires area $A\ge F/{\sigma }_f$. Its mass $m=\rho AL$ is therefore at least $FL/({\sigma }_f/\rho )$. Maximizing ${\sigma }_f/\rho$ minimizes mass under this simple strength-limited model. A material with higher strength can still be worse if its density increases more. Mechanical fatigue, buckling, joining and temperature limits can impose additional constraints not captured by this single ratio.