Selecting materials

In one lineRank materials by the combination of properties the job rewards — a performance index — not by the biggest number on a datasheet.

Leads toSpecifying materials · Case sizing

Why this matters

Material choice is usually the largest single lever on mass, cost and risk, and it is made early, when changing your mind is cheap. It is also where designs most often go wrong for a simple reason: they get sized on the wrong number. A datasheet’s room-temperature typical yield is not the minimum strength at the service temperature, in the load direction, for the product form you can actually buy. This page is about making the right comparison first; the specification page is about pinning the result down on a drawing.

The idea: a performance index

Think of it like…

Choosing a minibus by fuel used per passenger

If you need to move eight people, the car with the best miles per gallon is not the answer — a two-seater sports car would win that. What matters is miles per gallon per seat: the combination of properties that serves the job. A performance index is the same idea for materials. For a light, strong pressure vessel it is strength divided by density, not strength alone.

Ashby’s method turns a design requirement into that index in four steps:

1. Function — what does the part do? (Contain pressure, carry a bending load, conduct heat.)
2. Objective — what are you minimising or maximising? (Usually mass, sometimes cost or deflection.)
3. Constraints — what must it achieve no matter what? (Not yield at proof, not burst, survive 150 °C, be weldable.)
4. Free variables — what are you allowed to change? (Wall thickness, section size, the material itself.)

Write the objective, use the constraint to eliminate the free variable, and what remains is a group of material properties. Maximise that group.

Worked derivation — a light motor case

A thin cylinder of radius $r$ and length $L$ holds pressure $p$. Its mass is $m = 2\pi r L\,t\,\rho$, and the wall must carry the hoop stress without exceeding the allowable $\sigma$: $t = p\,r/\sigma$. Substituting the free variable $t$:

$$m = 2\pi r^2 L\,p\;\cdot\;\frac{\rho}{\sigma} \qquad\Longrightarrow\qquad \text{maximise}\;\; M = \frac{\sigma}{\rho}$$

Everything before the dot is fixed by the design; only $\rho/\sigma$ belongs to the material. That is the specific strength — the same index as a light tie in tension — and it is exactly why the case-sizing page reports $p_bV/W$.

Shape changes the index

One objective, three shapes, three indices

Shape changes the answer. A tie only needs area, so modulus pays off one-for-one. A beam or panel gets stiffness from its depth cubed, so a lighter, less stiff material can simply be made thicker — which is why aluminium, magnesium and composites win for panels even when steel wins for ties.

For a beam of square section with a stiffness target, deflection goes as $1/(E\,I)$ with $I \propto A^2$; eliminating the area gives $m \propto \rho/E^{1/2}$. For a panel of fixed width, $I \propto t^3$, so $m \propto \rho/E^{1/3}$. Taking logs turns each index into a straight line on a log–log property chart:

$$M = \frac{E^{1/2}}{\rho} \;\;\Longrightarrow\;\; \log E = 2\log\rho + 2\log M$$

— a line of slope 2. Every material on that line performs identically for the job; every material above it does better. That is the dashed guideline on the chart below.

Think of it like…

Contour lines on a map

A property chart is a map of the material world. The families are countries — metals over here, polymers down there, composites in a long thin island. A performance index is a set of contour lines drawn across the whole map: walking along one changes the material but not the performance; walking uphill, perpendicular to them, makes the part better.

Fracture needs its own index

Strength-per-weight rewards the strongest alloys — but toughness generally falls as strength rises. Two fracture indices capture the other side:

• Yield before break, $K_{IC}/\sigma_y$ — the critical crack size at yield scales with its square: $a_c = \frac{1}{\pi}\left(\frac{K_{IC}}{Y\sigma_y}\right)^2$. Large is forgiving.
• Leak before break, $K_{IC}^2/\sigma_y$ — the largest wall a pressure vessel can have and still leak before it breaks (see the leak-before-break screen).

On their own these pick soft, tough metals that are hopeless for a light case. Used together with the mass index they expose the real trade: how much toughness you give up for each kilogram saved.

The chart

Pick what you are designing, choose families, apply must-haves, then click any point (or table row) to make it the reference the guideline runs through. Hover for values.

Material selection chart

What are you designing?

Families

SteelsStainlessTitaniumAluminiumNickel & cobaltMagnesiumCopperRefractoryCompositesCarbon & ablativesPolymersElastomers

Must have

Service temp at least (°C)

Weldable only Hide family-default toughness (fracture indices)

SteelsStainlessTitaniumAluminiumNickel & cobaltMagnesiumCopperRefractoryCompositesCarbon & ablativesPolymersElastomers

Ranking by the selected index

Typical room-temperature values for screening and ranking, not design allowables (shared with REF·DB v1). Strength is 0.2% proof for metals, failure stress for ceramics and ablatives, and fibre-direction strength for composites — a real laminate is far lower — so cross-family rankings involving composites flatter them. Toughness is indicative.

Worked example — choosing a case material

Function: contain chamber pressure. Objective: minimum mass. Constraints: weldable, service temperature at least 150 °C, metallic (for this example). Free variable: wall thickness.

1. Rank by $\sigma_y/\rho$ (the chart’s default). The top eight, in kN·m/kg:

   #	Material	$\sigma_y/\rho$	$K_{IC}/\sigma_y$ (√m)	Crack at yield, $Y = 1.12$
   1	Maraging 18Ni-300	233	0.043	0.47 mm
   2	Ti-6Al-2Sn-4Zr-2Mo	218	0.066	1.09 mm
   3	AerMet 100	218	0.067	1.13 mm
   4	Maraging 18Ni-250	211	0.059	0.89 mm
   5	Ti-6Al-4V	199	0.085	1.84 mm
   6	Al-Li 2195	194	0.057	0.83 mm
   7	Ti-6Al-4V ELI	187	0.120	3.68 mm
   8	D6AC	185	0.048	0.59 mm

2. Check fracture. The lightest choice, maraging 300, tolerates a crack of only about half a millimetre at yield — close to the limit of reliable inspection. Maraging 250 gives up 10% of the mass index for nearly twice the crack size. Ti-6Al-4V ELI costs 20% of the mass index against maraging 300 but tolerates a crack nearly eight times deeper. That is the yield-before-break trade in numbers.

3. Check what the chart can’t see. D6AC ranks eighth on mass and low on toughness, yet it has flown on many large motors. The reasons are off the chart: large forgings and plate are available, heat treatment and welding are well understood, the indicative price in the database is roughly a quarter to a third of maraging steel’s, and decades of test data back it. Titanium brings cost and availability in large sections; maraging steels carry cobalt price and supply risk.

4. Composites. Switch on the composites family and filament-wound carbon/epoxy lands far above every metal — but on fibre-direction strength. A real filament-wound case is a laminate whose wall strength is a fraction of that number, and it brings its own failure modes (matrix cracking, impact damage, stress rupture). It still usually wins on mass, which is why so many modern cases are composite.

What the example teaches

An index ranks; it doesn’t decide. Use the mass index to find the candidates, a second index to expose what they cost you, and judgement for everything the chart can’t plot — supply, process, cost, compatibility and heritage.

Where this stops

Screening data, not allowables

The chart uses typical room-temperature values shared with REF·DB v1. Designs are sized on statistically based allowables (for example MMPDS A- or B-basis) for the specific product form, thickness, grain direction and temperature, from your organisation’s approved data.

• Temperature. Strength and toughness at the service temperature, including after any thermal exposure, decide the case. The max-temperature filter only screens.
• Product form and direction. Plate, forging and extrusion differ, and the short-transverse direction of thick plate can be much weaker and less tough than the rolling direction. See specifying materials.
• Environment. Stress-corrosion cracking, hydrogen embrittlement and compatibility with propellant, fluids and coatings remove candidates that the chart ranks highly.
• Cost and availability are real constraints, not afterthoughts — and they change.

References and further reading

• M. F. Ashby, Materials Selection in Mechanical Design, 5th ed., Butterworth-Heinemann (2016) — performance indices and property charts.
• M. F. Ashby & D. R. H. Jones, Engineering Materials 1, 5th ed., Butterworth-Heinemann (2019).
• Metallic Materials Properties Development and Standardization (MMPDS) — statistically based design allowables for aerospace metals.
• ASM Handbook, Vol. 1 (Properties and Selection: Irons, Steels) and Vol. 2 (Nonferrous Alloys).

Note

Screening and educational material. Final material selection is governed by your programme’s requirements, approved allowables and the responsible materials and stress authorities.