What a Property Actually Is
By now you have met the big idea of materials science and engineering: internal structure sets properties, and processing sets the structure. But we have been using the word property loosely. Let us pin it down. A material property is the material's own response to some imposed stimulus — push on it and it deforms, put a voltage across it and a current flows, heat one end and warmth travels to the other. Crucially, a property is defined so that it belongs to the material itself, not to the particular lump you happened to test.
That last point is the whole trick, and it is why the structure-property-processing-performance paradigm even works as a tool. A force in newtons is not a property, because a thick bar carries more force than a thin one made of the same stuff. So we divide the force by the cross-section to get stress (force per area), and the stretch by the original length to get strain. Now the number is about the material. That is why a data sheet says "Young's modulus = 200 GPa," a clean material fact, rather than "this particular beam pushes back with 5000 newtons."
Every property is measured under a defined test — a standard specimen, a defined temperature, a defined loading rate. Change the test and you can get a different number for the same material, which is honest to remember: a property is a response under stated conditions, not a magic constant carved into the atom.
Six Families of Properties
Textbooks sort every property into six families, according to the kind of stimulus. Mechanical: response to a force — stiffness, strength, toughness, hardness. Electrical: response to a voltage — resistivity, and for a semiconductor the band gap, the energy step an electron must jump to conduct. Thermal: response to heat — thermal conductivity and expansion. Magnetic: response to a magnetic field — read off a hysteresis loop. Optical: response to light — refractive index, colour, transparency. Deteriorative: response to a chemical environment — corrosion and degradation over time.
The single most common beginner mistake is to blur the mechanical family into one vague idea of "strong." Stiffness, strength, and toughness are three genuinely different things. Stiffness (Young's modulus) is how hard the material resists stretching elastically. Strength is the stress at which it stops springing back and starts to yield or break. Toughness is how much energy it can soak up before it fractures. A material can be high in one and low in another: a ceramic like window glass is stiff and strong but so brittle it has almost no toughness, while a coil of annealed copper is soft (low strength) yet enormously tough — it bends and bends without snapping.
The Two Bulk Descriptors: Density and Cost
Beyond the response-to-a-stimulus properties, two plain numbers dominate almost every real decision. The first is density, mass per unit volume. It flows straight out of the atomic picture from the previous guide: heavy atoms packed tightly give high density. Steel is about 7.8 g/cm^3, titanium 4.5, aluminum 2.7, most polymers near 1.0, and a foam far below that. In any design where you must carry the material's own weight — an aircraft, a bicycle, a car door, a prosthetic limb — density is not a minor detail, it is often the property you fight hardest to lower.
The second is cost and availability. Materials span an astonishing price range: mild steel is under a dollar per kilogram, aluminum a few dollars, titanium around twenty, and carbon-fibre composite thirty or more. Availability matters just as much as price — the whole history of the ages of materials, from Stone to Bronze to Iron to today's Silicon age, is really the story of which materials humans could actually win from the earth and shape. A material that is wonderful but scarce or impossible to process is, in practice, not available to you.
Comparing Properly: Normalize, Do Not Just Pick the Biggest
Here is the mental shift that turns a list of numbers into engineering judgement: there is no single best material. Steel is stiffer than aluminum, so a beginner picks steel — until you remember steel is also three times denser. The honest question is stiffness per unit weight, the specific stiffness E divided by density. Do the arithmetic: steel is 200 / 7.8 = about 26, and aluminum is 70 / 2.7 = about 26 as well. They are essentially equal. A tie-rod that must not stretch weighs about the same whether you make it from steel or aluminum; the aluminum one just ends up thicker.
The catch — and this is where it gets genuinely interesting — is that the right index depends on the shape of loading. A rod pulled in tension cares about E over density. But a panel or a beam bent in bending cares about the square root of E over density. Recompute: square-root(200) / 7.8 = about 1.8 for steel, but square-root(70) / 2.7 = about 3.1 for aluminum, and for wood along the grain about 4.9. Now light materials win decisively. That single change of exponent is exactly why bikes, aircraft wings, and diving boards are made of aluminum, wood, and carbon-fibre rather than steel.
Material Density E Strength Cost E/density
(g/cm^3) (GPa) (MPa) ($/kg) (specific stiffness)
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Mild steel 7.8 200 250 ~0.8 26
Aluminum alloy 2.7 70 300 ~2 26
Titanium alloy 4.5 110 900 ~20 24
Alumina (ceramic) 3.9 380 ~300 * ~5 97
CFRP (along fibres) 1.6 150 1500 ~30 94
Polyethylene 0.95 1 ~25 ~1.5 1
Oak (along grain) 0.7 12 ~50 ~1 17
* a ceramic's 'strength' is set by its worst flaw -> use a Weibull
distribution, not one number (see next section)A group like E over density is called a performance index, and plotting one property against another for every material at once gives an Ashby chart — the working map of materials selection. On such a chart each material is a bubble, families cluster into regions, and a straight line of constant index sweeps across to show you the winners for your particular job. It replaces "which is strongest?" with the far better question "which gives the most of what I need per unit of what I want to spend — weight, cost, or both?"
Where a Single Number Lies — and How to Choose Anyway
Be honest about the limits of any property number. A ceramic quoted as "300 MPa strong" is misleading, because a brittle material fails from its single worst flaw, and every specimen has a different worst flaw — so its strength is really a scatter band described by Weibull statistics, not one value. Worse, some behaviours are not single numbers at all: fatigue strength depends on how many cycles, creep depends on how long and how hot, and corrosion depends entirely on the environment. A material that is inert in dry air can dissolve in salt spray.
This is why engineers never load a part right up to its quoted strength. They divide by a factor of safety — often two or more — to leave room for the flaw they did not see, the overload they did not expect, and the data-sheet number that was an average. Selection, then, is not "find the biggest number" but a disciplined narrowing.
- State the function, constraints, objective, and free variables: what must the part do, what must it not violate (a stiffness, a temperature), what do you want to minimize (weight or cost), and what are you free to change (the material, the dimensions).
- Translate the objective into a property or a performance index — for a light, stiff tie-rod that is E over density; for a light, stiff panel it is square-root(E) over density.
- Screen: draw the hard limits (must survive 300 degrees C, must not corrode in seawater) and throw out every material that fails them, no matter how good its index.
- Rank the survivors by the index on an Ashby chart, and read off the top few candidates.
- Check the finalists against the whole context the numbers hid — a factor of safety, real cost and availability, how it will be joined and shaped, and its end of life through recycling and a life-cycle assessment.