Engineered materials.

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Stress and strain

In the Unit on bonding and structure we saw that layers of atoms in a metal slip over each other, and that foreign atoms of another size stop the slipping. Engineers test a material in a tensile testing machine: it stretches a standard specimen and measures how hard it resists.

Two numbers describe the test. Strain is how far the specimen has stretched, as a percentage of its length. Stress is the force on each unit of its cross-section, in pascals: a megapascal, MPa, is a newton on every square millimetre. At first they are in proportion, as in a spring: that is Hooke’s law, $\sigma = E\varepsilon$$\sigma =$$E\varepsilon$⁠, and $E$⁠, Young’s modulus, says how stiff the material is. For mild steel E = 200 GPa.

In the Scene Mild steel is stretched by 0.1%. What is the stress in it?

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Past the yield

Beyond a certain stress, about 275 MPa for mild steel, the metal yields: the grains of its crystal start to slip, and it stretches on without much more force. In the Scene the steel has been stretched by 12%. Predict what happens when you set Grips to Let go.

Step 3 of 5

Brittle but strong

A composite is built of two materials: stiff fibres that carry the pull, and a polymer glue that shares it out between them. The Scene shows Carbon fibre. Try raising Stretch past 1%, then compare with Mild steel: the carbon fibre snaps all at once, with no warning, while the steel stretches by more than a quarter of its length before it breaks.

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Strength for weight

Tensile strength is the highest stress a material can take. In an aircraft, a bicycle or a car the weight matters too, so engineers compare specific strength: the tensile strength divided by the density. For mild steel it is about 55 kN·m/kg, that is 430 MPa divided by 7.85 g/cm³.

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Springing back

NiTi, an alloy of nickel and titanium, behaves differently. Zoom inside is at Inside. When it is stretched, the rows of atoms in its crystals switch from a cubic structure to another, longer and sheared, so it stretches up to about 6% without any atoms slipping. In the Scene it has been stretched by 5%. Predict what happens when you set Grips to Let go.