Properties of Materials

Knowing the mechanical properties of materials is essential for designing and manufacturing technical objects. In order for a technical object to fulfill its overall function and to withstand the various constraints to which it is subjected, it is important to select the appropriate materials.

The following table displays the main mechanical properties.

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Most metals and alloys possess excellent ductility. This is why these materials are frequently selected for the production of wires or cables.

Materials such as wood and ceramics are not ductile since they do not stretch. Generally, materials that are ductile are also malleable.

Ceramics, several metals and their alloys, as well as some plastics, have good hardness. These materials are found in abundance in the construction industry as finishing materials, because their surface remains intact over time. The hardness of different woods varies according to their species. For example, maple (a hardwood) is harder than pine (a conifer).

The hardness of materials also makes it possible to choose the appropriate tool in order to make a notch or a cut. In fact, the hardness of the tool used must be greater than that of the material being cut.

Several plastics and some composite materials have good elasticity.

Elasticity should not be confused with malleability and ductility. In fact, these last properties imply that the material can deform and then retain its new shape. A ductile or malleable material cannot be elastic, since it cannot return to its initial shape by itself.

Ceramics such as glass or porcelain are materials with high brittleness. When subjected to mechanical stress, ceramics crack or break before being deformed.

Repetitive constraints on a material can increase its brittleness. The greater the force of the constraints, the less the material can withstand the repetitions. This is referred to as mechanical fatigue. For example, when a paper clip is unfolded and folded several times, it eventually breaks.

In general, materials that are brittle are neither ductile nor malleable, but they can be stiff.

Most metals and alloys have good malleability. This is why these materials are frequently selected for food packaging and jewellery.

To be malleable, the material must retain the new shape it has been given. For example, a crushed aluminum can will retain its new flattened shape. In contrast, a crushed sponge will return to its original shape. The sponge is not malleable, it is instead elastic.

Generally, materials that are malleable are also ductile.

Plastics and certain composite materials are examples of materials with great resilience. These materials tend to absorb the energy of an impact with little or no deformation. This is why they are excellent choices for protective equipment, sports equipment, and children's toys.

Materials that have effective resilience are not brittle.

Materials with good stiffness are found in all types of objects. For example, walnut (wood), plexiglass (plastic), and plywood (composite material) are stiff. These materials retain their shape, that is, they do not bend, stretch, or curl. This makes these materials good choices for structures that support high loads such as buildings, houses, and bridges.

Generally, materials that are stiff are neither ductile nor malleable, but they can be brittle.

When designing a technical object, the overall function of the object must be considered, but also the factors that are likely to degrade over time. Being aware of the non-mechanical properties of materials helps guide an appropriate selection.

These non-mechanical properties can be either chemical or physical.

The following table displays the main non-mechanical properties of the materials.

Metals and alloys are generally excellent electrical conductors. This means that electric current can easily flow through them.

Materials with low electrical conductivity are electrical insulators. Woods and ceramics are examples.

In general, materials that have good electrical conductivity also have good thermal conductivity.

Metals and alloys have excellent thermal conductivity because they easily conduct heat.

Heat is transferred through a material by conduction. If one end of a material is heated, the heat transfer to the other end depends on the thermal conductivity. The higher it is, the faster and more effectively the material spreads the thermal energy initially provided.

Ceramics have low thermal conductivity as they hardly transmit heat. They are thermal insulators.

In general, materials that have good thermal conductivity also have good electrical conductivity.

Plastics and many composite materials are lightweight (low density). Sporting goods, children's toys, and even outdoor equipment are all examples where lightness is an asset, as it makes the objects easier to use and carry.

Oxygen, salts, and smoke are substances that can cause corrosion of a material. The presence of corrosion is unwanted since when it forms, it degrades the material and reduces the material’s properties. Over time, it can erode and tear through the material. The presence of water or moisture accelerates the corrosion of a material.

The use of plastic materials, composite materials, or certain alloys which have an effective resistance to corrosion is sensible for avoiding degradation of objects.