Imagine pressing on a steel ruler, bending a rubber eraser, scratching a ceramic coffee cup, or tossing a plastic container. Each of those items responds in a unique way. It may seem easy to describe the results by saying the ruler is “stronger” or the cup is “harder,” but those words actually mean different things depending on context. The strength, stiffness, hardness, and toughness of a material all have their own distinct meanings that are only occasionally linked. If you can differentiate between each of these material properties, you will find property tables, stress-strain curves, and even material selection activities easier to digest.
Strength defines the amount of stress (force per unit area) that a material can sustain before it yields or fractures. A material can be strong and yet still easily bend because that same thin steel strip, for example, can be made of a strong material, yet still be capable of significant deflection. When you’re reviewing a property table, ensure you are checking for the correct type of strength. Yield strength tells you when permanent deformation will begin, and ultimate tensile strength is the maximum stress a sample can withstand in a tensile loading test. Both can tell you only a piece of the story regarding how a part might behave in the field.
Stiffness measures how much a material resists elastic deformation. Stiff materials deflect very little when loaded and return to their initial shape when the loading is removed, if loading within the elastic limits is maintained. For a stress-strain curve, stiffness can be related to the slope in the initial, nearly-linear, part of the curve. Materials such as glass, ceramics, and metals exhibit high stiffness. Stiffness alone is unrelated to toughness, though, since a brittle material could be quite stiff. Rubber is much less stiff because it can stretch considerably under modest stress. The geometry of a material also matters, because even a lower-stiffness material can resist more bending, as a thicker plastic may resist greater bending than a thin metal strip.
Hardness quantifies the resistance to plastic deformation that is localized to small surface areas. Hard materials can be used to make cutting edges and bearings and parts subject to abrasive wear conditions. Hardness should not be confused with strength alone. A steel surface that is hardened is very stiff and strong as well, but it may also have lower toughness. Hardness alone does not imply toughness, as some brittle, yet hard ceramic tiles, may chip or shatter when subjected to an impact stress. The value of the hardness for a material is determined from indentation testing in an industrial setting. Therefore, using a key or a lighter to scratch another material or a surface is not a reliable means to assess hardness, nor should it be used to scratch the surfaces of parts, especially coatings that might then become damaged by the test.
Toughness is the ability of a material to absorb energy prior to fracture, and it accounts for how resistant to stress it is and how able the material is to stretch. Materials can undergo some deformation in a tensile test or an impact test before fracture occurs and thus absorb a certain amount of strain energy. A less brittle material may survive falling from a certain height, whereas a more brittle, but harder material is more prone to fracture. On a stress-strain curve, the area under the stress-strain curve from zero to the point of fracture represents toughness. While the ductility of the material is associated with toughness, the two terms are not synonyms. Ductility describes the extent to which a material can permanently deform prior to breaking, and thus may only partially describe the toughness of the material if only part of the stress-strain curve is being analyzed.
You can do your own assessment on safe, everyday items and record your findings in your notebook. For example, choose several items made from different material families, such as a metal spoon, a silicone utensil, a plastic lid, and an intact ceramic cup. Do not break or damage the objects; just observe them and describe the degree to which they bend easily when force is applied, whether the surface has already been scratched, and if the geometry of the objects will impact your observations. Be sure to be specific when describing each of your property observations in this example and avoid mixing them up. You might observe, for example, that the spoon may feel stiff because it bends little, but its stiffness cannot be judged separately from its thickness and curved shape. You could observe that the glass mug is relatively hard, but it could also be brittle and fail under low loads.
In the absence of a more refined selection, when you look at properties, ask yourself instead, Will the part experience static loading? Will the component bend or deflect? Will the component be impacted? Will the part wear or scratch? What will be the geometry, temperature, environment, and history of the process of the part? When selecting for application, the decision-maker will need to be willing to make a compromise for some of the values for the four material properties, since the highest number for a single property is rarely going to lead to a successful selection. The best sign that you are gaining competency with understanding these properties is when you can describe a component design and the resistance it must demonstrate against different types of loading to succeed, with the added evidence you could provide to back up that argument.