Mechanical properties are the properties that a material exhibits when forces are applied to it. Examples include strength, ductility, modulus of elasticity, toughness, and fatigue limit. Mechanical properties determine the suitability of a material for a particular application and can impact the physical shape, weight, and service life of products produced from it.
Other important material properties include physical and chemical properties. Physical properties such as density and thermal and electrical conductivity are important criteria in product design. Chemical properties affect a material’s ability to withstand a corrosive environment and the biocompatibility of a material with the human body.
Strength is an important mechanical property of any structural component. Strength is the ability of material to resist stress without failing. Materials with a high strength-to-weight ratio like titanium can be used to produce lighter-weight durable medical implants with a smaller profile.
The modulus of elasticity is a measure of a material’s stiffness. People sometimes confuse stiffness and strength (and toughness), but each describes a different mechanical property. Stiffness is a measure of the material’s resistance to deformation when force is applied. Stiffer materials will deform less and ductile materials will deform more. However, a material can be stiff but not strong. Glass and chalk are two examples of this. In contrast, a material can be strong but not stiff. Rubber is a good example: how much force is required to snap a rubber exercise band?
Ductility is a measure of the amount of deformation a material will undergo when subjected to tensile strength without fracturing. This is different than the property of having low stiffness, which describes the resistance of a material to deformation. Ductility is typically thought of as the ability of a material to be pulled into a wire.
Toughness describes the amount of energy a material can absorb prior to fracture. In other words, it is a measure of the amount of work that must be done to break the material. Mathematically, it is the area under the stress-strain curve. In general, materials that are stiff and ductile have a higher toughness rating. A greater amount of force must be applied over a larger distance to break them. An example is ductile steel. Glass, however, is a material that is stiff but not tough. It requires significant stress to cause a small amount of deformation, but it only requires a small amount of stress to cause fracture. This, of course, applies only to bulk glass with normal defects; glass fiber in a composite matrix is a different matter. Chewed gum is a material that is ductile but not tough. Very little force is required to stretch the material, even over a large distance.
The fatigue limit is a critical stress level for ferrous material. If the stress is maintained below the fatigue limit, the product will essentially have an infinite fatigue life. Steel and titanium have a fatigue limit, but nonferrous materials like aluminum and copper do not and will eventually fail even from low stress amplitudes.
