What is the difference between the hardness, toughness, resilience, and stiffness of materials – quora gas kinetic energy formula

Toughness is the ability of a material to absorb energy when impacted. It is tested with impact Charpy or Izod testing machines by measuring how much a predetermined weight will rise after impacting and breaking the piece under test. The height correlates to how much energy the weight has left in its motion so it is an indication of how much energy the material can absorb.

Materials known to be very tough are stainless steels and titanium alloys. Materials known to be very fragile (the opposite of tough) are ceramics such as glasses or porcelain. The reason a ceramic plate shatters when dropped while your spoon will maybe only bend, is the difference in toughness between the two materials.

Toughness usually goes in the opposite direction of hardness, that is if a material is very hard it is usually very fragile. Diamonds are fragile even though they are hard. Aluminum is tough but not hard at all. The goal of every metallurgist would be to obtain a tough yet hard material. It is the most sought upon dream of structural materials.

The most resilient materials are metallic glasses, for example, which have a yield stress almost identical to the ultimate tensile stress . A more known highly resilient material is spring steel. Examples of non resilient materials are pure annealed copper or pure aluminium. Their yield stress is so low they barely store any elastic energy.

Stiffness is the tendency of a material to react with a small deformation when the material is stressed. It is measured with Young’s Modulus, which is the angular coefficient, or slope, of the linear stress-strain curve. This property depends directly on the bond type between the atoms. The stronger the bond, the higher the modulus (or the stiffness).

Stiff materials are usually ceramics or high melting point refractory metals such as diamond (with 1000-1200 GPa), Tungsten Carbide (with over 600 GPa) and Osmium (with 550 GPa). Soft materials are plastics such as low density poly-ethylene (LDPE, with 0,5-0,6 GPa) and Nylon (around 3 GPa).

In the stress-strain graph below the brittle material would be the stiffest, it could be tungsten carbide, for example. The strong material could se spring steel. The ductile material an aluminum alloy and the plastic material high density poly-ethylene.

Hardness is related with the yield strength of material. So in graph of stress vs strain, if material is very hard its graph will almost parallel to y-axis(stress). This shows that material will not go for deformation before failure. They are brittle in nature.

Toughness is energy absorbed during load impact. It is area between stress vs strain curve. It is opposite of hardness. Here material can go for long deformation before failure and its yield strength will be less. They are ductile in nature.

Resilience is energy absorbed during load impact till its elastic region in stress vs strain curve. It means that material will absorb load impact without causing any deformation in it till its elastic region. It is area under elastic region of graph.

Stiffness is related to hardness. It is ratio of stress to strain. So we can say that it is slope of stress vs strain till elastic limit and that is nothing but the modulus of elasticity. More stiffness means less deformation. Here materials have high yield strength and brittle in nature.

Hardness – Resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation.

Hardness has a variety of meanings. To the metals industry, it may be thought of as resistance to permanent deformation. To the metallurgist, it means resistance to penetration. To the lubrication engineer, it means resistance to wear. To the design engineer, it is a measure of flow stress. To the mineralogist, it means resistance to scratching, and to the machinist, it means resistance to machining. Hardness may also be referred to as mean contact pressure. All of these characteristics are related to the plastic flow stress of materials.

Resilience – Resilience is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading. Proof resilience is defined as the maximum energy that can be absorbed within the elastic limit, without creating a permanent distortion.