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Mechanical Properties of Materials: Toughness, Hardness and Stiffness

It is critical to understand the mechanical properties of a material before deciding it for an engineering application. The mechanical properties of materials are those that influence its mechanical strength and ability to be molded into a desired shape. Some of a material’s typical mechanical qualities include strength, toughness, stiffness, hardness, hardenability, brittleness, malleability, ductility, resilience and fatigue.

Basically a material is defined as a substance or combination of various substances, which constitutes to an object. Talking about the properties of a material, a material has numerous types of properties like physical, optical, thermal, electrical, mechanical, chemical, magnetic and atomic etc. But in material science, we are mainly concerned with 2 of them, physical properties and mechanical properties of materials.

Physical properties are those properties by which the state of system can be defined and such quantities are measurable too. Physical properties of a material include its shape, size, color, volume, area and density etc. On the other hand, mechanical properties of materials are the properties which describe a material and change in its physical properties on the application of external force. These properties include, hardness, ductility, toughness, strength, stiffness, modulus of elasticity, brittleness, elasticity, creep, viscosity, plasticity etc. Here we will discuss the difference between toughness, hardness and stiffness of a material.

What is toughness?

Toughness of a material is described as the ability of the material to absorb the applied force (impact) without being fractured, at some specified temperature. It can also be defined as the material’s ability to absorb energy before being plastically deformed. A material having high strength and high ductility will have more toughness as compared to the material with low strength and high ductility. There is difference between ductility and toughness of a material. Ductility measures the amount of plastic deformation before fracture. So if a material has high ductility, it doesn’t mean it is highly tough. If a material can be bent easily without breaking then it is tougher than a material that will break instead of bending. Tough materials are not easily breakable and can absorb more energy than other materials before fracture. They can even withstand with high pressures. The amount of energy per unit volume determines the numerical value toughness. A tough substance is one that can withstand repeated hits without rupturing. The ability of a material to absorb energy without cracking is commonly defined as toughness. Toughness is measured by the area under the stress strain curve.

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Unit of toughness

The unit of toughness is that of energy per unit volume i.e. Joule / meter cube.

Toughness of a material depends upon some factors, which are given below.

Strain rate: Toughness of a material will decrease if the rate of load is increased.

Temperature: Toughness of a material decreases if the temperature is decreased.

Notch effect: A material will exhibit good toughness if the uniaxial stress is applied.

What is stiffness?

Elasticity or stiffness is defined as the tendency of a material to return to its original shape and size when external force is removed. In other words, it can be defined as the resistance that the material offers as the result of an applied stress. Basic concept of stiffness is from flexibility, as both are opposite. So if a material is more flexible, it will have less stiffness. Stiffness formula is given as

k = F / δ

Where F is applied force on material and δ is the displacement that is caused due to applied force. The relation between young’s modulus and stiffness is given as

k = (E . A) / L

Where A is the cross-sectional area and L is the length of that part of material considered.

Unit of stiffness

Stiffness formula is given as

Stiffness = force / deformation

Thus the unit of stiffness is newton per meter. On the other hand, flexibility is given by deformation per unit force. So its unit is opposite to that of stiffness i.e. meter per newton.

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What is hardness?

It is ability of a material to resist permanent (plastic) deformation. It is the resistance to plastic deformation that is induced by the mechanical indentation. Plastics, ceramics, concrete and wood are harder than other materials. Hardness of a material is characterized by its strong intermolecular forces. Hardness can be measured by various manners as it is of various types.

What is Indentation Hardness?

Ability of material to oppose the dent which may be caused due to punch of external hard (sharp) objects.

Scratch Hardness: It is the ability of material to resist the scratches on the outer surface layer of that material due to external applied force.

Rebound Hardness: Also termed as dynamic hardness. It is determined by the height of “bounce” of a diamond tipped hammer dropped from a fixed height on the material.

It is a material’s ability to resist permanent form change as a result of external stress. Scratch Hardness, Indentation Hardness, and Rebound Hardness are three different types of hardness tests.

Hardness is determined by scratching, bouncing, and indentation. The most frequent method of describing hardness is indentation hardness. There are a variety of procedures for conducting these tests, depending on the material.

Unit of hardness

Newton /mm2 is the SI unit of hardness. Hardness is also measured in Pascal, although hardness should not be confused with pressure.

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