Mechanical Properties of Materials

The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and loads. The topic of the mechanical properties of materials is of great industrial importance in the design of tools, machines, and structures. These properties are structure sensitive in the sense that they depend upon the crystal structure and its bonding forces (at the microstructural level), especially upon the nature and behavior of the imperfections which exist within the crystal itself or at the grain boundaries. In this article, we will discuss 13 main mechanical properties of the metals, these are;

  1. Strength
  2. Elasticity
  3. Plasticity
  4. Ductility
  5. Malleability
  6. Brittleness
  7. Stiffness
  8. Hardness
  9. Creep
  10. Fatigue
  11. Resilience
  12. Toughness
  13. Weldability

These properties can be well understood with help of a stress-strain diagram (Given below). The Stress-Strain diagram is plotted with the help of a tensile test.

 

Stress Strain Curve

 

Now we shall discuss the 13 different mechanical properties of materials;

1) STRENGTH: Strength is defined as the ability of a material to resist the externally applied load. The internal resistance offered by a material to externally applied forces is called stress.

The capacity of resisting external loads by metal and to withstand destruction under the action of external loads against these stresses is known as strength.

The stronger the material, the greater the load it can withstand this property of material, therefore, determines the ability to withstand stress without failure.

Strength varies according to the type of loading like tensile, compressive, shearing, and torsional strengths. The maximum stress that any material can withstand before destruction is called it’s UTS or ultimate tensile strength (Point ‘D’ is the ultimate tensile strength (UTS) shown in the above figure). The tenacity of the material is its ultimate strength in tension.

 

2) ELASTICITY: Elasticity is defined as the property of a material to regain its original shape after removal of the externally applied load. We can take an example of a rubber band, whenever we pull a rubber band it gets elongated i.e. it’s shape gets deformed but when we remove the load the rubber band comes back to its original shape. Hence we can say that a rubber band is an elastic material or rubber band exhibits the property of elasticity.

When the external forces are removed it can also be referred to as the power of the material to come back to its original position after deformation. It can be used as an important application for building precision instruments like Springs or structures etc.

Any material will exhibit the elasticity property up to a certain load which is called as the elastic limit of that material (the region between point ‘O’ and ‘A’, in the above stress-strain diagram, is the elastic range, it is also known as the proportional limit. Beyond point ‘A’ permanent deformation of the material will start). If we keep on applying the external load beyond the elastic limit, the material will be permanently deformed i.e. the material will not be able to regain its original shape even after the removal of the external load.

 

3) PLASTICITY: Plasticity is defined as the property of material under which the material is not able to regain its original shape even after the removal of the load i.e. the material permanently gets deformed.

In other words, It is the ability or tendency of a material to undergo some degree of permanent deformation without its failure.

Plastic deformation takes place only after the elastic limit of material has been exceeded. This property is important in forming, shaping, extruding and many other hot or cold working processes materials such as clay lead, etc are plastic at room temperature and steel is plastic at forging temperature this property generally increases with an increase in temperature of materials.

This property of the material is required in forging in stamping images on coins and in on mental work.

 

4) DUCTILITY: Ductility is termed as the property of a material that enables it to be drawn into the thin wire with the application of tensile load.

The ductility is usually measured in terms of percentage elongation and percent reduction in the area which are often used as empirical measures of ductility.

In general, materials that possess more than 5% elongation are called as ductile materials

The ductile material commonly used in engineering practice in order of diminishing ductility a mild steel copper aluminum, nickel, zinc, tin, and lead.

 

5) MALLEABILITY: Malleability is the ability of the material which enables it to be flattened into thin sheets under applications of heavy compressive forces without cracking (by hot or cold working), which means it is a special case of ductility which permits materials to be rolled or hammered into thin sheets.

A malleable material should be plastic but it is not essential to be so strong.

The malleable materials commonly used in engineering practice in order of diminishing value wrought iron, copper and aluminum, lead steel, etc are recognized as highly malleable metals.

 

6) BRITTLENESS: Brittleness is the opposite of ductility. It is the property of breaking of a material with little permanent distortion the materials having less than 5% elongation and the loading behavior are said to be brittle materials.

Brittle materials when subjected to tensile loads snap off without giving any sensible elongation glass, cast iron, brass and ceramics are considered as brittle material thus brittleness is the property of a material to snap off without giving any sensible elongation when subjected to tensile loads.

 

7) STIFFNESS: Stiffness is defined as the ability of a material to resist deformation under stress. The resistance of a material to elastic deformation or deflection is called stiffness or rigidity.

Material that suffers slight or very less deformation and the load has a high degree of stiffness or rigidity for instance suspended beams of steel and aluminum may both be strong enough to carry the required load but the aluminum beam will sag or deflect further which means the steel beam is stiffer or more rigid than the aluminum beam.

If the material behaves elastically with linear stress-strain relationship under Hookes law its stiffness is measured by Young’s modulus of elasticity. The higher is the value of Young’s modulus, the stiffer is the material in tensile and compressive stress. It is called the modulus of stiffness or modulus of elasticity in shear. The modulus of rigidity is usually 40% of the value of young’s modulus for commonly used materials in volumetric distortion the bulk modulus.

 

8) HARDNESS: Hardness is defined as the ability of a metal to cut another metal.

A harder metal can always cut output impression to the softer metals due to its hardness

It is a very important property of metals and has a wide variety of meanings it embraces many different properties such as resistance to wear, resistance to indentation, resistance to scratches, resistance to deformation and machine mobility, etc.  Diamond is the hardest known material naturally.

9) CREEP: When a metal part is subjected to high constant stress at a high temperature for a longer period of time it will undergo a slow and permanent deformation which is known as creep. If the material will be continuously subjected to high stresses at higher temperature crack can be formed which may further propagate towards failure called creep failure.

 

10) FATIGUE: Fatigue is the failure of a material due to cyclic or repeated loading. The intensity of the load may be very less than the ultimate tensile stress, but due to the repeated or cyclic action of the load, the crack initiates and propagates which leads to the fatigue failure.

The fatigue process leads to Macroscopic and microscopic discontinuities (at the crystalline grain scale) as well as component design features that cause stress concentrations (holes, keyways, sharp changes of load direction, etc.) are common locations.

 

11) RESILIENCE: It is the amount of energy which a body can absorb without permanent deformation.

 

12) TOUGHNESS: The amount of energy that a material can absorb without breaking is called the toughness of that material. In other words, it is the ability of a material to absorb energy and deform plastically without fracturing.

 

Difference between toughness and resilience:

On application of external load on any material, in general, the material exiibits elasticity then it reaches the plasticity stage and after plasticity the material fails or breaks. Toughness is the energy absorbed without fracture of the material (i.e. the energy absorbed during elastic stage + the energy absorbed during the plastic stage before failure). However, Resilience is the energy absorbed during the elastic stage only i.e. the energy absorbed without permanent deformation of the material.

 

13) WELDABILITY: Weldability is not a main mechanical property, but it is very important when the material needs to be welded. Weldability is the ability of a material to be welded and retain its properties after the welding. If a material can be welded very easily with other materials, in any position, and able to retain the specified properties then we can say that the weldability of that material is good.

Carbon equivalent plays a very crucial role in determining the weldability of steel. In general, A material with a carbon equivalent less than 04% is considered as good weldability, Any material with a carbon equivalent between 0.4% to 0.5% is considered as limited weldability material and any material having a carbon equivalent of more than 0.5% is considered as poor weldability material. This is summarised below

  • Up to 0.4%: Good Weldability
  • Between 0.4 to 0.5%: Limited Weldability
  • Above 0.5%: Poor Weldability

 

Please watch the below video to understand more about the mechanical properties of the materials:

 

References and for further reading;

1. Wikipedia: Click here to read

2. thengineerspost.com

 

This article is written by:

MR. ABHISHEK SRIVASTAVA 

B.TECH (MECHANICAL ENGG.)

  • 4+ Years of Industrial Experience
  • 2+ Years of Teaching

E-Mailabhi.allahabad122@gmail.com

Sandeep Anand

I am a Mechanical Engineer with more than ten years of work experience in the field of welding and NDT.

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