Mechanical properties of materials

There are various Mechanical properties of any material out of which we will discuss almost all important properties here.

Proportional limit - it is defined as the highest level of his dress up to which the stress is proportional to strain.

Elastic limit - it is defined as the highest stress up to which the material is within the elastic deformation region. Note that elastic limit is higher than the proportional limit. Also in the elastic limit cannot be easily determined while the proportional limit can be determined easily experimentally.

Yield stress - it is defined as the constant level of stress at which material exhibits extensive permanent deformation without significant strain hardening.

Yield point elongation takes place due to return it is stopping and releasing of dislocations why carbon interstitial in the Steel lattice so it has wave likes it has shown in figure.

Note that lower yield point is considered as yield stress of material for safety of engineering design.

Proof stress - some materials like Copper Nickel and Aluminium alloys do not exhibit a well-defined easily. Hence in case of such materials proof stress is taken as an index of yield stress of the material.

Proof stress is defined as a s trees which results in a specified amount of permanent strain in a material. For example in 0.2% proof stress indicates stress which results in 0.2 % of permanent strain in the material.

Resilience - it is defined as the energy absorbed buy material within elastic deformation zone. It is given by the area under load vs elongation diagram of to elastic limit.

Modulus of resilience is defined as resilience for unit volume.

For more resilience a material should have high elastic limit and low Young's modulus. Resilience is an important property for springs, diaphragms and wires used in musical instruments.

Toughness - It is defined as the energy absorbed by the material prior to fracture or before fracture. It is given by the total area under the load vs elongation diagram.

Toughness is an important property required in materials for forming applications.

Modulus of toughness is defined as the toughness per unit volume.

Ductility - It is defined as the ability of a material to exhibit extensive permanent deformation under the action of tensile forces. In fact it is the material property by which it can be extended into long thin wires. It is an important property required in materials for wire drawing applications. The percentage elongation in length is considered as the index of ductility of a material.

Malleability - it is defined as the ability of a material to exhibit extensive permanent deformation under the action of applied compressive forces. In fact it is the property of a material by which it can be transformed into thin sheets. Percentage reduction in area of cross section is taken as an index of malleability of a material.

Strength Coefficient and strain hardening coefficient

In permanent deformation region, the relationship between true stress and true true strain is given in figure below.

Strength Coefficient is defined as the true stress at a true strain of Unity. Greater the value of string Coefficient more will be the strength of material.

Strain hardening exponent is an index of toughness of material and resistance to necking generally its value is lies between 0-1.

Stiffness - it is defined as the resistance offered by a material to elastic deformation elastic deflection. It is determined from the slope of linear portion of the true stress true strain diagram is also called as Young's modulus. Greater is the Young's modulus more will be the stiffness of material.

Hardness - it is defined as resistance to permanent deformation by indentation or abrasion on the surface.

There are three important test to measure the hardness of a material. These are

Brinell hardness test

Vickers hardness test

Rockwell hardness test

Out of the above three The Rock hardness test is the fastest test.

There are two important impact test known as charping test and azod test. These tests are performed to find the dynamic toughness of material.

Creep

Creep is defined as the floor and progressive permanent deformation which occurs in material at constant stress and at a temperature approximately equal to 0.4 times the melting temperature. Creep is an important phenomenon in materials such as nuclear reactor components gas and steam turbine blades jet engine components and filament in bulb.

Creep is also defined as the highest stress that a material can be the stand for a specified length of time without exceeding a specific permanent deformation at a given temperature.

There are three regions of creep

Primary creep region

Steady state creep region

Tertiary creep region

In primary creep region slope of the graph decreases because a strain hardening dominates the softening effect.

In the steady state creep region the slope of the graph remains constant we call a strain hardening and softening effect balance each other.

Whereas in tertiary creep region slope of the graph increases because of thing effect of temperature will dominate the strain hardening effect.

For better creep resistance

1. Use materials having high melting point

2. Use coarse grain structures

3. Use single crystal materials

Materials having higher oxidation resistance usually exhibits better creep resistance.