MECHANICAL PROPERTIES OF MATERIALS

The properties of materials when subjected to stresses and strains are called “mechanical properties”.

STRESS & STRAIN

Engineering Strain

Mechanical Properties of Solid

Strength:

Ability to withstand various forces to which it is subjected during a test or in service.
Various type are Tensile strength, Compressive strength, Shear strength, Bending strength, Torsional Strength

Elasticity

Property of a material to regain its original shape and size after the removal of load

Plasticity

Property the enables the formation of permanent deformation in a material

Toughness

Toughness is the ability to absorb energy up to fracture (energy per unit volume of material).
A “tough” material has strength approximated by the area under the stress-strain curve indicates the toughness.

Brittleness

Lack of ductility is brittleness

Malleability

Property of a material that can be drawn into thin sheet without rupture by hammered or rolled.

Hardness

Resistance of a material to penetration.

Creep

It is the slow plastic deformation of metals under constant stress at high temperature.

Fatigue

The failure of the material under repeatedly applied stress is called fatigue.

Tensile test

The tensile test of a metal is generally performed to determine: 

• Proportional limit and elastic limit 
• Yield point 
• Ultimate tensile strength 
• Percentage elongation and reduction of cross sectional area

STRESS STRAIN DIAGRAM

Elastic Region (Point 1 –2)

• The material will return to its original shape after the material is unloaded( like a rubber band).
• The stress is linearly proportional to the strain in this region i.e obey Hook’s law

Point 2 : Yield Strength 

 A point where permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.) 

Tensile Strength (Point 3)

The largest value of stress on the diagram is called Tensile Strength(TS) or Ultimate Tensile Strength
(UTS)
It is the maximum stress which the material can support without breaking.

 Fracture (Point 5)If the material is stretched beyond Point 3, the stress decreases as necking and non-uniform deformation occur.
Fracture will finally occur at Point 5.

Hardness Test

• Hardness test measures the resistance to penetration of the surface of a material by a hard object 
• Quantitative hardness techniques have been developed where a small indenter is forced into the surface of a material. 
• The depth or size of the indentation is measured, and corresponds to a hardness number. 
• The softer the material, the larger and deeper the indentation (and lower hardness number).

Brinell’s hardness Test

Deformation of solid

• Deformation is the change in dimensions of forms of matter under the action of applied forces 
• Stress is a measure of the force required to cause a particular deformation. 
• Strain is a measure of the degree of deformation. 
• Elastic Modulus: stress /strain
• The elastic modulus determines the amount of force required per unit deformation.
• A material with large elastic modulus is difficult to deform, while one with small elastic modulus is easier to deform. 

Type of Metal Deformation:

a)Elastic Deformation:

• It is the deformation which disappears when the load is removed.
• For elastic deformation , the strain is nearly proportional to stress.
• Temporary deformation:

b)Plastic Deformation:

• It is the deformation which persists even after the load is removed
• Take place after the elastic deformation has stopped.
• Permanent deformation

Tribology in Manufacturing

• Tribology is the science and technology of friction , lubrication and wear
• Tribology is the study of rubbing, or “the study of things that rub
• Tribology is a branch of mechanical engineering and material sceince
• Tribology plays an important role in manufacturing. 
• In metal-forming operations, friction increases tool wear and the power required to work a piece. This results in increased costs due to more frequent tool replacement, loss of tolerance as tool dimensions shift, and greater forces required to shape a piece. The use of lubricants which minimize direct surface contact reduces tool wear and power requirement.
• Necessary for the proper selection of tool and die materials

Application: gear, bearing etc

Friction

• It is defined as the resistance to relative motion between two bodies in contact under a normal load. 
• It plays an important role in metalworking and manufacturing processes. 

Law of friction : 
Friction force(F)=µ *Normal Load (N)

where, µ = coefficient of friction

Range of coefficient of friction in metal working processes

Theory of friction

The theory is based on the observation that two clean and dry surfaces, regardless of how smooth they are, contact each other (junction) at only a fraction of their apparent area of contact.
Maximum slope of the hills on these surfaces ranges typically between 5o and 15o

Wear

•Wear is defined as the progressive loss or removal of material from a surface.
•Effect of wear changes the shape of the tools and die & thus consequently affect the size and quality of the part produced

Wear Classification:

a) Adhesive Wear( Scratching)

• When tangential force is applied between two contact bodies, shearing can take place causing adhesive wear. 
• "The harder material scratches the softer material."

Wear Volume (V) =K *sliding distance(L)*Normal force between surface (W) /indentation hardness of softer body(H)
Where, K= wear coefficient

b) Abrasive wear

• It is caused by a hard , rough surface sliding across another surface.

c)Corrosive wear :

• Oxidation or chemical wear

d)Fatigue wear(surface fracture wear)

e)Erosion wear

Caused by loose abrasive particles abrading a surface..(turbine wear)

Lubrication

The role of a lubricant is to:

• Reduce Friction

• Prevent / Minimize Wear

• Transport Debris away from Interface

• Provide Cooling