Friday, 6 April 2018

MECHANICAL PROPERTIES OF MATERIAL,EVERY MECHANICAL ENGINEER MUST KNOW !!!

The mechanical properties of a material are those which effect the mechanical strength and ability of material to be molded in suitable shape. 

Some of the typical mechanical properties of a material are listed below-

#1. Strength:

The ability of material to withstand load without failure is known as strength. If a material can bear more load, it means it has more strength. Strength of any material mainly depends on type of loading and deformation before fracture. According to loading types, strength can be classified into three types.

a. Tensile strength:
b. Compressive strength:
3. Shear strength:

According to the deformation before fracture, strength can be classified into three types.

a. Elastic strength:
b. Yield strength:
c. Ultimate strength:

#2. Homogeneity:

If a material has same properties throughout its geometry, known as homogeneous material and the property is known as homogeneity. It is an ideal situation but practically no material is homogeneous.

#3. Isotropy:

A material which has same elastic properties along its all loading direction known as isotropic material.

#4. Anisotropy:

A material which exhibits different elastic properties in different loading direction known as an-isotropic material.

#5. Elasticity:

If a material regain its original dimension after removal of load, it is known as elastic material and the property by virtue of which it regains its original shape is known as elasticity.

Every material possess some elasticity. It is measure as the ratio of stress to strain under elastic limit.

#6. Plasticity:

The ability of material to undergo some degree of permanent deformation without failure after removal of load is known as plasticity. This property is used for shaping material by metal working. It is mainly depends on temperature and elastic strength of material.

#7. Ductility:

Ductility is a property by virtue of which metal can be drawn into wires. It can also define as a property which permits permanent deformation before fracture under tensile loading. The amount of permanent deformation (measure in percentage elongation) decides either the material is ductile or not.

Percentage elongation = (Final Gauge Length – Original Gauge Length )*100/ Original Gauge Length

If the percentage elongation is greater than 5% in a gauge length 50 mm, the material is ductile and if it less than 5% it is not.

#8. Brittleness:

Brittleness is a property by virtue of which, a material will fail under loading without significant change in dimension. Glass and cast iron are well known brittle materials.

#9. Stiffness:

The ability of material to resist elastic deformation or deflection during loading, known as stiffness.  A material which offers small change in dimension during loading is more stiffer. For example steel is stiffer than aluminum.

#10. Hardness:

The property of a material to resist penetration is known as hardness. It is an ability to resist scratching, abrasion or cutting. 

It is also define as an ability to resist fracture under point loading.

#11. Toughness:

Toughness is defined as an ability to withstand with plastic or elastic deformation without failure. It is defined as the amount of energy absorbed before actual fracture.

#12. Malleability:

A property by virtue of which a metal can flatten into thin sheets, known  as malleability. It is also define as a property which permits plastic deformation under compression loading.

#13. Machinability:

A property by virtue of which a material can be cut easily.

#14. Damping:

The ability of metal to dissipate the energy of vibration or cyclic stress is called damping. Cast iron has good damping property, that’s why most of machines body made by cast iron.

#15. Creep:

The slow and progressive change in dimension of a material under influence of its safe working stress for long time is known as creep. Creep is mainly depend on time and temperature. The maximum amount of stress under which a material withstand during infinite time is known as creep strength.

#16. Resilience:

The amount of energy absorb under elastic limit during loading is called resilience. The maximum amount of the energy absorb under elastic limit is called proof resilience.  

#17. Fatigue Strength:

The failure of a work piece under cyclic load or repeated load below its ultimate limit is known as fatigue. The maximum amount of cyclic load which a work piece can bear for infinite number of cycle is called fatigue strength. Fatigue strength is also depend on work piece shape, geometry, surface finish etc.

#18. Embrittlement:

The loss of ductility of a metal caused by physical or chemical changes, which make it brittle, is called embrittlement.

Thursday, 5 April 2018

MACHINABILITY & MACHINABILITY INDEX EXPLAINED !!!



MACHINABILITY

Machinability is a term indicating how the work material responds to the cutting process. In the most general case good machinability means that material is cut with good surface finish, long tool life, low force and power requirements, and low cost.

MACHINABILITY INDEX

It is a numerical value that designates the degree of difficulty or ease with which a particular material can be machined.

The machinability index KM is defined by

KM = V60/V60R
where ,
  • V60 is the cutting speed for the target material that ensures tool life of 60 min,
  • V60R is the same for the reference material. Reference materials are selected for each group of work materials (ferrous and non-ferrous) among the most popular and widely used brands.
If KM Greater than 1, the machinability of the target material is better that this of the reference material, and vice versa. Note that this system can be misleading because the index is different for different machining processes.
Example: Machinability rating
The reference material for steels, AISI 1112 steel has an index of 1.
For a tool life of 60 min, the AISI 1045 steel should be machined at 0.36 m/s.
Hence, the machinability index for this steel is,
KM = 0.36/0.5 = 0.72.
This index is smaller than 1, therefore, AISI 1045 steel has a worse workability than AISI 1112.

WAYS OF IMPROVING MACHINABILITY INDEX:

The machinability of the work materials can be more or less improved, without sacrificing productivity, by the following ways :
• Favourable change in composition, microstructure and mechanical properties by mixing suitable type and amount of additive(s) in the work material and appropriate heat treatment.

• Proper selection and use of cutting tool material and geometry depending upon the work material and the significant machinability criteria under consideration.

• Proper selection and appropriate method of application of cutting fluid depending upon the tool – work materials, desired levels of productivity i.e., VC and so and also on the primary objectives of the machining work undertaken.

• Proper selection and application of special techniques like dynamic machining, hot machining, cryogenic machining etc, if feasible, economically viable and eco-friendly.

CHECK OUT WHY BUBBLES ARE DANGEROUS IN HYDRAULIC BRAKING SYSTEM !!

Why are air bubbles dangerous in a hydraulic brake system?
One of the most necessary conditions for a hydraulic system to function properly is that the hydraulic fluid must be incompressible.

Effect of air bubbles on a hydraulic system:-
  1. Loss of bulk modulus -Air is a compressible fluid. When air bubbles get into a hydraulic system, the force does not get transmitted properly.Also,free or entrained air in the hydraulic system reduces substantially the effective bulk modulus of the system. That is, an air-oil mixture appears to increase the compressibility of the fluid, making the system spongy. 
  2. Loss of horsepower — When an air pocket is present in an actuator, it is alternately compressed and relaxed as the actuator is cycled. Since the air pocket must first be compressed before the fluid can cause the actuator to move, power is consumed. Upon relaxation, the air pocket expands and rives fluid out. The stored power, therefore, is expended in driving fluid back into the reservoir and not in moving the actuator.
  3. Spongy control — Because fluids are considered to be basically incompressible, we expect great stiffness in a hydraulic system. That is, the positioning of an actuator should be immediate (rapid response) and precise. The larger the amount of free or entrained air, the spongier (softer, less stiff) the system.
  4. Loss of system fluid — One of the most serious conditions that can occur in a hydraulic system is the loss of reservoir fluid. The fluid level must be kept high enough to insure enough fluid for the pump intake, otherwise cavitation begins.



In that case, on pressing the brake pedal, one would feel the pedal like a jello. It would be as if you are pressing a sponge. The wheels would not lock instantly and the car would travel considerably long distance before the required braking effect is obtained.