Bachelor of Technology Mechanical

Industrial Material UOG Lecture # 04 By: Jahangir Rana

Mechanical Properties of metal

Mechanical Properties of metal

• There are so many properties by which a metal can test. Some are as follow:  Engineering stress & Engineering Strain           Shear Stress & Shear Strain Modulus of Elasticity Yield Strength Ultimate tensile strength Percent Elongation Percent Reduction in area Hardness Fatigue Ductility Forging And many more!!!!!!!!

Stress

(

σ

)

Stress is defined as force per unit area. It has the same units as pressure, and in fact pressure is one special variety of stress. However, stress is a much more complex quantity than pressure because it varies both with direction and with the surface it acts on.

• • • • • • σ = Fn / A where (1) σ = normal stress ((Pa) N/m2, psi) Fn = normal component force (N, lbf) A = area (m2, in2)

Strain

(ε)

Strain is defined as the amount of deformation an object experiences compared to its original size and shape. For example, if a block 10 cm on a side is deformed so that it becomes 9 cm long, the strain is (10-9)/10 or 0.1 (sometimes expressed in percent, in this case 10 percent.) Note that strain is dimensionless.

Types of stress

• • • • Compression stress Tension stress Normal stress Shear stress

Compression stress

• Stress that acts to shorten an object.

Tension stress Stress that acts to lengthen an object.

Normal stress Stress that acts perpendicular to a surface. Can be either compression or tensional.

Shear stress

• • Stress that acts parallel to a surface. It can cause one object to slide over another. Stress parallel to the plane is usually denoted "shear stress" and can be expressed as τ = Fp / A (2) • • • • where τ = shear stress ((Pa) N/m2, psi) Fp = parallel component force (N, lbf) A = area (m2, in2) It also tends to deform originally rectangular objects into parallelograms. The most general definition is that shear acts to change the angles in an object.

Shear stress

Flexural stress

Torsional stress

Shear Stress & Shear Strain

•

Shear stress

Shear force applied on the area over which shear force is acted divided by the original area. It is donated by τ.

Τ = S/A •

Shear strain

The shear strain is defined as the amount of the shear displacement ‘a’ divided by the height at which shear force is applied is called shear strain.

γ = a/h

Stress Strain Curve

Modulus of Elasticity

• It is defined as when the metal is deformed elastically or stress is directly proportional to the strain.

stress α strain stress E strain ‘E’ is young's modulus or modulus of elasticity. It is unit is pascal or PSI

Cont…

•

Yield Strength

It is strength at which metals or alloys shows the significant plastic deformation.

•

Ultimate tensile strength

The maximum value of stress on stress strain diagram is called ultimate tensile strength.

Ductility & % Elongation

• •

Ductility

It is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. The property of metal which permits it to be reduced in cross sectional area without fracture. In a tensile test, ductile metals show considerable elongation eventually failing by necking, with consequent rapid increase in local stresses. •

% Elongation

The amount of elongation that a tensile specimen undergoes during testing provide the value of ductility of metals. It is most commonly expressed % elongation.

% Elongation

% Reduction in area

Hardness

• • • It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied.

The greater the hardness of the metal, the greater resistance it has to deformation.

It is a measure of a material’s resistance to localized plastic deformation (e.g., a small dent or scratch).

Hardness Testing

• In quantitative hardness tests, a small indenter is forced into the surface of a material to be tested, under controlled conditions of load and rate of application.

• The depth or size of the resulting indentation is measured by an automated testing machine and converted to a hardness number.

• The softer the material, the larger and deeper the indentation, and the lower the hardness index number.

Advantages of Hardness Testing

Hardness tests are simple and inexpensive.

Ordinarily, no special specimen needs to be prepared, and the testing apparatus is relatively inexpensive.

Advantages of Hardness Testing

• The test is non-destructive – the specimen is neither fractured nor excessively deformed. A small indentation is the only deformation.

• Other mechanical properties estimated from hardness data.

may be

Rockwell Hardness Tests

• The most common method used to measure hardness.

• Several different scales may be utilized from possible combinations of various indenters and different loads, which permit the testing of virtually all metal alloys (as well as some polymers).

Rockwell Hardness Testing Machines

Rockwell Hardness Tests

Indenters include spherical and steel balls having various diameters, and also a conical diamond (Brale) indenter for testing the hardest materials.

Rockwell Hardness Tests

• With this system, a hardness number if determined by the difference in depth of penetration resulting from the application of a minor load followed by a larger major load.

• On the basis of the magnitude of both major and minor load, two scales are used: Rockwell and superficial Rockwell.

Rockwell Hardness Scales

Scale Symbol A B C D E F G H K Indenter diamond 1.588 mm ball diamond diamond 3.175 mm ball 1.588 mm ball 1.588 mm ball 3.175 mm ball 3.175 mm ball Minor Load = 10 kg Major Load (kg) 60 100 150 100 100 60 150 60 150

Brinell Hardness Tests

• A hard, spherical indenter is forced into the surface of the metal to be tested.

• The diameter of the hardened steel (or tungsten carbide) indenter is 10.00 mm.

• Standard loads range between 500 and 3,000 kg in 500 kg increments.

Brinell Hardness Testing Machine

Brinell Hardness Tests

• During a test, the load is maintained constant for a specified time (between 10 and 30s).

• The Brinell hardness number, HB, is a function of both the magnitude of the load and the diameter of the resulting indentation.

Fatigue

• •

Fatigue

is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading.

Theses failure which occurs under repeated or cyclic stressing are called fatigue failure.

Changes Occur During Fatigue Process

The structural changes, which occur during • • • • fatigue process are: Crack initiation Slip band crack growth Crack growth on metal due to high stress Ultimate ductile failure

Cont…

•

Crack initiation

During this stage, the early damage occurs and crack may no be seen.

•

Slip band crack growth

Plastic deformation in one direction and then alternatively in reverse direction cause grooves appear on the surface is called slip band growth.

Cont…

•

Crack growth on metal due to high stress

During last stage crack may grow in metal only a few grain diameter and in this stage, crack growth will define at relatively rapid rate.

•

Ultimate ductile failure

Finally when the crack covers a sufficient area, so that the remaining metal can not support the applied load then the metal ruptured by the ductile area.

Factors that affect fatigue-life

• • • Cyclic stress state. Surface quality Environment.

Forging

• It is another primary method for working metal into useful shapes. • • In the forging process the metal is hammered or pressed into the desired shape.

• Most forging operations are carry out in the hot conditions.

Some are carry out in the cold conditions ( room temperature)

Types of forging

• • There are to types of forging process Hammer forging Press forging • • Press fording is further divided in the following types Open die forging Close die forging

Cont…

• •

Open die forging

Open die forging is carried out b/w two flat dies with very simple shape such as vees or semi circular cavities.

It is producing useful large for parts such as steel shafts for electric steam turbines and generator • •

Closed die forging

In close die forging the metal to be forged is placed b/w two dies which have he upper and lower impression of desired shape.

Close die forging can be carried out by using a single pair of dies, or multiple impression dies.

OPEN DIE FORGING

Cont…

CLOSED DIE FORGING

Open die forging