Sunday, 20 October 2019

# Mechanical properties of Metals (33) -Types of strains, Material Selection,heat treatment

Mechanical properties of metals:If you are a design engineer from the field of Mechanical Engineering then you should has to know about Mechanical properties of metals so that those can be taken into consideration while designing.because you are the one, who can know about the need and according to that need, what type of material can be taken as dependent on you.

That’s the basic consideration every design engineer must follow in picking up on the best material depending upon the Mechanical properties of metals.

Considering this fact, I will be explaining about 31 Mechanical properties of metals that every Mechanical engineer must know in this article in a detailed manner. Read this article completely so that it can be used in Interviews also.

Tabular Column:

 1.Mechanical Properties of metals

## 31 Mechanical properties of metals:

The 31 Mechanical properties of metals that every Mechanical engineer must know was explained below.
1.Strength:
The ability of a material that can withstand to Mechanical load is called Strength of that particular material.
• A strength of the material depends upon the directions in which it is loaded
• Ex: Tensile Test
• From Tensile Test, we get Tensile Strength, ductility, fracture toughness, resilience, yield stress, etc.

2.Ductility:
The ability of a material that can undergo plastic deformation before failure is called Ductility.
Ductility (D)= % of Elongation in length or % of Reduction in Cross-section at breaking point.

Note: Ductility is always reported in % of elongation in length but not in % of the reduction in c/s area because a reduction in c/s section area is small and difficult to measure during testing.

% of the elongation at different Temperatures:
If you can see the % of elongation of a metal w.r.t.Low temperature(L.T),Room temperature(R.T) and High temperature(H.T) was
D(L.T)<D(R.T)<D(H.T)

3.Fracture Toughness:
The ability of a material that can absorb energy at the time of failure against fracture is called Fracture Toughness.
• By calculating the area under stress v/s strain curve up to the failure point, the fracture toughness of the material is determined.
• A material possesses high fracture toughness means it has a higher capacity to absorb more strain energy against failure.
• A brittle material possess lower the fracture toughness(F.T) compared to a ductile material because of the area under the curve
A Ductile > A Brittle
F.T Ductile > F.T Brittle
Considering an area of Stress v/s Strain curve at different temperatures is noted as follows.
A H.T > A R.T > A L.T
That implies

F.T H.T > F.TR.T > F.T L.T
If the material is tested at high temperature, the displacement of atomic planes is easy, implies it can produce more plastic strain, implies area under the curve will be more, implies Fracture Toughness is high.

4.Hardness:
The resistance offered by the material against mechanical deformation is called hardness.

5.Brittleness:
The ability of a material that can withstand mechanical load without plastic deformation is called Brittleness.

6.Stiffness:
The ability of a material that can resist mechanical deformation under stress is called stiffness.

7.Creep:
Time v/s Strain behavior of a material under Constant mechanical load is called Creep.

8.Fatigue:
Stress v/s no. of load cycles of the behavior of metal under changing mechanical load with Time is called as Fatigue.
• Fatigue is a danger than Creep

9.Malleability:
It is a capability of being extended or shaped by beating with a hammer or by the external pressure of rollers so that the output is in the form of thin sheets called as Malleability.

10.Plasticity:
The quality of being easily shaped or molded called Plasticity.

11.Elasticity:
The ability of an object or material to resume or regain its normal shape or original shape after being stretched or compressed called Elasticity.

12.Toughness:
It is the state of being strong enough in order to withstand adverse conditions or rough handling called Toughness.
or
It is also defined as the Area under P.(Delta) curve up to Failure is called as Toughness.

13.Tensile strength:
The resistance of a material to breaking under tension called Tensile Strength.

14.Compressive strength:
The resistance of a material to breaking under Compression is called Compressive strength.

15.Resilience:
The ability of a material that can absorb energy without undergoing any shape change called Resilience.
Ex: Steel
• By calculating the area under Stress v/s Strain curve up to an elastic point, resilience(elastic resilience) of the material will be determined.
These are the 15 Mechanical properties of metals that are explained in a detailed manner and the rest are below along with material selection with flowcharts.

16.Hardenability:

The hardenability of a metal alloy is the depth up to which a material is hardened after putting through a heat treatment process called hardenability.

17.Machinability:

It is a process of removing the layer from the surface of the workpiece so as to get a good surface finish on the outer surface of the material called Machinability.

In the most general case, good machinability means that material is cut with good surface finish, long tool life, low force, power requirements, and low cost.

18.Stress:

When a material is loaded with a force, it produces stress which then causes the material to deform.

or

Stress is defined as “force per unit area” – the ratio of applied force F to the cross-section area is called stress.

19.Youngs Modulus:

Young’s modulus is the ability of a material to withstand changes in length when under lengthwise tension or compression.

• It is denoted by E.
• It is also defined as the ratio of stress to strain.

20.Elastic deformation:

It is a change in the shape of a material at low stress that is recoverable after the stress is removed called as Elastic deformation of a material.

or

A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape called as the elastic deformation

21.Plastic deformation:

Plastic deformation is a process in which permanent deformation is caused by a sufficient load. It produces a permanent change in the shape or size of a solid body without fracture, resulting from the application of sustained stress beyond the elastic limit.

After plastic deformation, no elastic recovery takes place.

22.The factor of safety:

It is the load-carrying capacity of a system beyond the expected or actual loads.

or

the factor of safety is how much stronger the system is that it usually needs to be for an intended load.

The factor of safety is different for brittle and ductile materials.

23.Elastic Recovery:

It is the measure of the ability of an elastomer to return to its original shape when a compression load or external load is removed called Elastic Recovery.

24.Engineering stress:

Engineering stress is the applied load divided by the original cross-sectional area of a material. It is also called nominal stress.

25.True stress:

True stress is the applied load divided by the actual cross-sectional area (the changing area with respect to time) of the specimen at that load.

26.Poissons Ratio:

Poisson’s ratio is the ratio of transverse contraction strain or Lateral strain to the longitudinal extension strain(Linear strain).

27.Bulk Modulus:

It is the relative change in the volume of a body produced by a unit tensile or compressive stress acting uniformly over its surface.

28.Rigidity Modulus:

It is also known as shear modulus

It is defined as the ratio of shear stress to the shear strain.

29.Strain:

It is defined as the ratio of change in dimension to its original dimensions called a strain.

• It is denoted by “Ɛ”
• It is a Dimensionless parameter
• Strains the response of a system to applied stress.

Now let’s discuss the types of strains in a detailed way…

3 Types of Strain every Engineer must know:

When you are going for any Interview, every recruiter will ask some questions and out of those questions, this question was also on the list. As we know about only one strain i.e.change in dimension to its original dimension but, when the recruiter asks about the types of strain, we will be looking here and there.

Therefore, In this article, I will be explaining about 3 Types of strain along with their representation.

The detailed explanation of all these Types of Strain is presented below.

1.Normal Strain:

• It is defined as the ratio of change in dimension to its original dimension.
• It is due to the normal forces and it is represented by e or ε.
• It is a dimensionless parameter.

If we consider the dimension as Length then Change in length to its original length is the Normal strain and it is represented below.

Let “L” be the Length of the component and “dL” will be the Change in Length of the component then the representation of the Normal strain is

 e or ε = dL/L

2.Shear Strain:

• It is defined as the angular distortion between any two mutually perpendicular planes in radian’s is the shear strain.
• It is due to shear force or tangential force acting on a specimen.
• It is represented by gamma (γ).

Representation:

Consider a component which is subjected to equal and opposite forces on either face (AB&CD) and one end of the face(AB) is fixed and the other end(DC) is kept free. Due to the application of force on one of the face, the component gets distorted through an angle of φ to the shape ABC’D’.

Now Shear strain or deformation per unit length is of the component is = CC’ / CD = CC’ / BC = * radian

3.Volumetric strain:

• It is defined as the ratio of change in volume to its original volume.
• It is due to the normal forces only.
• It is a dimensionless parameter.

Representation

This means that the volumetric strain of a deformed body is the sum of the linear strains in three mutually perpendicular directions.

• Due to normal forces, there will be dimensional and volumetric changes.
• Due to shear, there will be distortion in shape without a change in volume.
• Stress depends on strain.strain is an independent parameter and stress depends on strain.

This is the complete explanation of all the 3 types of strain in a detailed manner. If you have any doubts, then you can ask us from the comments section.

Let’s discuss some other properties which are listed below…

3.homogeneous Property, Isotropic Property, Orthotropic Property & An-isotropic Property

In order to use any material for the engineering practice, you should have to know all the features related to it.which means we should have to know whether the component has a homogeneous property, Isotropic property, Orthotropic property or An-isotropic property.

So in order to know all these things, I had written an article about the difference between them and was presented below.

Homogeneous Property:

Def: At any point in One Direction, if the properties of the material were the same called Homogeneous Property.

Ex: Wood, Iron, Silver, Copper, Gold, etc.

Isotropic property:

Def: At one point, in any direction, if the properties of the material are the same called isotropic property.

Ex: very fine grained molecules,iron, copper,silver,brass etc.

• As the name of isotropic itself says that Iso is same and Tropic is directional property.
• All homogeneous materials need not be isotropic and vice versa however some homogeneous materials are isotropic also.

Orthotropic Property:

Def: At one point in mutually perpendicular directions, if the properties of the material are different, then it is called as Orthotropic Property.

• It is a perpendicular directional property.

Ex: Layered materials,wood,asbestos, graphite,mica,onion etc.

An-isotropic Property:

• At one point, in any directions, if the properties of the material are different called as an Anisotropic property.

• It is also called as Alletropic property.

Ex: Material with small cracks and voids, cracked glass, cracked metal, etc.

This is the complete explanation about the difference between homogeneous Property, Isotropic Property, Orthotropic Property & Allotropic Property or An-Isotropic Property.

This is the detailed explanation of the Mechanical properties of metals. Hope, this information is helpful to you.

Now we can discuss…

4.Material Selection with Flowcharts:

Material selection plays a vital role in the field of design. In this article, I will be explaining how to do the Material Selection in Mechanical Design with the help of Flowcharts along with the design thinking process with 6 phases of design. Apart from that, I will explain about Spring back in design and Determination of Shear center in the sections.

Flowcharts include design tools, Market Requirements, Material data needs, etc. The detailed explanation about the flowcharts for material selection in Mechanical design is as follows.

Flowchart for Material Selection in Mechanical Design:

The flowchart for Material Selection includes 3 steps which are as follows.

1.Design Tools

2.Market Need

3.Material Data needs

These three combined together and produce Product Specification.

1.Design Tools:

• Function Modelling
• Viability studies
• Approximate Analysis
• Geometric Modelling
• Simulations method
• Cost Modelling
• Component Modelling
• Finite Element Modelling
• DFM and DFA.

2.Market Need:

• Concept
• Embodiment
• Detail

3.Material Data Needs:

• Data for all materials, low precision, and detail.
• Data for a subset of materials with higher precision and detail.
• Data for one material with highest precision and detail.

The connection between these 3 steps as shown in the above figure.

Now, I will be explaining about the Design Thinking process along with the 6 Phases in Design.

5.Design Thinking-6 Phases in Design:

1. Recognition of need
2. Definition of problem
3. Synthesis
4. Analysis & Optimization
5. Evaluation
6. Presentation.

Explanation of 6 Phases in Design:

1.Recognition of need:

The recognition of the need must be checked during designing or planning a program.

One of the examples for Recognition of need is the transportation system in which different aspects are to be considered and are as follows.

Ex: Transportation Equipment

• Faster transportation system
• The transportation system should be Economical i.e initial cost and running cost.
• There should be lesser environmental pollution due to transportation.

2.Definition of the Problem:

All the specifications are to be specified while defining a problem like What is the output for this problem etc.

3.Synthesis, Analysis, and Optimization:

• Firstly, Synthesis is to be done and later Analysis is to be made on the required component so as to analyze the deformations, stresses, strains, etc present in the component.
• If the stress in the structure(which we had defined in the problem)is above the yield point, then it is a case of failure and we have to optimize it so that there should be no failure.
• If the stress induced in the structure is less than the yield point of the material then it is safe. There is no need to further optimize.

Now let’s enter into our 3rd concept i.e. Spring back in design.

6.Spring Back in Design:

At the end of every metalworking operation, when the pressure on the metal is released, then there exists an elastic recovery by the material and the total deformation will get reduced a little called as Spring back in design.

• Spring back depends on yield point strength of the metal.
• Compensation can be made on spring back.

Compensation:

To compensate the spring back, the cold deformation must always be carried beyond the desired limit by an amount equal to spring back.

Let’s enter into our last concept i.e. location of shear center in sections.

How to determine the location of the shear center in the sections?

The shear center is also known as torsional axis, is an imaginary point on a section, where a shear force can be applied without inducing any torsion. In general, the shear center is not the centroid. In this article, I am going to explain how to determine the location of the shear center in the sections in a detailed manner.

The shear center is defined as the point on the beam section where the load is applied and no twisting moment is produced.

• At the shear center, resultant of the internal forces passes.
• For symmetrical sections, the shear center is the center of gravity for those sections.
• We can place the loads at the shear center where there is a sliding problem in those sections.

7.Heat Treatment Process:

Modification of grain size in a steel component by either heating or cooling process so that the strength of the Steel can be changed is called a heat-treatment process.

Every heat treatment process contains three steps

1.Heating

2.Holding and

3.Cooling

1.Heating:

Small grains will combine and form larger grains.

2.Holding:

All grains will turn into uniform shape and size.

3.Cooling:

Based on the rate of cooling, the final grain size is produced.

Note 1:

If a Steel component is heated to a high temperature followed by Rapid cooling process implies large grains will break down into small grains and thereby the hardness increases.

H1<H2

Note 2:

If a Steel component is heated to a high temperature followed by slow cooling process => small grains will combine and forms larger grains implies hardness decreases.

## Types of Heat Treatment Process:

1. Hardening
2. Annealing
3. Normalising
4. Tempering
5. Martempering
6. Austempering
7. Case hardening
8. Surface Hardening

The Various Types of Heat Treatment Processes are discussed below.

1.Hardening:

• Applicable to Steels with C%=(0-2.11)%
• Obtained hardness in steel components by heating followed by rapid cooling process.
• Due to Rapid cooling process or quenching, the component will undergo very fast cooling process the smallest grains will be produced which possess the highest hardness also and it is called as Martensite Phase.
• During the hardening process, the outer surface of the component will come in contact with the Quench medium ⇒ the outer envelope will be converted into martensite immediately ⇒ outer envelope produce martensite Phase.

The other seven types of Heat Treatment Process are presented soon.