Material Science & Engineering : Mechanical Properties of Materials

Here in this article we are providing study notes on Mechanical Properties of Material

Mechanical Properties of Materials and their Testing:

Mechanical properties help in finding the material’s behavior when subjected to mechanical stresses. The Properties include elastic modulus, ductility, hardness, and various measures of strength. Mechanical properties desirable to the designer, such as high strength, usually make manufacturing more difficult.
Testing of materials is a very important part of the from the point of view of design and manufacturing. It provides the information of material properties, helps in ensuring quality, helps in preventing failures, and also helps to make choices among different available materials.

Mechanical Properties of Materials:

(i) Ductility:

It is that properly of material due to which a metal piece can be drawn into wires of the thin section under tensioning effect.
Ductile materials are those which have at least 5% past elastic strain before fracture.
Tensile test is performed for finding the ductility.
Tensile testing:
This test is performed on the universal testing machines (UTM). In this test, the specimen is subjected to uniaxial tensile force in a controlled way until its failure. This test helps us in accessing the following properties ductility, yield strength, tensile strength, Young's modulus (E), and Poisson's ratio (μ).
Select the standard specimen and grip it in the crossheads with proper adjustment. While setting up the job, use the adjusting knob to make zero at lower points to zero to remove the dead weight of the lower table. Now, lock the job, fix the extensometer between the gauge length ( to find the extension), and apply the
Ductility: It relates the elongation during the tensile test of the material and it is defined as the percentage elongation.
Tensile strength: It is defined as the maximum load per unit cross-section area which the material can bear before breaking. It is given as:
Yield Strength: It is the strength of the material above which permanent deformation takes place in the material under stress.
Young's Modulus (E): It is also known as the modulus of elasticity (E) of the material and it represents the stiffness of the material. It is the measure of the regain of shape and size of the material on the removal of the load.

(ii). Brittleness: Brittleness

It is the property of the material by virtue of which it breaks without significant plastic deformation, when subjected to stress.
It is the lack of the ductility. Such metals don’t show necking before fracture.
Compression Test:
A compression test is also carried on the universal testing machine (UTM). Here, the load applied is compressive in nature and specimen is loaded till it fails.
The compression test is generally carried out for the Brittle materials.

(iii). Malleability:

It is that property of metal due to which a metal can be drawn into a thin sheet of negligible section by pressing/forging through the compression process.

(iv). Proof stress:

A proof stress is a level of stress at which a material undergoes plastic deformation.
More specifically, the proof stress is often defined as the point when the material undergoes an amount of plastic deformation equal to 0.2 percent.

(v). Elasticity:

It is that property of metals due to which original dimensions will be recovered offer loading within elastic limits the stress-strain curve may be linear or non-linear.

(vi) Resilience or Proof resilience:

Resilience is the ability of a material to absorb energy when it is deformed elastically and release that energy upon unloading. It is area under the load vs deformation curve within the elastic limit.
The modulus of resilience is defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion. It is area under the load vs deformation curve up-to elastic limit per unit volume which is same as area under the stress vs strain curve up to elastic limit.

(vii). Toughness:

It is the ability of a material to absorb energy and plastically deform without fracturing. Toughness is area under the load vs deformation curve up to fracture point.
Modulus of toughness: The modulus of toughness is the amount of strain energy per unit volume (i.e. strain energy density) that a material can absorb just before it fractures. The modulus of toughness is calculated as the area under the stress-strain curve up to the fracture point.
There are two types of impact tests named Izod and Charpy tests.
(1). Izod test:
- For Izod impact testing, the specimen is kept vertically as a cantilever beam. The specimen is kept in such a way that the notch side faces the striking hammer.
(2). Charpy test:
The Charpy test shows whether a metal is either brittle or ductile and it is used for predicting ductile to brittle transition.
In the Charpy test, the specimen is placed horizontally and fixed at both ends i.e. it is a simply supported beam. Striking hammer strikes from the opposite side of the notch.

(viii). Hardness:

Hardness is defined as the resistance of a material to local plastic deformation achieved from indentation of a predetermined geometry indenter onto a flat surface of metal under a predetermined load.
Hardness test are as follows:
1. Brinell Hardness test: In Brinell hardness test a steel or tungsten carbide ball is used to make a impression in the material under a specified load.

2.Vickers Hardness test: It uses the pyramid indenter of square shape and length of the diagonals of the indentation is measured to calculate the hardness number. It is suitable for very hard and tough materials.

Vickers hardness number is given by:

Where F is the applied force (in kg) and D is the average diameter of the diagonals measured.
3. Rockwell Hardness test: Rockwell test sues the diamond cone-shaped or spherical ball type of indenter for the indentation purpose. There are many scales in the Rockwell testing but C scale is the most commonly used scale and hardness on it is denoted as HRC.

Type of Metal Behavior:

(ix). Creep:

Creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.
It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Factors affecting creep are
(i). Magnitude of load
(ii). Type of loading (static or dynamic)
(iii). Time or age

(x). Fatigue:

Fatigue strength is the highest stress that a material can withstand for a given number of cycles without breaking.
Fatigue strength is affected by environmental factors, such as corrosion, wear, pitting etc.
The maximum stress that can be applied for a certain number of cycles without fracture is the fatigue strength.
Endurance limit:

An endurance or fatigue limit which is defined as the maximum stress below which the steel could presumably endure an infinite number of cycles without failure.
A simple rule of thumb calculation for the fatigue limit is one-half of the ultimate tensile strength.
Fatigue Test:
Fatigue is the permanent failure of the material due to fluctuating stresses and failure takes place below the yield point of the metal. The number of cycles at which failure occurs is measured and these can vary from a couple of hundreds to millions of cycles.

The failure of the specimen under rotating loading is termed fatigue failure. Rotating loading results in completely reversed stresses.
The results of the fatigue test are plotted as an S–N curve which is the graphical representation of stress amplitude and the number of stress cycles (N) before the fatigue failure on a log-log graph paper.

Fracture in Materials:

Ductile Fracture: A ductile fracture is a type of fracture characterized by extensive local plastic deformation i.e. necking. This usually occurs prior to the actual fracture.

Brittle Fracture: Brittle fractures occur with no apparent deformation before fracture; ductile fractures occur when visible deformation does occur before separation.

Cause of Brittle fracture:

(i). Notches

(ii). Laps, folds, flakes, large inclusions

(iii). Segregation, inclusions, undesirable microstructures, porosity, tears, cracks.

(iv). Cracks resulting from machining, quenching, fatigue, hydrogen embrittlement, liquid metal embrittlement.

(v). Residual stresses.

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