Impact hardness

  We have been speaking about tensile toughness. Tensile toughness can be defined as the resistance offered by material to plastic deformation i.e. the ability to resit indentation and penetration or abrasion. Here, the load is applied slowly and the strain rate is quite slow too.

But in real life materials are also subjected to sudden blows. The resistance offered by materials to such blows (or impacts) can be called as impact toughness.

Hard, strong materials with good tensile toughness also falter under sudden impacts and exhibit brittle nature and undergo brittle fracture. The brittleness of materials and the reliability of materials under impacts can be studied using Charpy test and Izod test.

The tests are described in the further sections of this blog. Please do take time to go through the same.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Rockwell Hardness Test

    The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load (60Kgf - 200Kgf) compared to the penetration made by a preload (10Kgf). There are different scales, which are denoted by a single letter, that use different loads or indenters. The result, which is a dimensionless number, is noted by HRX where X is the scale letter.

      The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load, and then noting the depth of penetration, vis a vis, hardness value directly from a dial, in which a harder material gives a higher number. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques. 

This method is widely used in Industry as the reading is available easily & quickly. 

The above info is taken from Wikipedia. Please do refer to them for more info. 

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Knoop Hardness Test

      The Knoop hardness test  is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes

    A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. 

The Knoop hardness HK or KHN is then given by the formula:

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, ideally 0.070279

P = load

        The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

       The above information is taken from Wikipedia. Please do visit eh same for more information.

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Brinell Hardness Test

       The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. 

     The objective of harness test is define the hardness number which represents an arbitrary quantity used to provide a relative idea of material properties. The hardness number derived in this test is called Brinell harness number and is designated as BHN

      For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation. The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. 

      The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation. The formula is shown in the picture shown below.

Where F = Force applied in kgF
          D = diameter of indenter
       

     This method is not used in industry since it is quite slow, deforms the specimen excessively and requires setup to calculate the depth of the indentaion..

    The above information has been taken from www.gordonengland.co.uk. Please do refer to them for more info.

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Hardness tests of materials

Hardness of a material refers to the resitance the material offers to permanent plastic deformation when an external force is applied. Wikipedia defines it as the measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied. 

In view of syllabus of material science, Impact hardness tests are important and necessary. Impact hardness refers to resitance offered by material when the force applied is impact in nature i.e. for short period with high magnitude. 

The four important tests covered in the syllabus are

• Rockwell Hardness test 
• Knoop Hardness test
•  Vickets hardness test
• Brinell's Hardness test

The hardness tests are performed since 

• They are easy, simple 
• The set up is in-expensive
• The test doesn't damage the entire specimen. Usually small specimen is sufficient
• Other physical properties can be told from this tests

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Abhishek Boinapalli 

Resilience of Material

Hello Everyone,

   Every wondered why objects like spring give back energy when they uncoil?? Well one of the reasons fro this behavior is resilience of material with which spring is manufactured. 

   Resilience of material is the ability of it to absorb energy when deformed elastically due to applied stress and return the energy back when unloaded. 

    Modulus of Resilience is the measure of this property and as per the wikipedia,  Modulus of Resilience can be calculated using the following formula: , where σ_y is yield stress, E is Young's modulus, and is strain.

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Stress Strain Curve, Mild Steel

Hello Everyone,

Those who are familiar with concept of Stress - Strain curve, please do continue with this post to understand about upper yeild strenght and lower yield strenght for mild steel. Those who are not familiar please take time to go through this article

From the image given here taken from etomica.org it is clear that materials like mild steel have two yield strenghs. The first called upper yield strenght and the second called lower yield strength.

Once the stress reaches the upper yield strength, the internal relaxation comes into play and the strain can be observed even at lower amount of stress. The stain is bound to osciallte between both the limits. The lower yield strength is about half the tensile strength of the material.  The explaination can be summarized as follows

   "At elastic limit, sudden yield happens & fall-off of load takes place. Hence material continues to defrom at lower load until material hardening sets in"

Answers.com says the reason for such behavior is Low carbon steels suffer from yield-point runout where the material has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drops dramatically after the upper yield point. If a low carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop Lüder bands.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Tensile Toughness

Toughness:

    Energy observed by material prior to fracturing is called toughness. It depends on both strength and ductility of the material in question. A pic from www.etomica.org is given below to show the relationship the three entities in question viz tensile toughness, ductility and strength.

 

From the figure, it can be concluded that tensile toughness is the are under the stress - strain curve. It is high if a material has high amount of strength and ductility. Materials with low ductility of low strength don't posses ample tensile toughness. 

The word toughness is usually used for tensile toughness.  In tesile toughness, the strain rate is relatively slow. There is another type of toughness called as impact toughness. Please do read about it here to understand the difference.

This post is made from the study material provided by IEI and from the www.etomica.org. Please do refer to them for more info

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards
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True Stress Vs Engg. Stress

Engineering stress assumes that the area a force is acting upon remains constant, true stress takes into account the variation in the cross sectional area as a result of the stress induced deformation (strain) of a material.

For example a steel bar in tension once it's yield point or stress is reached will start to "neck". Necking is the localised concentration of strain in a small region of the material, causing a reduction in cross sectional area at this point.

To calculate the engineering stress in the above case, the applied load is divided by the original cross sectional area, however the true stress would be equal to the load divided by the new deformed cross sectional area. Therefore true stress is likely to be significantly higher than engineering stress. Note that while the material is deforming elastically before thwe yield point is reached there will be some difference between true and enginnering stress (as the material is changing shape) but it will be much smaller than the difference after the yield point is reached.

A rock core in a uniaxial compression test will typically expand radially under loading. Therefore in this case, the engineering stress (based on the original diameter) will be larger than the true stress within the material.

The above info has been taken from Answers.com. Please do refer to them for more info.

Status Chapter 02, Defects in solids

Well Hello Everyone,

Hope your preparation for AMIE is going on at good pace. I studied a little about Crystal Defects Earlier and am posting notes here. The articles I posted here related to this chapter include:

• Question about Degrees of Freedom
• Cool PPT on Crystal Defects
• No. Of Atoms in Zinc Unit Cell
• Old Questions. Material Science. Chap. 02.
• Atomic Packing Factor
• Diffusion In Solids
• Line defects and Surface defects
• Point Defects in Crystal

• Status Chap 02
• Simple, Body Centered & Face Centered Cubic System..
• Crystal Systems. Bravias Lattices.
• Miller Index
• Prerequisites for understanding defects in crysta...
• Chap. 02. Defects in Crystals.

There is lot more to be covered. And all of it shall be done soon since the exams are fast approaching.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

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Status Chapter 03, Phase Diagrams

Well hello everyone,

   I worked a little on chapter Phase Diagrams. I have put up some posts related to the same on this blog. The posts mentioned below

• Introduction
• Basics of Phase diagrams
• Eutectic reaction
• Tie Line
• Old Questions, Material Science, Chap 3
• Phase diagrams & Lever Rule
• Triple Points & Gibbs Rule
• Presentation on Binary Isomorphous System
• Eutectic Systems

There is lot more to be covered and shall do the same at the earliest. The schedule for exams is out also. So gonna hurry up from now on.

Hope your preparation is going on at good pace

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Determination of yield Strength

Hello Everyone,

  In the previous articles, I told what is yield strength is? Now how to determine it is always a problem. Many a ductile materials get deformed (elastic and plastic). But the boundaries of deformation cannot be strictly defined due to hell lot of reasons. 

   So the Americans devised a plan to find out the yield strength. They define the same as the stress at which a predetermined amount of permanent deformation occurs. To find yield strength, the predetermined amount of permanent strain is set along the strain axis of the graph, to the right of the origin (zero). It is indicated in Figure as Point (D).

 

A straight line is drawn through Point (D) at the same slope as the initial portion of the stress-strain curve. The point of intersection of the new line and the stress-strain curve is projected to the stress axis. The stress value, in pounds per square inch, is the yield strength. It is indicated in Figure 5 as Point 3. This method of plotting is done for the purpose of subtracting the elastic strain from the total strain, leaving the predetermined "permanent offset" as a remainder. When yield strength is reported, the amount of offset used in the determination should be stated. For example, "Yield Strength (at 0.2% offset) = 51,200 psi."

 Notes for the above article is taken from www.engineersedge.com. Please do refer to them for more info

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards
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Relation between E, G and Poisson's Ratio

The definite relationship between Young's modulus, Shear modulus & Poissons ratio  is asked many a times in our old question papers though for two marks only.

So I thought I will put up the answer here:

Let young's modulus = E, Shear modulus = G, Bulk Modulus = K and

poisson's ratio = v

E = 3K(1-2v)

E = 2G(1+v)

the above relationship is taken from answers.com. Please do refer to them for further info.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

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Engg. Stress VS True Stress

     Engineering stress assumes that the area a force is acting upon remains constant, true stress takes into account the variation in the cross sectional area as a result of the stress induced deformation (strain) of a material.

For example a steel bar in tension once it's yield point or stress is reached will start to "neck". Necking is the localised concentration of strain in a small region of the material, causing a reduction in cross sectional area at this point.

To calculate the engineering stress in the above case, the applied load is divided by the original cross sectional area, however the true stress would be equal to the load divided by the new deformed cross sectional area. Therefore true stress is likely to be significantly higher than engineering stress. Note that while the material is deforming elastically before thwe yield point is reached there will be some difference between true and enginnering stress (as the material is changing shape) but it will be much smaller than the difference after the yield point is reached.

A rock core in a uniaxial compression test will typically expand radially under loading. Therefore in this case, the engineering stress (based on the original diameter) will be larger than the true stress within the material.

The above info is taken from Answers.com. Please do refer to them for more info

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

AllMyPosts

Stress And Strain

Stress

Stress is defined as "force per area".

Direct Stress or Normal Stress

Stress normal to the plane is usually denoted "normal stress" and can be expressed as

σ = F_n / A         (1)
where
σ = normal stress ((Pa) N/m², psi)
F_n = normal component force (N, lb_f)
A = area (m², in²)

Shear Stress

Stress parallel to the plane is usually denoted "shear stress" and can be expressed as

τ = F_p / A         (2)
where
τ = shear stress ((Pa) N/m², psi)
F_p = parallel component force (N, lb_f)
A = area (m², in²)

Strain

Strain is defined as "deformation of a solid due to stress" and can be expressed as

ε = dl / l_o = σ / E         (3)
where
dl = change of length (m, in)
l_o = initial length (m, in)
ε = unitless measure of engineering strain
E = Young's modulus (Modulus of Elasticity) (Pa, psi)

Hooke's Law -  Modulus of Elasticity (Young's Modulus or Tensile Modulus)

Most metals have deformations that are proportional with the imposed loads over a range of loads. Stress is proportional to load and strain is proportional to deformation expressed by the Hooke's law like

E = stress / strain = (F_n / A) / (dl / l_o)         (4)
where
E = Young's modulus (N/m²) (lb/in², psi)

Modulus of Elasticity or Young's Modulus are commonly used for metals and metal alloys and expressed in terms 10⁶ lb_f/in², N/m² or Pa. Tensile modulus are often used for plastics and expressed in terms 10⁵ lb_f/in² or  GPa


Please note: The above article is taken from www.engineeringtoolbox.com. Please do refer to them for further info.


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