Tensile Test, Part Two

Please read the previous part of this article here

The typical stress strain curve for ductile material is given here: 

The points to note from this picture are:

Elastic Limit

  The material is elastic till this point in the curve. The stress strain ratio is constant when the curve is linear within this zone. The material deforms in this zone but regains original shape and size once the load is removed. The working stress is always much below the Elastic Limit

Yield Point:

   Plastic deformation happens at this point. The deformation is permanent in nature and the original shape and size are not restored once the load is removed.

Creep:

   A small amount of creep may come into play due to sudden elongation of material. This effect of creep is not shown in this picture. Creep usually appears for negligible time and is not taken into account.

Ultimate Strength:

    Stress is necessary to obtain stain from the yield point onwards. Ultimate tensile strength (UTS), is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. This is the highest point in the curve

Fracture Point:

    Once the Ultimate stress is crossed, the material starts necking (i.e. non-uniform reduction in area of cross section in specimen). The material then breaks apart.

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The picture shown is taken from http://invsee.asu.edu/. Please do contact them for more info

Tensile Test, Part One

Tensile Test:

    Tensile test is a simple test, wherein the specimen in question is subjected to uni-axial load (pulled apart) till failure. This test is used to plot the stress - strain curve there by coming to conclusion about

• Yield point
• Elasticity limit
• Point of  Proportionality
• and lot more factors including, true breaking stress, fracture point load, ...

 

   A sample of specimen is taken, and is pulled apart in apparatus known as Universal testing machine. The length and cross section area of sample are decided as per our needs. Nomenclature of the specimen is shown in figure.

    Once the equipment is set up, the load on the specimen is gradually increased noting down the stress and strain levels till the point of rupture (i.e. fracture). A typical curve for ductile materials is shown here:

In figure,

the points to be noted include: 

• Proportionality zone .. i.e the zone where HOOK's law is valid
• The region where elasticity is exhibited
• The zone where material yields and plastic deformation happens
• The ultimate tensile strength & the uniform elongation of specimen till then
• The fracture point and the local necking which happens before fracture

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PS:  Some info has been taken from Wikipedia and from http://invsee.asu.edu/. Please do refer to them for more info

Continue reading this article here

Elasticity and Plasticity

Elasticity:

       External loads tend to deform materials from their original shape and size. Elasticity is the ability of a material to return to its original shape and size after removing the load applied. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied to the object, and disappears once the force is removed.This is so because the atoms in the metal change their position due to external stress but can't take new positions because the change of position is too small (or in acceptable range)

     Steels and other such materials are elastic over good range.

Plasticity:

     External loads tend to deform materials from their original shape and size. Plasticity is the ability of a material to retain the deformation even after external load is removed.In plastic deformation, the atoms in the material due to external force are displaced and take up new positions. They cannot come back to their natural positions once force is removed.

     This is very desirable property in machine tools. Lead has good plasticity at room temperature. Cast Iron has no plasticity even at very high temperature.    

PS: Some of the info here is taken from science.jrank.org & from Wikipedia. Please do refer to them for further info

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Poisson's Ratio

Poisson's ratio (ν), is the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to the applied load), to the extension or axial strain (in the direction of the applied load).

When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson's ratio ν (nu) is a measure of the Poisson effect

In the above picture, the stress is acting in X axis, but change in object is evident in Y and Z axes also. Poisson's effect is all about this change and Poisson's ratio is a measure of this effect and is given by 

For ideal material, the ratio is 0.5. But in general it ranges from 0.25 to 0.40.

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PS: Some of the info here is taken from Wikipedia. Please do consult them for more info

What are Mechanical Properties of Materials??

Hello Everyone,

From what I studied, I can come to conclusion that:

External loads are always applied on Materials during their service. The properties which describe the re-action of a material to those external loads are all classified as Mechanical Properties of materials.

It is important to ascertain the mechanical properties of material with standard laboratory tests in which the loads on the materials in real environment  are applied. This lets us determine behavior of materials and ensure we choose the right materials

 The important Mechanical Properties include:

• Ductility
• Hardness
• Elasticity and Plasticity
• Poission's ratio
• Creep
• Stress and Strain W.R.T Tensile stress, Shear Stress

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Mechanical Properties of Materials

Hello Everyone,

  Well I am a little tensed. Exam dates have been released. My preparation till now amounts to almost nothing and the pressure is huge. So started serious study from today morning itself.

  So stated with chapter called as Mechanical Properties of Materials. So the important topics to be studied here include:

• Tensile strength
• Poission's Ratio
• Yield strength
• Elastic and viscoelastic properties
• Creep, stress relaxation and impact.
• Fracture behaviour.
• Ductile fracture
• Griffith theory
• effect of heat treatment and temperature on properties of metals.

Well I stated at-last. Will see what is gonna happen at the D-Day

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Summer Schedule for 2011

The IEI has released dates for the examinations of Summer 2011. Please do find the same here:

Amie 2011 Summer Tt

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Question about Degrees of Freedom

Hello Everyone,

For those who are new here, this blog is about AMIE study materials, info & notes.Currently I am going through Material Science. So all I post here are related to Material Science for next few days.

I have been going through old question papers, wherein there is a peculiar question which can be stated as "Calculate degree's of freedom in water and ice under 1 atm pressure?".

I have some difficulty in answering this question. Please help me by telling me which among the following is right?

Answer 1)

Gibbs phase rule says F = C - P + 2. Here C = 1, P = 2 at 1 atm pressure hence F = 1

Answer 2)

Gibbs phase rule says F = C - P + 2 but since pressure is constant the formula becomes F = C - P + 1 hence F = 0 with C = 1 & P = 2. 

I am not sure of the answer. Please do help me to decide on the same by suggesting your answer and proof if any.

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