Histroy of Dutile Fractures

        Well, Hope everyone is preparing well for the Summer 2011 exams. Here are some of the cool facts about Ductile Fracture. The crack extension energy side of the Griffith equation applied only to "ideally brittle" materials. Believe me, it was not for lack of research that the materials research community failed to extend fracture theory into the very important field of ductile fracture. Some of the problems faced by Humanity due to Ductile fracture are given below:

Ships Break In Two!

This was an extremely serious problem in World War II, when over 250 ships fractured or cracked. Nineteen of these broke completely in two! Luckily, in some cases, fractures occurred in ships that were being outfitted and had never put to sea. All of the ship fractures and the two other examples that follow were in metals that were ductile, but just not tough enough.

The Great Boston Molasses Tank Disaster

One of the most famous brittle fractures was the Great Boston Molasses Tank Disaster in 1919. There was a tank of molasses, 90 ft in diameter and 50 feet high whose contents were supposed to have become rum. When the tank split, a wall of molasses advanced down the street. Many of the deaths and casualties occurred among people who were engulfed in their flats below the level of the street. There were 12 deaths and 40 injuries. Half a century later it was determined that the tank's steel was below its ductile/brittle transition temperature; the same problem as with the WWII merchant ships.

The Silver Bridge Collapse

A more recent brittle fracture disaster was the collapse of the Silver Bridge in West Virginia, in December 1967 in which 46 people perished as their cars plunged into the icy Ohio River. The National Bureau of Standards' metallurgists judged the bridge accident to be caused by stress-corrosion cracking resulting from long exposure to hydrogen sulfide vapor, H2S, from nearby paper mill digesters. The bridge failure is an example where the energy required to extend the fracture was reduced while the metal was in service.

With the benefit of 20/20 hindsight, the ship hull and molasses tank accidents occurred when the steel's energy required to extend the fracture at service temperatures was too low starting when the metal left the steel mills

The above information is taken from http://www.nhml.com/ Please do refer to them for more info. 

Fatigue Fracture

        Fatigue fracture is a fracture that occurs when a material is subjected to cyclic loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture.

        Rotating shafts, connecting rods, aircraft wings and leaf springs are some examples of structural and machine components that are subjected to millions of cycles of alternating stresses during service. Majority of fractures in such components is due to fatigue.

         Fatigue fracture occurs by crack propagation. The crack usually initiates at the surface of the specimen and propagates slowly at first into the interiors. At some critical stage, crack propagation becomes rapid culminating in fracture.

        The fatigue behavior can be understood from results of fatigue test, which are presented in from of S-N curves.  Samples of material are subjected to alternating stresses of different levels. The number of cycles of stress reversals N required to cause fracture is plotted against the applied stress level S. Some materials such as mild steel show a clearly defined fatigue limit. If the applied stress is below the fatigue limit, (aka Endurance Limit) the material will withstand any number of stress reversals. If materials don't show clearly defined limit, the fatigue limit is defined as stress that would cause failure after a specified number of stress reversals.

       The above info is taken from Material Science and Engineering by Raghavan and the picture shown here is taken from http://www.fea-optimization.com/. Please do refer to them for more info. 

Don't forget to grab a copy of Material Science and Engineering.

with warm regards

AllMyPosts

Griffith's Theroy: Mechanism of Brittle Fracture

      It has been observed that the stress required for a material, at which it fractures, is only a small fraction of cohesive strength. This discrepancy led Griffith to suggest that the low observed strengths were due to presence of micro-cracks, which act as the points of stress concentration. 

       According to the Griffith's criterion. the crack will propagate under the effect of a constant applied stress if an incremental increase in length produces no change in total energy of the systems. Mathematically the above criterion is explained as 

A proper explanation of the above theory is given as below by Wikipedia:

Fracture mechanics was developed during World War I by English aeronautical engineer, A. A. Griffith, to explain the failure of brittle materials. Griffith's work was motivated by two contradictory facts:

• The stress needed to fracture bulk glass is around 100 MPa (15,000 psi).
• The theoretical stress needed for breaking atomic bonds is approximately 10,000 MPa (1,500,000 psi).

        A theory was needed to reconcile these conflicting observations. Also, experiments on glass fibers that Griffith himself conducted suggested that the fracture stress increases as the fiber diameter decreases. Hence the uniaxial tensile strength, which had been used extensively to predict material failure before Griffith, could not be a specimen-independent material property. Griffith suggested that the low fracture strength observed in experiments, as well as the size-dependence of strength, was due to the presence of microscopic flaws in the bulk material.

      To verify the flaw hypothesis, Griffith introduced an artificial flaw in his experimental specimens. The artificial flaw was in the form of a surface crack which was much larger than other flaws in a specimen. The experiments showed that the product of the square root of the flaw length (a) and the stress at fracture (σ_f) was nearly constant, which is expressed by the equation:

An explanation of this relation in terms of linear elasticity theory is problematic. Linear elasticity theory predicts that stress (and hence the strain) at the tip of a sharp flaw in a linear elastic material is infinite. To avoid that problem, Griffith developed a thermodynamic approach to explain the relation that he observed.

      The growth of a crack requires the creation of two new surfaces and hence an increase in the surface energy. Griffith found an expression for the constant C in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the approach was:

• Compute the potential energy stored in a perfect specimen under an uni-axial tensile load.
• Fix the boundary so that the applied load does no work and then introduce a crack into the specimen. The crack relaxes the stress and hence reduces the elastic energy near the crack faces. On the other hand, the crack increases the total surface energy of the specimen.
• Compute the change in the free energy (surface energy − elastic energy) as a function of the crack length. Failure occurs when the free energy attains a peak value at a critical crack length, beyond which the free energy decreases by increasing the crack length, i.e. by causing fracture. Using this procedure, Griffith found that

where E is the Young's modulus of the material and γ is the surface energy density of the material. Assuming Eγ = 1 J/m² gives excellent agreement of Griffith's predicted fracture stress with experimental results for glass. = 62 GPa and

The above information is taken from Wikipedia. Please do refer to them for more info. Don't forget to review this copy of Material Science and Engineering book, which has info for all the syllabus of AMIE material science.

with warm regards

AllMyPosts

Brittle Fracture

In brittle fracture, no apparent plastic deformation takes place before fracture. In brittle crystalline materials, fracture can occur by cleavage as the result of tensile stress acting normal to crystallographic planes with low bonding (cleavage planes). In amorphous solids, by contrast, the lack of a crystalline structure results in a conchoidal fracture, with cracks proceeding normal to the applied tension.

The theoretical strength of a crystalline material is (roughly)

where: -

E is the Young's modulus of the material,

γ is the surface energy, and

r_o is the equilibrium distance between atomic centers.

On the other hand, a crack introduces a stress concentration modeled by

(For sharp cracks)

where: -

σ_applied is the loading stress,

a is half the length of the crack, and

ρ is the radius of curvature at the crack tip.

Putting these two equations together, we get

Looking closely, we can see that sharp cracks (small ρ) and large defects (large a) both lower the fracture strength of the material.

Recently, scientists have discovered supersonic fracture, the phenomenon of crack motion faster than the speed of sound in a material. This phenomenon was recently also verified by experiment of fracture in rubber-like materials.

The above info is taken from Wikipedia and from www.ubstech.com. Please do refer to the same 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 reagards

AllMyPosts

Brittle Fracture VS Ductile Fracture

Brittle Fracture:

• Caused due to high impact blows on the material
• Plastic deformation is zero or very very less
• Once crack is formed,  the crack is unstable in nature and propagates very rapidly.
• Crack propagates nearly perpendicular to the direction of the applied stress
• Crack often propagates by cleavage - breaking of atomic bonds along specific crystallographic planes (cleavage planes).

Ductile Fracture:

• Caused due to tensile forces acting on the material
• Necking can be observed i.e. excessive plastic deformation takes place
• Crack is stable i.e. once crack is formed, it resists propagation unless further stress is applied
• Micro-voids are formed fist, then by shear forces the crack propagates and results in fracture

Good material about about Fractures is available here & here

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

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

with warm regards

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.

with warm regards

AllMyPosts

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
AllMyPosts

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

AllMyPosts

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
AllMyPosts 

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

 with warm regards

AllmYPosts

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

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

with warm regards

AllMyPosts

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

with warm regards

AllMyPosts

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

with warm regards

AllMyPosts

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.

with warm regards

allmyposts

Eutectic Systems

       A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature. On a phase diagram the intersection of the eutectic temperature and the eutectic composition gives the eutectic point. Not all binary alloys have a eutectic point; for example, in the silver-gold system the melt temperature (liquidus) and freeze temperature (solidus) both increase monotonically as the mix changes from pure silver to pure gold.

The eutectic reaction is defined as follows:

        This type of reaction is an invariant reaction, because it is in thermal equilibrium; another way to define this is the Gibbs free energy equals zero. Tangibly, this means the liquid and two solid solutions all coexist at the same time and are in chemical equilibrium. There is also a thermal arrest for the duration of the reaction.

        The resulting solid macrostructure from a eutectic reaction depends on a few factors. The most important factor is how the two solid solutions nucleate and grow. The most common structure is a lamellar structure, but other possible structures include rodlike, globular, and acicula

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

with warm regards

allmyposts

Presentation on Binary Isomorphous System

I was surfing the web for some good presentation on Binary Isomorhous Systems. I found one here. I am embedding the same here:

with warm regards

almyposts

Eutectic reaction

Eutectic reaction:

A three-phase reaction in which, upon cooling, a liquid transforms to give two solid phases. 

e.g:                                 L ® α + b

The above information is taken from here

Tie Line

Tie Line:

An imaginary horizontal line (isotherm) spanning a two-phase region of an equilibrium phase diagram, terminating at the nearest phase boundaries on either side.

 Tie lines are important when using phase diagrams to predict the constitution of two-phase materials.

The above information is taken from here. 

Old Questions, Material Science, Chap 3

      I understand that AMIE is tough and its not enough to just read the study material. So I decided to scan through the old question papers. As of now I am studying the second chapter ( phase diagrams ). So I segregated the questions in this chapter for the benefit of all. Please do go through them and decide on what all topics to be studied for the exams.

with warm regards
Abhishek Boinapalli