Group members:Mohsin Ali MarwatShakil Ahmed AfridiHaider Ali AnsariQasim ZiaShahid Naseer

I n s ti t u t e O f S p a c e T e c h n o l o g y

Mechanics of Material

Experiment-01

Fatigue test of Aluminum Specimen

Objective:

The main objectives of fatigue are:

To know the life of material by using apparatus Fatigue Tester MT 3012 i.e. how much time will it take to fracture or become useless.

To know how much stress a material can with stand without fracturing. Material selection i.e. high strength of material are used to support high loads

and lower strength materials are used to support lesser loads To categories the material on the basis of strength to number of rotations by

using their S-N graphs. To know that from where actually the fracture started and where it ended.

Abstract:

A method for determining the behavior of materials under fluctuating loads. A specified mean load (which may be zero) and an alternating load are applied to a specimen and the number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. Data from fatigue testing often are presented in an S-N diagram which is a plot of the number of cycles required to cause failure in a specimen against the amplitude of the cyclical stress developed. The cyclical stress represented may be stress amplitude, maximum stress or minimum stress. Each curve in the diagram represents a constant mean stress.

Fatigue test gives us information about the selection of material. For example we will use high elastic materials if we want to support high loads. Similarly for less loading supports we will use lower elastic materials.

In this experiment we tested two specimens with different load application. We came to know that greater the load on the specimen in fatigue tester, lesser will be the number of cycles of specimen, larger will be the Static area of fracture piece and lower will be the fatigue life of that material. Similarly if the force/load applied on the specimen is smaller, greater will be the number of rotations, lesser will be the Static area and higher

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will be the fatigue life of that material. Thus we came to know that if load that is applied on the bridges, motor cars and some beams etc is lesser than the endurance limit then it will have greater fatigue life. Similarly if load applied to the system is larger than its endurance limit then a permanent change in a material occurs that is the fatigue life of that material becomes smaller.

Apparatus:

Fatigue Tester 3012, Aluminum Specimen.

Zero Button Counter 7 digits Locknut Loading control

Specimen

Spring Balance Micro switch Reset Button

Main Parts of Machine

Specimen Load Control Spring balance Lock nut Micro switch Reset button Counter 7 digits Zero button

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Keywords:

Fatigue

Stress

Specimen

Ultimate tensile strength

Endurance limit

Fatigue life

S-N curves

Residual stresses

Theory:

Testing of material is very important in material science and engineering. The day by

day growth of high strength materials is due to their study and testing. Fatigue test is

also one of the known tests that fulfill the needs of modern world. We can give the

approximate mathematical life of material. With the help of Fatigue Test we are able to

use proper material in suitable place. For example we used steel rods in the suspension

bridge to get high strength. If we would have used iron in place of it we would have

fatigue failure very soon because of the smaller magnitude of elasticity.

In fatigue test we apply high revolutions per second along with the application of force to

study the fatigue life of that material. The revolutions and force together make the

specimen to fracture. Once the fracture starts then it propagates like bacteria, a small

revolution and force application makes the specimen to break. If the edge length of a

fracture is sharp and narrow then it may be assumed that specimen will break soon but

if fracture edge length is not sharp then it will have some greater resistance towards

breaking. Formula written below gives the evidence for this conclusion

k1c = Yбf (Πa)1/2

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Fig: Crack propagation in the grain boundaries

Edge length

Fig: Fracture of a material having sharp edge length

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Stopping Cracks:

There are three stages of cracks i.e. starting, propagating and sudden crack. Once

fracture starts then it propagates to avoid this propagation we make two holes with the

help of a drill one near the point from where fracture starts and other near the point

where it finishes as shown in figure on next page. It is not recommended as the perfect

process for avoiding cracks as holes may result in the starting of new cracks. It is far

better to replace the cracked part entirely.

Fatigue Dependency:

There are some factors that affect fatigue life which are given below:

Type of material used.

Geometry of a material

Surface quality (polished surfaces have greater fatigue life than machined ones)

Direction in which load is applied

Environment effects (corrosion and erosion etc)

Size and distribution of internal defects

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Residual Stresses

Grain size (smaller grain size have greater fatigue life)

Temperature

There is a limit for material below which no fatigue failure can occur but if that limit is

accessed then fatigue failure starts. This limit is known as fatigue limit or Endurance

Limit.

Some Fatigue Failures

The 1968 Los Angeles Airways Flight 417 lost one of it's main rotor blades due

to fatigue failure.

The 1985 Japan Airlines Flight 123 crashed after the aircraft lost its vertical

stabilizer due to faulty repairs on the rear bulkhead.

The 1988 Aloha Airlines Flight 243 suffered an explosive decompression due to

fatigue failure.

The 1989 United Airlines Flight 232 lost its tail engine due to fatigue failure.

S-N Graphs

It is the physical representation between stress and number of rotations. Number of

rotations is independent which we take on x-axis while stress magnitude follows the

y-axis of S-N Graphs. Greater the number of rotations lesser will be fatigue life same is

the case greater the load application to the specimen lesser will be the time of

sustainability for a specimen. When the motor starts magnitude of load is greater as the

time proceeds load decreases and after some time sudden and abrupt change (zero) in

load occurs that is because of the reason that specimen last part is separated from its

entire body.

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Methodology:

Procedure:

Fatigue test involved the following steps while performing:

First of all we locked our specimen in the fatigue tester 3012-E machine.

Then we adjust the position of a specimen in the machine i.e. added up to the

limit desired.

After that we reset the load and stop watch to get accurate and precise readings.

After doing so we started machine which took some time to break (crack) the

specimen.

The time that machine took to cut a specimen into two pieces was noted.

Machine was then stopped and cut specimen was taken off.

The cut region was seen under compound microscope.

The end readings are then noted out.

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Calculations:

Rpm of fatigue testing machine = 3000 rev/min

No of cycles can be evaluated from the following formula:

No of Cycles = (rpm of motor) x (time in minutes)

= (3000rev/min) x (min)

Thus by multiplying the time of different specimens No of cycles are obtained as shown in the table.

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Readings:

S. No Limit Force

Original Force

Final Force

RPM of Motor

Time No of Cycles

units F(N) F(N) F(N) Rev/min T(min) No unit

1. 50 179 170 3000 5’16’’ 15850

2. 50 200 192 3000 2’36’’ 7200

3. 50 190 170 3000 6’1’’ 18050

Graphs:

Fig: S-N Graph for Specimen No-01

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Fig: S-N Graph for Specimen No-02

Result

After seeing the cut region of specimen under the microscope we observed that greater the force on the specimen greater will be the static area which in turn shows us that specimen will fracture soon. Similarly if force applied is smaller, then the specimen static area will also be smaller which shows that it will take greater time to fracture as compared to the previous one.

Observations:

There are two regions that appeared under microscope I.e. Static region and Fatigue region.

Static region: Black/dark central area

Fatigue region: Outer affected area due to cyclic loading

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Fig: Fractured Specimen due to Fatigue

Static Area Fatigue Area

Conclusion:

From “Fatigue Test” we may conclude that “The one and Only Allah is the perfect”. Everything in this world has certain limit after which it will become useless i.e. fracture or damage. Moreover constantly applied forces make material to lose their life. In addition to it we may also conclude that there is a limit of force as well. If material is acted with cyclic loadings under the sustainability limit of a material it will have greater fatigue life but if we apply greater cyclic loading than the sustainability limit of a material then it will have smaller fatigue life.

Sources of Errors

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Due to rude loading and array of specimen there are chances of error. Error is also possible due to real load that we supply to the specimen and the

load shown on the screen. Error is also possible due to the time difference that we note and the time of

breakage.

General precautions:

The specimen should be bolt tightly in chops to get true reading. You should be attentive during experiment to note the reading accurately. Do not touch the machine during operation. Put proper dress during experiment.

Safety Precautions:

Do not put the specimen immediately after it is broken because it may burn your skin due to hotness.

Make sure that the electric connections given to the machine are operating safely.

Make sure that safety cover is put on the specimen. Stand at some distance from the machine after that when it is started

Suggestions For Improvement:

If time gauge is installed on TERCO MT 3012 software then we have no need of time stopper.

When we start the fatigue test machine it has too much noise so we need some noise absorption tools which would decrease the noise pollution.

Definitions Of Keywords

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Fatigue:

Fatigue is the progressive and localized structural damage that occurs when a material

is subjected to cyclic loading.

Stress:

The average amount of force exerted per unit area. It is the internal resistance a

material offers to being deformed and is measured in terms of the applied load.

Specimen:

A specimen is a portion or quantity of material for use in testing, examination, or study

Ultimate tensile strength:

The maximum stress that a material can withstand while being stretched or pulled

before necking is called Ultimate tensile stress.

Endurance Limit:

The stress level below which a specimen withstand cyclic stress indefinitely without

exhibiting fatigue failure.

Static Area:

Black/dark central area of a specimen after it has been cut off due to cyclic loading.

Fatigue Area:

Outer affected area of specimen after it has been cut off due to cyclic loading.

Fatigue life:

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The number of cycles of stress that can be sustained prior to failure under a stated test

condition.

OR

The number of applications of a given stress to which a sample of metal can be

subjected before failing.

S-N curves:

Graph that shows relationship between stress and number of revolutions is called S-N

Graph.

Residual stresses:

Welding, cutting, casting, and other manufacturing processes involving heat or

deformation can produce high levels of tensile residual stress, which decreases the

fatigue strength.

References:

Text Book Mechanics of material, third edition, Beer Johnston and Dewolf.

www.Google.com

www.wikipedia.org

HOD (Now a days he is a Dean of IST) Notes, Principles of Material Science And

Engineering, smaister-02.

Own Grey Memory.

http://www.en.wikipedia.org/wiki/Fatigue_(material)

http://www.materialsengineer.com/CA-fatigue.htm

www.thefreedictionary.com

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