Fatigue Test

Jordan University of Science and

Technology

Faculty of Engineering

Industrial Engineering Department

Engineering Materials Laboratory

Exp. # 4

Fatigue Test

Student's name: Georgina Rezeq

Student's No.: 20122029020

Teacher: Dr.

Omar Bataineh

Contents:

Objectives ………. 1

Introduction ……….. 1

Procedure ………. 3

Analysis ……… 3

Discussion ……… 4

Conclusion ……… 5

References ………. 5

Results ………... 6

Data Sheet ………. 7

Objectives :

1. To understand the behavior of different materials under

fluctuating (cyclic or periodic) loads in service.

2. To differentiate the appearance of fatigue fracture from

other types of fractures.

Introduction

:

Fatigue is a progressive and localized structural damage

occurs when the material is subjected to cyclic loading. 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. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure .

Fatigue life is influenced by a variety of factors, such as temperature,

surface finish, microstructure, presence of oxidizing or inert

chemicals, residual stresses, contact (fretting), etc .

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-One of the machines used to determine the fatigue limit is the fatigue testing machine. In this machine a standard specimen is used where it is supported in a rotating chuck at one end and at the other end with a load (p) applied through a ball race. In this type of cantilever loading each point in the circumference of the specimen is subjected to alternate stress between tension and compression. To find the fatigue limit, different specimens are tested at different loading till failure occurs .

After performing the fatigue test, maximum stress that was obtained that caused the sample to break will be calculated using the formula :

I M y  

Where M is the bending moment (N.m) and I is the second moment of inertia (I = ) ( 64 4 4 m D 

) and y half the thickness of the sample (y=D/2 (m)) .

After calculating the Max stress it is plotted against number of cycles till failure occurred and S-N curve is obtained where the endurance limit (fatigue limit) is obtained .

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-Procedure

:

1. Select two materials, one shows a fatigue limit (ferrous

metal) and the other does not show this limit (non-ferrous metal).

2. Take at least two standard specimens from each material. 3. Mount the specimen in the machine and fix it properly,

then apply the required load by the loading system.

4. Make sure, that the reading of the revolution counter is

5. When the specimen is broken, read the number of

revolutions (cycles), as indicated by the revolution counter of the machine.

6. Determine the applied stress from the relation I

MY  

as explained above.

7. Tabulate the obtained results, together with the given

results from previous tests.

8. From these results, plot the S-N diagram for each tested

material, on the same graph paper.

9. Compare the obtained S-N diagram

with the diagram given in the lab handout and examine the

appearance of the fracture for each sample with the pictures also

provided in the handout .

Analysis

:

Use Steel standard specimen with diameter D = 9.57 mm. We convert this diameter to inches and we convert all dimensions to inches because the moment calculated in lb.in from the machine .

Convert 9.57mm to inches is equal 0.38 in .

The distance from the neutral surface :

y=D

2 = 0.38

2 =0.19

The second moment of area of the section : I =π D4 64 = π (0.384 ) 64 =0.001023

The applied stress to the first moment :

σ =MC I =

(240)(0.19)

0.001023 =44574.78

Same calculation to all but change the value of moment .

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-Discussion

:

The aim of this experiment is to understand the behavior of different materials under alternating (cyclic) loading, plus to learn how to differentiate the appearance of the fatigue fracture shape from other types of fractures caused by other types of loadings. Moreover, we are able to determine the brittleness and ductility of the material from the shape of its fatigue fracture and learned how the Ductility and Brittleness are related to fatigue .

We fined when we increase the moment on the specimen the stress on the specimen increase and the number of cycles to failure occurs decrease .we know that the stress is tension in top of the specimen and compression in other side of the specimen the stress in both side is the same value. This difference in tension and compression stress makes surface cracks and then the cracks propagate and failure occurs .

And we can notice from the S-N curve that as if we increase the stress, the number of cycles endured before failure will decrease .

The fatigue test takes long time because we need to break different specimen each time at different load and wait until fracture occur to get one point on S-N diagram which is the main and the important result of the fatigue test. To draw the S-N diagram minimum we need 50 points thus, 50 specimens and 50 fractures so it may take years to do that .

The fatigue test is also expensive if we compare it with other destructive tests that we did in the lab fatigue test need electrical power for long time to achieve our goal (S-N diagram) but it’s an important test to know the ability of the material in fatigue when we use it in a specific service work that may exposed to alternating loads .

Also we noticed that the cold worked steel have a definite fatigue limit while the worked Aluminum don’t have a definite fatigue limit .

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Conclusion

:

phenomenon that will cause a material to fail eventually under alternate loading, regardless of the number of cycles.

2- Fatigue is tested using the

fatigue testing machine and its value is obtained from plotting the S-N curve, where stress is calculated using the formula I

MY  

3- Fatigue affected by design, surface quality, material type,

grain size, direction of loading, grain size and temperature and other factors.

4- Cold working increases the fatigue strength.

5- Ductile materials have a lower fatigue limit than brittle

materials.

6- Fatigue fracture shape differs in structure from other

fractures used from other types of failures, as it consist of three zones ( initial crack zone, propagation zone and final fracture zone)

7- In ductile materials the propagation zone is larger than

References

:

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

2. Draper, John (2008). Modern Metal Fatigue Analysis. EMAS 3. Experiment # 4 Lab handout.

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-Results

:

Cold worked steel is a ferrous metal so we have a definite fatigue limit approximately in the upper graph . 4 5.4 5 5.5 6 5.6 0 0005 00001 00051 00002 00052 00003 00053 00004

munimulA dekrow dloc roF evruC N-S

Cold Worked Aluminum is a non-ferrous metal so, we note that we don’t have a definite fatigue limit, so the fatigue limit in this case spicified by

10 ×106

cycle and the corresponding SN≅

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