MEE30001 Materials AND Manufacturing 2 Lab 1 PDF

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5 10 15 20 MEE30001 MATERIALS AND MANUFACTURING 2 ____________________________________________________________________________________________________ 25 LAB 1: FATIGUE FAILURE OF METALS Prepared for: Dr. Rafiq Mirza Julaihi 30 GROUP MEMBERS: ID: Tasha Lai Sie Ming 100066170 Erna Fahmi 100060538 Sar...

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MEE30001 MATERIALS AND MANUFACTURING 2 ____________________________________________________________________________________________________

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LAB 1: FATIGUE FAILURE OF METALS Prepared for: Dr. Rafiq Mirza Julaihi

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GROUP MEMBERS:

ID:

Tasha Lai Sie Ming

100066170

Erna Fahmi

100060538

Sara Athynna Binti Mohd Kamal

100074393

1.0 Introduction 5

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Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses such as bridges, aircraft, and machine components. Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load. The term fatigue is used because this type of failure normally occurs after a lengthy period of repeated stress or strain cycling. Fatigue is important inasmuch as it is the single largest cause of failure in metals, estimated to comprise approximately 90% of all metallic failures; polymers and ceramics but not glasses are also susceptible to this type of failure. Fatigue is also catastrophic and insidious, occurring very suddenly and without warning.

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Furthermore, fatigue failure is brittle-like in nature even in normally ductile metals, in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks, and ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress.

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2.0 Objectives 25

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0 To determine the effect of stress concentrators on the fatigue life behaviour of metals. 0 To plot the S-N Curves for three different materials of Ck 35. 0 To identify the stress concentration effect, material microstructure property effect and experimental variability effect.

3.0 Theory Fatigue depends on the amplitude of the stress and the number of cycles. Fatigue testing involves the impact of cyclic stresses. There are several key parameters to characterize the fluctuating stress cycle.

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•

+

,

= 2

•

, =

−

2

• •

=

,

, =

−

Figure 1: Graph of stress against time

Specimen is subjected to a stress cycling at certain stress amplitude and the number of cycles to failure is counted. The data are then plotted on an S-N curve. 5

0 Fatigue limit

0 An older concept that defined a stress below which a material will not fail in a 10

fatigue limit. 0 Fatigue life

0 The number of cycles permitted at a particular stress before a material fails by 15

fatigue 0 Fatigue strength

0 The stress required to cause failure by fatigue in a given number of cycles, such as 500 million cycles 20

There will no fatigue failure if stress amplitude is less than fatigue limit, or Endurance Limit. The higher the magnitude of the stress, the smaller the number of cycles the material is capable of sustaining before failure.

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Figure 2: S-N graph

4.0 Experimental Apparatus 45

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Figure 3: WP140 Fatigue Testing Machine

4.1 Test Bars 5

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Figure 4: Simple representation of fatigue test apparatus 45

4.2 Materials

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Type 1 2 3

Test bars, Materials CK35 Curvature radius Surface roughness, (mm) 0.5 2.0 2.0

t

( )

4 4 25

Table 1: Test bars 55

Notes Small radius, smooth Large radius, smooth Large radius, rough

In all cases, the load F=170N corresponding to

a

=300 /

2

. Three samples are examined.

5.0 Experimental Procedures

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1. The sample is loaded in the fatigue testing rigs. 2. The fatigue testing of samples are conducted with three samples: a. Sample 1: Curvature radius of 0.5 mm with surface roughness of 4 µm b. Sample 2: Curvature radius of 2.0 mm with surface roughness of 4 µm c. Sample 3: Curvature radius of 2.0 mm with surface roughness of 25 µm 3. The number of revolutions to failure is recorded at each test specimen. 4. After all the three specimens have been tested, steps 1 to 3 are repeated with different amount of force (loads) as listed in the table below.

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No.

Number of load cycles for test bar under different loads Load in N Stress

in

/

2

a

1 2 3

200 170 150

400 340 300

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Table 2: Number of load cycles for test bar under different loads

6.0 Results 30

Load

Stress,

(N) 200 170 150

400 340 300

( / 2)

No. of cycles to failure, N Type 1 (Sample 1) 10776 24816 38405

Type 2(Sample 2) 12634 60269 85036

Table 3: Experimental results

Type 3(Sample 3) 13817 31678 108344

7.0 Discussion of Results 5

S-N Curve S-N curve is plotted according to the experimental results obtained for each respective test bar. 10

i) Specimen 1 20

S-N Curve for specimen 1 Stress (N/mm2)

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45 0 40 0

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35 0

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30 0 25 0

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20 0 15 0

45

10 0

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5 0 0

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

No of cycles to failur

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Figure 5: Graph of S-N Curve for specimen 1 2

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As seen from the graph, for a stress at 400 N/mm , test bar 1 fractures after 10776 cycles, 2 2 for a stress of 340 N/mm ; it fractures after 24816 cycles, and for a stress at 300 N/mm ; it fractures after 38405 cycles. Due to insufficient time, the experiment was not conducted 2 for a stress of 260 N/mm . Hence, it was assumed that the test bar may encounter the endurance limit before that stress value. ii) Specimen 2

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Stress (N/mm 2)

S-N Curve for specimen 2 450 400 350 300 250 200 150 100 50 0 0

10000

20000

30000 40000

50000

60000

No of cycles to failure, N

Figure 6: Graph of S-N Curve for specimen 2

70000

80000 90000

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It can be seen that an applied stress of 400 N/mm , the specimen fractures after 12634 cycles, 2 for a stress of 340N/mm ; the specimen fails after 60269 cycles; and for a stress of 300 2 N/mm , it fractures after 85036 cycles. Compared to test bar 1, test bar 2 has a larger notch radius and both test bars have the same surface roughness. Therefore, as the radius increases from 0.5mm to 2mm, the number of cycles to failure increases for each stress value respectively. iii) Specimen 3

S-N Curve for specimen 3 15

Stress (N/mm2 )

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45 0 40 0

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35 0

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30 0 25 0

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20 0 15 0 10 0 5 0 0 0 20 00 0 40 00 0 60 00 0 80 00 0 10 00 00 12 00 00

No of cycles to failure, N

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Figure 7: Graph of S-N Curve for specimen 3 2

It can be seen that an applied stress of 400 N/mm , the specimen fractures after 13817 cycles, 2 for a stress of 340N/mm ; the specimen fails after 31678 cycles; and for a stress of 300 2 N/mm , it fractures after 108344 cycles. 10

Test bar 3 has the same notch radius as test bar 2. However, its surface roughness is greater than that of test bar 2. Therefore, as the surface roughness increases, the number of cycles to failure for each stress value respectively. 15

Stress Concentration Effect

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Stress concentration is often known as stress raisers or stress risers. It is a location in an object where stress is concentrated. An object is strongest when the force is evenly distributed over its area. A reduction in area, mostly likely caused from a crack, results in localized increase in stress. A material can fail via a propagating crack, when a concentrated stress exceeds the materials theoretical value because most materials contain small cracks that concentrate stress. Fatigue cracks always start at stress raisers, therefore removing such defects increase the fatigue strength.

The causes of stress concentration are: geometric discontinuities, manufacturing defects, loading types, harsh environment, poor maintenance, surface roughness, surface treatments and grain size. 5

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i) Geometric discontinuities design/shape of an object is important as it can influence the stress concentration area. Any notch or edge can serve as a geometric discontinuity, such as holes, keyways, grooves, threads and many more. This is one of the reason why engineers and designers always opt for a smooth edge while designing and manufacturing parts. ii) Manufacturing defects is also another type of geometric discontinuity where stress concentrates and fatigue crack starts to initiate. Hence, engineers and designers are constantly developing new manufacturing techniques to prevent manufacturing defects. iii) Loading types – A mechanical part is often exposed to complex and random sequence of loads. Depending on the complexity of the geometry and the loading, one or more properties of the stress state need to be considered, such as the amplitude, mean stress, shear stress and load sequence. iv) Environmental conditions - Stress concentration can be affected due to environmental factors such as thermal fatigue and corrosion fatigue are present. v) Surface roughness – for many common loading situations, the maximum stress within a component structure occurs at its surface. Consequently, most cracks loading to fatigue failure originated at surface positions. Based on the experiment conducted, the number of cycles to failure decreases when the surface roughness of the material increases.

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Based on this experiment, Type 1 which have a curvature radius of 0.5mm, is sharper as compared to Type 2 and Type 3. Therefore, Type 1 samples have a higher stress concentration which the number of cycles to failure is lesser compared to Type 2 and Type 3. 35

Material Microstructure Property Effect

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Material microstructure properties refers t grain boundaries or grain size of the material. The grain size or microstructure of a material plays a crucial role in determining the fatigue life behaviour of a specific material. By controlling metallurgical variables, the fatigue strength can be improved. Microstructure can be controlled by: 0 Promote homogenous slip/plastic deformation through thermochemical processing. This could reduce stress concentration. 0 Heat treatment – to give a hardened surface but should avoid stress concentration 0 Avoid inclusions – stress concentration/fatigue strength

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By increasing tensile strength, fatigue strength can be improved. Some strengthening mechanisms are grain boundary strengthening, fibre strengthening and second phase strengthening and. As a result, it is proven that the grain size has its greatest effects on fatigue life in the low stress, high-cycle regime and for most metal, smaller grain size yield longer

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fatigue life. For metal, two fracture modes are possible, ductile and brittle. Classification is based on the ability of the material to experience plastic deformation. Ductile materials typically exhibit substantial plastic deformation with high energy absorption before fracture. For a brittle fracture, there is usually little or no plastic deformation with low energy absorption.

Based on this experiment, it has been proven that smoother surface yield longer fatigue life. Adding to this, materials composed of ferrite exhibits high fatigue properties relative to low ultimate tensile strength. Besides that, after studying and examining the failure areas of all specimens of the experiment, it was determined that all the fractures/failures were brittle.

Experimental Variability Effect 15

The fatigue behaviour of engineering materials is higher sensitive to a number of variables, namely: a) Mean Stress: 20

The dependence of fatigue life on stress amplitude is represented on the S-N plot. Such data are taken for a constant mean stress, often for the reversed cycle situation. Mean stress, however will also affect fatigue life. This influence may be represented by a series of S-N curves. As seen in the figure, increasing the mean stress level leads to a decrease in fatigue life. 25

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Figure 8: Demonstration of the influence of mean stress on S-N fatigue behaviour. 45

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b) Surface Effects: The maximum stress within a component or structure occurs at its surface for many common loading situations. Consequently, most cracks leading to fatigue failure originate at the surface positions specifically at stress implication sites. Therefore, it is observed that fatigue life is very sensitive to the condition and configuration of the component surface. Several factors influence fatigue resistance, the proper management of which will lead to an improvement in fatigue life. Those include design criteria as well as various surface treatments.

c) Design Factors: 5

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The design of a component can have a significant influence on its fatigue characteristics. Any notch or geometrical discontinuity can act as a stress raiser and fatigue crack initiation site. These design features include groove, holes, keyways, threads and so on. The sharper the discontinuity, the more severe the stress concentration. The probability of fatigue failure may be reduced by avoiding these structural irregularities or by making the design modifications whereby sudden contour changes leading to sharp corner are eliminated.

8.0 Conclusion Fatigue testing machine was used to create rotating bending stresses on the samples with different properties. The number of cycles to failure were recorded and analysed by using the S-N diagram. The analysis of S-N diagram for each type of sample has been discussed and the factors affecting such as stress concentration, microstructure properties and experimental variability effect was discussed to study the fatigue life behaviour of the materials. The difference in cycles to failure among the specimens were due to stress concentration effect due to the geometry and surface such as surface roughness.

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In conclusion, all objectives of the experiment have been achieved and therefore this experiment is deemed as a success....