Title: Failure Analysis

Objectives:

To determine the causes of failure of a given component by proposing a suitable

mechanical testing and metallographic examination and evaluating the tensile strength , hardness

and fatigue strength of the given component.

Introduction:

Failure analysis is a process where a few trouble-shooting and testing is performed to

collect and analyze the data to determine the cause of a failure. It can also be defined as the

inability of a component, machine or process to function properly. In this experiment, the given

component is steel. The component will undergo a few testing .To do a failure analysis, a step-

by-step flow chart must be performed. The flow chart is as follow.

Sample preparation (sectioning, mounting,

grinding, and polishing)

Macroscopic examination

Mechanical testing (Tensile strength, hardness, fatigue)

Initial observation

Metallographic examination.

1.0 Initial observation:

Initial Observation:

Tensile:

Trial Diameter (cm) Length (cm)

Before After Before After

1 5.36 4.98 2.7 3.2

2 5.36 4.94 2.7 3.3

Fatigue:

Trial Length (cm)

Before After

1 6.7 6.4

Steel used for tensile test.

Steel used for fatigue test.

2.0 Macroscopic examination:

Macroscopic examination is an extension of visual examination and evaluates quality and

homogeneity of the test sample indicating the flow of material during the forming or welding

process. Information for macro structural features can use to be assessing the properties of

material internal quality, presence of hydrogen flakes, chemical segregation, hard cases and

welds. Weld cross section examination is a most common macroscopic examination to reveal

internal discontinuities, weld profile, weld passes and sequence, extent of penetration and the

quality of weld.

Macroscopic examination is started by sectioning, mounting, grinding, polishing, and

etching of the sample. Etching is a chemical reaction on the surface of the sample, which allows

viewing the flow properties of the materials. Macroscopic examination is performed on the cross

section or longitudinal section of the sample. Microscopes is use to reveal a structure such as

grain flow lines and ingot patterns that are visible at the low magnification. Grain flow lines are

the result of fiber pattern observed in cold worked material and it showing the manner in which

the metal followed during the forming process.

3.0 Sample Preparation:

3.1 Sectioning:

The first step in preparing a specimen for metallographic analysis is to locate the area of interest.

Sectioning or cutting is the most common technique for obtaining this area of interest.

Sectioning is the process of cutting the material into smaller pieces, for analysis. It

exposes the internal surface ready for grinding and polishing to be observed under the

mircroscope. A cutting machine is used for sectioning. The water inside acts as a coolant to

prevent rusting of the blade.

Procedure:

1) A sample of steel cramped in the cutting machine.

2) Turn on the Sectioning Machine.

3) Slowly sliding down and attach the steel to the cutting wheel.

4) Close the safety cover.

3.2 Mounting:

Mounting is the process where the sample of the cut steel is encapsulated in resin. It

serves the purpose to ease handling and to preserve the microstructure of the sample. Epoxy resin

is used to adhere the sample and eliminate shrinkage gap. In addition, it does not react with the

sample and other solvents.

Apparatus:

1) Part Plastic Mould

2) Silicon mould Release

3) Hardener and epoxy resin chemical

Procedure:

1) Apply a thin layer of silicon mold release inside of the Part Plastic Mold and the lid.

2) Place the sample of the cut off steel in the center of the mold.

3) A ratio of 1:10 hardener and epoxy resin was measured by using a beaker and stirred well

before pour into the mounting cup.

4) The mixture was poured slowly and it was left for several days.

Precautions:

Ensure the ratio of the hardener and epoxy resin is poured as accurately as possible. The silicon

mold release must be applied to prevent the sticking of materials such as plastics, rubber and

waxes to molds. The mixture should be poured slowly to prevent air bubbles.

3.3 Grinding & polishing:

Grinding is to transports the swarf away from the platen and the specimen, lubricant that

use in grinding is water, and it act as a coolant to prevent heat buildup from friction. The purpose

for grinding and polishing is to transports swarf and dust away from the specimen, so that when

the specimen is undergo the etching, the grain flow lines can be easily to detected by the

microscope.

Apparatus:

Grinder polisher, sand paper (600 and 1000), mold, water, 3m diamat polycrystalline diamond,

1m diamat polycrystalline diamond

Procedure:

1. The mold was placed on the grinding machine. 2. The mold was grinded by sandpaper lubricated with water. 3. Next, the mold was transferred to the polishing machine. 4. Polishing wheels were fitted on the grinder polisher. 5. The polishing first wheel was lubricated with 3m diamat polycrystalline diamond and

then 1m diamat polycrystalline diamond on the second wheel.

6. The mold was polished with the first wheel for few minutes then transferred to polishing by second wheel.

The product after sample preparation which is also used in hardness testing.

4.0 Mechanical testing:

4.1 Tensile strength:

Introduction:

The tensile test is the simplest and most widely used test to characterize the mechanical

properties of a material. The test is performed using a benchtop machine shown above in the

figure above. The capacity of this machine is 10,000 pounds (tension and compression). The

specimen of a given material takes a cylindrical shape that is 2.0in long and 0.5in. in diameter in

its undeformed (no permanent strain or residual stress), or original shape.

Objective: to determine the limits of physical properties

Apparatus:

benchtop testing machine, steel sample .

Procedure:

1. The specimen was placed on the benchtop material testing machine. 2. All the data needed was keyed into the computer to start the testing. 3. The machine will slowly increase the tensile load. 4. After the material breaks, the dimension of the two parts of steel specimen was

measured.

5. The result for maximum tensile stress and load was displayed on the screen of computer.

Results:

After tensile test :

Before After

First steel D= 5.36mm

L=27mm

D=4.98mm

L=32mm

Second steel D=5.36mm

L=27mm

D=4.94mm

L33mm

The average diameter = 4.96mm

The average length = 32.5mm

The graph of force (N) against extension (mm)

The maximum force applied to the first and second steel is 10250 N and 10163N respectively

and thus, taking the average, the force is 10206.5N

=

==

=

= ( ) ( )

( )

=

=

0

2000

4000

6000

8000

10000

12000

0 2 4 6 8 10

Modulus elasticity E =

=

= 2.59GPa

=

=

=

=

Percentage elongation =

x 100 = 20.4%

Discussion:

The diagram above shows the evolution of ductile fracture. At the start of the tensile

pulling at (a) , it shows the initial necking where the diameter decreases. At (b) , small cavity are

formed and slowly is grows across the specimen at (c). At (d) , crack propagation occurs and

finally the specimen breaks and shears forming a cup and cone fracture. From the results

obtained, the average stress applied to the material is 528.23MPa , the modulus of elasticity is

2.59GPa, and a percentage elongation of 20.4%.

Modulus of elasticity, or better known as Youngs modulus, is the constant of proportionality. It is the measure of the stiffness of a material and the elasticity of the material

before experiencing plastic deformation. The Formula of modulus of elasticity is given by

E =

where is the shear stress and is the strain. The yield stress is the stress where a

material begins plastic deformation. While the ultimate tensile stress is the maximum stress that

a material can cope while being pulled with force before breaking.

Conclusion:

Youngs Modulus of 2.59GPa is required to break the material while the yield strength of

shows the significant plastic deformation. From this information, the limit of the physical properties of steel is determined.

4.2 Hardness.

Objective:

To determine the hardness of the material.

Introduction:

Hardness is the property of a material that plastic deformation. Deformation may refer to

as indenting, scratching, bending or cutting. In metals, ceramics and most polymers, the

deformation considered is plastic deformation of the surface. A Vickers hardness test machine is

used in this experiment to indent steel. The tip of the indenter, which is a diamond tip is strong

enough to indent the steel. The tip is forced to indent the metal surface and then it is being

withdrawn. The value shown on the machine indicates the value of hardness.

Vickers machine test.

Apparatus :

Mold with steel surface, Vickers machine.

Method:

1) A mold containing a steel surface was placed on the vice. 2) The indenter was lowered and adjusted to the visible green light on the steel surface. 3) The machine was started and the tip indents the surface. 4) After indenting, the indenter was withdrawn. 5) Through the microscope, the horizontal and vertical diameter of the dent was measured. 6) The value of hardness was computed and recorded. 7) Steps 1-6 were repeated to obtain the average value.

Results:

The microscopic view of the dent on the steel surface.

Trial d1 (m) d2 (m) dmean (m)

(d1 + d2)/2

Hardness(HV) Hardness(MPa)

1HV=9.807MPa

1 132.66 138.41 135.535 201.9 1980.03

2 127.31 134.26 130.785 216.8 2126.16

3 133.06 134.31 133.685 207.5 2034.95

Average 131.01 135.66 133.335 208.73 2047.05

HV = Vickers Hardness:

F= Load kgf

d= Arithmetic mean of the two diagonals, d1 and d2 in mm

Experimental Hardness:

F = 2 x 9.807 = 19.614

( )

= 2045 MPa

Theoretical Hardness:

2047.05 MPa

%

Getting the value d1 and d2

1. Measurement is started with two lines on one edge. 2. D1 is set fix at one edge. 3. Turn the knob to control the line of D2 to the opposite edge and press enter. 4. The value of vertical line and horizontal line (D2 and D1) is recorded as d1 and d2 in the

table.

The Vickers Hardness test was developed in England is 1925 and also known as the

Diamond Pyramid Hardness (DPH) test. The Vickers hardness test method consists of indenting

the test material with a diamond indenter, which means in right pyramid form with a square base.

A diamond of two tangents to the circle at the ends of a chord 3d/8 long intersect at 136 degrees

between opposite faces and also used to leave a mark in metal under a precisely applied force to

avoid impact. The diagonals of the impression have to be measured by using microscope and the

results will be calculated by using the formula.

Formula for Vickers Hardness:

D2 D1

D2 D1

D2 D2

D1

D1

The hardness values obtained each time has a varying value. The hardness of steel is

around 85HV to 740HV. The hardness varies with each different indenter is because different

position of the steel specimen will have different arrangement of grain, or there is some tiny flaw

compared to other location on the specimen. When location tested is fall on the flaw, the indenter

is more significant, so it affected the value.

Percentage difference is calculated to identify the successfulness of the experiment, as the

percentage is lower the successfulness of the experiment is closes. Percentage different that

calculated is 0.1002% which is almost perfect, so the experiment is successful and the hardness

of the steel specimen is determined.

4.3 Fatigue.

Introduction:

A metal subjected to a repetitive or fluctuating stress will fail at a stress much lower

than that required to cause fracture on a single application of load. Failures occurring under

conditions of dynamic loading are called fatigue failures, presumably because it is generally

observed that these failures occur only after a considerable period of service. Fatigue accounts

for at least 90 percent of all service failures due to mechanical causes.

Fatigue occurs when a material is subject to alternating or cyclic stresses, over a long period of

time. Examples of where fatigue may occur in a marine diesel engine are: crankshafts, valve

springs, turbocharger blades, piston crowns, bottom end bolts, piston skirts at the gudgeon pin

boss and tie bolts.

Stresses can be applied in three ways, torsionally, axially and by bending.

The symbol for stress is the Greek letter sigma and the units are force/ unit area i.e N/m2 or psi

(imperial)

Objective:

The objective of this experiment is to test the fatigue strength of a material subjected to repetitive

or cyclic stress.

Apparatus:

1- Stainless steel 2- Fatigue Testing Machine 3- Frequency inverter Three-phase motor 4- Loading scale 5- Limit switch 6- Specimen chuck holder

Method:

1. Fatigue Testing Machine was plugged into a 240VAC single

Phase 50Hz supply.

2. The specimen were Positioned into the shaft end first and then into the bearing end

That applies force.

3. The grips in the chuck were slowly tightened with the tee key provided.

4. The cycle counter was set to zero.

5. The required force was set by turning the load nut. (160N) which is (16 kg)

6. The main switch was turned on and the required frequency of the motor was set.

7. The motor was let to run until the specimen fail

8. The number of turns before failure was Recorded

9. The S-N curve was plotted on semi-log graph paper.

Results:

Steel

Force (N) No of cycles

to failure

Stress

MPa

160 800 152.6

Bending stress due to bending loads in a beam is defined as

Where

M = Moment = Force x distance = F x 0.0364 Nm

y = 0.0045m

I = 3.22 x 10-10 m4

Kt = 2 (constant)

0

20

40

60

80

100

120

140

160

180

200

0 5000 10000 15000 20000

Forc

e (

N)

Number of cycles to failure

S-N curve for steel

Series1

Discussion:

From the obtained results, a S-N curve is plotted. From the graph, it is known as when the

force applied to the material increases, the number of cycles to break the material to failure

decreases. The picture above shows the material after breaking. The fracture surface of the

specimens contains two different regions, which are smooth and rough. Smooth region is due to the rubbing action as crack propagates, while the rough surface area is formed by fast fracture

when load is too high for remaining cross-sectional area.

One of the factors that affect the number of cycles needed to break the material is the

temperature. Extreme high or low temperatures can decrease fatigue strength. Besides

temperature,, stress concentration, size effect, surface effect, combined stresses, cumulative

fatigue and sequence effect, metallurgical variables, and corrosion can affect the number of

cycles fracture. Increasing of the weight applied to the fatigue specimen results in a reduction in

number of cycles to failure.

We can then use the experimental results to construct an S-N curve The fatigue test is

normally conducted using at least 8-12 specimens in order to provide sufficient information for

the interpretation of fatigue behavior of the tested material. The S-N curve shows a relationship

between the applied stress and the numbers of cycles to failure, which can be used to determine

the fatigue life of the material subjected to cyclic loading. High applied cyclic stress results in a

low number of cycles to failure.

Limitations during fatigue test are inevitable. Difficulties in reading the dial gauge when

calibrating the specimen in the machine, human error as well as the accuracy of the gauge itself

could cause error in loading the part. One last possible error could be the number of cycles. The

level of uncertainty of the counter could cause error as well as the person reading the cycles, but

the possibility for 99 cycles to be missed always exists.

Conclusion:

The stress needed to break the steel is 152.6MPa when a force of 160N is applied after 800

cycles to fracture.

5.0 Metallographic examination:

The photomicrograph of specimen under 20x magnification.

Metals consist of small particles called atoms. Atoms after combination form molecules.

Combination of molecules or atoms is called grain. A grain is a portion of the material within

which the arrangement of the atoms is identical. The orientation of the grain may different but

arrangement of atoms is same. Grains are separated by boundaries called grain boundaries. They

are near with the boundaries. If they are too close to the boundaries call compression, if they are

far apart call tension. Microstructure of metals consists of many grain. Grains are very small,

cant be seen by eyes. Therefore, microscope is required. For seeing grain for any metal, a few operations have to perform on metal to prepare the sample, this technique is called

Metallographic examination.

From the photomicrograph taken above, some impurities is found under this specimen.

The impurities might be caused by the surface not being grinded enough. Flaws may happen

during sectioning, and thus affects the surface of the steel. Besides that, there are some grain

boundaries which appears in this specimen but we couldnt see the grain and the grain boundaries clearly based on we didnt go through the process of etching. Etching means that we use dilute acid to react with the surface of the sample. Therefore etching must be done to make

the grain and grain boundaries easier to be observed.

References:

Gordonengland.co.uk. 2013. Hardness Testing. [online] Available at: http://www.gordonengland.co.uk/hardness/ [Accessed: 1 Oct 2013].

Gordonengland.co.uk. 2013. Vickers Hardness Test. [online] Available at: http://www.gordonengland.co.uk/hardness/vickers.htm [Accessed: 1 Oct 2013].

Sp.se. 2013. Fatigue test of materials and structures. [online] Available at: http://www.sp.se/en/index/services/fatigue/sidor/default.aspx [Accessed: 2 Oct 2013].

PRACTICAL HARDNESS TESTING MADE SAMPLE by Elia E. LEVI www.aws.org (2003) PRACTICAL HARDNESS TESTING MADE SAMPLE by Elia E.

LEVI [Online] Available at:

http://www.aws.org/educators/Library/0000/000587.pdf [Accessed at 1/10/2013]

YEDITEPE UNIVERSITY ENGINEERING FACULTY MECHANICAL ENGINEERING me.yeditepe.edu.tr (n.d.) YEDITEPE UNIVERSITY ENGINEERING

FACULTY MECHANICAL ENGINEERING LABORATORY [Online] Available

at:http://me.yeditepe.edu.tr/courses/me402/lab%20manuals/hardness%20test.pdf

[Accessed at 1/10/2013]

HARDNESS TEST civil.eng.buffalo.edu (n.d.) HARDNESS TEST [Online] Available at:

http://civil.eng.buffalo.edu/cie616/2-LECTURES/Lecture%204a%20-

%20Material%20Testing/HARDNESS%20TEST.pdf [Accessed at 1/10/2013]

Science.jrank.org. 2013. Science Encyclopedia - JRank Articles. [online] Available at: http://science.jrank.org/pages/16981/metal-fatigue.html [Accessed: 2 Oct 2013].

Petrography.net. 2013. Sample Preparation Techniques. [online] Available at: http://www.petrography.net/samplepreparation.htm [Accessed: 2 Oct 2013].

Metallographic examination by: The Historical Metallurgy Society: Archaeology Datasheet No. 11 [Online]

Avaliable at: http://hist-met.org/hmsdatasheet11.pdf [ Accessed at 2/10/2013}

Materials and Methods by Harper CollinsCollege Publishers.(1993) [Online] Available at:

http://writing2.richmond.edu/training/project/biology/matmeth.html

About Grinding and Polishing www.struers.com (n.d.) About Grinding and Polishing [Online] Available at

http://www.struers.com/default.asp?top_id=5&main_id=23&doc_id=104 [Accessed on

2/10/2013]