Bahan Tugas Teknik

Oleh Beta8 08

13 tayangan
Bagikan artikel

Transkrip Bahan Tugas Teknik

Chapter 2

Mechanical Failure of Materials

Abstract This chapter describes the major causes of mechanical failure of the engineering components or
structure. Various level of materials performance is introduced. Failures due to fracture, fatigue, creep, wear and
corrosion have been explained in order to understand the common mechanical failure. A case study on the
failure analysis of an electrical disconnector has been presented with the recommendation to prevent the failure.
Keywords

Performance

level

of

materials

Mechanical

failure

Ductile–brittlefracture

Fracture

toughness Case study
Learning Outcomes
After learning this chapter student should be able to do the following:
Suggest the factors that influence the level of performance of a material Explain the major causes of mechanical
failure
Evaluate ductile-to-brittle transition phenomenon
Justify the safe use of materials for engineering application.

2.1 Introduction
Engineering materials don’t reach theoretical strength when they are tested in the laboratory. Therefore, the
performance of the material in service is not same as it is expected from the material, hence, the design of a
component frequently implores the engineer to minimize the possibility of failure. However, the level of performance of components in service depends on several factors such as inherent properties of materials, load or
stress system, environment and maintenance. The reason for failure in engineering component can be attributed
to design deficien- cies, poor selection of materials, manufacturing defects, exceeding design limits and
overloading, inadequate maintenance etc. Therefore, engineer should
M. A. Maleque and M. S. Salit, Materials Selection and Design,
SpringerBriefs in Materials, DOI: 10.1007/978-981-4560-38-2_2,
The Author(s) 2013

17

182 Mechanical Failure of Materials

Fig. 2.1 An oil tanker that fractured in a brittle manner by crack propagation around its girth (Callister 1997, 4e) (This material is
reproduced with permission of John Wiley & Sons, Inc.)

anticipate and plan for possible failure prevention in advance. Figure 2.1 shows a catastrophic failure of an oil
tanker that fractured in a brittle manner by crack propagation at the middle of the tanker.

2.2 Mechanical Failure
The usual causes of mechanical failure in the component or system are:
•Misuse or abuse
•Assembly errors
•Manufacturing defects
•Improper or inadequate maintenance
•Design errors or design deficiencies
•Improper material or poor selection of materials
•Improper heat treatments
•Unforeseen operating conditions
•Inadequate quality assurance
•Inadequate environmental protection/control
•Casting discontinuities.
The design of a component or structure often asks to minimize the possibility of failure. The failure of metals
is a complex subject which can only be dealt with fracture or other relevant phenomenon. Therefore, it is
important to understand the different types of mechanical failure i.e. fracture, fatigue, creep, corrosion, wear etc.

2.2 Mechanical Failure

19

The general types of mechanical failure include:
•Failure by fracture due to static overload, the fracture being either brittle or ductile.
•Buckling in columns due to compressive overloading.
•Yield under static loading which then leads to misalignment or overloading on other components.
•Failure due to impact loading or thermal shock.
•Failure by fatigue fracture.
•Creep failure due to low strain rate at high temperature.
•Failure due to the combined effects of stress and corrosion.
•Failure due to excessive wear.

2.3 Failure Due to Fracture
Fracture is described in various ways depending on the behavior of material under stress upon the mechanism of
fracture or even its appearance. The fracture can be classified either as ductile or brittle depending upon whether
or not plastic deformation of the material before any catastrophic failure. A brief description of both types of
fracture is given below.

2.3.1 Ductile Fracture
Ductile fracture is characterized by tearing of metal and significant plastic deformation. The ductile fracture may
have a gray, fibrous appearance. Ductile fractures are associated with overload of the structure or large
discontinuities. This type of fracture occurs due to error in design, incorrect selection of materials, improper
manufacturing technique and/or handling. Figure 2.2 shows the features of ductile fracture. Ductile metals
experience observable plastic deformation prior to fracture. Ductile fracture has dimpled, cup and cone fracture
appearance.
Fig. 2.2 Ductile fracture in aluminum and steel after tensile testing

202 Mechanical Failure of Materials

The dimples can become elongated by a lateral shearing force, or if the crack is in the opening (tearing)
mode. The fracture modes (dimples, cleavage, or inter- granular fracture) may be seen on the fracture surface
and it is possible all three modes will be present of a given fracture face.

2.3.2 Brittle Fracture
Brittle fracture is characterized by rapid crack propagation with low energy release and without significant
plastic deformation. Brittle metals experience little or no plastic deformation prior to fracture. The fracture may
have a bright granular appearance. The fractures are generally of the flat type and chevron patterns may be
present. Materials imperfection, sharp corner or notches in the component, fatigue crack etc. Brittle fracture
displays either cleavage (transgranular) or intergranular fracture. This depends upon whether the grain
boundaries are stronger or weaker than the grains. This type of fracture is associated with non- metals such as
glass, concrete and thermosetting plastics. In metals, brittle fracture occurs mainly when BCC and HCP crystals
are present.
In polymeric material, initially the crack grows by the growth of the voids along the midpoint of the trend which then
coalesce to produce a crack followed by the growth of voids ahead of the advancing crack tip. This part of the fracture
surface shows as the rougher region. Prior to the material yielding and necking formation, the material is quite likely to
begin to show a cloudy appearance. This is due to small voids being produced within the material. Ceramics are brittle
materials, whether glassy or crystalline. Typically fractured ceramic shows around the origin of the crack a mirrorlike region bordered by a misty region containing numerous micro cracks. In some cases, the mirror-like region may extend
over the entire surface. The difference between ductile fracture and brittle fracture is shown in Table 2.1.

2.3.3 Ductile-to-Brittle Transition
The temperature at which the component works is one of the most important factors that influence the nature of
the fracture. Sharp ductile-to-brittle transition (DBTT) is observed in BCC and HCP metallic materials as shown
in Fig. 2.3.
Table 2.1 The difference between ductile fracture and brittle fracture

Plastic deformation
Process flow
Crack
Warning signal
Shape
Strain energy

Ductile fracture
Extensive
Slowly
Stable
Imminent
Cup-and-cone
High

Brittle fracture

Little
Rapidly
Unstable
No
V or chevron
Less

2.4 Factors Affecting the Fracture of a Material
Fig. 2.3 Ductile–brittletransition temperature curve (Callister 2005)

21

Impact Energy

.,Cu,Ni)

FCC metals (e.g
BCC metals (e.g., iron at T < 914C) polymers
Brittle

More Ductile
High strength materials ( y>E/150)

Temperature
Ductile-to-brittletransition temperature

2.4 Factors Affecting the Fracture of a Material
The main factors those affect the fracture of a material are:
•Stress concentration
•Speed of loading
•Temperature
•Thermal shock.

2.4.1 Stress Concentration

In order to break a small piece of material, one way is to make a small notch in the surface of the material and
then apply a force. The presence of a notch, or any sudden change in section of a piece of material, can vary
significantly change the stress at which fracture occurs. The notch or sudden change in section produces what
are called stress concentrations. They disturb the normal stress distribution and produce local co-generations of
stress. The amount by which the stress is raised depends on the depth of the notch, or change in section, and the
radius of the tip of the notch. The greater the depth of the notch the greater the amount by which the stress is
increased. The smaller the radius of the tip of the notch the greater the amount by which the stress is increased.
This increase in stress is termed the stress concentration factor.
A crack in a brittle material will have quite a pointed tip and hence a small radius. Such a crack thus
produces a large increase in stress at its tip. One way of arresting the progress of such a crack is to drill a hole at
the end of the crack to increase its radius and so reduce the stress concentration. A crack in a ductile material is
less likely to lead to failure than in a brittle material because a high stress concentration at the end of a notch
leads to plastic flow and so an increase in the radius of the tip of the notch. The result is then a decrease in the
stress concentration.
222 Mechanical Failure of Materials

2.4.2 Speed of Loading
Another factor which can affect the fracture of a material is the speed of loading. A sudden blow to the material
may lead to fracture where the same stress applied more slowly would not. With a very high rate of application
of stress there may be insufficient time for plastic deformation of a material to occur under normal conditions, a
ductile material will behave in a brittle manner.

2.4.3 Temperature
The temperature of a material can affect its behavior when subject to stress. Many metals which are ductile at
high temperatures are brittle at low temperatures. For example, steel may behave as a ductile material above,
say, 0 LC but below that temperature it becomes brittle. The ductile–brittle transition temperature is thus of
importance in determining how a material will behave in service. The transition temperature with steel is
affected by the alloying elements in the steel. Manganese and nickel reduce the transition temperature. Thus
for low-temperature work, a steel with these alloying elements is to be preferred. Carbon, nitrogen and phosphorus increase the transition temperature.

2.4.4 Thermal Shocks
When hot water is poured into a cold glass it causes the glass to crack which is known as thermal shock. The layer of glass in
contact with the hot water tends to expand but is restrained by the colder outer layers of the glass, these layers not heating up
quickly because of the poor thermal conductivity of glass. The result is the setting up of stresses which can be sufficiently
high to cause failure of the brittle glass.

2.5 Griffith Crack Theory and Fracture Toughness
In 1920, Griffith advanced the theory that all materials contain small cracks but that a crack will not propagate
until a particular stress is reached, the value of this stress depending on the length of the crack. Any defect
(chemical, inhomogeneity, crack, dislocation, and residual stress) that exists is considered as Griffith crack, i.e.
an in-homogeneity that can cause stress concentration which can be developed to failure at particular value of
stress. Fracture toughness can be defined as being a measure of the resistance of a material to fracture, i.e. a
measure of the ability of a material to resist crack propagation. Stress intensity factor (SIF) is another way of
considering the toughness of a material in terms of intensity factor at the tip of a crack that is required for it to
propagate. The parameter stress concentration factor, KI (for mode I) is the ratio of the maximum stress in the
vicinity of a notch, crack

2.5 Griffith Crack Theory and Fracture Toughness
Fig. 2.4 Fracture toughness behavior: effect of material’s thickness
Plane-stress condition

23

Plane-strain condition

Kc
toughness
K1c
Fracture

Thickness

or change in section to the remotely applied stress. The stress intensity factor, K is used to determine the fracture
toughness of most materials which is a measure of the concentration of stress at crack front under some
consideration.
Severe fracture occurs when this SIF reaches to a critical value as denoted by K c. The relationship between
KI and Kc is similar to the relationship between yield strength and tensile strength whereby K c is greater than KI.
Therefore, Kc is the maximum value that can withstand by the material without any final fracture and depends
on both type of materials and its thickness. The smaller the value of K cmeans the less tough the material. The
critical stress intensity factor Kc is a function of the material and plate thickness concerned. The thickness factor
is because the form of crack propagation is influenced by the thickness of the plate. The effect of thickness on
the value of the critical stress intensity factor is shown Fig. 2.4.
At large thickness, the portion of the fracture area which has sheared is very small, most of the fracture being
flat and at right angles to the tensile forces. This lower limiting value of the critical stress intensity factor is
called the plane strain fracture toughness and is denoted by K 1c. This factor is solely a property of the material.
It is the value commonly used in design for all but the very thin sheets; it being the lowest value of the critical
stress intensity factor and hence the safest value to use. The lower the value of K 1c means the less tough the
material is assumed to be. Table 2.2 shows difference between stress intensity factor (SIF) and fracture
toughness (FT).

2.5.1 Factors for Fracture Toughness
Factors those affect fracture toughness are described as follows:
2.5.1.1 Composition of the Material
Different alloy systems have different fracture toughness. Thus, for example, many aluminium alloys have
lower values of plane strain toughness than steels. Within each alloy system there are, however, some alloying
elements which markedly reduce toughness e.g. phosphorus and sulphur in steels.
Table 2.2 Difference between stress intensity factor and fracture toughness
Stress intensity factor
Stress intensity factor, another way of considering the toughness
of a material is in terms of the intensity factor at the tip of a crack
that is required for it to propagate

Fracture toughness
Fracture toughness can be defined as being a measure of the resistance
of a material to fracture

Material will fail at maximum stress
The stress intensity factor, K is used to determine the fracture toughness
of most materials which is a measure of the concentration of stress
at crack front under some consideration
In a flawed material, as the stress is applied the crack will propagate

As the thickness increase fracture toughness will decrease and reaches
a constant value
Fracture toughness is a measure of the ability of a material to resist
crack propagation
Fracture toughness depends on the materials geometry and properties

24

aterials of Failure Mechanical 2

2.5 Griffith Crack Theory and Fracture Toughness

25

2.5.1.2 Heat Treatment
Heat treatment can markedly affect the fracture toughness of a material. Thus, for example, the toughness of
steel is markedly affected by changes in tempering temperature.

2.5.1.3 Service Conditions
Service conditions such as temperature, corrosive environment and fluctuating loads can all affect fracture
toughness.

2.6 Failure Due to Fatigue
Metal fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating
stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is
most severe. Figure 2.5 shows typical S–N curve for the fatigue strength of a metal.
The process of fatigue consists of three stages:
•Initial crack formation
•Progressive crack growth across the part
•Final but sudden fracture of the remaining cross section.

2.6.1 Prevention of Fatigue Failure
The most effective method of improving fatigue performance is improvements in design. The following design
guideline is effective in controlling or preventing fatigue failure:

Fig. 2.5 Schematic of S–Ncurve showing increase in fatigue life with decreasing stresses

262 Mechanical Failure of Materials

•Eliminate or reduce stress raisers by streamlining the part or component.
•Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes.
•Prevent the development of surface discontinuities during processing.
•Reduce or eliminate tensile residual stresses caused by manufacturing.
•Improve the details of fabrication and fastening procedures.

2.7 Failure Due to Creep
Creep occurs under certain load at elevated temperature normally above 40 % of melting temperature of the
material. Boilers, gas turbine engines, and ovens are some of the examples whereby the components experiences
creep phenomenon. An understanding of high temperature materials behavior over a period of time is beneficial
in evaluating failures of component due to creep. Failures involving creep are usually easy to identify due to the
deformation that occurs. A typical creep rupture envelop is shown in Fig. 2.6. Failures may appear ductile or
brittle manner due to creep. Cracking may be either transgranular or intergranular, if creep testing is done at a
constant temperature and load, actual components may experience damage or failure at various temperatures and
loading conditions.
In a creep test, a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is
then measured over a period of time. The slope of the curve, shown in Fig. 2.7 is the strain rate of the test during
stage II or the creep rate of the material. Primary creep (known as stage I) is a period of decreasing creep rate.
Primary creep is a period of primarily transient creep. During this period deformation takes place and the
resistance to creep increases until stage II. Secondary creep (or stage II) is a period of approximate constant
creep rate. Stage II is referred to as steady state creep. Tertiary creep (stage III) occurs when there is a reduction
in cross sectional area due to necking or effective reduction in area due to internal void formation. Subsequently,
increase in creep rate leading to the creep fracture or stress rupture.

strain,
INCREASING T



tertiary
0t

primarysecondary

elasticT < 0.4 T m
0

time

Fig. 2.6 Creep rupture envelop

2.7 Failure Due to Creep
Fig. 2.7 Strain rate (typical creep curve) of material under creep test

27

Design Problem 1
The following data apply to extruded and cold rolled nickel alloy (Nimonic 80A) at 750 LC.
Given data:
Young’s modulus = 140 GPa
0.2 % proof stress = 450 MPa (minimum)
Elongation to fracture = 25% (short term tensile test)
Mean coefficient of thermal expansion (20–750 LC range) = 15.8 9 10-6
The stress to cause a (plastic) creep strain in 3,000 h is
Stress (MPa) 110 130
160
Strain (%)
0.1 0.2
0.5

Estimate the coefficient n in a power law representation between stress and strain rate. What would be the
total change in length of a bar of 50 mm initial length at 20 LC, when held at a stress of 150 MPa?
Solution
Strain (%)
0.1
0.2
0.5

Stress r (MPa)
110
130
160

Strain rate e, (% h-1)
3.33 9 10-5
6.67 9 10-5
1.67 9 10-4

Log(e)
-4.48
-4.18
-3.78

Log r
2.04
2.11
2.20

The creep rate is related to stress by e ¼ Arn ! logðeÞ ¼ logðAÞ þThe slope of the plot in Fig. 2.8 provides n =
4.3.
(a) Raising temperature to 750 LC, Thermal strain = 15.8 9 10-6 20) = 1.15 %
n logðrÞ.
9 (750 -

28
Fig. 2.8 Log strain rate versus log stress

Log (strain rate, % hr-1)

2 Mechanical Failure of Materials
-3.7Log strain rate
-3.8
Linear (Log strain rate)
-3.9
-4
-4.1
-4.2
-4.3
-4.4

y = 4.3000x -13.2591
R² = 0.9997

-4.5
2

2.05

2.1

2.15

2.2 2.25

Log (stress, MPa)

Applying
stress of 150 MPa and using r=E ¼ e ! Elastic strain
150 140 103 ¼ 0:1 %
Increase
in
total strain = Thermal ? Elastic
component
strains = 1.15 ? 0.1 = 1.25 %
For a 50 mm long bar, the extension = 0.63 mm.
(b)
For stress ¼ 150 MPa LogðrÞ ¼ logð150Þ ¼ 2:18
¼
of
Using graph or the linear regression

y ¼ 4:3x 13:2591; Log ðeÞ ¼

3:9! e ¼ 1:26 104 % h1
After period of 3,000 h, material will
3; 000 ¼ 0:38 %
New total strain = 1.25 ? 0.38 = 1.63 %
Extension = 0.82 mm

creep and e ¼ 1:26 104

Design Problem 2 (Creep Life estimation)
The creep rupture properties of nickel alloy (Nimonic 105) are shown in Fig. 2.9. Using Fig. 2.9, estimate the
maximum operating temperature of a gas turbine blade made out of this material which is to withstand a stress
of 150 MPa for a duration of 10,000 h.
What would be the new design life if the turbine engine ran 40 LC hotter?
Solution:
Larson–Miller Parameter = 27.5 (when r = 150 MPa)
T(20 ? log t)/1,000 = 27.5
T(20 ? log 10,000)/1,000 = 27.5
T = 1,146 K = 873 LC
New design life if operating T goes up by 40 LC

2.7 Failure Due to Creep

29

Fig. 2.9 Stress versus Larson–Miller parameters graph

T = 1146 ? 40 = 1186 K 1186(20 ? log t)/1000 = 27.5 20 ? log t = 23.2
t = 1539 h

2.8 Failure Due to Corrosion
Corrosion of metallic materials occurs in a number of forms which differ in appearance. Failure due to corrosion
is a major safety and economic concern. Several types of corrosion are encountered in metallic materials, among
those: general corrosion, galvanic corrosion, crevice corrosion, pitting, intergranular, stress corrosion etc. This
can be controlled using galvanic protection, corrosion inhibitors, materials selection, protective coating and
observing some design rules.
Corrosion is chemically induced damage to a material that results in deterio- ration of the material and its
properties. This may result in failure of the com- ponent. Several factors should be considered during a failure
analysis to determine the effect of corrosion in a failure. Examples are listed below:
•Type of corrosion
•Corrosion rate
•The extent of the corrosion

•Interaction between corrosion and other failure mechanisms.
302 Mechanical Failure of Materials

As the corrosion is a normal and natural process it can seldom be totally prevented, but it can be minimized
or controlled by proper selection of material, design, coatings, and occasionally by changing the environment.
Various types of metallic and nonmetallic coatings are regularly used to protect metal parts from corrosion.

2.9 Failure Due to Wear
Wear may be defined as damage to a solid surface caused by the removal or displacement of material by the
mechanical action of a contacting solid, liquid, or gas. It may cause significant surface damage and the damage
is usually thought of as gradual deterioration. Types of wear: abrasive and erosive wear, surface fatigue,
corrosive wear, fretting etc. The main feature in wear failure:
•Removal of material and reduction of dimension as a mechanical action
•Wear takes place as a result of plastic deformation and detachment of materials over a period of time.
Adhesive wear has been commonly identified by the terms galling, or seizing. Abrasive wear, or abrasion, is
caused by the displacement of material from a solid surface due to hard particles or protuberances sliding along
the surface. Erosion, or erosive wear, is the loss of material from a solid surface due to relative motion in contact
with a lubricant that contains solid particles. More than one mechanism can be responsible for the wear
observed on a particular part.

2.10Failure Analysis of an Electric Disconnector: Case Study
2.10.1 Introduction
This section will describe a case study result on the failure analysis of an electric power station disconnector (Maleque and
Masjuki 1997). At the end of this section a recommendation is made to overcome the catastrophic failure of the component.
At the initial investigation it was found that the fractured disconnector for a 500 kV substation (Fig. 2.10a) was failed during
installation that cause of failure is unknown. Therefore, thorough destructive examinations were performed to eluci- date the
causes of the failure. An inspection was conducted for the evidence offailure on site which tells nothing promising about
what could have caused the current failure of the Disconnector switch. However, the following features were observed:

•Detail specification of the break disconnector
•Driving mechanism of the disconnector
•Installation procedure of the disconnector.

2.10Failure Analysis of an Electric Disconnector: Case Study

(a)(b)
(c)

31

(d)

Fig. 2.10 a Fractured part of disconnector. b Crack pattern at the end of the fractured surface. c Extensive cracking at the bolt area. d
Crack propagation at the inferior

2.10.2 Scope of Analysis
•Analysis of the failed components
•Determination of the cause and mode of failure
•Recommendation for corrective measure.

2.10.3 Visual Examination
The fractured part of the disconnector was cleaned properly and dye penetrant was applied. Cracks were found
in various locations as follows:
•At the edge of the broken part as shown in Fig. 2.10a. In Fig. 2.10b, few cracks can be seen which were very
close to the fractured surface, having extensive cracking.

322 Mechanical Failure of Materials

•A wide and long crack nearby the fractured surface were found (as shown in Fig.2.10c, d).
•Some voids or pored radiating from the surface of the part were also observed.
The nuts bolted with the hole had an interference fit where the bolts behave as integral parts. However,
during installation or after installation, high force might acted on the component (such as base or blade) even if
there is little movement or misalignment of the bushbar. Therefore, fracture or failure occurs towards downward
direction.
From the nature and distribution of the cracks, and the appearance of the cracks surface of the disconnector,
it can be suggested that:
•no plastic deformation
•rupture is downward
•surface is porous
•misalignment or mishandling of the component.

2.10.4 Metallographic Examination
The following sections were prepared for microstructural investigation:
•cross section of the blade of the disconnector
•two sections from the vicinity of the fractured part.
Before metallurgical examination, the specimens were polished and etched according to standard procedure
and the microstructures were observed under optical microscope. In the photograph of the component (refer to
Fig. 2.11) it can

Fig. 2.11 Optical micrograph of materials. Showing a-aluminum dendrites, acicular silicon and primary silicon plates (9100)

2.10 Failure Analysis of an Electric Disconnector: Case Study

33

be seen that the structure consist of a-aluminum dendrites, acicular silicon and primary silicon plates. Few
inclusions and voids were noticed from the micro- structure of the specimen which causes inferior mechanical
properties of the disconnector material.

2.10.5 Mechanical Properties
2.10.5.1 Hardness Test Result

Tests were carried out using 10 kgfv and 5 kgf load. The hardness value of the material, close to the fractured
edge, was around 40 HV5. However at most of the places, the hardness was about 72 HV10.
The microhardness test was carried out on polished surface specimen. The test result is shown in Table 2.3.
For the area near to the fractured layer, its hardness is lower and the hardness increased as it when goes towards
bulk area, at a distance around 4 mm from the fractured edge. Almost constant microhardness value was
obtained away from the edge.

2.10.5.2 Tensile Properties
The tensile properties of the Disconnector switch are given in Table 2.4. The test was done according to BS18
(1987). From the test result it can be seen that some of the parameters comply with requirements given in
Aluminium Standard and Data Hand Book (1984). The percentage elongation was about 4 which is quite below
the requirements. This is possibly because of the very brittle nature of the material. The breaking load was 9.65
kN.
2.10.5.3 Compressive Strength
The compressive test result is also shown in Table 2.4. It can be noticed that the material compressed by 22.68
% when the applied load was 28,000 kg f. At the transverse direction no remarkable change occurred when the
same amount load was applied.
Table 2.3 Microhardness test results of 500 kV disconnector material
Distance from edge (mm)
1
2
3
4

Load (gf)
100
100
100
100

VHN
67.07, 72.1, 67.07

74.04, 73.19
75.68, 68.58
78.84, 75.68

Average VHN
68.75
73.56
72.13
77.26

Table 2.4 Mechanical properties of 500 kV disconnector material
Specimen Breaking load Proof stress
Tensile strength
Elongation
(kN)
(0.2 %)
(MPa)
(%)
1
9.5965.4
121.54
4.06
2
10.1162.11
127.96
4.12
3
9.2556.34
119.74
3.82
Average
9.6561.28
123.08
4.00

Reduction of area
(%)
1.72
1.22
2.15
1.70

Young’s modulus
(MPa)
5899.14
7673.42
6716.95
6763.17

Compressive
test
24.18
21.18
22.04
22.47

Charpy impact
test (J)
2.95
3.03
3.03
3.00

2.10 Failure Analysis of an Electric Disconnector: Case Study

35

Fig. 2.12 Charpy fracture surface showing the shiny, granular surface which is the characteristics of brittle fracture

2.10.5.4 Fracture Surface
The charpy V-notch Impact Energy test results are shown in Table 2.4 (at the last colum). The average value of
the change in potential energy was 3.00 J. From Table 2.4, it is obvious that the Disconnector head is below
capacity of the absorbed energy as far as the impact energy is concerned.

The fracture surface was very shiny, having granural appearance (refer to Fig. 2.12). However, the resulting
fracture surface was relatively flat without large undulation or gross irregularities.
2.10.5.5 Chemical Composition
From chemical analysis test the material of the Disconnector seemed to be AlSiMg alloy.
The analysis result is given in Table 2.5 and was obtained from optical emis- sion spectrometry.
Table 2.5 Microhardness test results of 500 kV disconnector material
Element

Analysis (%)

Mg

0.55

Si

9.63

Fe

0.37

Cu

0.04

Mn

0.14

Zn

0.06

Al

Remainder

362 Mechanical Failure of Materials

From Table 2.5, it can be seen that all the chemical contents are within spec- ification except Si. The Si
percentage is quite high (9.63 %) for this type of material and seemed to be decreased the ductility of the matrix
and thus, enhanced the brittle characteristics of the material.

2.10.6 Discussion on the Findings
2.10.6.1 Mode of Failure
•failure of the disconnector occurred due to brittle fracture
•the major cracks on the vicinity of the fracture show that they were formed during installation and were not new
cracks.
2.10.6.2 Contributory Factor
The macro and micro examination tests showed that some inclusions and voids are distributed throughout the
matrix which seemed to be flattened. The macroscopic examinations also shows that the color of at the fracture
surface and around it is shiny and having granular appearance. The fracture surface seems to be flat without
large undulation or gross irregularities which show that brittle fracture had occurred. This is probably because of
high percentage of silicon content in the alloy. The high amount of Si resulted in the aluminium wrought alloy
becoming brittle.
Many cracks were found at the edge of the bolt which is an undesirable feature because it elevated the local
stress level and might initiate and propagate crack. The presence of indentation mark closed to the fracture
surface as well as edge of the bolt resulting from misalignment seems to be one of the probable cause of failure.
Fracture occurred with no plastic deformation and proceeded along crys- tallographic planes.

2.10.6.3 Conclusion and Recommendation
The disconnector had failed by brittle fracture due to insufficient impact energy due to installation. The hardness
of tensile properties and chemical composition (except silicon) are within specification. However, the Si
percentage is quite high and contributes significantly to the brittle nature of the Disconnector material. The
presence of inclusions, indentation mark at the edge of the bolt as aggravate the mechanical strength as well
stress level at the point of fracture.
It is recommended that the percentage of Si of the alloy be reduced as it promotes brittleness of the material.
Adopting some metallurgical strategy is important in order to avert or rather reduce brittleness of the material. It
is also
2.10 Failure Analysis of an Electric Disconnector: Case Study

37

suggested to handle the disconnector and other supporting components carefully during installation and refer to
maintenance manual closely.

2.11 Summary
Engineering materials don’t reach theoretical strength when they are tested in the laboratory. The usual causes
of failure of engineering components can be attrib- uted to: design deficiencies, poor selection of materials,
manufacturing defects, exceeding design limits and overloading and inadequate maintenance. Flaws produce
stress concentrations that cause premature failure in the component. Sharp corners in generally produce large
stress concentrations leading to premature failure. Creep failure depends on both temperature and stress.

2.12 Tutorial Questions
2.1.What are the main factors that influence the level of performance of a part or component? What are the causes of
failure of engineering components?
2.2.Explain the difference between stress intensity factor and fracture toughness.
2.3.Draw and explain the effect of thickness on fracture toughness behavior of materials.
2.4.Define and show both fatigue limit and fatigue strength using S–N diagrams.
2.5.Write down the common types of mechanical failures that encountered in engineering components or structures.
2.6.List down the differences between ductile and brittle fracture. Explain theductile-to-brittle phenomenon. Support
your answer with suitable diagram.
2.7.Ti–6Al–4V and aluminium 7075 alloys are widely used in making lightweight engineering structures. The fracture toughness of Ti-6Al-4 V and aluminium 7075 alloys are 55 MPa
m1/2 and 24 MPa m1/2 respectively. The
NDT equipment can only detect flaws larger than 3 mm in length. For the design of a structure that is subjected
to a stress of 400 MPa,
(1)Calculate the critical crack length of both materials.
(2)Make a comment on the safe use of material for the structural applications.
2.8.AISI 4340 and Maraging 300 steels are being considered for making engineering structure. The fracture toughness of AISI 4340 and Maraging 300 steels are 50 and 90 MPa
m1/2 respectively. The NDT equipment can only
detect flaws larger than 3 mm in length. For the design of a structure that is subjected to a stress of 600 MPa,
(1)Calculate the critical crack length of both materials.
(2)Make a comment on the safe use of material for designing a structural component.

382 Mechanical Failure of Materials

Fig. 2.13 Fracture of an oil tanker

2.9.Explain what is meant by fracture toughness. Explain the terms stress intensity factor K, critical stress intensity
factor Kc and plane strain fracture toughness K1c.
2.10.What factors can affect the values of the plane strain fracture toughness?
2.11.Secondary creep rate, where r is stress, Q is activation energy, R is uni- versal gas constant, T is temperature in
degrees absolute, D and n are
material constants. From laboratory tests on a Nickel alloy the value of n is found to be 3. The secondary creep
rate is 3 9 10-10 s-1 at stresses of 18 and 4 MPa at temperatures of 627 and 777 LC respectively. Determine the
values of D and Q. Use the equation to find the stress which will produce the same value of at a temperature of
727 LC.
2.12.Figure 2.13 shows the fracture of an oil tanker. Explain why and how such kind of fracture phenomenon occurs?
2.13.What do you mean by fracture toughness?
2.14.What are the main features of brittle material fracture surface? Discuss theductile-to-brittle transition temperature
(DBTT) with the help of diagram. Figure below shows a severe failure of the Titanic ship. What is your
recommendation to overcome such kind of failure?
2.15.Explain the fatigue limit and fatigue life for safe-life fatigue of the engi- neering materials. Support your answer
with diagrams.

References
Anon (1984) Aluminum standard and data hand book. Aluminium Association, Washington, DC BS 18: (1987) British Standard
method for tensile testing of metals
Callister WD (1997) Materials science and engineering: an Introduction, 4th edn, Wiley, NY Callister WD (2005) Materials science
and engineering: an introduction, 6th edn. Wiley, NY Maleque MA, Masjuki HH (1997) Failure analysis of 500 kV HAPAM
DISCONNECTOR report. Technical report submitted to Transmission Technology (M) Sdn Bhd., Sept 1997

http://www.springer.com/978-981-4560-37-5

Judul: Bahan Tugas Teknik

Oleh: Beta8 08


Ikuti kami