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ANSWER SCRIPT UNIVERSITI MALAYSIA PERLIS

Peperiksaan Semester Satu

Sidang Akademik 2012/2013

Januari 2013

ENT 145 – Materials Engineering

[Kejuruteraan Bahan]

Masa : 3 jam

Please make sure that this question paper has TWELVE (12) printed pages including this

front page before you start the examination. [Sila pastikan kertas soalan ini mengandungi DUA BELAS (12) muka surat yang bercetak termasuk muka

hadapan sebelum anda memulakan peperiksaan ini.]

This question paper has SIX (6) questions. Answer ALL questions in Section A and

THREE (3) questions in section B. Each question contributes 20 marks. [Kertas soalan ini mengandungi ENAM (6) soalan. Jawab SEMUA soalan Bahagian A dan TIGA (3)

soalan Bahagian B. Setiap soalan menyumbang 20 markah.]

Note : Some formulas and tables are given in the Appendices [Nota: Beberapa rumus dan jadual diberikan dalam lampiran]

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Part A – Answer ALL questions [Bahagian A – Jawab semua soalan]

Question 1 [Soalan 1]

(a) Bohr atomic model has some significant limitations that explain several

phenomena involving electrons. Therefore, a resolution was reached with a wave-

mechanical atomic model. Describe the importance of additional refinements that

resulted from the wave-mechanical atomic model. [Model atom Bohr telah didapati mempunyai beberapa batasan untuk menjelaskan beberapa

fenomena mengenai electron. Oleh itu, satu kebezajelasan telah dicapai dengan model atom

gelombang-mekanikal.Terangkan mengenai maklumat tambahan yang penting yang didapati

daripada model atom gelombang ] (3 Marks / Markah)

ANSWER:

Important refinements resulting from the wave-mechanical atomic model are (1)

that electron position is described in terms of a probability distribution, and (2)

electron energy is quantized into both shells and subshells—each electron is

characterized by four quantum numbers

(b) Sketch and explain the main differences between ionic, covalent and metallic

bonding. [Lakar dan jelaskan perbezaan utama antara ikatan ion, ikatan kovalen dan ikatan logam]

(6 Marks / Markah)

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(c) Atom X has an atomic radius of 0.1345 nm and density of 12.41 g/cm3.

[Atom X mempunyai jejari atom 0.1345 nm dan ketumpatan 12.41 g/cm3.]

(i) Determine the crystal structure for atom X. Justify you answer.

[Nilai struktur kristal bagi atom X. Justifikasikan jawapan anda.]

(6 Marks / Markah)

(ii) Determine the atomic packing factor (APF) for crystal structure in (i).

[Tentukan faktor pemadatan atom bagi struktur Kristal pada (i).]

(5 Marks / Markah)

ANSWER:

(i)

(ii)

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Question 2 [Soalan 2]

(a) Low-alloy steel can be categorized into low-carbon steel, medium-carbon steel

and high carbon steel. For each of the low-alloy steel, describe the properties and

its typical applications. [Keluli beraloi rendah dibahagikan kepada keluli berkarbon renda, keluliberkarbon sederhana

dan keluli berkarbon tinggi. Untuk setiap keluli beraloi rendah, terangkan sifat dan

penggunaannya.] (4 Marks / Markah)

Low carbon steel < 0.25wt%C Bridge,towers,auto

structures sheet

Medium carbon steel 0.25-0.6wt% C Piston,gears,etc

High carbon steel 0.6 -1.4 wt% C Turbine,furnace etc

(b) A portion of the iron – iron carbide (Fe-FeC) phase diagram is shown in Figure 1.

Consider 3.0 kg of austenite containing 1.15 wt% C, cooled to below 725 oC.

[Satu bahagian bagi gambarajah fasa besi-besi karbida ditunjukkan dalam Gambarajah 1.

Pertimbangkan 2.0 kg austenite yang mengandungi 1.15 wt% C, di sejukkan ke bawah 725oC.]

(i) Determine the proeutectoid phase. [Tentukan fasa proeutektoid.]

(2 Marks / Markah)

(ii) Calculate the mass of total ferrite and cementite form. [Kira jisim bagi jumlah ferit dan cementit yang terbentuk.]

(3 Marks / Markah)

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(iii) Calculate the mass of the pearlite and proeutectoid phase form. [Kira jisim bagi pearlite dan fasa proeutektoid yang terbentuk .]

(3 Marks / Markah)

(c) A FCC iron-carbon alloy initially contains 0.2 wt% C is carburized at an elevated

temperature and in an atmosphere where the surface carbon concentration is at 1.0

wt%. If after 49.5 h the concentration of carbon is 0.35 wt% at a position 3.5 mm

below the surface of the alloy, analyze the temperature at which the treatment was

carried out. Refer to table in Appendices. [Satu besi-karbon aloi pada awalnya mengandungi 0.2 wt% C disusukkarbonan pada suhu ternaik

dan atmosfera dimana kepekatan permukaan pada 1.0wt%. Jika selepas 49.5 jam kepekatan

karbon menjadi 0.35wt% pada kedudukan 3.5mm dibawah permukaan aloi itu, nilai suhu dimana

rawatan dilakukan. Sila rujuk jadual pada lampiran.]

(8 Marks / Markah)

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Question 3 [Soalan 3]

(a) Discuss the differences between brittle and ductile metals with respect to its

tensile stress-strain behaviour.

[Bincangkan perbezaan antara logam rapuh dan logam mulur berdasarkan kepada sifat tegangan

tegasan-terikan.]

(4 Marks / Markah)

ANSWER:

(b) A cylindrical specimen of hypothetical metal alloy has a diameter of 8.0 mm. A

tensile force of 1000 N produces an elastic reduction in diameter of 2.8 x 10-4

mm.

Compute the modulus of elasticity for this alloy, given that the Poisson’s ratio is

0.30. [Satu spesimen bagi logam aloi hipotesis berbentuk silinder mempunyai diameter 8.0 mm. Daya

tegangan sebanyak 1000N menghasilkan pengurangan anjal bagi diameter sebanyak 2.8 x 10-4

mm. Kira modulus anjal bagi aloi ini, diberi nisbah Poisson’s ialah 0.30.]

(6 Marks / Markah)

Ductile Metal experience plastic

deformation upon fracture

Brittle Metal very little or no

plastic deformation

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(c) Table 1 shows a list of materials and their mechanical properties. Each of the

material will be tested as a cylindrical rod specimen with 100 mm long and

having a diameter of 10 mm. If the tensile load is 27.5 kN, answer the following

questions. [Jadual 1 menunjukkan senarai bagi bahan dan sifat mekanikalnya. Setiap bahan tersebut akan

diuji sebagai satu specimen rod silinder dengan panjang 100 mm dan diameter 10 mm. jika beban

tegangan ialah 27.5 kN, jawab soalan-soalan berikut.]

(i) From Table 2, choose the material(s) that will not experience plastic

deformation. Justify your choice(s). [Daripada Jadual 2, pilih bahan yang tidak akan mengalami ubah bentuk plastik.

Justifikasi pinlihan anda.]

(6 Marks / Markah)

(ii) By referring answer in (i), select the material(s) that will not experience a

diameter reduction of more than 7.5 x 10-3

mm. [Dengan merujuk kepada jawapan di (i), pilih bahan yang tidak akan mengalami

pengurangan diameter tidak lebih daripada 7.5 x 10-3

mm.]

(4 Marks / Markah)

Table 1

[Jadual 1]

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Question 4 [Soalan 4]

(a) Discuss pearlite, spheroidite and martensite with respect to microstructures and

mechanical properties.

[Bincang pearlit, spherodit dan martensit berdarkan kepada mikrostruktur dan sifat mekanikal.]

(6 Marks / Markah)

Microstructure Mechanical Properties

Pearlite Alpha ferrite + cementite/ layer

which alternate with one another

Less ductile than spherodite. Harder

than spherodite.

Spherodite Alphaferrite + cementite / sphere-

shaped particles

extremely ductile. softest and weakest.

Martensite Alphaferrite + cementite / needle

shaped grains

Extremely hard but brittle

(b) The complete isothermal transformation diagram of 0.76 wt% C steel alloy is

shown in Figure 2. [Gambarajah lengkap penjelmaan suhu untuk bagi keluli aloi 0.76 wt % C ditunjukkan dalam

Gambarajah 2.]

(i) Develop the heat treatment process for producing a microstructure of 100

% martensite structure. [Bangunkan proses rawatan haba untuk menghasilkan mikrostruktur bagi 100% struktur

martensit.] (5 Marks / Markah)

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(ii) Develop the heat treatment process for producing a microstructure of 50%

bainite and 50% martensite structure. [Bangunkan proses rawatan haba untuk menghasilkan mikrostruktur bagi 50% struktur

bainit dan 50% struktur martensit.] (5 Marks / Markah)

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(iii) Alloy in question (i) is tested for bending fatigue test. It has experienced a

fracture which occurs in brittle manner. Propose a suitable solution to

improve mechanical properties of the alloy. Justify your answer. [Aloi pada (i) diuji bagi ujian lenturan lesu. Daripada ujian tersebut, aloi mengalami

patah di mana patah berlaku dalam betuk rapuh. Cadangkan satu penyelesaian yang

bersesuaian untuk memperbaiki sifat mekanikal aloi tersebut. Justifikasikan jawapan

anda.]

(4 Marks / Markah)

ANSWER:

Ductility of martensite may be enhanced by heat treatment process known

as tempering. Process of heating a martensitic steel to temperature

between 250-650 C for 1h and slowly cool to room temperature.

Question 5 [Soalan 5]

(a) Pitting, intergranular corrosion and stress corrosion are example of metallic corrosion. [Bopeng, kakisan antara butir, kakisan tegasan adalah contoh bagi kakisan logam.]

(i) Describe conditions under which these corrosions occur.

[Jelaskan dibawah keadaan apa berlakunya kakisan.]

(3 Marks / Markah)

Pitting – Downward propagation of small pits and holes

Intergranular – Corrosion along grain boundaries often where precipitate

particles form

Stress corrosion - Corrosion at crack tips when a tensile stress is present and

corrosive environment

(ii) Discuss the measures that may be taken to prevent and control these corrosions.

[Bincangkan pengukuran yang akan diambil untuk mencegah dan mengawasinya kakisan

tersebut]

Pitting –

Alloying with 2 % molybdenum enhance their resistance

Intergranular –

Lower the carbon content

Alloying with other metal, ex. Titanium, which tendency to form carbide greater

Stress corrosion-

Lower magnitude of stress and increase cross sectional area

(6 Marks / Markah)

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toughness of 35 MPa√m. Fracture occurred at a stress of 250 MPa when the maximum

internal crack length is 2.0mm. [Satu komponen kapal terbang telah difabrikasi daripada aloi aluminum yang mempunyai keliatan

patah terikan satah 35MPa√m. Patah terjadi pada tegasan 250 MPa bila panjang retak dalaman

maksimum ialah 2.0mm.]

(i) Determine the value of dimensionless correction factor, Y. [Tentukan nilai bagi faktor pembetulan tak berdimensi, Y.]

(5 Marks / Markah)

(ii) If a stress level is increased to 325 MPa and the maximum internal crack length is

reduced to 1.1 mm, predict whether any fracture will occur. Compare your answer with

existing data. Justify your answer. [Jika aras tegasan ditingkatkan kepada 325MPa dan panjang retak dalaman maksimum

dikurangkan kepada 1.1mm, anggarkan sama ada patah akan terjadi. Bandingkan jawapan anda

dengan data yang tersedia.Justifikasi jawapan anda.] (6 Marks / Markah)

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Question 6 [Soalan 6]

(a) A specimen 760 mm long of an S-590 alloy is to be exposed to a tensile stress of 80

MPa at 815 o

C. Figure 3 shows stress versus steady-state creep rate for S-590 alloy at

four temperatures. Determine its elongation after 5,000 hour. Assume that the total of

both instantaneous and primary creep elongations is 1.5 mm. [Satu spesimen panjang 760 mm bagi aloi S-590 akan didedahkan kepada tegasan tegangan sebanyak

80MPa pada suhu 815oC. Gambarajah 3 menunjukkan tegasan berlawanan keadaan-mantap kadar

rayapan aloi S-590 pada empat suhu. Tentukan pemanjangannya selepas 5,000 jam. Anggap bahawa

jumlah bagi pemanjangan rayapan ketika dan pemanjangan rayapan utama ialah 1.5 mm. ]

(7 Marks / Markah)

…10/

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(b) Figure 4 shows stress amplitude versus logarithm cycles to failure for a cast iron. [Gambarajah 4 menunjukkan amplitud tegasan berlawanan logaritma kitar untuk patah bagi besi

tuang.]

(i) Determine the fatigue limit for this alloy.

[Tentukan had lesu bagi aloi tersebut.] (3 Marks / Markah)

Answer:

From graph, fatigue limit is 193 MPa

(ii) Estimate the fatigue lifetimes at stress amplitudes of 250 MPa and 175 MPa. [Anggarkan masa hayat lesu pada amplitud tegasan untuk 250 MPa dan175 MPa.]

Answer:

The fatigue lifetime at a stress amplitude of 250 MPa is about is about 90,000

cycles.

At 175 MPa the fatigue lifetime is essentially an infinite number of cycles since

this stress amplitude is below the fatigue limit.

(4 Marks / Markah)

(iii) Data from Figure 4 is to be used for bending rotating test. This material will be used as an

automobile axle rotates at average rotational velocity of 750 rpm. Predict the maximum

lifetimes of continuous driving that are allowed for stress levels of 250 MPa and 150

MPa. [Data dari Gambarajah 4 akan digunakan untuk ujian lenturan berputar. Bahan ini akan

digunakan sebagai satu gandar kenderaan yang berputar pada purata halaju putaran sebanyak

750 rpm. Anggarkan masa hayat maksimum panduan berterusan yang dibenarkan untuk aras

tegasan 250 MPa dan 150 MPa. ] (6 Marks / Markah)

Answer:

For a stress level of 250 MPa, the fatigue lifetime is approximately 90,000 cycles.

This translates into (9 × 104 cycles)(1 min/750 cycles) = 120 min.

For a stress level of 150 MPa, the fatigue lifetime is essentially infinite since we

are below the fatigue limit [193 MPa].