chap2_mechanical testing and microscopy.pdf

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  • 8/18/2019 Chap2_Mechanical testing and microscopy.pdf

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    Mechanical testing and

    microscopy

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    Types of mechanical testing

    การทดสอบสมบัตท าง ก ล ขอ ง วัสดปร ะ เภท ตางๆแยกตามประเภทของแรงกระทา : (a) แร ง ดง (Tension), (b)แร ง อัด (Compression), (c) แรงกดบนผว (Indentation hardness), (d) แร ง ดัดบนคานท ตดกับกาแพง 

    (Cantilever bending), (e) แร ง ดัด 3 จด (Three-point bending), (f) แร ง ดัด 4 จด (Four-point

    bending) (g) แรงบด (Torsion)

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    การทดสอบแรงดงดวยเคร องทดสอบอเนกประสงค 

    การเคล �อนท �ของครอสเฮดผานระบบสกร

    ช นงานทดสอบ

    อปกรณตดตามระยะ

    เซลลวัดแรง

    ครอสเฮด

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    เคร องทดสอบอเนกประสงค (Universal testing machine)

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    Some examples of specimens before and after tensile testing and a

    typical arrangement for threaded-end specimens are shown below.

    ช นงานทดสอบแรงดง ทั งกอ น แล ะ ห ลัง ก าร ท ดส อ บ (

    ซาย)

    โลหะ (

    ขวา)

    พอลเมอร 

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    Types of test specimens

    ช นงานทดสอบสมบัตท าง ก ล 3 ประเภท: (a) ช นงานผวเรยบไมมรอยบาก, (b) ช นงานมรอยบาก (Notched) (c) ช นงานท มรอยแตก (Precracked)

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    การเปล ยนแปลงร  ปรางของวัสดเม อไดรับแรงด ง 

    L

     A0 

    L0 

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    Tensile test

     A unidirectional force is applied to a

    specimen in the tensile test by means of the

    moveable crosshead

    Engineering stress = σ = F/A0 (MPa)

    (1 Pa = 1 N/m2)

    Engineering strain = ε = (l-l0)/l0 

    Stress-strain curve for gray cast iron in

    tension showing brittle behavior

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    กราฟแสดงความสัมพันธระหวางความเคนและความเครยดทางว ศวกรรมของวัสดทั วไป 

    ความตานแรงดง 

    ความเคนประลัย 

        ค    ว    า    ม    เ    ค          น

    ความเครยด

    ความเคนจดคราก 

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    กราฟแสดงความสัมพันธระหวางความเคนและความเครยดในชวงแรกท ความชันของกราฟมลักษณะเปนเสนตรง หรอวัสดมการเปล ยนแปลงร  ปรางแบบยดหย น 

    เม �อมแรงกระทา

    ความชันของกราฟ = มอดลัส 

    0

      ความเครยด

        ค    ว    า    ม    เ    ค          น

    เม �อไมมแรงกระทา

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    กราฟแสดงวธการหาคามอดลัสแบบแทนเจนตและแบบซแคนต  

    ∆σ

    ∆ɛ มอดลัสแบบซแคนต ระหวางจดเร �มตนและความเคน σ1

    σ1

    σ2

    ∆σ

    ∆ɛมอดลัสแบบแทนเจนต ณ ความเคน σ

    2

    ความเครยด

        ค    ว    า    ม    เ    ค          น

    =

    =

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    กราฟแสดงปรากฏการณจดครากท มทังจดครากบนและจดครากลาง 

    ความเครยด

        ค    ว    า    ม    เ    ค          น   จดครากลาง

    σy

    จดครากบน

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    กราฟแสดงตาแหนงของขดจากัดการแปรผันตรง  

    ตวามเครยด

        ค    ว    า    ม    เ    ค          น

    σy   ขดจากัดการแปรผันตรง

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    Elastic properties

    • Modulus of elasticity (Young’s modulus) = E =σ

    /ε (GPa or psi)

    • This relation ship is Hooke’s law and can be seen from stress-

    strain curve (elastic region)

    Schematic of the engineering stress-strain

    curve of a typical ductile metal that

    exhibits necking behavior

    Engineering stress-strain curve and geometry

    of deformation typical of some polymers

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    The stress-strain curve for an aluminium alloy Comparison of the elastic behaviour of

    steel and aluminium

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    Necking behaviour

    Deformation in a tension test of aductile metal: (a) unstrained, (b)

    after uniform elongation and (c)

    during necking

    Fractures from tension tests on (left) rot-

    rolled AISI 1020 steel and (right) gray cast

    iron

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    กราฟแสดงความสัมพันธระหวางความเคนและความเครยดของวัสดเหนยวและวัสดเปราะ 

    0 B   B’

    A’

    Aวัสดเปราะ

    วัสดเหนยว

        ค    ว    า    ม    เ    ค          น

    ความเครยด

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    กราฟแสดงการหาคามอดลัสของรซเลยนซ 

        ค    ว    า    ม    เ    ค          น

    ความเครยด0.002

    σy

    ɛy

    พ นท �ใตกราฟของสามเหล �ยม

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    กราฟแสดงการหาคาความแกรงของวัสด  

    0

    วัสดเหนยว

        ค    ว    า    ม    เ    ค          น

    ความเครยด

    คาความแกรง = พ นท �ใตกราฟความเคนและความเครยด

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    True stress and true strain

    Fig. 20 The relationship between the

    true stress-strain diagram and the

    engineering stress-strain diagram

    True stress (σt)

    σt = F/Ai 

    True stain (εt)

    εt = ln (li/l0) 

    • It is often not requiring true stress and true strain because when

    the material exceed the yield strength, it deforms.

    • This leads to failure of component because it no longer has the

    original intended shape.

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    กราฟแสดงความเคนจรงและความเครยดจรงเปรยบเทยบกับความเคนและความเครยดทางวศวกรรม 

        ค    ว    า    ม    เ    ค          น

    ความเครยด

    จดท �เกดการคอด

    จดท �เกดการคอด

    ความเคนและความเครยดทางวศวกรรม

    ความเคนและความเครยดจรง

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    Engineering measures of ductil ity

    • Ductility: a measure of the amount of plastic deformation that a

    material can withstand without breaking

    • There are two approaches to measure ductility.

    % Elongation = (lf -l0)/l0 x 100

    % Reduction in area = (A0-Af )/A0 x 100

    where lf  = the distance between gauge marks after the specimen

    breaks

     Af  = the final cross-sectional area at the fracture surface

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    Trends of tensile behaviour for different materials 

    Tensile properties for some engineering metals

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    Mechanical properties for polymers at room temperature

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    Tensile properties for various SiC reinforcements in a 6061-T6 aluminum

    matrix

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    Engineering stress-strain curves from

    tension tests on three steels

    Engineering stress-strain curves from

    tension tests on three aluminum alloys

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    Effect of temperature

    The effect of temperature (a) on the stress-strain curve and (b) on the tensile

    properties of an aluminium alloy

    • Tensile properties depend on temperature.

    • Hot working (deformation of material at a high temperature takesadvantage of the higher ductility and lower required stress

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    Effect of strain rate

    Effect of strain rate on the ultimate tensile strength of copper for tests at various

    temperatures

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    Compression test

     A typical arrangement for a compression test is shown in the below figure.

    Uniform displacement rates in compression are applied in a manner similarto a tension test, except the direction of loading

    Compression test in a universal testing machine using a

    spherical-seated bearing block

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    Some examples of compression specimens of various materials both

    before and after testing are shown as the following figures.

    Compression specimens of metals (left to

    right): untested specimen, and tested specimen

    of gray cast iron, aluminum alloy 7075-T651,

    and hot-rolled AISI 1020 steel

    Untested and tested 150 mm diameter

    compression specimens of concrete with

    Hokie limestone aggregate

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    Trends in compressive behaviour

    Initial portions of stress-strain curves

    in tension and compression for 7075-

    T651 aluminum

    Stress-strain curves for plexiglass (acrylic,

    PMMA) in both tension and compression

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    Hardness test

    Brinell hardness test

    Plastic deformation under a Brinell

    hardness indenter

    • The Brinell hardness number is defined

    as HB and can be calculated as follows.

    HB = 2P/[ πD{D-(D2-d2)0.5}]

    • HB can also be found from table for

    hardness values of materials

    elsewhere.

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    Vickers hardness test

    The Vickers hardness number (HV) is defined as below.

    HV = (2P/d2)sin(α/2)

    Vickers hardness indentation

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     Approximate relative hardness of various

    metals and ceramics

     Approximate relationship between UTS and Brinell

    and Vickers hardness of carbon and alloy steels

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    Vickers hardness and bending strengths for some ceramics and glasses

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    Rockwell hardness test

    The Rockwell hardness number (HV) is

    defined as below.

    HRX = M-(∆h/0.002)

    Where ∆h = h2-h1 

    M = the upper limit of the scale

    Rockwell hardness indentation made by application

    of (a) the minor load and (b) the major load, on a

    diamond Brale indenter

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     Commonly used Rockwell hardness scales

    Brinell and Rockwell hardness indentations

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      Approximate equivalent hardness numbers

    and UTS for carbon and alloy steels

    Hardness correlations and

    conversions

    σu = 3.45 (HB) MPa

    where σu = the ultimate tensile

    strength

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    Impact test

    The two most common tests are Charpy V-notch and Izod tests.

    Specimens and loading configurations for (a) Charpy V-notch and (b) Izod tests

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    Charpy testing machine, shown with

    the pendulum in the raised position

    prior to its release to impact a

    specimen

    Broken Charpy specimens

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    Properties obtained from the impact test

    Results from a series of Izod impact tests

    for a supertough nylon thermoplastic

    polymer

    The Charpy V-notch properties fro a BCC

    carbon steel and a FCC stainless steel.

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      Variation in Charpy V-notch impact energy

    with temperature for normalized plain

    carbon steels of various carbon contents

    Temperature dependence of Charpy V-

    notch impact resistance for different alloy

    steels of similar carbon content all

    quenched and tempered to HRC 34

    B di t t

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    Bending test

    (a) The bend test often used for measuring the strength of brittle materials and (b) the

    deflection (δ) obtained by bending

    • By applying the load at three points and causing bending, a tensile

    force acts on the material opposite the midpoint. Fracture begins at this

    location

    • Flexural strength (modulus of rupture)

    Flexural strength = 3FL/2wh2 

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    Stress-deflection curve for MgO obtained from a bend test

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    Fl li

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    Flow lines

    • Flow lines caused by the concentration altering during the cooling

    cycle results in ‘banding’ where the different metal constituentsoccur at different concentrations.

    Flow line in forging highlighted after sectioning and etching

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    Mi B i

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    2109504 Adv. Phy. Met. I 51

    © J.Paul Robinson

    Microscope Basics

    • Standard required the following

     –  real image formed at a tube length

    of 160mm

     –  the parfocal distance set to 45 mm

     –  object to image distance set to 195

    mm

    Focal length

    of objective

    = 45 mm

    Mechanical 

    tube length 

    = 160 mm

    Object to

    Image

    Distance

    = 195 mm

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    The Conventional Microscope

    Focal length

    of objective

    = 45 mm

    Object to

    Image

    Distance

    = 195 mm

    Mechanical

    tube length

    = 160 mm

    Upright Scope

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    2109504 Adv. Phy. Met. I 53

    Upright Scope

     Brightfield

    Source

     Epi-

    illumination

    Source

    Inverted Microscope

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    2109504 Adv. Phy. Met. I 54

    Inverted Microscope

     Brightfield

    Source

     Epi-

    illumination

    Source

    Typical inverted microscope

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    2109504 Adv. Phy. Met. I 55

    Typical inverted microscope

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     An example of SEM

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    Scanning coils

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    Scanning coils

    • In SEM the specimen is traversed by an electron beam.

    • Movement is achieved by scanning (raster) coils in the microscopecolumn controlled by a scan generator.

    • Primary beam is electronically deflected over a given area of the

    specimen:

    • Raster Pattern of the primary electron beam is synchronised with

    the scanning pattern of the cathode ray tube yielding a point-to-

    point translation.