(Sifat Mekanik)

download (Sifat Mekanik)

of 54

description

Material Teknik

Transcript of (Sifat Mekanik)

Chapter 6

1SIFAT MEKANISChapter 6-Materi kuliah :Konsep tegangan dan regangan.Sifat elastis dan sifat plastis material.Uji tarik.Kurva tegangan regangan dan intepretasinya.Chapter 6-

21. Kondisi awal2. Beban kecil3. Beban dilepasElastic means reversible!

DEFORMASI ELASTISChapter 6-DEFORMASI ELASTIS

Deformasi elastis bersifat reversibelChapter 6-DEFORMASI PLASTIS (LOGAM)

Deformasi plastis bersifat tetap (permanen)Chapter 6-4 Tegangan tarik, s: Tegangan geser, t:

Satuan tegangan :N/m2 atau lb/in2KONSEP TEGANGANChapter 6-5 Tegangan tarik (kabel)

Tegangan geser (poros)

Note: t = M/AcR here.Ski lift (photo courtesy P.M. Anderson)KONDISI PEMBEBANAN

Chapter 6-

6 Beban tekan :

Note: compressivestructure member(s < 0 here).(photo courtesy P.M. Anderson)(photo courtesy P.M. Anderson)KONDISI PEMBEBANANChapter 6-7 Tegangan tarik 2 sumbu: Tekan Hidrostatik:

Pressurized tank

s < 0h(photo courtesyP.M. Anderson)(photo courtesyP.M. Anderson)KONDISI PEMBEBANANChapter 6-8 Regangan tarik : Regangan lateral: Regangan geser :

Regangan merupakan besaran tanpa satuan.KONSEP REGANGANChapter 6-CONTOH SOAL 1Sebatang tembaga dengan panjang 305 mm ditarik dengan tegangan sebesar 276 MPa, jika deformasi yang terjadi adalah deformasi elastis, hitung resultan elongasinya.JAWAB :Pada deformasi elastis, hubungan antara tegangan dan regangan adalah :Elongasi (l) dapat dihitung dengan persamaan :

Chapter 6-CONTOH SOAL 1Gabungan dari kedua persamaan tersebut, didapat :

Dengan memasukkan harga harga : = 276 MPal0= 305 mmE properti tembaga = 110 x 103 MPaHarga l dapat dicari.......

Chapter 6-CONTOH SOAL 2Sebuah tegangan tarik diberikan pada sebuah batang kuningan yang berdiameter 10 mm. Hitung beban yang diperlukan untuk menghasilkan perubahan diameter 2,5 x 10-3 mm, jika deformasi yang terjadi adalah deformasi elastis :JAWAB :

Chapter 6-Ketika gaya F diberikan, spesimen akan mengalami perpanjangan arah z dan mengakibatkan pengecilan diameter, d = 2,5 x 10 -3 mm dalam arah x.

Regangan arah z dapat dihitung dengan persamaan:

Chapter 6-Tegangan yang diberikan dapat dihitung dengan Hukum Hooke :

Gaya yang harus diberikan :

Chapter 6- Contoh spesimen uji tarik9 Tipe uji lain : -- tekan : bahan getas (misal : beton) -- torsi : poros, tabung silinder

Skema uji tarik

Adapted from Fig. 6.2, Callister 6e. Adapted from Fig. 6.3, Callister 6e. (Fig. 6.3 is taken from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, p. 2, John Wiley and Sons, New York, 1965.)UJI TEGANGAN DAN REGANGANChapter 6-

Tensile Test Machine (Instron)Chapter 6-

ExtensometerGripSpecimenGripChapter 6-17 Maximum possible engineering stress in tension. Metals: occurs when noticeable necking starts. Ceramics: occurs when crack propagation starts. Polymers: occurs when polymer backbones are aligned and about to break.Adapted from Fig. 6.11, Callister 6e.TENSILE STRENGTH, TS

Chapter 6-Necking: The localized decrease in diameter in a specimen near the rupture point.Elastic Deformation Region: The area of a stress - strain curve where the specimen will deform under load, yet return to its original shape when the load is removed.Plastic Deformation: Deformation that occurs once the object has been stressed past its elastic limit. The deformation is no longer reversible.TermsChapter 6-

Chapter 6-

Chapter 6-

GripGripTensile Test SpecimenChapter 6-

Chapter 6-

Necking Starts

Necking StartsChapter 6-Necking occurs as the sample leaves the elastic deformation region and begins to deform plastically.Necking

Chapter 6-26

Chapter 6-

Fracture Initiates at Necking AreaChapter 6-

Chapter 6-

Fracture is Complete at Necking AreaChapter 6-

Chapter 6-The classic cup & cone shape of a fairly ductile tensile fracture is visible here.

Chapter 6-

2 Ductile fracture is desirable! Classification:Ductile: warning before fractureBrittle: No warningAdapted from Fig. 8.1, Callister 6e.DUCTILE VS BRITTLE FAILURE

Chapter 6-Compare the material properties of these three metal samples. All three failed under tension

Chapter 6-Tensile Test Stress Strain DiagramThe applied stress versus the strain or elongation of the specimen shows the initial elastic response of the material, followed by yielding, plastic deformation and finally necking and failure. Several measurements are taken from the plot, called the Engineering Stress-Strain Diagram. These include:

Modulus of elasticity Yield strengthTensile strengthModulus of resilienceFailure stress Ductility ToughnessChapter 6-Modulus of elasticity - the initial slope of the curve, related directly to the strength of the atomic bonds.

Chapter 6-Menghitung modulus elastisitasModulus elastisitas (E)

10Daerah elastis

10Chapter 6-Yield strength, usually defined as the point at which a consistent and measureable amount of permanent strain remains in the specimen.

Chapter 6-0.2 % Offset Yield StrengthOffset Yield Strength

Defining the yield stress as the point separating elastic from plastic deformation is easier than determining that point. The elastic portion of the curve is not perfectly linear, and microscopic amounts of deformation can occur. As a matter of practical convenience, the yield strength is determined by constructing a line parallel to the initial portion of the stress-strain curve but offset by 0.2% from the origin. The intersection of this line and the measured stress-strain line is used as an approximation of the material's yield strength, called the 0.2% offset yield.

Chapter 6-

0.2 % Offset Yield StrengthChapter 6-

Tensile strength - the maximum stress applied to the specimen. Chapter 6-

Failure stress - the stress applied to the specimen at failure (usually less than the maximum tensile strength because necking reduces the cross-sectional area)Chapter 6- Plastic tensile strain at failure:19

Another ductility measure:

Note: %AR and %EL are often comparable. --Reason: crystal slip does not change material volume. --%AR > %EL possible if internal voids form in neck.

Adapted from Fig. 6.13, Callister 6e.DUCTILITY, %ELChapter 6-

Ductility - the total elongation of the specimen due to plastic deformation, neglecting the elastic stretching (the broken ends snap back and separate after failure). Chapter 6-Ductility

% Elongation:% elongation is a measure of ductility, which is given by:

% elongation =100 * (Lf - Lo)/ Lo

where,Lo = Initial lengthLf = Final LengthChapter 6-Ductility% Reduction in Area:% reduction in area is a measure of ductility, which is given by:

% reduction in area =100 * (Ao - Af)/ Ao

where,Ao = Initial araeAf = Final areaChapter 6-

Modulus of resilience - the area under the linear part of the curve, measuring the stored elastic energy. Chapter 6-

Toughness - the total area under the curve, which measures the energy absorbed by the specimen in the process of breaking.Chapter 6- Energy to break a unit volume of material Approximate by the area under the stress-strain curve.20

TOUGHNESSChapter 6-14 Simple tension test:(at lower temperatures, T < Tmelt/3)

PLASTIC (PERMANENT) DEFORMATIONChapter 6-15 Stress at which noticeable plastic deformation has occurred.when ep = 0.002

YIELD STRENGTH, syChapter 6-True Stress and Engineering Stress:True stress is calculated by : = (applied force)/(area) =P/Awhere A=Actual area (actual area constantly decreases from its initial value)Substitution of the actual area into the equation gives a larger stress (true stress) than the engineering stress. Note that engineering stress uses the initial area, regardless of the change in diameter during the tensile test.TRUE STRESSENGINEERING STRESSSTRESSSTRAINChapter 6- An increase in sy due to plastic deformation.22 Curve fit to the stress-strain response:

STRAIN HARDENING EXPONENTChapter 6- Stress and strain: These are size-independent measures of load and displacement, respectively. Elastic behavior: This reversible behavior often shows a linear relation between stress and strain. To minimize deformation, select a material with a large elastic modulus (E or G). Plastic behavior: This permanent deformation behavior occurs when the tensile (or compressive) uniaxial stress reaches sy.24 Toughness: The energy needed to break a unit volume of material. Ductility: The plastic strain at failure.SUMMARYChapter 6-