Bölüm 6: Mekanik Özellikler

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1 Bölüm 6: Mekanik Özellikler
DEĞİNELECEK KONULAR... • Gerilme ve Genleme: What are they and why are they used instead of load and deformation? • Elastik davranış: When loads are small, how much deformation occurs? What materials deform least? • Plastik davranış: At what point does permanent deformation occur? What materials are most resistant to permanent deformation? • Tokluk ve Süneklik: What are they and how do we measure them?

2 Elastik Deformasyon d F F d 1. İlk durum 2. Küçük yük
3. Yükün kaldırılması F d bağlar geriniyor İlk duruma dönüş F d Doğrusal- elastik Doğrusal-Olmayan Elastik geri dönebilen! anlamına gelir

3 Plastik Deformasyon (Metaller)
1. İlk durum 2. Küçük Yük 3. Unload Düzlemler hala Kaymış durumda F d elastik + plastik Bağlar geriniyor & düzlemler kayıyor plastik F d doğrusal elastik plastik Plastik kalıcı! Anlamına gelir

4 Mühendislik Gerilmesi
• Çekme gerilmesi, s: Yüklemeden önceki İlk alan Alan, A F t s = A o 2 f m N or in lb • Kayma gerilmesi, t: Alan, A F t s = A o  Gerilme birimleri: N/m2 ya da lbf/in2

5 Genel Gerilme Durumları
• Basit çekme: kablo A o = kesit alanı (yük kaldırıldıktan sonra) F o s = F A s Kayak Telesiyeji (photo courtesy P.M. Anderson) • Burma (kaymanın bir şekli): Tahrik Mili M A o 2R F s c o t = F s A Not: t = M/AcR .

6 DİĞER GENEL GERİLME DURUMLARI(1)
• Basit basma: A o Balanced Rock, Arches National Park (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o s = F A Not: basma Yapı elemanı (s < 0 here). (photo courtesy P.M. Anderson)

7 DİĞER GERİLME DURUMLARI (2)
• İki-eksenli çekme: • Hydrostatik basma: Suyun içinde balık Basınçlı tank (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) s z > 0 q s < 0 h

8 Mühendislik Genlemesi (Birim Şekil Değiştirmesi)
/2 L o w • Çekme genlemesi: • Yatay genleme: e = d L o - d e L = w o • Kayma genlemesi: q x q g = Dx/y = tan y 90º - q Genleme her zaman boyutsuzdur. 90º Adapted from Fig. 6.1 (a) and (c), Callister 7e.

9 Gerime-Genleme Deneyleri
• Tipik bir çekme makinası • Tipik bir çekme numunesi Adapted from Fig. 6.2, Callister 7e. ölçü boyu specimen extensometer Adapted from Fig. 6.3, Callister 7e. (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.)

10 Doğrusal Elastik Özellikler
• Elastiklik Modülü, E: (Young modülü olarak ta bilinir) • Hooke Kanunu: s = E e s Doğrusal- elastik E e F basit çekme deneyi

11 Poisson oranı, n eL e - n e n = - • Poisson oranı, n: Birimler:
metaller: n ~ 0.33 seramikler: n ~ 0.25 polimerler: n ~ 0.40 Birimler: E: [GPa] ya da [psi] n: boyutsuz  > yoğunluk artar  < yoğunluk azalır (boşluklar oluşur)

12 Mekanik Özellikler Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metal Adapted from Fig. 6.7, Callister 7e.

13 Diğer Elastik Özellikler
simple torsion test M t G g • Elastik Kayma modülü, G: t = G g • Elastik kütle (Bulk) modülü, K: pressure test: Init. vol =Vo. Vol chg. = DV P P = - K D V o • İzotropik malzemeler için özel bağıntılar: 2(1 + n) E G = 3(1 - 2n) K

14 Young Modülü: Karşılaştırması
Grafit Seramikler Yarıiletkenler Metal Alaşımları kompozitler /elyaflar (fiberler) Polimerler 0.2 8 0.6 1 Magnesium, Aluminum Platinum Silver, Gold Tantalum Zinc, Ti Steel, Ni Molybdenum G raphite Si crystal Glass - soda Concrete Si nitride Al oxide PC Wood( grain) AFRE( fibers) * CFRE GFRE* Glass fibers only Carbon fibers only A ramid fibers only Epoxy only 0.4 0.8 2 4 6 10 00 1200 Tin Cu alloys Tungsten <100> <111> Si carbide Diamond PTF E HDP LDPE PP Polyester PS PET C FRE( fibers) FRE( fibers)* FRE(|| fibers)* E(GPa) Based on data in Table B2, Callister 7e. Composite data based on reinforced epoxy with 60 vol% of aligned carbon (CFRE), aramid (AFRE), or glass (GFRE) fibers. 109 Pa

15 Useful Linear Elastic Relationships
• Simple tension: • Simple torsion: a = 2 ML o p r 4 G M = moment = angle of twist 2ro Lo d = FL o E A d L = - n Fw o E A F A o d /2 L Lo w • Material, geometric, and loading parameters all contribute to deflection. • Larger elastic moduli minimize elastic deflection.

16 Plastic (Permanent) Deformation
(at lower temperatures, i.e. T < Tmelt/3) • Simple tension test: Elastic+Plastic engineering stress, s at larger stress Elastic initially permanent (plastic) after load is removed ep engineering strain, e plastic strain Adapted from Fig (a), Callister 7e.

17 Yield Strength, sy y = yield strength sy
• Stress at which noticeable plastic deformation has occurred. when ep = 0.002 tensile stress, s engineering strain, e sy p = 0.002 y = yield strength Note: for 2 inch sample  = = z/z  z = in Adapted from Fig (a), Callister 7e.

18 Yield Strength : Comparison
Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers Yield strength, s y (MPa) PVC Hard to measure , since in tension, fracture usually occurs before yield. Nylon 6,6 LDPE 70 20 40 60 50 100 10 30 2 00 3 4 5 6 7 Tin (pure) Al (6061) a ag Cu (71500) hr Ta (pure) Ti Steel (1020) cd (4140) qt (5Al-2.5Sn) W Mo (pure) cw Hard to measure, in ceramic matrix and epoxy matrix composites, since in tension, fracture usually occurs before yield. H DPE PP humid dry PC PET Room T values Based on data in Table B4, Callister 7e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered

19 Tensile Strength, TS TS engineering stress strain engineering strain
• Maximum stress on engineering stress-strain curve. y strain Typical response of a metal F = fracture or ultimate strength Neck – acts as stress concentrator engineering TS stress engineering strain Adapted from Fig. 6.11, Callister 7e. • Metals: occurs when noticeable necking starts. • Polymers: occurs when polymer backbone chains are aligned and about to break.

20 Tensile Strength : Comparison
Si crystal <100> Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers Tensile strength, TS (MPa) PVC Nylon 6,6 10 100 200 300 1000 Al (6061) a ag Cu (71500) hr Ta (pure) Ti Steel (1020) (4140) qt (5Al-2.5Sn) W cw L DPE PP PC PET 20 30 40 2000 3000 5000 Graphite Al oxide Concrete Diamond Glass-soda Si nitride H wood ( fiber) wood(|| fiber) 1 GFRE (|| fiber) ( fiber) C FRE A FRE( fiber) E-glass fib Aramid fib Room Temp. values Based on data in Table B4, Callister 7e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered AFRE, GFRE, & CFRE = aramid, glass, & carbon fiber-reinforced epoxy composites, with 60 vol% fibers.

21 Ductility x 100 L EL % - = • Plastic tensile strain at failure: e s Lf
o f - = • Plastic tensile strain at failure: Engineering tensile strain, e E ngineering tensile stress, s smaller %EL larger %EL Lf Ao Af Lo Adapted from Fig. 6.13, Callister 7e. • Another ductility measure: 100 x A RA % o f - =

22 Toughness • Energy to break a unit volume of material
• Approximate by the area under the stress-strain curve. very small toughness (unreinforced polymers) Engineering tensile strain, e E ngineering tensile stress, s small toughness (ceramics) large toughness (metals) Adapted from Fig. 6.13, Callister 7e. Brittle fracture: elastic energy Ductile fracture: elastic + plastic energy

23 Resilience, Ur 2 1 U e s @ Ability of a material to store energy
Energy stored best in elastic region If we assume a linear stress-strain curve this simplifies to y r 2 1 U e s @ Adapted from Fig. 6.15, Callister 7e.

24 Elastic Strain Recovery
Adapted from Fig. 6.17, Callister 7e.

25 Hardness • Resistance to permanently indenting the surface.
• Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. e.g., 10 mm sphere apply known force measure size of indent after removing load d D Smaller indents mean larger hardness. increasing hardness most plastics brasses Al alloys easy to machine steels file hard cutting tools nitrided diamond

26 Hardness: Measurement
Rockwell No major sample damage Each scale runs to 130 but only useful in range Minor load kg Major load (A), 100 (B) & 150 (C) kg A = diamond, B = 1/16 in. ball, C = diamond HB = Brinell Hardness TS (psia) = 500 x HB TS (MPa) = 3.45 x HB

27 Hardness: Measurement
Table 6.5

28 True Stress & Strain Note: S.A. changes when sample stretched
True Strain Adapted from Fig. 6.16, Callister 7e.

29 ( ) Hardening s e • An increase in sy due to plastic deformation.
large hardening small hardening y 1 • Curve fit to the stress-strain response: s T = K e ( ) n “true” stress (F/A) “true” strain: ln(L/Lo) hardening exponent: n = 0.15 (some steels) to n = 0.5 (some coppers)

30 Variability in Material Properties
Elastic modulus is material property Critical properties depend largely on sample flaws (defects, etc.). Large sample to sample variability. Statistics Mean Standard Deviation All samples have same value Because of large variability must have safety margin in engineering specifications where n is the number of data points

31 Design or Safety Factors
• Design uncertainties mean we do not push the limit. • Factor of safety, N Often N is between 1.2 and 4 • Example: Calculate a diameter, d, to ensure that yield does not occur in the 1045 carbon steel rod below. Use a factor of safety of 5. 1045 plain carbon steel: s y = 310 MPa TS = 565 MPa F = 220,000N d L o 5 d = m = 6.7 cm

32 Summary • 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. • Toughness: The energy needed to break a unit volume of material. • Ductility: The plastic strain at failure.

33 ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: