Chapter 6:Mechanical Properties

Transcription

1 Chapter 6:Mechanical Properties Why mechanical properties? Need to design materials that can withstand applied load e.g. materials used in building bridges that can hold up automobiles, pedestrians materials for space exploration materials for skyscrapers in the Windy City NASA materials for and designing MEMs and NEMs Space elevators? Chapter 6-1

2 ISSUES TO ADDRESS... Stress and strain: What are they and why are they used instead of load and deformation? Elastic behavior: When loads are small, how much deformation occurs? What materials deform least? Plastic behavior: At what point does permanent deformation occur? What materials are most resistant to permanent deformation? Toughness and ductility: What are they and how do we measure them? Chapter 6-2

3 Stress and Strain Stress: Pressure due to applied load. tension, compression, shear, torsion, and their combination. stress = σ = force area Strain: response of the material to stress (i.e. physical deformation such as elongation due to tension). Chapter 6-3

4 Tension Compression Shear Torsion Chapter 6-4

5 COMMON STATES OF STRESS Simple tension: cable F F Ao = cross sectional Area (when unloaded) σ = F A o σ σ Ski lift (photo courtesy P.M. Anderson) From Callister 6e resource CD. Chapter 6-5

6 COMMON STATES OF STRESS Simple compression: A o Canyon Bridge, Los Alamos, NM (photo courtesy P.M. Anderson) Balanced Rock, Arches National Park (photo courtesy P.M. Anderson) σ = F A o Note: compressive structure member (σ < 0 here). From Callister 6e resource CD. Chapter 6-6

7 COMMON STATES OF STRESS Hydrostatic compression: Fish under water (photo courtesy P.M. Anderson) σ < 0 h From Callister 6e resource CD. Chapter 6-7

8 Tension and Compression Tension Engineering stress = Engineering strain = σ = ε = F A o li l l A o = original cross sectional area l i = instantaneous length l o = original length Note: strain is unitless. o o = l l o Compression Same as tension but in the opposite direction (stress and strain defined in the same manner). By convention, stress and strain are negative for compression. Chapter 6-8

9 Shear Pure shear stress = τ = F A o Pure shear strain = γ = tanθ Strain is always dimensionless. Chapter 6-9

10 Elastic Deformation 1. Initial 2. Small load 3. Unload bonds stretch δ Elastic means reversible! F -a non-permanent deformation where the material completely recovers to its original state upon release of the applied stress. F δ return to initial Linearelastic Non-Linearelastic Chapter 6-10

11 Plastic Deformation (Metals) 1. Initial 2. Small load 3. Unload bonds stretch planes & planes still shear sheared δelastic + plastic δplastic F F Plastic means permanent! linear elastic δ plastic linear elastic δ Chapter 6-11

12 Typical tensile test machine Stress-Strain Testing Typical tensile specimen extensometer specimen Adapted from Fig. 6.2, Callister 7e. gauge length 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.) Chapter 6-12

13 Linear Elastic Properties Modulus of Elasticity, E: (also known as Young's modulus) F Hooke's Law: stress σ = E ε Modulus of elasticity (Young s modulus) strain Measure of material s resistance to elastic deformation (stiffness). σ Linearelastic E F simple tension test ε For metals, typically E ~ GPa Chapter 6-13

14 Note: some materials do not have linear elastic region (e.g. cast iron, concrete, many polymers ) Define secant modulus and tangent modulus. σ Tangent modulus = slope of the tangent line σ 2 σ 1 Secant modulus = σ = ε σ1 ε 1 ε 1 ε Chapter 6-14

15 Silicon (single crystal) (depends on crystallographic direction) Glass (pyrex) 70 SiC (fused or sintered) Graphite (molded) ~12 High modulus C-fiber 400 Carbon Nanotubes ~1000 If we normalize to density: ~20 times that of steel wire Density normalized strength is ~56X that of steel wire Chapter 6-15

16 Poisson Ratio So far, we ve considered stress only along one dimension Along z: tension ε z = l l o z x d o l o d o + d l o + l Along x: compression Isotropic x and y: Poisson ratio = υ = ε x = d d o ε y = ε x ε ε x y = ε ε z z Compression Elongation Relation between elastic and shear moduli: E = 2G(1+ν) Chapter 6-16

17 Poisson Ratio Poisson Ratio has a range 1 ν 1/2 Look at extremes No change in aspect ratio: w /w = l /l w /w ν = l /l = 1 l w Volume (V = AL) remains constant: V =0 or l A = - A l σ Hence, V = (l A+A l) = 0. In terms of width, A = w 2, and A = w 2 - (w+ w) 2 = 2w w + w 2 then A/A = 2 w/w + w 2 /w 2 in the limit of small changes A/A = 2 w/w then 2 w/w = - l/l ν = w w l / l / 2 1 ( l / l) = l / l = 1/ 2 Chapter 6-17

18 Poisson's ratio, ν Poisson's ratio, ν: ε L ε ν = L ε ε metals: ν ~ 0.33 ceramics: ν ~ 0.25 polymers: ν ~ ν Units: E: [GPa] or [psi] ν: dimensionless ν > 0.50 density increases ν < 0.50 density decreases (voids form) Chapter 6-18

19 Poisson Ratio: materials specific Metals: Ir W Ni Cu Al Ag Au generic value ~ 1/3 Solid Argon: 0.25 Covalent Solids: Si Ge Al 2 O 3 TiC generic value ~ 1/4 Ionic Solids: MgO 0.19 Silica Glass: 0.20 Polymers: Network (Bakelite) 0.49 Chain (PE) 0.40 Elastomer: Hard Rubber (Ebonite) 0.39 (Natural) 0.49 Chapter 6-19

20 Mechanical Properties 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. Chapter 6-20

21 Elastic Shear modulus, G: τ = G γ Other Elastic Properties τ G γ M simple torsion test Elastic Bulk modulus, K: P = -K V Vo Special relations for isotropic materials: G = E 2(1 + ν) K = P K E 3(1 2ν) V Vo P M P P pressure test: Init. vol =V o. Vol chg. = V Chapter 6-21

22 Young s Moduli: Comparison E(GPa) 10 9 Pa Metals Alloys Tungsten Molybdenum Steel, Ni Tantalum Platinum Cu alloys Zinc, Ti Silver, Gold Aluminum Magnesium, Tin Graphite Ceramics Semicond Diamond Si carbide Al oxide Si nitride <111> Si crystal <100> Glass -soda Concrete Graphite Polymers Composites /fibers Polyester PET PS PC PP HDPE PTFE LDPE Carbon fibers only CFRE( fibers)* Aramid fibers only AFRE( fibers)* Glass fibers only GFRE( fibers)* GFRE* CFRE* GFRE( fibers)* CFRE( fibers) * AFRE( fibers) * Epoxy only Wood( grain) 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. Chapter 6-22

23 Simple tension: Ao Useful Linear Elastic Relationships δ = FL o EA o F δ L = ν Fw o EA o δ/2 Simple torsion: α = 2ML o πr o 4 G M = moment α = angle of twist w o L o L o δ L /2 2r o Material, geometric, and loading parameters all contribute to deflection. Larger elastic moduli minimize elastic deflection. Chapter 6-23

24 Plastic (Permanent) Deformation Simple tension test: engineering stress, σ Elastic+Plastic at larger stress Adapted from Fig (a), Callister 7e. Elastic initially permanent (plastic) after load is removed ε p engineering strain, ε plastic strain A permanent deformation (usually considered for T<T m /3). Atoms break bonds and form new ones. In metals, plastic deformation occurs typically at strain Chapter 6-24

25 Tensile properties A. Yield strength (σ y ): the strength required to produce a very slight yet specified amount of plastic deformation. What is the specified amount of strain? Strain offset method 1. Start at strain (for most metals). σ y σ P 2. Draw a line parallel to the linear region. 3. σ y = where the dotted line crosses the stress-strain curve. P = proportional limit (beginning of deviation from linear behavior. Mixed elastic-plastic behavior Elastic region ε For materials with nonlinear elastic region: σ y is defined as stress required to produce specific amount of strain (e.g. ~0.005 for most metals). Chapter 6-25

26 Tensile properties Yield point phenomenon occurs when elastic-plastic transition is welldefined and abrupt. No offset methods required here. Fig Callister Chapter 6-26

27 Yield strength, σy (MPa) Yield Strength : Comparison Metals/ Alloys Steel (4140) qt Ti (5Al-2.5Sn) W (pure) a Cu (71500) Mo (pure) cw Steel (4140) a Steel (1020) cd Al (6061) ag Steel (1020) hr Ti (pure) a Ta (pure) Cu (71500) hr Al (6061) a Graphite/ Ceramics/ Semicond Hard to measure, since in tension, fracture usually occurs before yield. Polymers PC Nylon 6,6 PET humid PVC PP HDPE dry Composites/ fibers Hard to measure, in ceramic matrix and epoxy matrix composites, since in tension, fracture usually occurs before yield. σy(ceramics) >>σy(metals) >> σy(polymers) 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 10 Tin (pure) LDPE Chapter 6-27

28 Tensile Strength, TS Maximum stress on engineering stress-strain curve. TS σ y engineering stress strain engineering strain Metals: occurs when noticeable necking starts. Polymers: occurs when polymer backbone chains are aligned and about to break. Typical response of a metal Adapted from Fig. 6.11, Callister 7e. F = fracture or ultimate strength Neck acts as stress concentrator Chapter 6-28

29 True stress and strain TS engineering stress Typical response of a metal strain Recall: Engineering Stress = σ = F A o Notice that past maximum stress point, σ decreases. Does this mean that the material is becoming weaker? Necking leads to smaller cross sectional area! Original cross sectional area! True Stress = True Strain = σ ε T T = F A i l = ln l i o A i = instantaneous area l i = instantaneous length If no net volume change (i.e. A i l i = A o l o ) σ ε T T = σ (1 + ε ) = ln(1 + ε ) Only true at the onset of necking Chapter 6-29

30 Example problem Calculate/determine the following for a brass specimen that exhibits stress-strain behavior shown on the left. 1) Modulus of elasticity. 2) Yield strength. 3) Maximum load for a cylindrical specimen with d = 12.8mm. 4) Change in length at 345MPa if the initial length is 250mm. Chapter 6-30

31 Tensile strength, TS (MPa) Tensile Strength : Comparison Metals/ Alloys Steel (4140) qt W (pure) Ti (5Al-2.5Sn) Steel (4140) a a Cu (71500) Cu (71500) hr cw Steel (1020) Al (6061) ag Ti (pure) a Ta (pure) Al (6061) a Graphite/ Ceramics/ Semicond Diamond Si nitride Al oxide Si crystal <100> Glass-soda Concrete Graphite Polymers Nylon 6,6 PC PET PVC PP LDPE HDPE Composites/ fibers C fibers Aramid fib E-glass fib AFRE( fiber) GFRE( fiber) CFRE( fiber) wood( fiber) GFRE( fiber) CFRE( fiber) AFRE( fiber) wood ( fiber) 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. Chapter 6-31

32 Tensile properties C. Ductility: measure of degree of plastic deformation that has been sustained at fracture. Ductile materials can undergo significant plastic deformation before fracture. Brittle materials can tolerate only very small plastic deformation. Chapter 6-32

33 Ductility Plastic tensile strain at failure: Engineering tensile stress, σ smaller %EL larger %EL L %EL = f L o L o x 100 A L o o A f L f Adapted from Fig. 6.13, Callister 7e. Engineering tensile strain, ε Another ductility measure: A A f %RA = x 100 A o - o Chapter 6-33

34 Energy to break a unit volume of material Approximate by the area under the stress-strain curve. Engineering tensile stress, σ Toughness small toughness (ceramics) large toughness (metals) Adapted from Fig. 6.13, Callister 7e. very small toughness (unreinforced polymers) Engineering tensile strain, ε Brittle fracture: elastic energy Ductile fracture: elastic + plastic energy Chapter 6-34

35 Resilience, U r Ability of a material to store energy Energy stored best in elastic region U = ε r 0 y σdε If we assume a linear stress-strain curve this simplifies to U r 1 2 σ y ε y Adapted from Fig. 6.15, Callister 7e. Chapter 6-35

36 Elastic recovery after plastic deformation This behavior is exploited to increase yield strengths of metals: strain hardening (also called cold working). Chapter 6-36

37 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. most plastics brasses Al alloys easy to machine steels file hard cutting tools nitrided steels diamond increasing hardness Chapter 6-37

38 Rockwell Hardness: Measurement No major sample damage Each scale runs to 130 but only useful in range Minor load 10 kg Major load 60 (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 Chapter 6-38

39 Hardness scales Indentation with diamond pyramid tip Indentation with spherical hardened steel and conical diamond (for hardest materials) Indentation with spherical hardened steel or tungsten carbide tip. Chapter 6-39 Qualitative scale

40 Hardness: Measurement Table 6.5 Chapter 6-40

41 True Stress & Strain Note: S.A. changes when sample stretched True stress True Strain σt = F A i ε T = ln l ( l ) i o σ ε T T = σ = ( 1+ ε) ( + ε) ln 1 Adapted from Fig. 6.16, Callister 7e. Chapter 6-41

42 Hardening An increase in σ y due to plastic deformation. σ σ y1 large hardening σ y0 small hardening Curve fit to the stress-strain response: σ T = K( ε ) n T hardening exponent: n = 0.15 (some steels) to n = 0.5 (some coppers) true stress (F/A) true strain: ln(l/l o ) ε Chapter 6-42

43 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 x = n Σ x n n Standard Deviation s = n Σ ( x x ) i 2 n where n is the number of data points Chapter 6-43

44 Design or Safety Factors Design uncertainties mean we do not push the limit. Factor of safety, N σ working = σ y 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. σ 220, 000N π ( d 2 / 4) working 5 = σ y N d = m = 6.7 cm 1045 plain carbon steel: σy = 310 MPa TS = 565 MPa F = 220,000N d Chapter 6-44 Lo

45 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 σ y. Toughness: The energy needed to break a unit volume of material. Ductility: The plastic strain at failure. Chapter 6-45