I.Lecture outline A.Introduction to materials B.Solids 1.Form 2.Bonding 3.Hooke’s Law….stress, strain 4.Elasticity C.Material Strength D.Strength testing

I. Lecture outlineA. Introduction to materials

B. Solids1. Form

2. Bonding

3. Hooke’s Law….stress, strain

4. Elasticity

C. Material Strength

D. Strength testing

materials

How does processing influence structure?Why is this important????This will influence material properties….and ultimately performance

Engineering Innovation |Materials

materials

some of the things made possible/impacted by materials science…….



what is structure?what is the basis of structure??

a little chemistry is required at this point…….


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II. some chemistryA. protons, neutrons & electrons atom

1. what are atoms?

smallest subunit of an element

2. protons & neutrons nucleus

3. electron “cloud”

4. # of protons determines identity

5. # electrons = # protons (neutral)

6. Electrons arranged in shells

7. Electrons are the basis of materials properties

atom = stadiumnucleus = housefly on center

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II. some chemistryA. atoms

8. All atoms of a given element are identical

9. Atoms of different elements have different masses

10. a compound is a specific combination of atoms of >1 element

11. in a chemical reaction, atoms are neither created nor destroyed – only change partners to produce new substances

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12. Can we see them?

Yes

electron microscopy or scanning probe microscopy

Xe on Ni Au surface

http://www.almaden.ibm.com/vis/stm/gallery.html

Xe on Ni Au surface


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13. What can they do?

a. form bonds with other similar atoms – elemental substances (molecules, metals, network solids)

b. form bonds with atoms of other elements to make compounds

http://www.almaden.ibm.com/vis/stm/gallery.html

science’s quest for simplicity…..various combinations of the 100 elements make up all matter on earth

III. what holds the atoms in a crystal/ceramic/polmyer/elastomer together?........primary bonds

A. Covalent bonding1. Two or more atoms share electrons

2. Strong and rigid

3. Found in organics and sometimes ceramics

4. Strongly directional

5. E.g.methane CH4

6. C has 4 valence electrons; H has 1

7. Elemental solids e.g. diamond

8. Can be strong (diamond)

9. Can be weak (Bi)

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III. what holds the atoms in a crystal/ceramic/polmyer/elastomer together?........ primary bonds

B. ionic bonding1. Metal and non-metal

2. Metal gives up valence electron(s) to non-metal

3. Result is all atoms have a stable configuration…also an electrical charge

4. E.g. Na+Cl-

5. metal becomes +ly charged (cation); non-metal becomes –ly charges (anion)

6. Electrostatic attraction

7. Omnidirectional

8. Close-packed

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III. what holds the atoms in a crystal/ceramic/polmyer/elastomer together?........primary bonds

C. metallic bonding1. Hold metals and alloys together

2. Enables dense packing of atoms – reason why metals are heavy

3. Valence electrons (1, 2 or at most 3) not bound to a particular atom

4. Free to drift throughout the entire material –”sea of electrons”

5. Nonvalence electrons + atomic nuclei = ion core (net + charge)

6. Good conductors of electrons & heat

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III. what holds molecules together?........secondary bonds

2ndry bonds are physical bonds and are weaker than what we’ve just talked about

A. Hydrogen bonds1. Intermolecular attraction in which a H atom bonded to a small, electronegative

atom (N, O or F)is attracted to lone pair of electrons on another N, O or F

2. Weak

3. Due to charge distribution on molecule

4. Often seen in organic compounds

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III. what holds molecules together?........secondary bondsB. Van der Waals forces

1. Again, interactions are much weaker (~10kJ/mol) as compared to chemical bonds (100kJ/mol)

2. Forces arising from surface differences across molecules

3. Gecko feet: microscopic branched elastic hairs on toes which take advantage of these atomic-scale attractive forces to grip and support heavy loads

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Autumn et al. PNAS 2002, 99, 12252

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Takes a fictional superhero to bringnanotech to the under 5’s

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IV. structureA. What do I mean by structure?

1. Structure is related to the arrangement of a materials components

a. This could be on any length scale

b. Atomic, nano-, micro-, macro-

c. All of these length scales matter

2. Types of carbon (literally just carbon)

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Diamond Graphite C60 - FullereneCarbon nanotubes


V. propertiesA. A material trait in terms of the kind and magnitude of response to an

imposed stimulus1. e.g. sample subjected to force will experience deformation

2. A polished metal surface will reflect light

B. Categories of properties1. Mechanical, electrical, thermal, magnetic, optical & deteriorative

2. Each has a characteristic stimulus provoking a response

C. mechanical properties relate deformation to an applied load or force

D. mechanical properties include elastic modulus, strength

E. Electrical properties (conductivity) respond to an electric field

what causes differences in properties of materials???

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Many properties of a material are consequence of1. Identity of atoms that comprise them

2. Spatial arrangement of those atoms

3. Interactions between atoms

atomic structure and bonding are important

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Material properties

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Same material – aluminum oxide. Depending on structure (which is influenced by processing) materials are transparent, translucent, opaque


VI. Solid materialsA. Classification

1. Crystalsa. molecules attracted to one another try to cohere in a systematic way, minimizing

volume (dense materials)

b. stiff yet ductile (capable of large amounts of deformation without fracture)

2. Glasses/ceramicsa. materials whose high viscosity at liquid/solid point prevents crystallization –

amorphous

b. E.g. porcelain, SiO2, glass, cement

c. Stiff, strong, hard BUT very brittle and susceptible to fracture

d. insulators

3. Polymersa. materials built up of long chains of simple molecular structures… plastics and living

things

b. Low densities

c. Extremely ductile, pliable – can be formed into complex shapes

d. Soften/decompose at high T

4. Elastomersa. long-chain polymers which fold or coil e.g. artificial rubber

b. Totally elastic due to cross-linking

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VI. solid materialsElastomers

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UnstressedAmorphousTwisted, kinked, coiled

Elastic deformationPartial uncoiling, straighteningelongation

Removal of stress…..spring back


silly putty smash

silly putty pull

http://www.youtube.com/watch?v=uWYxc8xhihg

http://www.youtube.com/watch?v=383VnMCl5cY

VII. mechanical properties

Let’s think about spaghettiA. How easy is to break it by pulling (tension)?

B. Is thicker spaghetti easier or harder to break by pulling?

C. Theory says that force needed increases with cross sectional area

D. How easily will it buckle if you compress the ends?

E. Depends on force, material strength, length and thickness of spag1. A longer piece buckles easier than a shorter piece

2. Thinner piece buckles easier than a thicker piece

F. How easily will it bend if you push perpendicular?

G. Is it tension, compression?

H. Deflection depends on force, material strength, length of span, area of spaghetti

1. Larger force, larger deflection

2. For a given force, longer pieces bend easier

3. For a given force, thin pieces bend easier

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spaghetti crop


VII. mechanical properties

How do engineers figure in the picture?

2 concepts: stress and strain

structural engineers: determine stress/strain distributions in objects subjected to well-defined loads (beams in bridges)

materials/metallurgical engineers: produce materials that will have the desired mechanical properties

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VII. mechanical propertiesA. first need to define stress and strain

1. stress is related to the force or load applied to a material

a. stress = = force/original area

b. from figure: = F/A0 (units?)

F: newton = kg m / s2

= F/A0 = N/m2

pascal = N/m2

MPa = 106 Pa, GPa = 109 Pa

from figure: = F/A0 Pa or F/A0 x 10-6 MPa

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VII. mechanical propertiesA. first need to define stress and strain

2. strain is related to the response of the material to the applied force

a. strain = ε = change in length over original length Δl/l0

b. strain is unitless but m/m (or in/in) may be used;

strain can be expressed as a %

c. 2 types: elastic & plastic strain/deformation,

(i) elastic strain exists only while stress is applied;

elasticity

(ii) plastic strain does not disappear upon removal of

stress; plasticity

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VII. mechanical propertiesD. End up with a stress-strain curve

1. provides huge amount of information about material properties

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VII. mechanical propertiesD. End up with a stress-strain curve

2. Initial part of curve is especially interesting…..

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Yield strength

Yield strength:Load required to go fromelastic-plastic deformation


VII. mechanical propertiesE. Hooke’s Law and Young’s modulus, E

1. stress () and strain (ε) are proportional under certain conditions (low stress)

a. = εelE Hooke’s Law

b. E - Young’s modulus, modulus of elasticity, stiffness, resistance to elastic deformation (GPa or psi)

c. physical meaning of E being large?

higher E implies greater stiffness

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Material range of EMetal 45 – 400 GPaCeramics 60 – 500 GPaPolymers 0.01 – 4 GPaSpaghetti 4.8 GPa

VII. mechanical propertiesF. microscopic description of elastic deformation

1. strain manifests as small changes in interatomic spacing of bonds

2. |E | is a measure of resistance to separation of adjacent atoms/ions/ molecules (i.e. it is related to bonding forces)

ordr

dFE

Or differences in E are due to differences in bonding!

In other works microscopic (bonding) determines macroscopic (E)

Also as T increases, E generally decreases

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VII. mechanical propertiesG. Young’s modulus, E for different materials

1. Values of E for ceramics are similar to metals; for polymers E is lower

Why?

5. As temperature increases, E diminishes_?__

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VII. mechanical propertiesH. tensile strength (TS)

1. maximum load / initial area

a. TS is the stress value at the maximum of the s-s curve, point M

b. corresponds to maximum stress sustainable by a structure in tension

c. if this stress is maintained, fracture will result

d. All deformation so far is uniform throughout speciman

2. at point M, neck formation occurs

3. stress is concentrated at M

4. fracture ultimately occurs at F

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2. mechanical properties of materialsmaterials


VII. mechanical propertiesI. ductility and elongation

1. ductility is the degree of plastic deformation at (prior to) failure

2. low or no ductility – brittle

3. ductility is quantified as % elongation, %EL

(i)

lf = length at fracture

l0 = initial length

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%100%

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llEL


VIII. Material strengthA. Tensile strength

1. How hard does something need to be pulled to break material bonds

2. Some examples:a. Steel piano wire = 450,000 psi

b. Aluminum = 10,000 psi

c. Concrete = 600 psi

B. Compression strength1. Materials fail in compression in many ways depending on geometry, support

a. Buckling – hollow cylinders e.g. tin can

b. Bending – long rod or panel

c. Shattering – heavily loaded glass

C. Yield strength1. Load required to cross line from elastic to plastic deformation

D. Ultimate tensile strength1. Maximum possible load without failure

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IX. Material testingA. Tensile strength… most common method

1. apply stress uniaxially along sample

2. continually increase force on ends

3. perform test until fracture (sample breaks)

4. measure force vs. sample elongation

5. tensile testing machine elongates specimen at a constant rate

6. applied load and resulting elongations are continuously and simultaneously measured

specimenextensometer

steel 1018 stress-strain


IX. Material testingAside…..

1. in stress-strain plots it appears that stress is decreasing between

M and F

2. it is not decreasing….any ideas what is happening?

3. cross-sectional area is decreasing in the necking region

4. results in a reduction in the load-bearing capacity of specimen

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IX. Material testingB. Euler buckling load, Pc

1. P load (MLT-2)

2. I moment of inertia (L4)

3. E Young’s modulus (ML-1T2)

4. L length (L)

5. 4 variables, 3 primitive dimensions = 1 dimensionless group

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IX. Material testingWhat if the material is very brittle….can we do a tensile test?

Tensile tests can’t easily be done on ceramics/brittle material because

A. Difficult to prepare and test samples with required geometry

B. Difficult to grip brittle materials without fracturing them

C. Ceramics fail very quickly (0.1% strain)

Transverse bending test is more usually employed

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IX. Material testingC. bending

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IX. Material testingC. Bending

1. At point of loading, top surface is in compression and bottom surface is in tension

2. Stress is computed from specimen thickness, the bending moment, and the moment of inertia of cross-section

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IX. Material testing

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minimizing moments of inertia to increase rates of rotation


IX. Material testingD. Compressive strength

1. what’s going to happen a beam (spaghetti) under compression?a. Will fail by crushing or buckling, depending on material and L/d

b. Crushing: atomic bonds begin to fail, inducing increased local stresses, which causes more bonds to fail

c. Buckling: complicated as there are many modes

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Documents

I.Lecture outline A.Introduction to materials B.Solids 1.Form 2.Bonding 3.Hooke’s Law….stress, strain 4.Elasticity C.Material Strength D.Strength testing