MSE 200 Unit 2

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safe working stress

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73 Terms

1

safe working stress

σw = σy/ N

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2

define ductility

plastic tensile strain at failure

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3

define tensile strength

maximum strength. point of fracture. metals- when necking starts, polymers- when backbone chains are about to break. max point on a graph

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4

yield in metals

by disl motion, increased by obstacles due to disl motion

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5

yield in ceramics

disl motion difficult, tends to fracture before disl motion

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6

yield on polymers

yes yield, no disl motion

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7

toughness

energy to break a unit volume of material, approx by area under the stress-strain curve

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8

resilience

ability to store energy, the integral of stress

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9

yield strength

place where noticeable plastic def occurs, σy

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10

elastic deformation

a 2-way arrow, not permanent

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11

modulus of elasticity

directly proportional to Eo

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12

hardness

localized plastic def, resistance to permanently indenting the surface

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13

large hardness means

resistance to plastic deformation or cracking in compression, better wear properties

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14

metals

disl motion easier due to non-directional bonds, close packing, and ion cores

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15

covalent ceramics

Si, Diamond. disl motion harder due to directional bonding

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16

ionic ceramics

NaCl. motion hard due to avoidance of ++/-- neighbors

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17

to strengthen materials

make disl motion harder by adding slip barriers

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18

ease of disl movement factors

crystal structure, class of material, temp of material, presence of barriers to slip (grain boundaries)

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19

slip system

combination of slip plane and direction

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20

FCC slip system

{111} plane,
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21

BCC slip system

thermally activated. {110} plane,
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22

HCP plane

{0001} only close packed plane, more brittle

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23

disl motion and burgers vector

for edge disl, disl motion is in the same direction as b

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24

resolved sheer stress

Tr, why crystals slip, θcosλcosθ

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25

Tr

Fs/ As

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26

condition for disl motion

Tr > Tcrss

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27

4 strengthening mechanisms

all reduce ductility. grain refining, alloying or solid sol strengthening, cold working or strain hardening, precipitation hardening

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28

grain refining

smaller grains in a greater boundary area. hall-petch eqn

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29

Hall-Petch eqn

σ yield = σo + Ky d ^-1/2

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30

solid solution strengthening/ alloying

lattice strains of impurities interact with disl movement, alloys are stronger than pure metals

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31

cold working (strain hardening)

metal becomes harder and stronger when it accumulates dislocations; dislocations entangle; density increases

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32

precipitation hardening

volume obstacles, disl cut through or go around large obstacles (precipitates)

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33

3 annealing stages

recovery, recrystallization, grain growth

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34

recovery

revert partially back to pre-cold working values; disl density decreases, no microstructural changes. some relief of internal strain by disl motion

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35

recrystallization

deformed grains are replaced by new grains, properties fully returned to pre-cold work.

new set of strain-free grains forms, and the material becomes softer and more ductile.

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36

recrystallization temp

temp 100% strain free grains in 1 hr. temp up time to anneal down. increased % cold working decreases temp

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37

grain growth

small grains disappear, unit volume decreases.

d^n - do ^n = kt

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38

v and 0.5

greater than 0.5, density increases. less, decreases and void forms

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39

E rankings big to small

ceramics then metals then polymers

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40

yield strength number

.002

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41

disl slip more difficult where

low atomic density plane

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42

polycrystalline metals

stronger than single crystals

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43

increasing temp

decreased yield strength and TS, increased %EL

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44

Tr

recrystallization temperature = point of highest rate of property change

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45

ductile fracture

occurs with plastic deformation, not crack sensitive, desirable, larhe %AR %EL

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46

brittle fracture

little to no plastic def, catastrophic, crack sensitive, small %AR %EL

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47

transgranular brittle fracture

breaking atomic bonds

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48

intergranular brittle fracture

along grain boundaries

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49

stress concentration

due to microscopic flaws or cracks, measured value smaller than predicted

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50

maximum stress

at the crack tip

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51

effect of stress raiser

higher in brittle materials; ductile has more uniform distribution of stress

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52

ductile materials

deform at the tip and blunt at the tip, sharp tips indicate larger stress conc

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53

plastic def occurs when

σm > σys, elastic strain energy is released when crack propogates

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54

fracture toughness Kc

resistance to brittle fracture when a crack is present. metals and alloys are the most forgiving

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55

plane strain

thickness > crack dimensions

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56

temp/strain rate and Kc

directly proportional

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57

microstructure and Kc

inversely proportional

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58

impact loading

makes material more brittle and decreases toughness

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59

impact loading rate proportions

directly to sigma y and Ts, inversely to %EL

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60

ductile to brittle transition temp DBTT

sharpest slope indicates; crystal structure impacts

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61

how to decrease impact energy

increase yield strength, decrease temp, increase strain rate

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62

impact test

qualitative

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63

fatigue

failure under cyclic stress

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64

fatigue life

number ot cycles to cause failure

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65

fatigue limit

no fatigue if S (stress amp) < S fatigue

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66

stress and fatigue life

increase stress levels decrease fatigue life. remove stress concentrations (sharp corners) to improve life

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67

creep

higher temperature test

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68

primary creep

slop decresaes with time, increase in strain hardening

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69

secondary creep

constant (steady-state) slope, competition between recovery and strain hardening

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70

tertiary creep

slope increases, accelerates to rupture

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71

creep minimized when

Tm up, E up, large grain size prevents sliding

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72

relationships between the direction of the applied shear stress and the direction of dislocation line motion

edge dislocation--parallel

screw dislocation--perpendicular

mixed dislocation--neither parallel nor perpendicular

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73

small-angle grain boundaries are not as effective in interfering with the slip process as are high-angle grain boundaries

not as much crystallographic misalignment in the grain boundary region for small-angle, and therefore not as much change in slip direction

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