MSE 200 Unit 2

safe working stress

safe working stress

σw = σy/ N

define ductility

plastic tensile strain at failure

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

yield in metals

by disl motion, increased by obstacles due to disl motion

yield in ceramics

disl motion difficult, tends to fracture before disl motion

yield on polymers

yes yield, no disl motion

toughness

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

resilience

ability to store energy, the integral of stress

yield strength

place where noticeable plastic def occurs, σy

elastic deformation

a 2-way arrow, not permanent

modulus of elasticity

directly proportional to Eo

hardness

localized plastic def, resistance to permanently indenting the surface

large hardness means

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

metals

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

covalent ceramics

Si, Diamond. disl motion harder due to directional bonding

ionic ceramics

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

to strengthen materials

make disl motion harder by adding slip barriers

ease of disl movement factors

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

slip system

combination of slip plane and direction

FCC slip system

{111} plane, <110> direction, 12 total

BCC slip system

thermally activated. {110} plane, <111> direction, 12

HCP plane

{0001} only close packed plane, more brittle

disl motion and burgers vector

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

resolved sheer stress

Tr, why crystals slip, θcosλcosθ

Tr

Fs/ As

condition for disl motion

Tr > Tcrss

4 strengthening mechanisms

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

grain refining

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

Hall-Petch eqn

σ yield = σo + Ky d ^-1/2

solid solution strengthening/ alloying

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

cold working (strain hardening)

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

precipitation hardening

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

3 annealing stages

recovery, recrystallization, grain growth

recovery

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

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.

recrystallization temp

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

grain growth

small grains disappear, unit volume decreases.

d^n - do ^n = kt

v and 0.5

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

E rankings big to small

ceramics then metals then polymers

yield strength number

.002

disl slip more difficult where

low atomic density plane

polycrystalline metals

stronger than single crystals

increasing temp

decreased yield strength and TS, increased %EL

Tr

recrystallization temperature = point of highest rate of property change

ductile fracture

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

brittle fracture

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

transgranular brittle fracture

breaking atomic bonds

intergranular brittle fracture

along grain boundaries

stress concentration

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

maximum stress

at the crack tip

effect of stress raiser

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

ductile materials

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

plastic def occurs when

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

fracture toughness Kc

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

plane strain

thickness > crack dimensions

temp/strain rate and Kc

directly proportional

microstructure and Kc

inversely proportional

impact loading

makes material more brittle and decreases toughness

impact loading rate proportions

directly to sigma y and Ts, inversely to %EL

ductile to brittle transition temp DBTT

sharpest slope indicates; crystal structure impacts

how to decrease impact energy

increase yield strength, decrease temp, increase strain rate

impact test

qualitative

fatigue

failure under cyclic stress

fatigue life

number ot cycles to cause failure

fatigue limit

no fatigue if S (stress amp) < S fatigue

stress and fatigue life

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

creep

higher temperature test

primary creep

slop decresaes with time, increase in strain hardening

secondary creep

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

tertiary creep

slope increases, accelerates to rupture

creep minimized when

Tm up, E up, large grain size prevents sliding

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

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