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MSE 200 Unit 2
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73 Terms
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1
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safe working stress
σw = σy/ N
2
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define ductility
plastic tensile strain at failure
3
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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
4
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yield in metals
by disl motion, increased by obstacles due to disl motion
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yield in ceramics
disl motion difficult, tends to fracture before disl motion
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yield on polymers
yes yield, no disl motion
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toughness
energy to break a unit volume of material, approx by area under the stress-strain curve
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resilience
ability to store energy, the integral of stress
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yield strength
place where noticeable plastic def occurs, σy
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elastic deformation
a 2-way arrow, not permanent
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modulus of elasticity
directly proportional to Eo
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hardness
localized plastic def, resistance to permanently indenting the surface
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large hardness means
resistance to plastic deformation or cracking in compression, better wear properties
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metals
disl motion easier due to non-directional bonds, close packing, and ion cores
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covalent ceramics
Si, Diamond. disl motion harder due to directional bonding
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ionic ceramics
NaCl. motion hard due to avoidance of ++/-- neighbors
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to strengthen materials
make disl motion harder by adding slip barriers
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ease of disl movement factors
crystal structure, class of material, temp of material, presence of barriers to slip (grain boundaries)
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slip system
combination of slip plane and direction
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FCC slip system
{111} plane,
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BCC slip system
thermally activated. {110} plane,
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HCP plane
{0001} only close packed plane, more brittle
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disl motion and burgers vector
for edge disl, disl motion is in the same direction as b
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resolved sheer stress
Tr, why crystals slip, θcosλcosθ
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Tr
Fs/ As
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condition for disl motion
Tr > Tcrss
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4 strengthening mechanisms
all reduce ductility. grain refining, alloying or solid sol strengthening, cold working or strain hardening, precipitation hardening
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grain refining
smaller grains in a greater boundary area. hall-petch eqn
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Hall-Petch eqn
σ yield = σo + Ky d ^-1/2
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solid solution strengthening/ alloying
lattice strains of impurities interact with disl movement, alloys are stronger than pure metals
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cold working (strain hardening)
metal becomes harder and stronger when it accumulates dislocations; dislocations entangle; density increases
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precipitation hardening
volume obstacles, disl cut through or go around large obstacles (precipitates)
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3 annealing stages
recovery, recrystallization, grain growth
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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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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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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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grain growth
small grains disappear, unit volume decreases.
d^n - do ^n = kt
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v and 0.5
greater than 0.5, density increases. less, decreases and void forms
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E rankings big to small
ceramics then metals then polymers
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yield strength number
.002
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disl slip more difficult where
low atomic density plane
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polycrystalline metals
stronger than single crystals
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increasing temp
decreased yield strength and TS, increased %EL
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Tr
recrystallization temperature = point of highest rate of property change
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ductile fracture
occurs with plastic deformation, not crack sensitive, desirable, larhe %AR %EL
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brittle fracture
little to no plastic def, catastrophic, crack sensitive, small %AR %EL
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transgranular brittle fracture
breaking atomic bonds
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intergranular brittle fracture
along grain boundaries
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stress concentration
due to microscopic flaws or cracks, measured value smaller than predicted
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maximum stress
at the crack tip
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effect of stress raiser
higher in brittle materials; ductile has more uniform distribution of stress
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ductile materials
deform at the tip and blunt at the tip, sharp tips indicate larger stress conc
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plastic def occurs when
σm > σys, elastic strain energy is released when crack propogates
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fracture toughness Kc
resistance to brittle fracture when a crack is present. metals and alloys are the most forgiving
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plane strain
thickness > crack dimensions
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temp/strain rate and Kc
directly proportional
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microstructure and Kc
inversely proportional
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impact loading
makes material more brittle and decreases toughness
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impact loading rate proportions
directly to sigma y and Ts, inversely to %EL
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ductile to brittle transition temp DBTT
sharpest slope indicates; crystal structure impacts
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how to decrease impact energy
increase yield strength, decrease temp, increase strain rate
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impact test
qualitative
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fatigue
failure under cyclic stress
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fatigue life
number ot cycles to cause failure
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fatigue limit
no fatigue if S (stress amp) < S fatigue
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stress and fatigue life
increase stress levels decrease fatigue life. remove stress concentrations (sharp corners) to improve life
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creep
higher temperature test
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primary creep
slop decresaes with time, increase in strain hardening
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secondary creep
constant (steady-state) slope, competition between recovery and strain hardening
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tertiary creep
slope increases, accelerates to rupture
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creep minimized when
Tm up, E up, large grain size prevents sliding
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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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*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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