Dental pt7 Flashcards by Ann My

Stress formula, unit

stress=F/cross sectional area (f/cm2)

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Def stress

force apply to material NO depends only on force ALSO shape (cross section)
inversely prop cross sectional area ; proportional to load

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Types stress

tensile
compressive
shear

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Charac of all 3 stresses

all present at 1
func under same rule - if stress > strength material -> structure fail

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What force when breakage occurs

tensile force

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Strain def

temporary elongation when interxs tensile stress -> longer

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Calc value strain

length increase/og length

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Elasticity def

material have constant ratio stress-strain

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Modulus elasticity def, entails

constant of constant ratio stress/strain
x2 stress -> x2 strain
diff material - diff charac - diff constant ratio -> modulus elasticity

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Proportional limit/ elastic limit

greater stress when stress prop strain

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Yield strength/ yield point

strength measure at stress w/ little strain

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Ultimate tensile strength

(stretched to max) -> stress at breakage

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Work hardening

metals stress > prop limit -> hardening
-> stronger
-> harder
-> > brittle

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Elongation

total strain (elastic, plastic) at fracture

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Charac modulus elasticity

inherent property
NO alter

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Plasticity

elastic/ prop limit exceeds
-> plastic behavior

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Types of plasticity material

Ductile - high plastic behavior/ permanent deform
Brittle - little/NO plastic behavior

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Toughness

entire A under stress-strain curve
how much E required -> fracture material

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Resilience

A under elastic reg
how much elastic E stored

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Fatigue

cycles loading, unloading -> fracture even w/o ultimate tensile strength
ex: mastication

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Newtonian fluid shows

constant viscosity
constant flow - INDEPENDENT strain rate

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Formula newtonian fluid

n=o/e
viscosity= shear stress/ strain - flow rate

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Types of fluids depends on flow when stress

Plastic (stress)
Pseudoplastic (shear)
Dilatant (pressure)

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Plastic fluid

min stress -> flow

Pseudoplastic fluid

increase shear -> increase flow

Dilatant fluid

increase pressure -> increase rigidity

Hardness

resistance material to indentation/penetration

Tests hardness material - Birnell hardness no. BHN

hardened steel ball measure dia -> test hardness

Tests hardness material - Vickers hardness no. VHN/ diamond pyramid hardness test

pyramidal diamond measure diagonals square indent -> avg 2 dimensions

Formula VHN

1.85 x (F/d1xd2)

Compare VHN - BHN

VHN=1.05 + BHN

Thermal prop dentin

thermal insulator

Thermal conductivity/ thermal diffusivity

rate heat flows thru material

Thermal conductivity K measured by

speed heat (calorie) travels thru thickness (1cm) when 1 side maintain at constant temp + 1 celcius higher (speed-thickness-heat)

Thermal diffusivity

how fast int surface heats up when ext heated

Formula thermal diffusivity

h= k/ (Cp +p) h: thermal diffusivity of material (mm2/sec) k: thermal conductivity Cp: heat capacity p: density

Thermal expansion in dental

some restorative material have coeff very diff than tooth -> can cause damage -> use porcelain, porcelain fused metal - same contraction rate on cooling from firing temp

Electrochem series

list elements based on tendency gain/lose e-

Reference standard electrochem series

potential of standard hydrogen electrode 0.000V

Ex metal large pos electrode potential

Gold Platinum

Effect metal w/ large pos electrode potential

resistant OX, corrosion

Formation electrolytic cell

large diff btw electrode potential 2 metals same solution -> form electrolytic cell -> discomf ex: Au, Al

Electric resistivity

resistance material to flow electric current

Formula resistance

R=p(l/A) R: resistance (Ohms) p: resistivty l: length A: cross sectional area

Ceramics def

any material have metallic comp + non metallic comp -> have crystalline/ amorphous structure

Common material in dental is also ceramic

dental cement porcelain gypsum

Sub groups ceramics

Crystalline cermics (Al2O3) - insulators ; high melting point ; britlle Inorg salts Glass - amorphous (CaO, Na2O, K2O)

Metals def

pos ions w/ metallic bond (Strong bond) crystalline structure (always solid crystal) e- cloud -> conduct electricity, heat soft ferromag prop

Sub groups metals

Pure metals - same elements (solid/liquid - Hg) Alloys - 1/more metallic comp - has new metallic prop Intermetallic - all metals

Effect Cu, Al in alloy

increase strength > susceptible - corrosion

Backbone polymer

C Si

Structure polymer

crystalline amorphous

Categories polymer

rigid polymer waxes elastomers

Charac poly

soft can become rigid heat resistant

Tg of rigid poly

Tg > room temp

Tg elastomer

Tg < room temp

Plasticizer use

uncross cross linked -> decrease Tg -> more elastic

Charac plasticizer

less water needed (b/c water form hydrophilic mono) poly shrinkage free mono -> toxicity (b/c abdsorption plasticizer neg charges form in mono -> repel)