Dental pt7 Flashcards
Stress formula, unit
stress=F/cross sectional area (f/cm2)
Def stress
force apply to material NO depends only on force ALSO shape (cross section)
inversely prop cross sectional area ; proportional to load
Types stress
tensile
compressive
shear
Charac of all 3 stresses
all present at 1
func under same rule - if stress > strength material -> structure fail
What force when breakage occurs
tensile force
Strain def
temporary elongation when interxs tensile stress -> longer
Calc value strain
length increase/og length
Elasticity def
material have constant ratio stress-strain
Modulus elasticity def, entails
constant of constant ratio stress/strain
x2 stress -> x2 strain
diff material - diff charac - diff constant ratio -> modulus elasticity
Proportional limit/ elastic limit
greater stress when stress prop strain
Yield strength/ yield point
strength measure at stress w/ little strain
Ultimate tensile strength
(stretched to max) -> stress at breakage
Work hardening
metals stress > prop limit -> hardening
-> stronger
-> harder
-> > brittle
Elongation
total strain (elastic, plastic) at fracture
Charac modulus elasticity
inherent property
NO alter
Plasticity
elastic/ prop limit exceeds
-> plastic behavior
Types of plasticity material
Ductile - high plastic behavior/ permanent deform
Brittle - little/NO plastic behavior
Toughness
entire A under stress-strain curve
how much E required -> fracture material
Resilience
A under elastic reg
how much elastic E stored
Fatigue
cycles loading, unloading -> fracture even w/o ultimate tensile strength
ex: mastication
Newtonian fluid shows
constant viscosity
constant flow - INDEPENDENT strain rate
Formula newtonian fluid
n=o/e
viscosity= shear stress/ strain - flow rate
Types of fluids depends on flow when stress
Plastic (stress)
Pseudoplastic (shear)
Dilatant (pressure)
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)
