Pres Res unit 1-materials Flashcards
Occlusal Force trend
decreases from molars to incisors
everyday chewing force
10N
movement of teeth during chewing
axial and horizontal
Force is defined by
point of application, magnitude, and direction of application
average max biting force
756N
Stress Strain Curves
A certain amt of force is required to move an object (rubberband) a distance deltaL; divide Force and deformation are devided by the cross-section of area and the length so that the force is independent of size since it changes
stress is equal to and units
force/area in Pascals (1MPa=1,000,000 Pa) Pa=N/m^2
shearing
forces on diff planes in opposite directions (i.e. force on top to left and force on bottom to right)
Torsion
twisting
Strain
no units, deformation/length; undergoes change in length
Elastic
deformation will not be permanent (rubber band)
Plastic
permanent deformation; relocation of atoms
universal testing machine
produces stress-strain curves
stress strain curve
will determine elasticity vs placitity
Elastic limit
looks at material, to determine the boundary between elastic and plastic
Proportional limit
use a machine to determine the boundary between elastic and plastic
Stress-strain curve
the stress value at which a material exhibits a specified permanent strain
ceramic materials
break before they permanently deform, such as ceramics
Ultimate tensile strength
the max stress a material can withstand before failure in tension
Fracture Strength
the stress at which a material fractures; not necessarily equal to ultimate strenght
Elastic Modulus/Young’s modulas
represents the stiffness within the elastic range E=theta/E=stress/strain
Stiffness of Cobalt chrome
good/stiff
Acrylic resin and composite resin stiffness
not very (relative)
Brittle materials in dentistry (not ductile)
Amalgam, composites, cements, and ceramics
Ductility
ability of a plastic to be deformed
Dutility trend
gold silver most
Lead tin least
Resilience
resistance to deformation., energy absorbed per V material
Toughness
resistance of a material to fracture, or energy absorbed
Stress strain comparisons
see charts
Brittle-straight line
Ductile- curved line
Taller line-strong
Shorter line-weak
Does not go far in x direction/high slope-stiff
Further in x-flexible (not much slope)
Tough- will bend a lot before breaking
Resielent- graph will not bend, stays elastic
Tough-graph will bend, becomes plastic but hard to fracture
Ultimate compression strenght
amt of stress to compress a material
impact strength
energy required to fracture a material under impact force
diametral compression test
indirect tensile test, test brittle materials for tensile strength
shear strength
max stress before failure in shear mode; used for bond strength testing
Bending creates
areas of compression (of side angle getting smaller); neutral axis on object; larger angle is under tension (more likely to fail)
cantilever Bending Test
tests bending, force applied down
Bending moment
force x distance (basically torque)
Flexural strength
denture materials and ortho wires, transverse strength, 3 point bend test
Torsion
twistin, important in endo files for root canals, shear stretch
Fatique
progressive fracture under repeating loading
Stress to cycles to failure
shows endurance limit, may fail over long periods of small or larger stress
Factors Affecting Fatique
environment: temp, humidity, pH
Surface features
Material history
Fracture Toughness
A measure of material’s resistance to a brittle fracture when a crack is present; good for brittle materials (flaws in metal are more forgiving than ceramics)
Viscoelasticity
has elastic and flow qualities; some elastic and plastic deformation is possible;
Creep
Viscoelastc property-time dependent strain of a material under a static/constant stress
Stress Relaxation
Viscoelastic- reduction in stress in a material subjected to constant strain- think ortho bands which apply less force after application
Hardness
resistance to permanent surface indentation
Brinell Hardness Test
used for metals; hardened steel ball indenter is used and diameter is measured
Rockwell Hardness Test
Used for metals and plastics; a ball or cone used
Vickers Hardness Test
microscopic, for metals and plastics-softer material loses out-porcelain hardest
Knoop Hardness Test
micro indentation test , rhombic based pyramidal diamond
Wear
loss of material; relocation
Bruxism
pathological form of wear
Abrasive two body wear
to surfaces rubbing, grinding
abrasive three body
food between two teeth
corrosive wear
wear coupled with a chemical attack
Rheology
study of flow of matter
Viscosity
fluids resistance to flow
newtonian fluid
viscosity remains constant (water)
Plastic
no flow until a certain shear stress is reached (yield point) toothpaste
Psuedoplastic
decreasing viscosity as shear rate increases (stir faster and it gets thinner/easier)
Dilatant
viscosity increases with a shear rate (harder to mix as you stir)
Thixotropic
Time dependent decreasing in viscosity, has yield point, curve is not reversible (ketchup bottle)
Burnishing
closing the metal margin of a metal crown-recovery of elastic strain but NOT plastic
Barcol
additional hardness test-resin composites
Shore
used for rubber
