properties of materials Flashcards
mechanical properties use
used to indicate how a material or component will respond to use
what are many materials supplied as
2 compounds mixed together
what do you need to consider in material
unmixed compounds
during mixing
set materials
mechanical properties
stress and strain
fatigue
hardness
abrasion resistance
stress and strain catagories
elastic limit plastic flow fracture strenght yeild stress ultimate tensile strength ductility resilence fracure toughness youngs modulus
loading types
compression
tension
shear
what is stress
cross sectional area that is action on a material
equation for stress
F/As (Force/original cross area)
units
Mpa = newtons per square meter
strain
fractional change in the dimensions cause by the force
no units as it is a ration of lengths
strain equation
(L1-L0)/L0
L0 original length
L1-L0 change in length
youngs modulus
how bendy something is
what happens as you increase stress
you increase strain
elastic area
can go back to its original shape even when stressed
plastic flow
material will break once this is reached
toughness
amount of energy a material can absorb to the point of fracture
resilience
amount of energy a material can absorb without undergoing any plastic deformation
ductility
amount of plastic strain at fracure
brittle material
no plastic region
brittle plastuc
will frature at lower strengths
still no plastic region
elasomer
huge plastic region
will deform a lot, but not break until after plastic reion
fatigue properties
given as fatigue life or fatigue limit or fatigue strength
fatigue life
number of cycles to failure
fatigue limit or strength
cyclic stress required to cause failure for a set number of cycles
what can accumulation of stress casue
crack propagation leading to failure
hardness
ability to withstand surface indentation by a compressive force
what can hardness be measure by
indentation techniques or scratch test
what does a scratch test measure
indicates material resistance to wear
what is hardness directly proportional to
size of the indentation and therefore is given a number
large for soft materials
- small for harder materials
chemical and physical properties
elasticity viscosity viscoelascity durability and degredation thermal properties adhesion colour and aesthetics biological properties
thermal properties
thermal conductivity
thermal diffusivity
thermal expansion
exothermic reactions
elasticity viscosity viscoelascity indications
working times
setting times
durability and degredation
solubility
corrosion
erosion
elastic materials
strain when stretched
instantaneously return to original state once stress removed
viscous material
resists flow and strain linearly with time when stress is applied
viscoelastic material
have elements of both elastic and viscous materials
- exhibit time dependant strain
solubility
extend to which a material dissolves in a fluid
erosion - general and in dentistry
materia is removed and transported away therefor both chemical and mechanical components
in dentistry
- used to describe destruction of enamel and dentine by acid attack, which can be compounded by mechanical forces such as brushing
durability
ability to withstand an environment
corosion
deterioration of a material electrochemical processes (can be considered destructive oxidation)
the 3 types of degradation can lead to
leaching of constituents into oral envrionment
- these leaked constituents must be safe locally and systemically
thermal conductivity
Rate of heat flow per unit of temperature gradient under steady state conditions
- property of metal to conduct heat
when does conductor occur
when there is a temperature gradient
the temp gradient is directional i.e. from hotter to colder
thermal diffusivity equation
Thermal conductivity /(p CP)
p= density
CP= heat capacity
what happens when transient heat is applied to teeth/dental material
a proportion will be expended rating the temperature of the tooth/material
the remainder will be conducted to the pulp
- materials with low thermal diffusivities are generally preferred
what is thermal expansion. due to
increased amplitude of atomic/molecular vibrations due to the absorption of heat energy
importance of thermal expansion
need to ensure restoration material won’t expand otherwise tooth under stress may fracture
thermal expansion in solids
often stated as linear coefficient of thermal expansion
which is fractional length change per degree of temperature change
- metals expand the most due to the sea of e- that can move about
exothermic reaction
heat is generated
transferred to the surrounding tooth tissue
need to be careful with the pulp
