properties of materials Flashcards
What properties need to be considered?
Unmixed compounds ~storage conditions, shelf life During mixing ~heat generation, setting time, viscosity Set material ~strength, toughness, solubility
Why are mechanical properties important?
Indicates how material will respond to applied forces
Ensures it can function in mouth- occlusal forces and changes in temp
Material usually has minimum value it must achieve ISO standards eg. Mean flexural strength>300MPa
What are some mechanical properties?
Stress and strain ~elastic limit, plastic flow, Young’s elastic modulus, fracture strength, yield stress, Ultimate tensile strength, ductility, resilience, fracture toughness Fatigue Hardness Abrasion resistance (wear)
What types of loading are there?
Tension- pulling Compression- squashing Shear- rubbing over Torsional- twisting Bending- combination stress, tension and compression
What is stress?
The force per unit cross-sectional area that is acting on a material
(Mega) pascals, MPa
Stress=force/original cross area
What is strain?
Fractional change in the dimensions caused by force
Strain=change in length/original length
What is resilience?
The amount of energy a material can absorb without undergoing any plastic deformation
What is toughness?
The amount of energy a material can absorb to the point of fracture
What is ductility?
The amount of plastic strain at fracture
What is elastic modulus?
A measurement of the stiffness, (mega or gigs) pascals, MPa or GPa
What is fatigue?
Loads are too small to cause catastrophic failure
Accumulated stress can cause crack propagation leading to failure
Fatigue life, limit, strength
What is hardness?
Ability to withstand surface indentation by a compressive force
How is hardness measured?
Indentation techniques
~Knoop, Vickers, Brinell or Roackwell
~Directly proportional to size of indentation
~Hardness number, large for soft, small for hard
Scratch test
~hardness indicates resistance to wear
What are some chemical and physical properties?
Elasticity, viscosity and viscoelasticity ~working and setting times Durability and degradation ~solubility, corrosion and erosion Thermal properties ~conductivity, diffusivity, expansion and exothermic Adhesion Colour and aesthetics Biological properties
What is elasticity?
Strain when stretched and instantaneously return to original state once stress is removed
What is viscosity?
Resist flow and strain linearly with time when a stress is applied
When load is released they don’t immediately return to original shape
What is viscoelasticity?
Elements of elasticity and viscosity therefore exhibit time-dependent strain
What is durability?
The ability to withstand an environment
What is solubility?
The extent to which a material dissolves in a fluid
What is erosion?
Material is removed and transported away- chem and mech
~in dentistry- acid attack and then brushing
What is corrosion?
Deterioration of a material, usually metal w environment
Electrochemical
Destructive oxidation
What is thermal conductivity?
Rate of heat flow per unit of temperature gradient under steady state conditions
What is thermal diffusivity?
When transient heat is applied a proportion will be expended raising the temperature of the material whereas the remainder will be conducted
What is thermal expansion?
An increase in amplitude of atomic/molecular vibrations due to the absorption of heat energy
Materials expand
Linear coefficient
What happens in an exothermic reaction?
Heat is generated and transferred
