Physical And Mechanical Properties Flashcards
How do you establish the most important property of a material?
Once you have defined the task a material has to fulfill
What are the two simple modes of stress?
Tension and compression
What what is tensile strength?
Stretching force
What is compressive strength?
Compressive force
Which forces are a combination of compressive and tensile modes?
Torsion, shear and flexible modes
What is torsion?
Twisting
What is shear strength?
Ability to resist forces that cause material’s internal structure to slide against itself
What is flexural strength?
Bending
Why is the tooth a complex material to test the strength of?
It is a composite - made up of multiple components
What does a universal testing machine do?
It deforms the sample tested in a known and controlled way whilst monitoring the force
What does anistrophic mean?
Having a physical property which has a different value when measured in different directions
Where are the loads directed in enamel?
Along the the length of the prism
What form will the material being tested in the UTM need to be in?
A regular and reproducible specimen
What could affect if a tooth is used for a sample?
The history of the tooth. - damage, etc
Describe what is shown in the elastic region
Initially the stress is proportional to strain
Deformation is recoverable
What is shown in the plastic region?
Permanent deformation
What is the point where the elastic and plastic regions meet?
The elastic limit, proportional limit or yield point
What is the highest value for stress termed?
Strength
What is Young’s modulus?
The slope of the elastic region - modulus of elasticity
What is a complaint material?
A material with a low stiffness
What does a higher Young’s modulus mean?
The stiffer the material
Which tooth tissue is easily deformed?
PDL - relatively easily elastic ally deformed.
What is aberfraction of the tooth?
Where cyclic loading had lead to enamel spooling at the neck of the tooth
Fatigue is a major problem in regards to which device?
Posts - used to fix crowns to devitalised teeth which have their pulps extracted
What affects the highly ware resistant enamel?
Our diets and bad oral habits
What assumptions can we make about hard materials?
That they have higher ware resistance and are more abrasive
Name and explain a hardness scale
- Mohs’ hardness scale
- 1 to 10 talc to diamond
- scratch one material with another - the less hard material is scratched
How do we define fracture toughness?
The Griffith Equation
What does the growth of a crack/fault lead to?
Failure
All materials have flaws/cracks
How do you increase rapture toughness?
Make the path of crack more difficult
How does the structure of the tooth increase fracture toughness but what is the issue with the nature of the tooth?
Cracks are deflected from common paths
Tooth is anisotropic - higher fracture toughness on one plane as oppose to another
Explain the know test vickers test
An indent with equal length diagonals and cracks emanating from their tips are measured
Notoriously hard to do correctly
How do you test fracture toughness with a biaxial flexure rig?
- indent is placed opposite top loading piston
- sample loaded till it fails maximum load
- other variable then used to calculate fracture toughness
Unfortunately cant generate big enough disks of enamel or dentine
How does an impact tester work?
- hammer strikes specimen
- specimen will break on absorbing some energy from the hammer
- hammer travels less high
- measure difference in pendulum travel - calculate energy absorbed -> impact energy
Which wavelengths do (near) white teeth reflect?
All wavelengths- reasonably well
Why does the inside colour of the tooth effect its appearance?
Enamel is quite translucent - esp at tips
What does value mean?
How light or dark a colour is
What is important to make sure of when matching shades/colours?
Quality of light source - natural north light - flat and even - possible Edith electric light
