Fundamentals 2 Flashcards
What are mechanical properties?
Change of material dimensions due to external forces, such as mastication
What is viscosity?
Resistance of a fluid to flow.
Viscosity measures __________. The higher the viscosity, the ___________ and _______ resistance to flow
internal friction
internal friction, greater
Thiccer materials are most viscous
Higher viscosity materials behave like ______ while lower viscosity materials behave like ______
solid
liquid
What is the shear thinning phenomena?
It describes the affect on viscosity of a material if subjecting to stress.
Shear Thinning describes a ______ in viscosity as stress is ______.
Is this Newtionian?
Decrease
Added
no
What word is used to describe materials whose viscosity is unaffected by external stress?
Newtionian
Shear Thickening describes a ______ in viscosity as stress is ______.
Is this Newtonian?
increase, stress
NO
Shear Thinning examples
- Squeezing ketchup out of a bottle
- Squeezing toothpaste out
- Blood in our body as it flows
High to low viscosity describes ______.
The vice versa describes what?
Shear thinning
Shear thickning
Examples of Shear Thickening
Cornstarch
Moonsand
Why is viscosity important?
- Viscosity can change during preparation of materials
- Dispensing of dental material into tissue so it reaches pits and fissures
- Consistency and Manipulation ability of material
What is the ideal viscosity of impression materials?
Low viscosity outside the mouth so it captures all anatomy but increasing viscosity in the mouth so it sets!
What is viscoelasticity?
Elastic + Viscous Behavior
Springy + Dampening behavior where materials behaves like solid and fluid
What is the difference of the Stress/Strain curve between fully elastic materials and viscoelastic ones?
Fully Elastic Materials will have the same linear path for load and unload.
Viscoelastic Materials will have a slight convexity for the load and a slight concavity for the unload.
Viscoelastic materials ______ energy when stress is________ . As a result the Stress Strain curve is ________.
What shape does it form?
lose , loaded-unloaded.
NOT proportional
A Hysteresis
Describe the strain vs time graphs for materials with the following properties.
Elastic
Viscoelastic
Viscous
Elastic- It will look like a rectangle. Increase in strain on y-axis, strain will be consistent with constant stress. When stress is removed strain will drop right back to zero.
Viscous- Strain will gradually increase linearly (sloped) as stress is applied. Strain will be consistent with constant stress. HOWEVER, when stress is removed, strain does not recover. ( Sloped line + 0 slope line
Viscoelastic - Stress will cause a convexed increase in strain. The elastic property will attempt to recover the material when stress is reduced caused a vertical decrease in strain. However, the viscous property will prevent full elastic behavior and cause a concave decrease. Looks like Shark fin
The more viscoelastic, the more ______ behavior.
Viscous
Purely elastic materials dissipate energy when load is applied then removed.
True or False
FALSE
Plastic deformation results in a _________ of energy which is uncharacteristic of _____ materials.
Loss
Elastic
Viscous materials loose energy through the ______ cycle because the ______________.
How can we quantify this energy?
loading/unloading
energy lost during loading is never fully recovered
Area in between hysteresis!
What is creep?
Effect of constant stress over TIME on strain
Think coat hanger.
Strain will increase in a convex curve
Stress will remain constant (Slope 0 horizontal)
What is stress relaxation?
Effect of constant strain over time on stress.
Stress will decrease in a convex fashion
Strain will remain constant (Slope 0)
What are the 5 viscoelastic dental materials?
- Dentin
- Enamel
- Cells
- Impression Material
- SOME acrylic dentures
