Properties of Materials Flashcards
When we talk about dental materials, do we talk about stress or force more?
Stress
What is stress?
When you put a force on a body there will be a reaction back?
How is stress measured?
The force and area to which it is applied are measured and stress is calculated from the ratio of force per area
What is the ratio of force per area measured in?
Pascals = 1N/m2
What are the types of stress?
Tension, Compression, Shear, Torsion, Bending
What is tension?
Elongation forces are directed away from each other in the same straight line. Molecules resist being pulled apart = resistance
What is compression?
Elongation forces are directed toward each other in the same straight line. Molecules resist being forced more closely together = resistance
What is a shear stress?
Forces directed parallel to each other but NOT along the same straight line. One portion sliding past the other = resistance
What is torsion?
Twistin forces = resistance
What is bending?
Results from a bending moment = resistance
What is strain?
Deformation of a material caused by stress
How is deformation measured?
As a unitless value or as percentage. Deformation = deformation/original length
What is the proportional limit?
The greatest stress sustained w/o deviation from the linear proportionality of the stress and strain curve
What is the elastic region Vs plastic region?
Elastic is reversible strain. Plastic is irreversible or permanent strain (stress past a point where the material will not rebound to its original shape)
What is elastic limit?
The greatest stress sustained w/o permanent deformation. Exceptions are super-elastic materials
How does proportional limit and elastic limit compare in linearly elastic material?
They can stand the same stress
What is yield strength?
The stress at which the material begins to function in a plastic manner. Limited strain has already occured
Which stress is higher, yield strength or proportional limit?
Yield strength
Is permanent deformation a bad thing in the oral cavity?
No not always
What is constructive permanent deformation? Destructive?
Ortho wires, partial clasps. Destructive is fillings, crowns, bridges
What is ultimate strength?
The highest point on the stress/strain curve. Point of greatest strength
What is fracture strength?
Stress at which material fractures
Does a material always fracture at the point of greatest stress?
No, may elongate excessively reducing the cross-sectional area causing a reduction in stress
For most dental alloys and ceramics subject to tension, how does Ultimate Strength vs Fracture Strength compare?
They are very similar
What is elongation?
Deformation that results from the application of tensile stress. Indicates the workability of alloy. Expressed as a %
What is elastic modulus?
The measure of elasticity. Stress/strain = elastic modulus. Measure of how rigid or how flexible a material is
How does the percentage of elongation affect ductility?
Higher elongation means a more ductile material, as in gold margins can be burnished
What forces are responsible for the property of elasticity?
Interatomic/intermolecular forces
What does a higher value of elastic modulus (E) mean?
The higher the value of E, the more stiff or rigid the material
What is resilience?
Resistance of a material to permanent deformation
What toughness?
Resistance of a material to fracture. Amount of energy needed to cause fracture
What is toughness a combination of?
Combination of yield strength, ultimate strength, and strain. Increasing any of these, increases toughness
What is fracture toughness?
Characterizes the behavior of materials with cracks or flaws
How do defects affect fracture toughness?
Generally weaken a material and may result in fractures at stresses well below the yield stress.
Where do fractures occur more?
With brittle materials that have no ability to deform and redistribute stresses
What is a diametral compression test?
A test of tensile properties of brittle materials
What is a diametral test good for?
For brittle material such as amalgam, cements, ceramics, plaster, and stone
Do brittle materials have both elastic and plastic properties?
Yes, but the plastic properties will be small
What is fatigue strength?
The stress at which a material fails under repeated loading. Depends on Magnitude of the load and the Number of loadings
What is Endurance Limit?
Stress at which the material can be loaded an infinite number of times w/o failing
What is rate of loading?
How fast a material is deformed or how fast the stress is applied. Viscoelasticity - is important for many materials (alginate elastomeric impression materials, waxes, amalgam, polymers)
What is viscoelastic?
Materials w/ mechanical properties DEPENDENT on loading rate
What is viscosity?
The resistance of a fluid to flow.
What is the unit of viscosity?
Poise
What is creep?
Increase in strain (deformation) in material under constant stress (amalgam)
What is tear strength?
Resistance to tearing forces. Important for polymers in thin sections (like when impression material gets into a sulcus). Depends on rate of loading. Rapid loading = higher values
