Week 4 Flashcards
Is the yield point high or low for stiff materials? Flexible materials?
stiff = high
flexible = low
Describe the stress-strain curve for brittleness
Closer to 90 = strong
Further from 90 = weak
Describe the stress-strain for brittle and ductile materials
brittle = no plastic deformation
ductile = plastic deformation
Compare and contrast the stress-strain curves for stiff, ductile, flexible, and brittle materials
The measure of energy required to permanently deform a material
resilience
Where is resilience on a stress-strain curve?
up to the point of elastic limit
the measure of energy required to fracture a material
toughness
Where is toughness on a stress-strain curve?
the entire under the curve up to the point of fracture
Which material is tougher? Black “ductile” or gray “brittle”?
ductile because the area under the curve will be larger
the amount of deformation that a material can withstand before rupture
elongation
Compare toughness for a brittle fracture and a ductile fracture
brittle = small toughness
ductile = large toughness
What are the three different testing methods that can generate a stress-strain curve?
- tension
- compression
- bending
True or false: The tensile and compressive strength (yield) of a material may be significantly different.
True
What do the behaviour of materials depend on?
loading mode
What is Hooke’s Law?
strain in a solid is proportional to the applied stress within the elastic limit of that solid
How is Hooke’s Law related to the force of a spring?
When comparing ceramics and polymers for use in a hip prosthesis. What will become deformed more from a 20 kg load, a polymer or ceramics?
polymer since it is less stiff
Regarding dental materials, which material has the highest ultimate tensile strength?
zirconia
Weakening of a material caused by repeated applied loads, such as mastication.
cyclic loading (fatigue properties)
During cyclic loading, maximum stress values that cause damage may be (much more/much less) than the strength of the material.
much less
What’s an example of a material that has failed due to cyclic loading?
airplane engine failture
material failed due to repeated low magnitude loads
Describe the general trend behind the endurance limit
the less stress you have = the more cycles you get before failure
Example of graph
each point corresponds to a failed specimen so you can see that when the specimen was subjected to a stress close to 480 MPa, after that the material fails (first 3 dots at the top)
if you lower the stress amplitude to 350 for 10,000 cycles, the material will hold for that long which is 9,000 more cycles than the first 3 dots.
Now if you reduce stress to 200 the sample will resist 1,000,000 cycles
The resistance of a fluid to flow
viscosity
Describe the relationship between viscosity and internal friction
higher viscosity = higher internal friction = greater resistance to flow
Describe the behaviour of low and high viscosity materials.
Low viscosity = behaves like a fluid
High viscosity = behaves like a solid
Describe the graph of different viscosity behaviours:
- shear thickening
- normal
- shear thinning
What is shear thinning?
viscosity decreases after external force is applied
*solid goes to liquid like
What are some examples of shear thinning?
ketchup
toothpaste
blood
What is shear thickening?
viscosity increases after external force is applied
*liquid goes to solid like
What is the difference between load-unload cycles in fully elastic and vicoelastic properties?
Describe the stress strain curves for elastic, viscous and viscoelastic properties.
What are the two viscoelastic effects?
- Creep = constant stress
- Stress relaxation = constant strain
What are the 6 surface properties?
measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion
hardness
What is the brinell hardness test (BHN)?
- used for low to medium hardness metals and non-metals
- a hard ball is pressed into specie surface with a load of 500, 1500, or 3000 kg
What is the rockwell hardness test?
- cone shaped indenter is pressed into specimen using an initial load of 10kg (“initial)
- following, a major load f 150 kg is applied, causing indenter to penetrate beyond it’s initial position (“major)
What is the vickers hardness test?
What can we calculate from hardness values?
elastic modulus
When a drop of liquid is placed on a surface, it may either spread out or remain in the form of a droplet
contact angle
What are the two types of wettability?
- good wetting
- bad wetting
Describe the surface accuracy for hydrophobic and hydrophilic materials
High degree of surface detail and surfaces get more “wet”
hydrophilic
Low degree of surface detail and liquid doesn’t “wet” the surface sufficiently
hydrophobic
What is an example of superhydrophobic and self cleaning?
lotus leaf
contact angle exceeds 150 degrees
What happens to dirt on lotus leaves?
smooth surfaces
- redistributed by water
rough surfaces
- adhere to the droplets and are removed when droplets roll off
Give examples of superhydrophobic surfaces
When do you want a material to be hydrophobic?
antibacterial
