3. Failure Of Dental Materials In The Oral Environment 2 Flashcards
Dental biomaterials
2
what does a elastic modules define?
2
The stiffness of a material
Describe what each of the lines of the graph tell us
Comment on the labels on x and y axis
What are each of these materials
A brittle ceramic
B strong ceramic
C metal
D metal
E strong polymer
F weak polymer
Define resilience
the capacity of a material to absorb energy when it is deformed elastically. The area under the curve up to the elastic limit.
Amount of energy material can absorb without undergoing any permanent deformation
Define toughness
the amount of energy that a material can absorb before rupturing. The total area under the stress - strain curve.
Toughness vs resilience on stress strain curve
Describe the features of the materials on these graphs 1-6
Define Poissons ratio
What does poissons ratio indicate?
What does a tensile force result in?
What does a compressive force result in?
What do brittle materials show?
Measure of poisson effect where a material expanses in directions perpendicular to direction of compression
If these reactions occur in the elastic limit, the ratio between lateral and axial strain is called the Poisson’s ratio.
Possions ratio represented by what letter?
Which region is poissons ratio applicable in?
Equation for poissons ratio?
Typical ranges ?
Typical range for ceramics, metals, polymers, elastomers?
Represented by v
Applicable only in Elastic Deformation Region
• Axial
• Lateral
ν = lateral/axial strain
• Defined in elastic deformation region
Elastic Modulus and Poisson’s Ratio Effects
What value of stress in comparison to breaking stress leads to failure not occurring?
Define fracture toughness
Fracture toughness - quantitative way of expressing a materials resistance to crack propagation
There are standard values for a given material for fracture toughness
Fracture toughness is NOT the same as toughness
To assess the effect of small cracks/defects that appear when you process / manufacture a specimen, we have to understand concept of fracture toughness
Brittle materials often fracture suddenly , well below ideal fracture stress
• Local stress σ
• If σ ≥ σ c then failure will occur
• The fracture toughness Kc is the critical value of K
• When K = Kc there is catastrophic crack propagation
• Kc is a material property – independent of testing conditions
How do we determine fracture toughness?
To determine fracture toughness, a specimen is taken of a known dimension and a crack is created with a known dimension - eg crack length and shape and then is subjected to loading and stress
When stress exceeds critical stress, then crack propagates there’s fracture and the specimen breaks
Crack growth-Stress intensity factor
The resistance of a material towards crack propagation
The stress intensity factor at the tip of the crack is given by k= Yσ √πa
Y: is the shape factor
σ: the controlling stress
a: the crack length
Fracture toughness is determined using notched specimens and it effectively gives a value of the work in creating two new surfaces when cracking occurs
Fracture toughness
What is k ?
Typical fracture toughness values?
Relationship between stress intensity and fracture toughness?
What are the different modes of failure?
What is most common mode of failure?
What are some methods of limiting crack propagation in dentistry?
All ceramic crown failure
What causes crowns to fail?
How can fracture toughness of all ceramic crowns be improved?
Enamel cracks?
Dynamic loading
What does it involve?
Why is it important?
Mechanical properties mentioned so far are all conducted under static mode
Static test results provide info on how materials fail on continuous loading but give limited info on what happens to material when it is subjected to dynamic load
Dynamic loading involves intermittent loading/cyclic loading at lower stresses and defined frequencies
What is fatigue?
Accumulation of small amounts of intermittent stress is known as fatigue
Fatigue = a process of progressive localised plastic deformation which occurs in a material subjected to cyclic stresses at high stress concentration locations , that results in cracks or complete fracture after a sufficient no of fluctuations
Fatigue can lead to failure
Many dental materials are subjected to intermittent stresses over a long period of time
These stresses are much lower than the fracture stresses but it can still lead to failure of a material
How can fatigue testing be done?
Fatigue testing can be done subjecting the specimen to cyclic loading over a range of loads
The number of cycles required to cause failure is counted
Stress is plotted as a function of the log of the cycles required to cause failure, known as S-N curves
Fatigue can be determined using S N curves. S - stress amplitude
N - number of cycles
A test material is subjected to known magnitude and frequency and the no of cycles to failure are recorded
Can determine fatigue life of material
Can also choose specific no of cycles and determine the level of cyclic stress that can a cause fracture that gives you the fatigue limit
Fatigue testing part 2
What are the two types of behaviour?
Example of catastrophic failures?
Fractured specimen looks like:
What are the Thermal properties of materials?
Coefficient of thermal expansion
What is it ?
Formula?
What letter is coefficient of thermal expansion represented by?
Typical values for materials?
What coefficient of thermal expansion is desired?
Do we want a to be similar or different ?
Linear coefficient of thermal expansion is defined as the fractional increase in length of a body for each degree rise in temperature
What we would like in a filling material is that it should have a coefficient of thermal expansion that is similar to tissue it is replacing
So important to find out coefficient of thermal expansion before use especially as a restorative material
If alpha is similar to each other then it is fine but if value of alpha of the tooth and the material are different, a small marginal gap can form
This can cause a problem because with repeated cycles of this occurring with hot/cold stimulus, you can get marginal leakage which can eventually cause failure
Example of Failure due to coefficient of thermal expansion
Thermal conductivity
What is it?
Equation?
Comparative thermal properties of tooth structure
Why is thermal conductivity important?
Thermal conductivity is defined as the quantity of heat ( Q) transmitted through a unit thickness (L) in a direction normal to a surface of unit area ( A) due to a unit temperature gradient (ΔT) under steady state conditions and when the heat transfer is dependent only on the temperature gradient.
Importance - Important as good conductors can transmit heat
Insulators will not transmit heat
If you take metallic restoration eg amalgam and it is close to the pulp then any change in temp will allow heat to transferred towards the pulp and therefore can cause some damage
Important to select material by considering not only mechanical properties but also thermal properties
Thermal diffusivity
Definition?
Equation?
Importance?
Thermal Conductivity & Thermal Diffusivity
Thermal conductivity
DEFINITON?
Equation?
Glass Transition temperature Tg
Which materials is it relevant to ?
Compare tg of a material to temp in mouth ?
Thermal properties - which materials are each of the thermal properties relevant to?
Optical properties
What are the 3 optical properties?
Hue
Chroma
Value
Optical properties
Define hue
Define chroma
Define value
Translucency and opacity
Tooth Colour and Layering of Restorative Resin Based Composites
Summary
