Failure of Materials Flashcards
What kind of environmental stresses does a restorative material face in the oral cavity
-Mechanical forces
Max occlusal forces range from 200-3500N
Molars more than incisors
- Variation in pH
- Variation in temperature
- Moist conditions
Describe briefly the properties of enamel and dentine
-Enamel:
Hard and brittle
Wear resistant
Cracks but does not generally fail
-Dentine support
Soft and compliant
Lots of moisture and organic component
-Teeth deform in function
Axial loading leads to buccal-lingual and mesial-distal expansion
Describe the structure of enamel and dentine
Enamel:
- Highly mineralised crystalline structure
- 98% inorganic matter
- Hydroxyapetite is the largest mineral constituent
- Consists of rods, rod sheath and cementing inter-rod in some areas
- Run from EDJ to external surface
- Rods interwined, densely packed and run a waxy course approximately perpendicular to the EDJ
Dentine:
- 70% mineral and acellular
- Hydroxyapatite crystals
- 30% organic content as water, collagen and mucopolysaccharides
- Dentine tubules extend from external surface to the pulp
- Fluid can flow through these tubules
Harder to bond to dentine than it is to enamel
Ideal restorative material should be:
- Biocompatible
- Exhibit similar properties to enamel and dentine
- Ability to perform in the oral environment
- Assist in tissue regeneration or repair of missing/damaged tissues
-Oral environment exposed to temperature, pH and stresses
Desired list of properties of restorative materials:
- Restoration remains integral and in place
- Restore occlusion and withstand masticatory forces
- Aesthetics maintained over a long period of time
- Prevents formation of caries and recurrence
- Restores aesthetics
- Provides patient comfort and restores function
Definition of stress
- Defined as force/area
- Pa units
- When an external force is applied on a test specimen an internal force, equal in magnitude but opposite in direction is set up in the body
Definition of strain
- External force is applied on a test specimen it results in a change in the dimension
- Change in length/original length
Force application directions
Axial Tensile Force- Elongation Axial Compressive Force- Compression Shear Force- Shear Twisting Motion- Torsion Bending Movement- Bending
When do materials fail
- Physical failure
- When the critical stress is exceeded
- Magnitude of the critical stress depends on the loading conditions
-In general, a material loaded in shear has a lower critical stress than one loaded in tension
-Elastic Modulus definition, units, what it signifies and ideal elastic modulus of a restorative material
- Gradient of the stress-strain curve at any point before the proportional limit (elastic zone)
- Stress/strain measured in MPa or GPa
- aka Youngs Modulus
- Measure of the stiffness of a material within the elastic range
- How much it elongates after you place a stress on it
- Material should have a high elastic modulus especially posteriorly because you want them to withstand as much stress without straining much
What happens if you unload a material whilst its still in elastic deformation
-It will go back to its original size and shape
What happens past the yield point
- Plastic deformation
- Material does not return to its original size and shape after loading
What properties of the material affect its Young’s Modulus
-Interatomic and intermolecular forces of material
Stronger force- stiffer, more rigid material
Same in compression and tension
Independent of heat treatment
- Stiffness depends on dimensions
- But if Young’s modulus is known by using standard specimens, then it is possible to determine stiffness of any structure from that material
Clinical importance of yield point for dental materials
- Yield point maximum stress before permanent deformation occurs
- Orthodontic wires
Give the approximate elastic modulus of enamel, dentine, resin based composite, porcelain, Pd-Ag alloy, Zirconia and Alumina
At least know which ones are similiar and the general order
Enamel- 80-90GPa Dentine- 15-18 Resin Based Composite- 10-18 Porcelain-70-80 Pd-Ag Alloy- 180-200 Zirconia- 210 Alumina- 340
Measuring tensile strength
- Most useful test
- Sample of material stretched in uniaxial direction
- Carried out at a constant strain rate (constant rate of extension)
- Load measured from a load cell
- Elongation corresponding the applied load is measured simultaneously
- Stress and corresponding strain can therefore be calculated
- Stress-strain curve can therefore be constructed
- Lots of properties can be determined
- Easy to analyse
- Difficult specimen preparation
- Alignment is crucial
Indications for compression test
- If material is too brittle, tensile test is difficult to carry out
- Compression test used instead
How is a compression test carried out and alternative test
- Basically compress the item but
- Barrelling can occur which leads to an increase in cross sectional area as the stress increases
- Very complex stress pattern is produced in the material
- Cannot be analysed easily
- Interpretation of compression tests is very difficult
- May not yield accurate results as the frictional forces at the contact points are not uniform across the specimen
- Use a compromise test instead
- Indirect tensile strength measured
- Measurement of diametral tensile strength
- Disc of material subjected to compressive load
- Load applies to disc results in tensile strength in direction perpendicular to the applied load
- Commonly used on brittle dental materials because it is simple and provides more reproducable results than a tensile test
Difference in compressive and tensile strength between ceramics and composites
- Composites have higher tensile strength than compressive strength
- Ceramics have high compressive strength than tensile strength
How to measure flexural strength and clinical significance
- Bending test
- Place stress in the centre of the rod
- Work out how much stress it can withstand before it bends
- Fracture often initiated from the side of the specimen which is in tension
- Important for bridges
Clinical significance of hardness
- Resistance to abrasion
- Ease of cutting and polishing
- Strong relationship between hardness of a material and its ultimate tensile strength
Hardness Test
- Use of an indenter or cutting tool harder than the material itself
- If in the shape of a ball (Brinell), asymmetric diamond (Knoop), symmetric diamond (Vickers) or a cone (Rockwell)
- Indenter pushed into the material for a given amount of time, leaving behind an impression of the indentor
- Size of the impression depends on the hardness of the material
- Smaller the indent, the higher the hardness
- Indent must excees the local yield strength of that material- plastic deformation
Examples of Vickers Hardness approximate values
Enamel, Dentine, Acrylic Resin, Dental Amalgam and porcelain
Enamel- 350 Dentine- 60 Acrylic Resin- 20 Dental Amalgam- 100 Porcelain-450
Difference in cracking and hardness test between enamel and dentine
-Enamel has higher hardness than dentine
Enamel is hard and brittle
Small indent left with cracks at the point of indent
Enamel structure orients cracks
Dentine is soft and compliant (less stiff)
Larger indent (same load)
No cracking occurs
What does the elastic modulus of a material define
-Stiffness
Approximate elastic modulus of enamel, dentine, resin based composite, porcelain, Pd-Ag alloy, zirconia and alumina
Enamel: 80-90 Dentine: 15-18 Resin Based Composite: 10-18 Porcelain: 70-80 Pd-Ag alloy: 180-200 Zirconia: 210 Alumina: 340
Just know approximate order and that resin based and dentine are similiar but significantly lower than enamel
Resilience and toughness definitions
Resilience:
- Capability of a material to absorb energy when it is deformed elastically
- Area under the curve up to the elastic limit
Toughness:
- Amount of energy that a material can absorb before rupturing
- Total area under the stress-strain curve
Difference between fracture toughness and toughness
FT is a property that governs the critical stress at which a crack in a brittle solid becomes unstable and propagates
Toughness is the amount of energy a material can absorb before rupturing
Nitinol testing and use in dentistry
- Alloy of nickel and titanium used in orthodontic wires
- Tensile testing performed to characterize the strength and ductility of a material
- Tested in tension
- As you add tensile forces to the material, it transforms from austenite into stress induced martensite up to 6% strain
- Then, you unload it and the material reverses into austenite
- The loading plateau at 3% strain is different to the unloading plateau at 2.5% strain as they are different materials
-This stress hysteresis is the basis of orthodontic wires
How is strength of a material shown on SS curve
- How tall the line is
- How much stress it can withstand
How is stiffness of a material shown on SS curve
- Elastic modulus
- Gradient
- Higher the gradient, the more stiff
How is flexibility of a material shown on a SS curve
- Elastic modulus
- Gradient
- Higher the gradient, the less flexible
How is the brittleness/ductility of a material shown
- Dependent on the gradient after the proportional point
- Draw a line of the same gradient as the elastic modulus from the fracture point
- The greater the strain, the more ductile
How is toughness of a material shown on a SS curve
-Area under the entire curve
How is resilience of a material shown on a SS curve
-Area under proportional limit of the curve
Poisson’s ratio
- When a material is placed under axial loading (either tensile or compression), the stress also generates strain in the axial direction
- However, it also generates strain in the lateral direction
- If you compress a rod, it will shrink in length but will increase cross sectional area
- Ratio between lateral and axial strain IN THE ELASTIC LIMIT is known as Poisson’s ratio
- Ratio indicates that the change in cross-section is proportional to the deformation in the elastic range
- Brittle materials show little permanent reduction in cross-section during tensile test situations that more ductile materials
- Lateral:Axial Strain
- So the higher the ratio, the more it laterally strains IN THE ELASTIC REGION
- Poisson’s ratio is the same in compression or tension
Poisson’s ratio approx values for metals, ceramics, polymers and elastomeres and significance of a higher one
- Most materials 0.2-0.4
- Ceramics and metals 0.25-0.3
- Polymers and elastomeres 0.3-0.4
Important in design and stress distribution studies
Higher poissons = more strain laterally
The effects of elastic modulus and Poisson’s ratio on sealant bonded to enamel
- Sealant has a significantly lower elastic modulus than enamel
- Sealant has a slightly higher Poisson’s ratio than enamel
- During mastication (under compression), the sealant will strain more laterally than the enamel
- This causes shear at the interface between enamel and the sealant
Difference between real materials and ideal materials
- Real materials display non ideal behaviour
- If the material is isotropic and homogenous, then the breaking stress of the material would be x(critical stress)
- If the stress applied is less than the critical stress, than failure will not occur
- However, real materials are not ideal
- They are often anisotropic (physical properties of material depend on in which direction it is loaded. eg. wood stronger along the grain)
- They are non-homogenous
- They contain defects
The concept of stress concentration
- If the stress is greater than the critical stress then failure will occur
- The fracture toughness is defined as Kc
- When K=Kc there is catastrophic crack propagation
- Kc is a material property, independent of testing conditions
- Fracture toughness is not the same as toughness
Stress Intensity Factor
-The resistance of a material towards crack propagation
- Stress intensity factor at the tip of the crack
- Depends on the shape factor, the controlling stress and the crack length
- FT is the critical stress intensity factor of a sharp crack where propagation of that crack suddenly becomes rapid and unlimited
- Fracture toughness is determined using notched specimens and it effectively gives a value of the work in creating two new surfaces when cracking occurs
- FT is the ability of a material to resist fracture
Relationship between stress intensity and fracture toughness
- Similiar to the difference between stress and tensile stress
- Stress intensity, KI, represents the level of stress at the tip of the crack
- Fracture toughness is the highest value of stress intensity that a material under very specific conditions can withstand without fracture
How can you improve fracture toughness in dental materials
- Remember crack intensity factor at the tip of the crack depends on shape factor, controlling stress and crack length
- Shape factor and controlling stress will often be constant and not able to change
- However addition of rubber particles in PMMA dentures or addition of alumina into ceramic crowns limit crack propagation by limiting the crack length
Does enamel-dentine debonding occur
No
-Despite being so different in properties it does not occur
-Enamel supported by dentine, and if dentine is lost cracks can occur in the enamel
Concept of dynamic loading and significance in dentistry
- Many materials are subjected to fluctuating or intermittent stresses in the mouth
- Difficult to measure this
- Can be mechanical or thermal loading
- A material subject to repeated cyclic loading may fail after a number of cycles fail even though the maximum stress in any cycle is significantly lower than the failure stress of the material
- Determination of relationship of stress level and number of cycles to failure/deformation permits the estimation of the reliability of a material/restoration
Fatigue of a material and fatigue testing
- Accumulation of small amounts of intermittent stresses is known as fatigue
- Can lead to failure
- Fatigue testing done by subjecting the specimen to cyclic loading over a range of loads
- Number of cycles required to cause a failure is counted
- Stress is plotted as a function of the log of the cycles required to cause failure, known as S-N curves
- Stress and number of cycles
2 types of behaviour that occur with fatigue testing
- Number of cycles on x axis
- Stress on y
1) As the number of cycles increases, the allowable load decreases
2) Some materials exhibit an endurance limit, below which the material can be subjecting to an indefinate number of cycles without fracturing
3 optical properties and definitions
-Hue
Basic colour
-Chroma
Strength or intensity of the hue
-Value
How light or dark that colour is
Translucency and opacity definitions
- Transparent material allows light to be transmitted without distortion or change in colour of an object
- For example glass
- Translucent material allows light transmission but with scattering and loss of definition and true colour of the object
- Opaque material permits only scattering of the incident light and no transmission of light
Names of the important thermal properties of dental materials
Ideal thermal properties for various dental materials
-Coefficient of thermal expansion
Ideally similiar to enamel and dentine for direct restorative materials
-Glass transition temperature
High for restorative and denture base materials
37 degrees for soft lining materials
-Thermal conductivity and thermal diffusivity
Low and act as an insulator for direct restoratives
Medium for denture bases so pt doesnt burn themself
-Polymerisation exotherm
Low to not cause damage to the pulp
Definition of coefficient of thermal expansion and material approximate values for enamel, dentine, acrylic resin, composite resin and amalgam
Significance in dentistry
-Fractional increase in length of a body for each degree rise in temperature
- Enamel- 11.4
- Dentine- 18
- Acrylic resin 90
- Composite resin 25-60
- Amalgam 25
- May get ingres of oral fluids due to debonding
- Secondary caries
- Porcelain has a much lower CTE than metals so may lead to delamination and stresses
Thermal conductivity definition and enamel/dentine/metals specific examples
- HeatDistance/AreaTemp.Gradient
- 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 (deltaT) under steady state conditions and when the heat transfer is dependent only on the temperature gradient
- Basically how much heat does a material conduct
- Enamel- 0.0022cal/sec/cm2
- Dentine- 0.0015cal/sec/cm2
- Metals- 0.1/0.9cal/sec/cm2
Difference between thermal conductivity and thermal diffusivity
-Thermal diffusivity more important than conductivity in dentistry
- A measure of the rate at which a temperature disturbance at one point in the body travels to another point
- How long is the material gonna stay warm
-Expressed by the relationship
H=K/CpP
Where K is the coefficient of thermal conductivity, P is the dentisty and Cp is the specific heat at constant pressure
WHEREAS Thermal conductivity (K) is defined as the rate of heat flow per unit temperature gradient
Glass transition temperature definition
-Applicable to linear amorphous polymers mainly
-When a polymer is heated,
segmental motions of the chains increase and finally overcome the interactions
- The glassy brittle stage progresses to a rubbery and less rigid form whilst the modulus drops rapidly over a narrow range of temperatures
- Composites for dental restoration must have a Tg higher than the maximum temperature in the oral cavity in order to preserve the material’s physical and mechanical properties
- Composites undergo polymerisation to set and the determination of Tg can indicate if there is inadequate curing
