2 - Failure Of Materials Flashcards
Materials in the mouth
How are restorative materials subjected to a harsh oral environment?
Mechanical forces
What do occlusal forces range from?
Force of first and second molars?
Force of Incisal ?
Maximum occlusal forces may range from 200 -3500 N
First & second molars~ 400-800N
Incisal forces~ 150-300N
These forces also increase in growing children
What is the tooth’s structure?
Describe enamels structure
Describe dentines structure
What should the features of Ideal Restorative Materials be?
What are the desired properties of restorative materials ?
Failure of materials
Why do materials fail?
Do dental restorations last forever?
Have limited lifespan
Replacement of failed restorations constitutes a large part of operative work in dental practice
Disadvantage of dental restorations?
Replacing a restoration may lead to an increase in cavity size
Weaken the tooth
Limited lifespan
Mechanical Properties of Biomaterials
What are the different types of forces a material experiences in the mouth?
Mechanical Properties of Biomaterials
What does compression result in
Why might physical failure of a material occur?
Mechanical properties of biomaterials
What forces are veneers of porcelain subject to
What do we use to describe mechanical properties of materials?
Stress strain relationship
What is stress?
Stress: 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
• STRESS= FORCE/AREA UNITS= Pascal (Pa)
(Textbook says area is cross sectional area?)
What is strain?
How do we test dental materials mechanical properties using stress strain relationship
We apply load at a uniform rate and deformation should also occur at a uniform rate
Can use universal testing machine to determine these mechanical properties
Stress strain curves
1) what do they show?
2) what does the straight line region demonstrate?
3) what does the gradient of the straight line region demonstrate?
4) what does the region under the straight line demonstrate?
5) What is the yield point?
6) what does region under curved part of graph indicate?
7) how does failure point compare in a more brittle material?
8) what is elastic limit ?
9) what is yield stress?
10) what is ultimate stress?
11) what is ultimate strain?
1) demonstrate relationship between stress and strain
2) straight line - linear relationship between stress and strain. As strain increases, stress increases proportionally
3) gradient of the straight line = elastic modulus of material
4) area under straight line = elastic region
Materials in this region will recover from any strain and no change in dimension
(Elastic deformation occurs)
5) yield point = also known as elastic limit . Point beyond which material will experience permanent deformation and change in dimension
On the graph, point the graph is no longer a straight line
6) area under curved - plastic region
Here materials suffer permanent change in dimension. Material will deform and extend until it fails - failure point (End of graph )
Plastic deformation occurs
Has exceeded elastic limit
7) more brittle materials - failure point is closer to yield point
8) elastic limit is the maximum stress a material can withstand without permanent deformation
9) calculate by finding yield point and then going across to y axis and is that value of stress
10) calculate by finding failure point and going across to y axis and is that value of stress
11) calculate by finding failure point and going down to x axis and is that value of strain
Hookes law ?
Young modulus?
Link?
Elastic deformation
Elastic deformation is represented by which part of the stress strain curve?
What is Young’s modulus?
What is proof stress?
Elastic deformation - straight line part
Young’s modulus - gradient of straight line part of graph
It is a measure of stiffness of a material
Proof stress = Point of which a particular degree of permanent stress occurs in the sample, also called offset yield stress
Stress needed to create typically 0.1-0.2% plastic deformation
Elastic deformation
Plastic deformation
What is Youngs Modulus / Elastic modulus?
Measure of stiffness of a material within the elastic range (the linear portion of the curve) - KEY DEFINITON
It is also a mechanical property that measures the stiffness of a solid material
Defines relationship between stress and strain
What is elastic modulus determined by?
What does stiffness depend on?
What do we need to know about a material to calculate its stiffness?
Which of these graphs is strongest?
Which is most brittle?
Which is most ductile?
Which is most stiffest?
Why is it important to know the yield point?
Elastic modulus comparisons
Tensile strength
What is tensile strength ?
Maximum amount of tensile stress a material/specimen can withstand before failure - either deformation or breaks if brittle
Ability of a material to resist a force that tends to pull it apart
How do we test tensile strength ?
Test specimen placed in universal testing machine
Uniform load applied to pull apart and tensile strength can be determined
Is it easy to determine tensile strength?
Compressive strength
What is compressive strength?
How do we determine compressive strength?
Maximum compressive load specimen can withstand without undergoing failure (what failure means was mentioned on prev fc - deformation or breakage)
Determined using cylindrical specimens with dimensions in accordance to ISO or ASTM standards
Is tensile test easy to carry out on brittle material?
FROM TEXTBOOK NOT LECTURE
For brittle materials tensile test is difficult to carry out and results show high degree of scatter
An alternative is compression test - more easily performed on brittle materials
What happens to brittle materials when maximum compressive strength is exceeded?
What happens to ductile materials when maximum compressive strength is exceeded?
Compressive strength - Amalgam failure?
What is true stress?
Do compressive strengths calculated from equations yield accurate results ? Why / why not?
Do ceramics have higher compressive strength or tensile strength?
Do composites have higher compressive strength or tensile strength ?
What is diametrical compressive strength also known as Indirect Tensile strength ?
How tested?
What is it widely used for and why?
Why is it widely used?
It is a property for characterising dental composite restorations
Cylindrical specimen loaded onto universal testing machine BUT axis is changed , cylinder turned on axis and indirect tensile strength
Widely used especially for dental composites as you can prepare small specimens
How do we test flexural strength ?
What is flexural strength of a material?
What are flexural forces?
Is high or low flexural strength desired and why or why not?
BENDING TEST ( or flexure test ) USED TO TEST FLEXURAL STRENGTH
Flexural strength - Ability of a material to resist deformation under load
For materials that deform significantly but do not break, the load is yield typically measured at 5 percent deformation of outer surface
Flexural forces are result of forces generated in clinical situations and dental materials need to withstand repeated flexing, twisting and bending
High flexural strength is desirable as once materials are under action of chewing stresses may induce permanent deformation
What is the difference between flexure test and bend test?
Difference between flexure test and bend test is between type of material used and info we have obtained from them
Flexure test designed to measure bend strength of brittle material
Bend test designed to measure crack resistance of ductile material
How is bending test carried out?
Two types of bending test:
Centre loading test / 3 point test OR 4 point bending test
Centre loading test - specimen supported by 2 spherical supports. Centre point loading and bending is measured
4 point bending test - 2 supports but instead of loading at the centre using one knife edge, 2 knife edges used to bend / load specimen
Surface defects become more apparent with this test
Formula for bending strength - don’t need to memorise but do need to understand
Would test bending in a bridge
What is hardness?
Why is hardness important?
How can hardness measurement be defined ?
Resistance to permanent surface indentation
Sufficient hardness ensures restorations are resistance to surface scratching during mastication or abrasion
Important in dentistry as it governs ease of cutting, polishing, resistance to abrasion in service
Hardness measurement can be defined as macro, micro or nano- scale according to the forces applied and displacements obtained .
How is hardness measured ?
Vickers hardness of materials?
Vickers indentations near and across the Tooth DEJ
