Failure of material in the oral environment Flashcards
(44 cards)
Define resilence.
The capacity of a material to absorb energy when it is deformed elastically.
How is resilence measured on a stress strain graph?
The area under the curve up to the elastic limit:
What does the resilience zone indicate about the stiffness of a material?
- Bigger zone = more flexible
- Therefore the resilence zone is the same as the modulus of elasticity:
What 2 strains occur under axial loading?
- Axial strain
- Lateral strain
What does a tensile force to do the area of cross-section and original length?
- Reduction of cross-section.
- Increase in original length
What does a compressive force to do the area of cross-section and original length?
- Increase in cross-section
- Reduction of original length
What is Poisson’s ratio and what does it indicate?
- The ratio between the lateral and axial strain in the elastic limit.
- That the change in cross-section is proportional to the deformation in the elastic range.
Why must elastic modulus’ and poisson’s ratio of the material replacing the tissue be similar to the tissues around it?
So deformation rates are similar with both the materials and tissues to prevent shear forces e.g. sealant and enamel:
What is the unit for fracture toughness and when does a fracture occur?
- Kc
- When Kc = K (critical value)
- Then catastrophic crack propagation occurs.
What is the crack growth-stress intensity factor?
The resistance of a material towards crack propagation
What specimens are used to determine fracture toughness?
Notched specimens
What value does fracture toughness give?
A value of the work in creating 2 new surfaces (when cracking occurs)
What is the relationship between stress intensity and fracture toughness?
- The stress intensity, KI represents the level of “stress” at the tip of the crack
- The fracture toughness, KIC, is the highest level of stress intensity that a material under very specific (plane-strain) conditions that a material can withstand without fracture.
What are the 3 modes of failure in dental materials?
Mode I - tensile crack opening KIc.
Mode II - sliding crack opening (in plane shear) KIIc.
Mode III - tearing crack opening (anti-plane shear) KIIIc
How can the critical value of
K be stopped to avoid the crack?
Both Y (shape) and stress are constant as the crack propagates.
Some methods to limit crack propagation include:
- Addition of rubber particles in PMMA dentures act as a barrier to cracks.
- Same with alumina into ceramic crowns.
- What part of the tooth stops cracks from propagating in the enamel?
dentine
What is dynamic loading?
Where materials are subjected to fluctuating or intermittent stresses.
How can a material fail with stresses less than the maximum stress being exerted?
With repeated cyclic loading.
What does determination of the relationship of stress level and number of cycles to failure allow an estimation of with the material?
The reliability of material.
Define fatigue in material testing.
Accumulation of small amounts of intermittent stress.
Explain the clinical significance of determining fatigue behaviour of direct restorative materials.
It allows for an estimation of the reliability of the material.
The stress level and number of cycles to failure can be used to determine the material’s endurance limit.
How can fatigue testing be done?
- By subjecting the specimen to cyclic loading over a range of loads.
- The number of cycles required to cause failure is counted.
How is stress plotted when calculating fatigue and what is the curve called?
- Plotted as a function of the log of the cycles required to cause failure against stress amplitude.
- Known as S-N curves:
Give 4 thermal properties of materials in the oral cavity.
- Coefficient of Thermal Expansion
- Glass transition temperature
- Thermal Conductivity and Thermal Diffusivity
- Polymerisation exotherm
