Mechanical Properties I and II Flashcards
- Describe the importance of the mechanical properties of dental biomaterials
to properly select materials and design devices and appliances in dentistry
- List the objectives in measuring mechanical properties of materials
- Know the fundamental properties (strength, elasticity, etc.)
- Determine how those properties hold up in conditions the material will be used (impact, fatigue, abrasion…)
State the 1st Classifications of mechanical properties:
o Bulk Properties
o Surface Properties
State the 2nd Classification of mechanical properties:
sub-classifications of bulk properties
fundamental properties
applied properties
1st Classification of Mechanical Properties DEFINE Bulk:
consider whole mass of material
(strength, elasticity)
1st Classification of Mechanical Properties DEFINE Surface:
hardness (indentation) and wear (abrasion)
2nd Classification of Mechanical Properties
How to obtain Fundamental Properties:
Fundamental: obtained from test applying steadily increasing load (stress-strain)
2nd Classification of Mechanical Properties DEFINE applied properties, 3 examples
Applied properties: specific conditions in service, such single sudden force (impact), repeated low loads (fatigue), time dependent properties (viscoelasticity).
Define Stress
force per unit area from externally applied forces, units: Pa/MPa.
Units for Stress
Pa/MPa
State the 3 Fundamental Types of Stress
Tension, Compression, Sheer
Define Tension (a fundamental type of stress)
force perpendicular to resisting area
Define Compression (a fundamental type of stress)
force perpendicular to resisting area, sense of load opposite to that of tension
Define Shear (a fundamental type of stress)
force parallel to resisting area
State the 2 combined types of stress: combination of fundamental stresses
Torsion and Flexure/Bending
Do stress concentrations increase or decrease on notches, grooves, cracks, surface regularities?
The stress concentrations increase - that is why we avoid sharp corners in teeth cavity preparations.
Ceramics and glasses (smooth surfaces) are sensitive to surface roughness because of _____ _______ at the top and bottom of topographical irregularities
stress concentrations
Define Strain
describes relative deformation or change in shape and size with respect to initial length (dimensionless)
State the 2 Types of Strain
Elastic and Plastic
Elastic is ____, Plastic is ______.
Elastic is recoverable, Plastic is permanent
Recording an impression - _____ on insertion (____ deformation). ______ on displacement from tissue (______ deformation)
Plastic on insertion (permanent deformation). Elastic on displacement from tissue (recoverable deformation
Define Elastic Limit
Stress at which material changes from elastic to plastic strain. Stresses equal or higher than the elastic limit produce permanent/plastic deformation in the material.
Ductility is measured by % of ______
elongation
High % of elongation means
it’s a ductile material
Low % of elongation means
it’s a brittle material
Define Ultimate Tensile Strength
stress at which the material breaks the last point in a stress-strain curve
** REVIEW THE STRESS STRAIN CURVE
First part of graph is linear (Elastic strains), slope of linear part of the graph is Modulus of Elasticity or Young’s modulus. Material follows Hook’s law.
Remember 1: flexibility is a combination of the notion of less rigid but also more extend of deformation in the elastic region of a material.
High modulus = more rigid material, Low modulus: less rigid material
Define Maximum Strength
- Maximum stress the material undergoes during stress-strain test - Also called generically ‘strength’.
Define Resilience
- Amount of energy a material can absorb without permanently deforming.
Resilience on the Stress Strain Graph
- Area under the stress-strain graph in the elastic portion of it.
Define Toughness
- Total amount of energy a material can absorb until fracture.
Toughness on the stress strain graph
- Area under the total stress-strain graph.
Big ole bold reminder on the summary page:
Fundamental properties of a specific material can not be related between them, i.e. you need to know the individual values of each of them to have the total mechanical description of the material. Most importantly, you can not infer any mechanical property associated in the plastic region of the material by knowing the mechanical properties of that same material in the elastic region.
5 Applied Properties
Impact, Fatigue Strength, Hardness, Abrasion, Viscoelasticity
A material should be tested for its mechanical response while suffering an _____
impact. and the energy the material absorbs is the measured property in impact test.
Define Impact
Measure of brittleness under application of forces at high speed.
Fatigue Strength
Repeated low forces (bellow elastic limit) that bring the material to an unexpected fracture
Number or repetitions/cycles of load recorded until fracture depending of the value of the repeated stress
Also Fatigue Strength
Relating to Fatigue Strength, if material is cyclically loaded with stresses lower than this value the material will ____ _____.
never break.
Define Hardness
ability for material to resist indentations, resistance to be scratched
Define Abrasion
loss of anatomic form from surface or restoration
Abrasion results in __ of the material
wear
Wear is the ___ _______ property to be reliably measured. Depends so much on conditions in service.
most difficult
Define Creep (in relation to Viscoelasticity)
slow deformation with time under constant stress
Define Stress Relaxation (in relation to Viscoelasticity)
material constantly deformed, may show drop in stress to keep deformation with time.
Viscoelasticity is ____ dependant.
TIME dependant. Viscous part is released/shown when constant stress or strain is applied for a certain period of time.
Viscoelasticity is a combination of _____ and _____ behavior.
a combination of viscous and elastic behavior.
List the Five Types of Stress
Tension, Compression, Sheer, Torsion Flexure/Bending
- List three standard methods of measuring hardness
?
State Hooke’s Law
see pic
Point out on a stress-strain graph the point of transition from elastic to plastic strain
see pic
Equation to measure Stress (Pa)
Force/Area = N/m2 = Pa
Strain Problem Example:
A rod of 100mm (Li) stretched to 102mm (Lf) what is the strain
Change in L divided by Li
2/100 = 2%
“what happens when a regular shaped pieve of material is placed on a testing machine and a gradually increasing load (either compressive or tensile) is applied to it?
Keep applying and it keeps deforming and displacing the material until the material breaks. (mechanical energy/mechanical work disforms until the response is to break).
The machine measures load/force nad displacement/change in length.
