Property of materials
Strength depends on what factors: (4)
1. <b>strain rate</b> 2. the shape of the test speciment 3. the surface finish (control flaws) 4. the environment in which material is tested
Name three properties that are related to heardness:
1. Compressive strength 2. proportional limit 4. ductility
Give 4 reasons why shear stress often is not the cause of fractures in oral environment.
1. most restorations have rough, <b>curved</b> surfaces 2. tensile strength is <b>below</b> shear strength value 3. force must be applied exactly adjacent to the
Three types of stress in dentistry:
1. tensile stress 2. shear stress 3. compressive stress
Three common methods for measurement of ductility:
1. the <i>percent elongation</i> <b>after</b> fracture. 2. the <i>reduction in area </i>of the tensile test specimen 3. the max # of bends performed in a cold bend test.
Two important aspects of microscopic flaws in brittle materials:
1. the stress intesntiy at these flaws increases with the length of the flaw, specially when it is oriented prependicular to the direction of tensile stress 2. flaws on the surface are associated with higher stresses than are flaws of the same size in interior regions.
A value of 0.3 for Poisson's ratio is typical. Thus, the shear modulus is usually about ___% of the elastic modulus.
38%
average biting force
756 N
Which of the following has more resistence? a. Dental porcelain b. Amalgam c. Resin-based composite d. Poly e. Alumnia ceramic
????
Toughness
Ability of a material to absorb elastic energy and to deform plastically before fracturing; measured as the total area under a plot of tensile stress vs. tensile strain.
Stress concentration
Area or point of significantly higher stress associated with a structural discontinuity such as a crack or pore or a marked change in dimension of a structure.
Why is the ultimate tensile strength sometimes less than the maximum stress?
Because of the reduction in area, the force required to increase deformation actually decreases.
Strain
Change in length per unit initial length. Units are in %
Compressive strength
Compressive <b>stress</b> within a compression test specimen at the point of fracture. note: compressive stress -Ratio of compressive force to cross-sectional area perpendicular to the axis of applied force.
Elastic strain
Deformation that is recovered upon removal of an externally applied force or pressure.
Why do prostheses sometimes fail under a very small force, even though the strength of the prosthetic material is relatively high?
Flaws (could be microscopic)
Pressure
Force per unit area acting on the external surface of a material.
Flexural strength
Force per unit area at the point of fracture of a test specimen subjected to flexural loading.
Flexural stress (Bending stress)
Force per unit area of a material subjected to flexural loading.
Stress
Force per unit area within a structure subjected to an external force or pressure (See Pressure).
Units of Modulus of elasticity
Giganewtons per square meter (GN/m²), or gigapascals (<b>GPa</b>)
Percent elongation
Maximum amount of plastic strain a tensile test specimen can sustain before it fractures (See Ductility).
Shear strength
Maximum shear stress at the point of fracture of a test specimen.
Proportional limit
Maximum stress at which stress is proportional to strain and above which plastic deformation occurs. in this case: PL= 1020 MPa
Strength
Maximum stress that a structure can withstand without sustaining a specific amount of plastic strain (yield strength) or stress at the point of fracture (ultimate strength).
Plastic deformation occurs when the ______________ within the prosthesis is exceeded.
Plastic deformation occurs when the <b>elastic stress limit (proportional limit)</b> within the prosthesis is exceeded.
the stress above which stress is no longer proportional to strain.
Proportional limit
True stress
Ratio of applied force to the actual cross-sectional area; however, for convenience stress is often calculated as the ratio of applied force to the initial cross-sectional area.
Compressive stress
Ratio of compressive force to cross-sectional area prependicular to the axis of applied force.
Shear stress
Ratio of force to the original cross-sectional area parallel to the direction of the force applied to a test specimen.
Ductility
Relative ability of a material to deform physically under a <b>tensile</b> stress before it fractures
Brittleness
Relative inability of material to deform plastically
Elastic modulus
Relative stiffness of a material; ratio of elastic stress to elastic strain
fatigue failure
Repeated stresses resulting in fractures. the fracture due to fatigue occurs at stress levels well below tensile strength.
Hardness
Resistance of a material to plastic deformation typically measured under an indentation load.
Stress vs. force
Stress = Force / area stress units - MPa
Mechanical properties are expressed most often in units of:
Stress and/or strain
When stress is induced in a two-unit cantilever bridge, where you would expect the tensile and compressive stress develop?
Tension would develop on the occlusal half where as compression would develop in the gingival half.
The area bounded by the elastic region is a measure of ________, and the total area under the stress-strain curve is a measure of ____________.
The area bounded by the elastic region is a measure of <b>resilience</b>, and the total area under the stress-strain curve is a measure of<b> toughness</b>.
Resilience
The relative amount of elastic energy per unit volume released on unloading of a test specimen.
Toughness increases with increases in _____ and _________.
Toughness increases with increases in strength and ductility.
(T/F) Stiffness of a dental prosthesis can increase by increasing its thickness.
True however, the <b>elastic modulus</b> is always a constant. never changes! it describes material's <b>relative</b> stiffness.
Maximum stress that is required to cause fracture is called _________.
Ultimate strength
diametral compression test
a test for tensile strength that is only used for materials that exhibit predominantly elastic deformation and little or no plastic deformation (brittle).
Tensile strength for: a. Dental porcelain b. Amalgam c. Resin-based composite d. Poly e. Alumnia ceramic
a. Dental porcelain → 50-100 MPa b. Amalgam → 27-55 MPa c. Resin-based composite → 30-90 MPa d. Poly → 60 MPa e. Alumnia ceramic → 120 MPa
which tooth structure has the hgiher <b>propotional limit</b> and <b>modulus of elasticity</b>? a. enamel b. dentin
a. enamel
When a metal had been stressed beyond its proportional limit, the hardness and strength of the metal ________ at the area of deformation. a. increases b. decreases
a. increases although ductility of the metal ↓↓
A low elastic modulus and a low tensile strength suggest ___________ resistance. a. low impact b. high impact
a. low impact
Which is more stronger in tesnion? a. enamel b. dentin
b. dentin dentin → 50 MPa enamel → 10 MPa
Which one is tougher? a. enamel b. dentin
b. dentin is tougher! the more flexible material is the tougher it is. enamel is brittle
Shear stress is calculated by dividing the force by the area ________ to the force direction. a. perpendicular b. parallel
b. parallel brackets for braces is a good example
A _______ stress is caused by a load that tends to stretch or elongate a body. a. compressive b. tensile c. shear
b. tensile
in most cases fracture occurs because of the _____ stress component. a. compressive b. tensile c. shear
b. tensile
From mechanical property point of view, why isn't it a good idea to burnish amalgam, composites, ceramics and nonresin luting agents?
because they have <b>low</b> or <b>zero</b> percent elongation.
How can resilience of two materials be compared?
by comparing the <b>area</b> underneath the elastic region. ↑ the area = ↑ resilience
which of the dental restorative materials have the highest resilience?
composites followed by porcelain, PMMA, amalgam and alumina.
___________represents the ability of a material to sustain a large permanent deformation under a <b>tensile</b> load before it fractures.
ductility
How does polshing of brittle material help their strength?
flaws on the surface are associated with higher stresses than are flaws of the same size in interior regions. Thus surface finishing of brittle materials such as ceramics, amalgams, and composites is extremely important in areas subjected to tensile stress.
if stress/strain value is very small the materials are said to be:
flexible The <b>maximum flexibility</b> is defined as the flexural strain that occurs when the material is stressed to its proportional limit.
endurance limit
line below which the material will NEVER fail under an infinite amount of cycles: IDEAL!!!!
For brittle materials that exhibit only elastic deformation and can sustain no plastic deformation, stresses at or slightly above the ________________ result in fracture.
maximum elastic stress (proportional limit)
The ability of a material to sustain considerable permanent deformation without rupture under <b>compression</b>, as in hammering or rolling into a sheet, is termed ____________.
mealibility
The fracture toughness of dentin varies by a factor of 3 as a function:
of enamel rod orientation.
As the interatomic spacing increases, the internal energy increases. As long as the stress is <b>not greater than the proportional limit</b>, this <u>energy</u> is known as__________.
resilience the term resilience is associated with 'springiness'
The __________ of a material is defined as the average level of stress at which a material exhibits a certain amount of initial plastic deformation or at which fracture occurs in test specimens of the same shape and size.
strength
Poisson's ratio
the ratio of horizontal extension to vertical compression. Most engineering materials have values of approximately <b>0.3</b>
What does the straight line in the strain-stress curve represent?
the region represents <b>reversible</b> elastic deformation, because the stress remains below the proportional limit.
Materials with a high elastic modulus have: a. high strength values. b. low strength values.
they could have either. they are independent of each other.
a different property, __________, is used in such cases when the proportional limit cannot be determined with sufficient accuracy.
yield strength is a property that represents the stress value at which a small amount (0.1% or 0.2%) of plastic strain <b>has occurred</b>. Does not exist for very brittle materials
Yield strength
• The <b>stress</b> at which a test specimen exhibits a specific amount of plastic strain (0.2%) • The YS at 0.2% strain offset from the origin is 1536 MPa in this case.
the restorative material should exhibit a moderately ____ elastic modulus and relatively ____ resilience, thereby limiting the elastic strain that is produced.
↑ elastic modulus ↓ resilience/springiness
