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### Brittleness

Relative inability of material to deform plastically

### Compressive stress

Ratio of compressive force to cross-sectional area prependicular to the axis of applied force.

### 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.

### Ductility

Relative ability of a material to deform physically under a <b>tensile</b> stress before it fractures

### Elastic strain

Deformation that is recovered upon removal of an externally applied force or pressure.

### Elastic modulus

Relative stiffness of a material; ratio of elastic stress to elastic strain

### Flexural strength

Force per unit area at the point of fracture of a test specimen subjected to flexural loading.

### Hardness

Resistance of a material to plastic deformation typically measured under an indentation load.

### Percent elongation

Maximum amount of plastic strain a tensile test specimen can sustain before it fractures (See Ductility).

### Pressure

Force per unit area acting on the external surface of a material.

### Proportional limit

Maximum stress at which stress is proportional to strain and above which plastic deformation occurs.

in this case: PL= 1020 MPa

### Resilience

The relative amount of elastic energy per unit volume released on unloading of a test specimen.

### Shear stress

Ratio of force to the original cross-sectional area parallel to the direction of the force applied to a test specimen.

### Shear strength

Maximum shear stress at the point of fracture of a test specimen.

### 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.

### Stress

Force per unit area within a structure subjected to an external force or pressure (See Pressure).

### 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.

### Strain

Change in length per unit initial length.

Units are in %

### 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.

### 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.

### 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.

### Mechanical properties are expressed most often in units of:

Stress and/or strain

### Three types of stress in dentistry:

1. tensile stress
2. shear stress
3. compressive stress

strength

### 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

### 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)

### Stress vs. force

Stress = Force / area

stress units - MPa

b. tensile

b. tensile

### 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

### 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

### 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.

### 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.

### (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.

### Which one is tougher? a. enamel b. dentin

b. dentin is tougher!

the more flexible material is the tougher it is. enamel is brittle

### 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.

### Units of Modulus of elasticity

Giganewtons per square meter (GN/m²), or gigapascals (<b>GPa</b>)

38%

### 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.

### 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'

### How can resilience of two materials be compared?

by comparing the <b>area</b> underneath the elastic region.

↑ the area = ↑ resilience

### 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>.

### the restorative material should exhibit a moderately ____ elastic modulus and relatively ____ resilience, thereby limiting the elastic strain that is produced.

↑ elastic modulus
↓ resilience/springiness

### Poisson's ratio

the ratio of horizontal extension to vertical compression.

Most engineering materials have values of approximately <b>0.3</b>

### Maximum stress that is required to cause fracture is called _________.

Ultimate strength

### the stress above which stress is no longer proportional to strain.

Proportional limit

### 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.

### 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

### 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 ↓↓

### 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).

### fatigue failure

Repeated stresses resulting in fractures. the fracture due to fatigue occurs at stress levels well below tensile strength.

### endurance limit

line below which the material will NEVER fail under an
infinite amount of cycles: IDEAL!!!!

### 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

????

### The fracture toughness of dentin varies by a factor of 3 as a function:

of enamel rod orientation.

### Toughness increases with increases in _____ and _________.

Toughness increases with increases in strength and ductility.

b. dentin

dentin → 50 MPa
enamel → 10 MPa

a. enamel

756 N

### 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.

ductility

mealibility

### 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.

### Name three properties that are related to heardness:

1. Compressive strength
2. proportional limit
4. ductility

### 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)

### 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.

### 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.

a. low impact

### which of the dental restorative materials have the highest resilience?

composites followed by porcelain, PMMA, amalgam and alumina.

Example: