Mechanical Properties of Dental Materials

Diametral Compression Test

(also called indirect tensile test) useful for testing brittle materials to obtain a tensile strength not valid if the material deforms significantly before failure (should fracture along the center)

Flexural Strength

(transverse strength) 3 point bend test -denture based materials -long bridge spans

Stress Relaxation

*FORCE DECAY* the reduction in stress in a material subjected to constant strain useful in evaluating orthodontic modules or chain some modules may sustain forces longer than others (ex: rubber band pulled apart -- as you continue to hold it apart, it will require less and less force to keep it stretched out)

What does the consistency of food affect?

-The number of chewing strokes -Frequency of tooth contact -The degree of gliding contact

Dental Importance: Hardness

-some relation to wear -hardness affects ability to finish or polish a material, which is desired esthetically -hardness gives indication to resistance to inservice scratching (scratches may decrease fatigue strength and lead to failure)

Chewing carrots

0.9 mm contact glide

What defines force?

1. Point of application 2. Magnitude 3. Direction of application

Thixotropic: 3 Characteristics

1. time-dependent decrease in viscosity 2. yield point (similar to plastic) 3. Not reversible shear stress/strain rate curve (structure alteration) New ketchup bottle: tap bottle to get flow, flows...next use: do not have to tap

Chewing meat

1.75 mm contact glide

Incisor

150 N (34 lbs)

Cuspid

200 N (45 lbs)

Bicuspid

300 N (67 lbs)

First and Second Molar

400-800 N (90-180 lbs)

Average Maximum sustainable biting force

756 N (170 lbs)

Strain

A body undergoes deformation when a force is applied to it change in length per unit length of the body when it is subjected to a stress deformation/original length (delta L/L)

Microtensile bond strength

Alternative to shear bond testing Typically used for adhesives

Brittle Dental Materials

Amalgam Composites Cements Ceramics

Marginal Ditching

At restoration/tooth interface

Types of Stress: Bending Moment

Bending

Pseudoplastic

Decreasing viscosity as shear rate increases easier to mix with increased stirring speed silicone impression materials: flow through syringe some endodontic cements

Gender force

Men > Women

International unit of Force

Newton

Types of Stress: Shear

Shear

Torsion Curve

Similar to stress-strain curves -linear portion (elastic strain) followed by a non-linear portion (permanent strain) permanent angular rotation if angular rotation exceeds the value at the end of the linear portion (most materials) LARGER wires are stiffer in torsion (slope of initial linear portion) but *linear portion is less* (permanently deformed at lower angular rotation)

Rheology

Study of the flow of matter

Torsion

TWISTING important in endodontic files and reamers rotated in the root canal produces a shear stress, from zero at the center and increasing radially outward to a *maximum at the surface*

Impact Strength

The energy required to fracture a material under an impact force popular for comparison of denture base materials

Fracture Toughness

The measure of a material's resistance to fracture when a crack is present useful for brittle materials because failure is frequently flaw dependent (fail due to crack propagation) --example: pulling apart a piece of paper normally versus when there is already a rip in it

Types of Stress: Twisting Moment

Torsion

Age force

Young adults > children

Viscosity

a fluid's resistance to flow Units: pascal x second = *poise (p)* high viscosity -- flows slowly (think syrup) low viscosity -- flows quickly (think water) many dental materials are in the liquid state at some time

Wear

a loss of material resulting from removal and relocation of materials through the contact of two or more materials several different types of wear, but one of the most common with respect to dentistry and dental materials is *abrasive wear*

Elastic Modulus

also called modulus of Elasticity or Young's modulus represents the stiffness within the elastic range E = stress/strain a stiffer material = will have steeper line a more flexible material = will have a more flat line

Angle of Bending

angle from normal resting position measured to displacement (angle underneath the baseline position)

What loads are produced during chewing

axial and horizontal loads (tooth movement in all directions) AXIAL > HORIZONTAL

Occlusal Forces

biting force decreases from the molar region to the incisors

Burnishing

closing the metal margin of a metal crown recovery of elastic strain but not of plastic strain

General values in a chart are dependent on what:

composition, material history, and other factors

Additional Hardness Tests: Barcol

dental materials applications include measuring resin composites

Stress

dental materials may experience different types of stress dependent upon loading, geometry, etc.

Stress-Strain Curve

elastic region = the deformation is recoverable plastic region = the deformation is permanent

Types of Stress: Axial (tensile)

elongation

S/N PLot = (Stress vs. Cycles to Failure)

endurance limit: repeat stress below the limit will last forever

Factors affecting fatigue

environmental agents: -temperature -humidity -pH -aqueous media -biological substances surface feature -smooth -rough material history

Abrasive Agents

food during mastication toothpaste between tooth and toothbrush

Viscosity characteristics

for liquids that do NOT set, viscosity decreases with increasing temperature and is independent of time for liquids that DO set, viscosity increases with increasing temperature and also increases with time HIGH viscosity = will set quicker

Stress-Strain curve

force versus deformation --> helps to form the stress-strain curve converging three lines onto one curve -- example

Bending Moment

force x distance between the applied force and the fulcrum (Force x Length)

Stress

force/area units= N/m^2 = pascal (1 MPa = 1,000,000 Pa) if a biting force of 400 N is concentrated on the amalgam restoration having a cross-sectional area of 4mm^2, the stress developed would be 100 MPa

Force

generated by one body pushing or pulling on another

Viscoelasticity

having appreciable and conjoint viscous and elastic properties behavior is intermediate between that of an elastic solid and a viscous liquid combination of elastic and plastic deformation and elastic recovery when stresses are eliminated *important in impression materials and other polymer-based materials* THINK MEMORY FOAM - eventually will return to normal over time

Vickers Hardness test

micro indentation test used for metals and plastics the indenter is a 136 degree pyramidal diamond that forms a square indent Vickers Hardness Number is reported (VHN or HV) **Inverted pyramid shape**

Knoop Hardness Test

micro indentation test used for variety of materials the indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent Knoop Hardness Number is reported (KHN or HK)

Plastic

no flow until a certain shear stress is reached (yield point) example: tooth paste **initial stress required to get flowing**

Angle of contact glide - is affected by?

occlusal morphology

Bruxism

pathological form of wear

Fatigue

progressive fracture under repeated loading stresses frequently vary cyclically (mastication) failure occurs below the single load strength (static) *Failure typically due to crack propagation* (example: breaking paper clip -- bending back and forth repeatedly... add up over time)

Torsional Moment

shear force x distance between location of force and center (r or radius) Force x Radius (r)

Types of Stress: Axial (compression)

shrinkage

Bending Moment-Angular Deflection Curves

similar in shape to stress-strain curves -linear portion (elastic strain) followed by a non-linear portion (permanent strain) permanently bent if bending angle exceeds the value at the end of the linear portion (most materials) LARGER wires are stiffer (slope of initial linear portion) an enter into permanent strain at lower bending angles

Compressive Strength

stress-strain curve similar to those for testing in tension may be created for materials tested in compression

Creep

the *time-dependent plastic strain* of a material under a static load or constant stress (even if below elastic limit) occurs in materials at temperatures above about 1/2 melting temperature ---historically, a problem in amalgams

Ductility

the ability of a material to be plastically deformed how much deformation before fracture

Stress Relaxation Curve

the force needed to keep each extended decreases over time, but the top material exhibits a larger, more rapid decrease

Proportional limit

the greatest stress that a material will sustain without a deviation from the linear proportionality of stress to strain essentially the same as elastic stress, BUT differs by definition looking at the curve

Shear Strength

the maximum stress that a material can withstand BEFORE failure in shear mode Used frequently in bond strength testing -porcelain/metal interface -interface between tooth and orthodontic brackets, composite resins, and adhesives

Ultimate Compressive Strength

the maximum stress that a material can withstand before failure in compression is also a factor in determining stress-bearing capability in a restorative material

Ultimate Tensile Strength

the maximum stress that a material can withstand before failure in tension =strength of a material

Elastic Limit

the maximum stress that a material will withstand without permanent deformation looking at the material

Toughness

the resistance of a material to fracture also thought of as energy absorbed per volume of material

Resiliency

the resistance of a material to permanent deformation important in orthodontic wires may be thought of as energy absorbed per volume of material (how much energy to permanently deform the material??)

Hardness

the resistance to permanent surface indentation different types of tests are based upon indenter material, geometry, and load each type has a different scale, but in general - smaller indentations correspond to higher hardness values...signifying harder materials

Fracture strength

the stress at which a material fractures NOT necessarily equal to ultimate strength *Brittle material --> Fracture Strength usually equals Ultimate Tensile Strength*

Yield Strength

the stress value at which a material exhibits a specified permanent strain usually 0.2% is used

Abrasive Wear: Two Body

two surfaces rubbing together -the harder surface may indent or cut away the softer material from the other surface tooth to tooth contact

Brinell Hardness Test

used for METALS a hardened steel ball indenter is used and the diameter of indentation is measured Brinell hardness number is reported (BHN or HB)

Rockwell Hardness Test

used for metals and plastics a ball or metal cone indenter is used and the depth of indentation is measured based upon indenter geometry and load rockwell hardness number is reported (RHN or HR)

Additional Hardness Tests: Shore

used for rubber (denture liners and mouth protectors)

Normal Chewing

usually <10 N

Dilatant

viscosity increases with shear rate harder to mix with increased stirring (shear rate) example: some denture-based resins

Newtonian Fluid

viscosity remains constant example: water some dental cements and impression materials

Corrosive Wear

wear coupled WITH chemical attack issue in wear of composites, especially posterior restorations (higher stresses)

Abrasive Wear: Three Body

when objects (abrasive agents) are between the two surfaces (that do not have to be in contact)