# mechanical properties of dental materials Flashcards

what is the biting force like from back to front of the mouth? does the force change for bridges and dentures?

Maximum biting forces decrease from the molar to the incisor region

The average biting forces on the first and second molars are about 580 N

The average forces on bicuspids, cuspids, and incisors are about 310, 220, and 180 N, respectively

Patients exert lower biting forces on bridges and dentures than on their normal dentition

what types of stresses are there? (5)

Types of stress: compressive, tensile, shear, twisting moment, and bending moment (flexure)

how do you calculate the compressive or tensile stress?

Compressive or tensile Stress = Force / Area

this is force over original area if you want to calculate it that way.

if you are calculating it using the force over instantaneous area, the area will begin to get smaller and smaller as more stress is applied and it gets closer to fracture.

LOOK AT THIS IN THE BOOK

for stress-strain curves how are the proportional limit, the elastic limit, and the yield point calculated?

they are calculated experimentally. You always want to be under the yield stress, so if you designed the material you would always want to be at about 80% of the yield point.

what is the proportional limit?

Proportional limit: the stress on the stress-strain curve when it ceases to be linear, or when the ratio of the stress to the strain is no longer proportional

what is the yield strenght?

Yield strength: stress at some arbitrarily selected value of permanent strain, such as 0.001, and thus is slightly higher than the proportional limit

what is ductility and malleability?

Ductility and malleability: The percentage of elongation and compression are measures of ductility and malleability respectively

ductility is tension

malleability is compression

what is brittleness?

Brittleness: relative inability of a material to deform plastically

e.g. porcelain, glass

SHOULD I WATCH THE TENSILE TEST VIDEO FROM THE LECTURE?

yes

what is resilience?

in the stress versus strain diagram it is when it is linear, the area under that part

Resilience: the measure of energy required to deform a material permanently

for the stress, strain diagram what is the toughness?

it is the area under the linear plus the non-linear portion.

Toughness: the measure of energy required to fracture a material

Two materials may have the same resilience. One has a high yield stress and low corresponding strain, such as a composite dental material. The other has a lower yield stress, but a higher corresponding strain, such as unfilled acrylic

why are stress-strain curves important?

Stress-strain curves are important for materials in which the strain is independent of the length of time that the load is applied

why is the strain-time curve important?

Strain-time curves are more useful than stress-strain curves when the strain is dependent on the time that the material is subjected to a load. For example, alginate and elastomeric impression materials, dental amalgam, etc…

how is a strain-time curve read?

The curve represents the application of a compressive load at t1 causing an instantaneous vertical increase in strain

The load was maintained until t2, with the strain gradually increasing. This increase is the result of a combination of viscoelastic strain (time dependent but recoverable) and viscous flow (time dependent but not recoverable)

Upon removal of the load at t2, the strain instantaneously dropped due to elasticity. Then continued gradual decrease in strain occurred due to the recovery of viscoelastic strain

A permanent strain remains with time

what is the clinical value of strain-time curves?

If the load has been applied for a longer time than t2, or the magnitude of the load has been greater, the amount of permanent strain would have been more.

Clinically, the shorter the time and the lower is the load applied to the impression material, the lower the permanent strain and the more accurate is the impression