MECHANIAL PROPERTIES

PART 2

Dr/ Asmaa Abdel-Hakeem Metwally

Lecturer of Biomaterials
 ENERGY RELATED PROPERTIE
I-RESILIENCE
 The amount of energy absorbed by structure when it
is stressed P.L. Or the amount of energy necessary
to deform a material to proportional limit
 Or Resistance of the material to permanent
deformation under sudden impact.

 Called stored energy because when the load is

removed its released causing complete recovery of
deformation it may described as spring back potential

 It depend on P.L and E

It is measured graphically by the area
under the elastic (linear) portion of the
stress strain curve.
Area of triangle=
½ base x height =
(½ strain x stress ) at P.L=
½ m x MN =m MN/m³
m m²
= energy per unit volume

Resilience
 DENTAL APPLICATION
 Resilience has particular importance in
the evaluation of orthodontic wires . It
determines the magnitude of the force
that can be applied to the tooth and how
far the tooth can move before the spring is
no longer effective.
 Elastomeric soft liners absorb considerable
amounts of energy without being
permanently distorted when stressed and the
energy stored is released when the material
springs back to its original shape after
removal of the applied stress.
 Therefore, these materials act as cushion
between the hard denture base and soft
tissues to reduce masticatory forces
transmitted by prosthesis to the underlying
tissues.
 II-TOUGHNESS

 It's the amount of energy absorbed by the

material when its stressed to point just before
fracture point
 It depend on Strength and ductility or

 malleability of the material.

 It is measured by the total area under

 there stress strain curve(elastic and plastic)

 Unite is mMN/m³=energy per unit volume

 Dental importance:- mica crystal in

 ceramic
 Difference between resilience and toughness
Toughness is the total amount of energy a material can absorb
up to the point of fracture.
Whereas resilience represents the energy required to stress a
material to it’s proportional limit OR The maximum amount of
energy a material can absorb without undergoing permanent
deformation.

Proportional limit
Stress

Stres TOUGHNESS
RESILIENCE
s

Strain Strain
Toughness of the ductile materials is higher
than toughness of brittle materials
 III-FRACTURE TOUGHNESS

 the amount of energy required to fracture sample

with crack Or It is the ability of the material to
resist fracture through its resistance to crack
propagation.
 its proportional to the energy consumed in plastic
deformation so brittle material has low fracture
toughness and ductile material has high one due
to cracks has no effect on ductile material because
they able to plastic deform
CLINICAL SIGNIFICANCE

 Design basis against this type of failure can be

made to increase the life time of the material
 In composites The presence of glass particles
increase the fracture toughness because glass
will stop crack propagation
.
 In porcelain The addition of zirconia’s particles
increase the fracture toughness because
zirconium particles absorb the energy needed for
crack propagation
 In amalgam low copper amalgam has greater
fracture toughness than high copper
IV-IMPACT STRENGTH

 Is the amount of energy absorbed to fracture material

when subjected to sudden Force.
 Fracture happend due to crack propagation by the
energy of the sudden impact(impact strength cant
initiate cracks)
 The units are joules (J=Nm)
 Dental application :-
 -It predict the behavior of the material under such
condition,
 -Fracture of complete denture when dropped on a
floor corrected by using rubber modified acrylic resin
denture base
 IMPACT TEST

Izod test
Charpy test
CANTILEVER BENDING

 A beam of a material is fixed at one end and subjected

to load at the other end.
 bending moment=force x distance,
 A plot of bending momentum versus angle can
obtained and have the similar appearance of stress
strain curve
 an instrument permanently bent if the bending angle
exceed the value at the end of linear proportion of the
curve
 Dental consideration:
 fixed free bridge
 endodontic file and reamers
 TRANSVERSE STRENGTH TEST, THREE POINT
BENDING(3PB), FLEXURAL STRENGTH, MODULUS OF
RUPTURE(MOR)

This test is sensitive to surface condition specially on

tensile side
 From this test
 Stress= 3x load x length

 2x width x thickness²
 Deformation = load x length³
 4x E x width x thickness³
 Dental application:-
THE DIAMETRAL COMPRESSION TEST
(INDIRECT TENSILE TEST , BRAZILLIAN TEST )
 This test involves diametric compression of a disc with a
thickness one-half its diameter between two plates until fracture
occurs. The compressive force introduces tensile stresses normal
to the diameter under load, and failure is tensile in character.
 The tensile strength is calculated P

as =2P / DT
 Where,
 P  load Tension
Fracture plane
 D  Diameter of the disc
 T  Thickness of the disc

P
 TEAR STRENGTH

 The resistance to tearing

 It depend on the rate of loading (strain rate
dependent )
 Important in impression materials especially
in interproximal
 Tear strength increase by removing

 the impression with sharp snap parallel

 to the long axis of the teeth
 FATIGUE

 Fatigue Is the fracture of a material when

subjected to repeated cyclic small stresses
(dynamic fatigue) or constant load (static
fatigue) below the proportional limit.(called
catastrophic fracture)
 depend on:- the magnitude of the load

 the number of loading cycle repetitions.

 The mechanism:- repeated application of small

stress to an object causes tiny microcracks
which grows and coalesce to form macrocracks
until the material breaks. (surface condition as
roughness and sharp angles promote fracture)
 Fatigue tests performed by subjecting
 a specimen to alternating cyclic stress

 application below the yield stress until

 fracture occurs

 The curve show that when stress is high

the material fail at low no. of cycle

Stress

Fatigue strength

10³ 104 105 106 cycles

endurance limit, Fatigue limit is the
stress can be applied an infinite no of times
without causing material failure
Endurance limit of restoration should be
higher than masticatory force
DENTAL APPLICATION
SURFACE MECHANICAL PROPERTIES
I-Hardness

 resistance of the material to scratching, permanent

indentation or penetration
 Its cant be determined from stress strain curve

 important to dentistry.

 1-cutting, finishing, and polishing, e.g cobalt

chromium its
 2-natural teeth should not be opposed by porcelain,

 3-model and die materials

 HARDNESS TEST

1. Brinell hardness test

2. Vickers hardness test

3. Knoop hardness test

4. Rockwell hardness test

5.

6. Shore A hardness test

 VICKERS HARDNESS TEST

 Indenter is Diamond pyramid shape with angle 136º

and square based
 Measured by VHN = load /the area of indentation.
 micro-indentation
 Used for;- Ductile material E .g Gold castings
 Brittle material, Tooth structure, very hard
materials.
 Advantage:
 Measuring the hardness of very small
 areas
 Disadvantage:
 Not suitable for elastic materials
SHORE A HARDNESS TEST
SHORE A HARDNESS TEST


 The instrument consists of a blunt – point indenter 0.8mm

diameter, that taper to a cylinder 1.6mm.
 Indenter is attached by a lever to a scale that is graduated
from 0-to 100 units to masseur the depth of penetration.
 The usual method is press down firmly and quickly on the
indenter and record the maximum reading.
 If indenter completely penetrates the sample reading of 0
is obtained if no penetration occurs Reading of 100 units
results.
 Is used in the rubber industry to determine the
relative hardness of Elastomers and rubber
viscoelastic material. Evaluate soft denture liners ,
mouth protectors and maxillo- facial Elastomers.
 WEAR

 Wear is a surface loss of material resulting from

mechanical action
 Wear is usually undesirable due to produce shape
changes in the object that can affect function and
wear can produce particles that can elicit an
inflammatory response
 Under controlled conditions during finishing and
polishing, wear can be very useful.
 Wear dentally may result from mechanical
abrasion (improper use of tooth brush),
physiological attrition (mastication), or
pathological conditions (bruxism)
 Rheology, Creep and Flow:
 Rheology: the study of the flow of the material

 Viscosity: the resistance to flow of the material.

Caused by internal frictional forces in the
liquid.

 Units:
 Centipoise(CP) which= Mpa.Sec or MN sec/m

 Shear stress/ shear strain rate diagram( the slope of

the line)
 VISCOELASTICITY

 .
 describes materials that exhibit characteristics of
both elastic solid & viscous fluids.
 Viscoelastic materials are strain-rate dependent.


 Ideal elastic solid Ideal viscous fluid Ideal anelastic Arranged on series Viscoelastic material
Arranged on parallel

t1

t0
Strain

Strain

Strain

Strain

Strain
t0 t1 t0 t1 t0 t1 t0 t1 t0 t2 t1
Time Time Time Time Time

At time of Instantaneous strain Strain increases linearly Strain increases non- Instantaneous strain Instantaneous strain
load linearly
application
(t0)
At time of Instantaneous Strain remains constant Non-linear decrease of Instantaneous decrease Instantaneous decrease of
load removal disappearance of strain strain down to zero of strain strain (elastic portion),
(t1) then strain decreases non-
linearly (anelastic portion),
then strain remains
constant (viscous portion)
Ideal elastic solid model :Strain is independent
of the rate of loading or the length of time
in which the load was applied
Strain

t0 t1
Time
Ideal viscous fluid model:
Total strain directly proportional to the total
time of loading
Strain

t0 t1
 Ideal anelastic model (Parallel):

 non linear increase of strain on a given stress

with time
 non linear but complete gradual recovery after
removal of stress if there is enough time
 (time dependent)
Strain
Elastic & viscous (series model):
Strain

t0 t1
Time
Viscoelastic material model:
Strain

t0 t2 t1
Time
 removing the elastic impression should be with
sharp snap parallel to the long axis of the teeth due
to more permanent strain occur in these
impression with longer time applied during
removal of them from patient mouth (load
application)
 •Also they should be given a time to recover before die
can be poured


Creep:
Def: Time dependant plastic deformation of a hardened material
to static or dynamic stress occurring near melting
temperature of the material.
Importance:
 Most metallic and ceramic restorations do not creep in the
oral environment.
 1-Dental amalgam creep WHY ?
 it contain component with melting T slightly above mouth
T
 in class II lead to lose of its marginal adaptation (marginal
overhang) .loss of contact , loss anatomy, gingival
inflammation
 2-Sagging in ceramometalic restoration
Flow:
Def: It is analogous to creep but for amorphous materials, e.g.
waxes.
IMPORTANT NOTES

 Failure of the material below P.L may be

due to
 Fracture as in,
 -Fatigue

 -Impact strength
 or
 Deformation as

 -Creep
 -wear
ENERGY RELATED PROPERTIES

 Resilience,
 Toughness,
 Impact strength
 Dynamic related properties

 Fatigue,
 Creep,
 Impact strength ,
 Wear
 STRESS CONCENTRATING FACTORS

 Failure of dental restorations usually results from locally high

stress in specific areas although the average stress in the
structure are normal.
 Areas of high stress concentration may caused by:
 •Surface flaws, such as porosity and roughness
 •Interior flaws, such as voids
 •Marked change in contour (such as point of attachment of
clasp arm to a partial denture and axio-pulpal line angle in
restoration)
 •Large difference in modulus of elasticity between bonded
structures
 •Large difference in coefficient of thermal expansion between
bonded structures.
 •Point contact
 TO MINIMIZE AND REDUCE RISK OF
CONCENTRATION :CLINICAL FAILURE

 Polishing the surface to reduce surface flaws

 Use high quality material(with little internal
flaws) or use higher bulk of the material.
 close matching in modulus of elasticity and
coefficient of thermal expansion between bonded
structures
 internal line angles should be rounded.

 Restorations and appliance should be designed so

that the force of mastication are distributed as
uniformly as possible.
 In summary
 Three interrelated factors should be considered
in selection of dental materials;
 1-Type of the material(material choice),

 2-Geometry of restoration and underlying

structure,
 3-Component design.