Download By applying a tensile or compressive load beyond the elastic limit

SGD Orthodontic: Material
Hamzi, Zulkhairi, Azizul, Haziq, Aishah, Anis, Asmat, Masyitah
Lecture outline: material

Wire fracture

Mechanics of spring

Bauchinger effect

How does the material affect stability and the stiffness of the
component?
Wire fracture

Small loads → the stress is below the elastic
limit of the material, reversible elastic strain
occurs that disappears completely when
specimen is unloaded.

High stress


A ductile material begins to undergo irreversible
plastic or permanent deformation
A brittle material will fracture without any
significant permanent deformation
Stainless steel wire

Orthodontic wire are generally shaped by
bending and the wire should possess sufficient
ductility to resist fracture during this bending
procedure.

The amount of residual ductility remaining in a
wire depends in part on the ductility used up
in its manufacture.



Possible to carry out soldered repair to an adam’s clasp if
a fracture has occurred at tip of arrowhead.
This is an unusual place of fracture unless the wire been
overworked during construction.
The arrowhead should be cleaned, fluxed and flushed with
solder. Other attempts are not worth and replacement is
more sensible.


On ocassion, a clasp that has broken where the wire
crosses the embrasure may be cut away to leave one
intact arrowhead, which can be pinched closed with a pair
of pliers so no sharp end remains.
The arrowhead may adjusted to provide retention. This is
useful where retention provided by other wire is fairy
good and especially when appliance is not going to be
worn for much longer.



For broken springs, the remains of the spring are cut away
and a recess drilled into the fitting surfaces of acrylic
baseplate.
A replacement spring may be bent up and embedded into
this space using small amout cold cured acrylic.
In case palatal finger spring, the presence of guard wire
will help to hold new spring in place during this
procedure.
Mechanics of spring
Mechanics of spring
Force= (deflection)(radius4) @ F=dr4/l3
length3
•
Force:
–
–
•
Single rooted: 25-40g
Excess force: delay movement, overload anchorage &
discomfort.
Deflection:
–
–
–
Common spring activation: 3mm
Greater activation -> pt insert it incorrectly -> unwanted
movement
Smaller activation -> force applied decrease -> wanted
tooth movement (1-2mm/month)
BAUSCHINGER EFFECT

Named after German Engineer, Johann Bauschinger.

Applies to very small deformations.
May be stated as follows
“By applying a tensile or compressive load
beyond the elastic limit, the elastic limit for
compression or for tension, respectively, is
reduced considerably, and the more the load
exceeds the elastic limit, the greater the
reduction”
0
Compressive
stress
Compressive strain
Tensile stress
In this graph, lets treat tensile
stress and strain as POSITIVE
and compressive stress and
strain as NEGATIVE
Tensile strain
Tensile stress
If an annealed specimen is
loaded from 0 to B beyond its
elastic limit,
designated by point A,
B
A
and unloaded,
SeT
Its condition is represent by C.
Note that the elastic limit of the
material in tensile is given by SeT
0
C
Compressive
stress
Compressive strain
Tensile strain
Tensile stress
If the same specimen is next
loaded in compression, it
follows the path CDE,
where D is the elastic limit point
on the compression curve,
so that the elastic limit in
compression is now S’eC
According to Bauschinger effect,
S’eC < SeT
B
A
SeT
0
Compressive strain
C
D
Compressive
stress
E
S’eC
Tensile strain
Tensile stress
If an annealed specimen instead
of being loaded in tension and
then in compression, as stated
above, was directly loaded in
compression,
The elastic limit in compression of
the annealed material should be SeC ,
And would be equal to magnitude
to SeT .
B
A
SeT
0
Compressive strain
SeC
C
D
F
Compressive
stress
E
S’eC
Tensile strain
Hence, SeC = SeT ,
and S’eC < SeT
and S’eC < SeC

Similar reasoning will happen if the annealed
specimen was initially loaded in compression
past the elastic limit, unloaded and loaded next
in tension.

The resulting elastic limit in tension would be
smaller than the elastic limit of annealed
material in compression

Whereas Bauschinger effect was originally
stated in terms of the elastic limit, the
discussion of this effect in the literature has
involve the use of terms elastic limit and yield
strength interchangeably.

The reason for this anomaly lies in the elastic
limit and the yield point being located very
close to each other on the stress-strain curve.
The important thing is that one
should not lose sight of the
fact that Bauschinger effect
applies to VERY small strains
only.
How does the material affect stability
and the stiffness of the component?
How does the material affect stability and
the stiffness of the component?

The stability ratio of a spring in mechanical terms :
Stiffness in the direction of unwanted displacement
Stiffness in the intended direction of tooth movement

The spring must be guided so that its action is exerted
only in the appropriate direction by:
 Place
the spring in an undercut of the tooth so that it does
not slip occlusally during activation
 Use a guide to hold the spring in its position during
activation
 Bond an attachment to the tooth surface to engage the
spring
In Practice

High stability spring eg. Finger spring


Low stability spring eg. Buccal canine retractor


Straightforward to adjust/movement
Difficult to position precisely on the tooth to be
moved
The spring should be adjusted so that the
point of application will give the desired
direction of tooth movement.

Self supported spring


These springs are made up of thicker wire to avoid
distortion by the patient
Supported spring

These springs are made up of thinner wire , a
guidewire maybe provided. Alternatively, they maybe
supported by an additional sleeve or ‘boxed’ of
acrylic – to ensure adequate stability
References


1. Kerr, W.J.S. (1984) Appliance breakages. British journal
of Orthodontics, 11: 137-142
2. Munns, D. (1971) An analysis of broken removable
appliances. Proceedings BSSO, 45-48
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