Tensile properties of orthodontic C. C. Twelftree, M.D.S., B.Sc.Dent.(Hons.),* G. J. Cocks, Ph.D.,** and M. R. Sims, B.D.S., M.Sc.D., F.R.A.C.D.S.*
wire B.Sc., M.Sc.,
Adelaide, South Australia
T
he mechanical properties of an arch wire are an important consideration in the construction of an orthodontic appliance. For example, the Begg lightwire appliance* uses resilient round wires and light elastic forces to accomplish the required tooth movement. Incorporated into the arch form are certain characteristic bends which, as the wire is pinned to the teeth, become activated; that is, stresses are produced within the wire and these generate forces which act on the teeth. The magnitude and continued application of the resolved sum of these forces are vital for the efficient functioning of the appliance.2 For maximum control of anchorage and efficient tooth movement, it is necessary for the arch wire to remain active for many months. The appliance is continuously subjected to masticatory forces, so the wire must be sufficiently resilient to resist permanent deformation and maintain its activation. Therefore, the elastic properties of the wire are of vital importance for the efficient functioning of the appliance. This investigation is concerned with a preliminary examination of the tensile properties of a number of wires commonly used for construction of a Stage 1 arch wire in the Begg orthodontic appliance. Emphasis is directed toward the measurement of those properties which most closely relate to the clinical effectiveness of an arch wire. Terminology
A tensile test is normally used to determine the mechanical properties of orthodontic wire.5 The specimen is slowly loaded along its long axis, and the load is plotted as a function of specimen extension. The results are converted to stress and strain which are independent of the geometry of the sample. The stress-strain curve for orthodontic wire has the general shape shown in Fig. 1. In the regime where stress is proportional to strain, the material behaves elastically. If the load *Department **Materials Adelaide.
682
of Dental Engineering
Health, Group,
University Department
of Adelaide. of
Chemical
Engineering,
University
of
Volume
72
Number
6
Temile
Fig.
properties
5i ~olhstklly
AY 04%
Pzprowl~umit
BXllilbnbMStrmeth
1. Schematic
stress-strain
of orthodontic
FmolStreu
curve
for
orthodontic
20
wire.
40
00 TIME
Fig.
2. Tensile
Fig. 3. Premium;
test
result
Stress relaxation C, Special Plus;
with
zero
properties D, Special;
683
wire
19 (h)
suppression. of wire over E, Dentaurum;
a
3-day F, Yellow
period. A, Premium Elgiloy; 0, Unisil.
Plus;
8,
is removed at any point during elastic deformation, the strain would return to zero. The ratio of stress to strain in this linear portion of the curve (PZ/OZ) is the elastic modulus (Young’s modulus). The value of stress where direct proportionality between stress and strain ceases is known as the proportional limit (PZ) . As the specimen is strained beyond its elastic limit, the stress increases to a maximum value termed the ultimate tensile strength (BX) which, for orthodontic wire, may be equated with the breaking strength. Deformation of the wire beyond the proportional limit involves both elastic and plastic strain. On release of the load, the elastic strain is recovered but the sample will exhibit permanent deformation. Orthodontic wire in the as-received condition is heavily deformed, so the transition from elastic to plastic deformation is not accompanied by a marked change of slope in the stress-strain curve. In this situation, a proof stress is defined4 as being that stress required to produce a specified permanent strain.
684
Twelftree,
Am. J. Orthod. December 1977
Cocks, and Sims
TO determine the proof stress, a line parallel to Young’s modulus is constructed from a point representing a small plastic strain (for example, 0.1 per cent) and the magnitude of the stress at the point of intersection of this line with the stressstrain curve is termed the proof stress (AY) . In a specimen which has been deformed and held in a fixed position, the stress may diminish with time, even though the total strain remains constant. This phenomenon is referred to as stress relaxation. Materials
and
testing
procedure
The mechanical properties of orthodontic wire are determined not only by the inherent properties of the material but also by the thermomechanical treatment used in their manufacture. Wires with a nominal diameter of 0.406 mm. (0.016 inch) were used in this investigation. Seven wires were tested: Wilcock* Premium Plus, Premium, Special Plus, and Special grades; Unitekt Unisil; Dentauruml Remanit Super Special Spring Hard; and Rocky Mountain Yellow Elgi1oy.s In accordance with the manufacturers’ recommendations, specimens of Yellow Elgiloy were heattreated at 500° C. for 10 minutes before testing. Ten&e testing. Wires were tested at a constant nominal strain rate of 0.005 min.-l and a temperature of 20 -I 2O C. in an Instron Universal testing machine Model TT-D. Specimen strain was recorded by two techniques. The first series used cross-head movement as a measure of specimen extension. Zero suppression in 5 kg. steps was used to magnify the load (Fig. 2). Geometric constructions were used to determine the load at which the curve first deviated from a straight line. The stress corresponding to this load was the proportional limit. Proof stress determinations were made by constructing offsets at 0.02, 0.05, and 0.10 per cent strain. The ultimate tensile strength was calculated from the load at which fracture occurred. The second series of tests used an extensometer to accurately measure strain for the determination of the modulus of elasticity. Stress relaxation tests. Specimens were inserted into the Instron and the crosshead was lowered until a load of 20 kg. was registered. This load was selected to optimize the sensitivity and acctiracy of the stress relaxation measurements. A load-time record was plotted on a chart recorder. The change in stress was measured at intervals over a 5-day period, using duplicate specimens of each wire. An extensometer (sensitivity of 2 x 1O-5 strain) ensured that the total strain remained constant during testing. Temperature was controlled at 20 + 2O C., and the effect of temperature changes was taken into account in plotting of the stress relaxation curves. Corrections were also made for relaxation in the testing apparatus. Materiak
*A.
J. Wilcock
tUnitek SDentaurum, $Rocky
Scientific
Corporation, Pforzheim,
Mountain
and
Engineering
Monrovia, Dental
Equipment,
Melbourne,
Calif.
Germany. Products
Company,
Denver,
Colo.
Australia.
Tensile properties Table I. Tensile properties
of orthodontic
wire
685
of wires Ultimate
Wife
Modulus of elasticity
Proportional limit
Mean
Mean
S.D.*
Premium Plus 174,000 5,600 2,510 Premium 170,000 6,300 1,920 Special Plus 164,000 4,300 1,530 Special 150,000 5,000 1,480 Unisil 153,000 8,800 1,620 Dentaurum 160,000 5,800 1,460 Elgiloy yellow 169,000 8,500 1,380 All units are MNm-2 (meganewtons per “S.D. = Standard deviation.
0.02%
S.D.
proof
stress
Meun(
SD.
33 63 2,610 73 2,150 13 32 11 1,710 98 1,740 129. 82 95 1,810 43 1,710 43 219 1,460 220 square meter).
0.05% proofstress Mean
S.D.
2,700 42 2,350 48 1,940 50 1,920 130 2,030 99 1,900 34 1,560 217
0.10% proof
stress
Mean
S.D.
2,830 2,520 2,160 2,180 2,270 2,120 1,670
29 23 63 117 98 18 214
tensile strength Mew
3,070 2,870 2,790 2,630 2,850 2,680 2,000
S.D.
15 3 12 77 62 5 37
To obtain some indication of the effect of long-term loading on elastic properties, 160 cm. lengths of Wilcock Special Plus and Unisil wirea were loaded for 112 days under a dead load of 30 kg. at 22O C. After this time, the effect of the stress on the original form of the wire was observed. Rerults
Tensile tests. Table I shows the average values of Young’s modulus, proportional limit, proof stress, and ultimate tensile strength determined from a minimum of six tegts on each wire. Stress relaxation tests. Stress relaxation measurements were performed on all wires subjected to tensile tests. The mean values of stress determined from duplicate tests are shown in Fig. 3. These values were found to be reproducible to within + 8MNm-“. No stress relaxation was detected in Wilcock Premium Plus, Premium, and Speoial Plus over the 3-day period, but in the other wires some stress relaxation could be detected and it would be expected from Fig. 3 that this behavior would continue for longer periods. The results of the longer-term dead-load tests are recorded in Fig, 4. Fig. 4, A shows 160 cm. lengths of Special Plus and Unisil immediately after removal from the spool on which they are supplied, and Fig. 4, B shows the two lengths after testing. Special Plus maintained its original coil size, whereas Unisil showed a significant increase in curvature of the coil shape. Both wires maintained their coil shape after loading to 30 kg. for 1 minute, so that the changes in coil shape reported above are dependent on time. Stress relaxation in Unisil is one possible explanation for these observations, but further investigation of the elastic properties of plastically deformed orthodontic wires is necessary.
The wires tested in our studies exhibit a range of values of elastic modulus, proportional limit, proof stress, and ultimate tensile strength. A similar observation has been recently reported by Masson and Ware.5 In addition, the ratio of proportional limit to ultimate tensile strength was found to vary between ap-
686
Twelftree,
Am. J. Orthod. Deoember 1977
Cocks, and Sims
SPECIAL PLUS
UNISIL
f
\
\
ltlcm ‘0 8
0A Fig.
4. Appearance
of
wires
before
test
(A) and
after
loading
at
30
kg.
for
112
days
(6).
proximately 0.55 and 0.82, supporting the conclusion of Wilkinson6 and Masson that this ratio for orthodontic wires is not constant. The parameters and data recorded in Table I may be useful in the selection of the appropriate wire for a particular application. We believe that while the ultimate tensile strength is useful for the broad classification of orthodontic wires, it would be desirable to include the proportional limit or a proof stress value in standards71 8 to provide an additional basis for comparison. The stress relaxation tests indicated that some orthodontic wires exhibit detectable amounts of relaxation over 3 days when loaded initially to 20 kg. and that the amount of stress relaxation varies from one wire to another. The clinical significance of this phenomenon has not been established. However, since the efficiency of the Begg orthodontic appliance depends on the maintenance of applied forces, without removal of the arch wire for periods of up to 3 months,’ this is a process which must be investigated. The work is being extended to include oral temperatures, different initial loads, and longer periods of time. Conclusions
1. Wires with a range of tensile properties are available for construction of a Stage 1 arch wire in the Begg orthodontic appliance. 2. Over a period of 3 days, a detectable amount of stress relaxation occurs in some orthodontic wires loaded initially to 20 kg. Discussions Adelaide, are
with D. acknowledged
R. Miller, Professor with pleasure.
of
Materials
Engineering,
University
of
REFRRENCES
1. Begg, P. R., and Kesling, P. W. B. Saunders Company. 2. Munday, M.: An analysis of 3. Skinner, E. W., and Phillips, 1967, W. B. Saunders Company. 4. Masson, R. J.: An evaluation thesis, University of Sydney, 5. Masson, R. J., and Ware, A. 4: 53-61, 1975.
C. : Begg
orthodontic
archwire, R. W.:
Begg J. Orthod. 5: 57-70, 1909. The science of dental materials,
of 0.016 inch (0.406 1969. L.: Physical properties
theory
mm.)
and
technique,
orthodontic
of orthodontic
Philadelphia,
1971,
ed. 6, Philadelphia light
wires,
wires, Aust.
M.D.&. Orthod.
J.
Tensile properties
of orthodontic
wire
687
ti. Wilkinson, J. V.: Some metallurgical aspects of orthodontic stainless steel, AM. J. ORTHOD. 48: 194-206, 1962. 7. British Standard 2056.: Rust, acid and heat resisting steel wire for springs, London, 1953, British Standards Institution. 8. Australian Standard T32.: Resilient orthodontic wires, Sydney, 1965, The Standards Association of Australia. 9. Begg, P. R., and Kesling, P. C.: The differential force method of orthodontic treatment, AM. J. ORTHOD. 71: l-39, 1977.
It is impossible That must be be of service:
to give absolute left to the judgment (1) If there is any
directions what Tooth or Teeth ought of the operator; but the following one Tooth very much out of the row,
to be pulled out. general hints may and all the others
regular, that Tooth may be removed, and the two neighboring ones brought closer together. (2) If there are two or more Teeth of the same side very irregular, (as for instance, the second incisor and cuspidatus) and it appears to be of no consequence, with respect to regularity, which of them is removed, I should recommend the extraction of the farthest back of the two, viz. the cuspidatus; because, if there should be any space, not filled up, when the other is brought into the row, it will not be so readily seen. (3) If the abovementioned two Teeth are not in the circle, but still not far out of it, and yet there is not room for both; in such a case I would recommend the extraction of the first bicuspis, although it should be perfectly in the row, because the two others will then be easily brought into the circle; and, if there is any space left, it will be so far back as not to be at all observable. (John Hunter, Surgeon Extraordinary to the King and Fellow of the Royal Society: The Natural History of the Human Teeth, ed. 2, London, 1788.)
wire B.Sc., M.Sc.,
Adelaide, South Australia
T
he mechanical properties of an arch wire are an important consideration in the construction of an orthodontic appliance. For example, the Begg lightwire appliance* uses resilient round wires and light elastic forces to accomplish the required tooth movement. Incorporated into the arch form are certain characteristic bends which, as the wire is pinned to the teeth, become activated; that is, stresses are produced within the wire and these generate forces which act on the teeth. The magnitude and continued application of the resolved sum of these forces are vital for the efficient functioning of the appliance.2 For maximum control of anchorage and efficient tooth movement, it is necessary for the arch wire to remain active for many months. The appliance is continuously subjected to masticatory forces, so the wire must be sufficiently resilient to resist permanent deformation and maintain its activation. Therefore, the elastic properties of the wire are of vital importance for the efficient functioning of the appliance. This investigation is concerned with a preliminary examination of the tensile properties of a number of wires commonly used for construction of a Stage 1 arch wire in the Begg orthodontic appliance. Emphasis is directed toward the measurement of those properties which most closely relate to the clinical effectiveness of an arch wire. Terminology
A tensile test is normally used to determine the mechanical properties of orthodontic wire.5 The specimen is slowly loaded along its long axis, and the load is plotted as a function of specimen extension. The results are converted to stress and strain which are independent of the geometry of the sample. The stress-strain curve for orthodontic wire has the general shape shown in Fig. 1. In the regime where stress is proportional to strain, the material behaves elastically. If the load *Department **Materials Adelaide.
682
of Dental Engineering
Health, Group,
University Department
of Adelaide. of
Chemical
Engineering,
University
of
Volume
72
Number
6
Temile
Fig.
properties
5i ~olhstklly
AY 04%
Pzprowl~umit
BXllilbnbMStrmeth
1. Schematic
stress-strain
of orthodontic
FmolStreu
curve
for
orthodontic
20
wire.
40
00 TIME
Fig.
2. Tensile
Fig. 3. Premium;
test
result
Stress relaxation C, Special Plus;
with
zero
properties D, Special;
683
wire
19 (h)
suppression. of wire over E, Dentaurum;
a
3-day F, Yellow
period. A, Premium Elgiloy; 0, Unisil.
Plus;
8,
is removed at any point during elastic deformation, the strain would return to zero. The ratio of stress to strain in this linear portion of the curve (PZ/OZ) is the elastic modulus (Young’s modulus). The value of stress where direct proportionality between stress and strain ceases is known as the proportional limit (PZ) . As the specimen is strained beyond its elastic limit, the stress increases to a maximum value termed the ultimate tensile strength (BX) which, for orthodontic wire, may be equated with the breaking strength. Deformation of the wire beyond the proportional limit involves both elastic and plastic strain. On release of the load, the elastic strain is recovered but the sample will exhibit permanent deformation. Orthodontic wire in the as-received condition is heavily deformed, so the transition from elastic to plastic deformation is not accompanied by a marked change of slope in the stress-strain curve. In this situation, a proof stress is defined4 as being that stress required to produce a specified permanent strain.
684
Twelftree,
Am. J. Orthod. December 1977
Cocks, and Sims
TO determine the proof stress, a line parallel to Young’s modulus is constructed from a point representing a small plastic strain (for example, 0.1 per cent) and the magnitude of the stress at the point of intersection of this line with the stressstrain curve is termed the proof stress (AY) . In a specimen which has been deformed and held in a fixed position, the stress may diminish with time, even though the total strain remains constant. This phenomenon is referred to as stress relaxation. Materials
and
testing
procedure
The mechanical properties of orthodontic wire are determined not only by the inherent properties of the material but also by the thermomechanical treatment used in their manufacture. Wires with a nominal diameter of 0.406 mm. (0.016 inch) were used in this investigation. Seven wires were tested: Wilcock* Premium Plus, Premium, Special Plus, and Special grades; Unitekt Unisil; Dentauruml Remanit Super Special Spring Hard; and Rocky Mountain Yellow Elgi1oy.s In accordance with the manufacturers’ recommendations, specimens of Yellow Elgiloy were heattreated at 500° C. for 10 minutes before testing. Ten&e testing. Wires were tested at a constant nominal strain rate of 0.005 min.-l and a temperature of 20 -I 2O C. in an Instron Universal testing machine Model TT-D. Specimen strain was recorded by two techniques. The first series used cross-head movement as a measure of specimen extension. Zero suppression in 5 kg. steps was used to magnify the load (Fig. 2). Geometric constructions were used to determine the load at which the curve first deviated from a straight line. The stress corresponding to this load was the proportional limit. Proof stress determinations were made by constructing offsets at 0.02, 0.05, and 0.10 per cent strain. The ultimate tensile strength was calculated from the load at which fracture occurred. The second series of tests used an extensometer to accurately measure strain for the determination of the modulus of elasticity. Stress relaxation tests. Specimens were inserted into the Instron and the crosshead was lowered until a load of 20 kg. was registered. This load was selected to optimize the sensitivity and acctiracy of the stress relaxation measurements. A load-time record was plotted on a chart recorder. The change in stress was measured at intervals over a 5-day period, using duplicate specimens of each wire. An extensometer (sensitivity of 2 x 1O-5 strain) ensured that the total strain remained constant during testing. Temperature was controlled at 20 + 2O C., and the effect of temperature changes was taken into account in plotting of the stress relaxation curves. Corrections were also made for relaxation in the testing apparatus. Materiak
*A.
J. Wilcock
tUnitek SDentaurum, $Rocky
Scientific
Corporation, Pforzheim,
Mountain
and
Engineering
Monrovia, Dental
Equipment,
Melbourne,
Calif.
Germany. Products
Company,
Denver,
Colo.
Australia.
Tensile properties Table I. Tensile properties
of orthodontic
wire
685
of wires Ultimate
Wife
Modulus of elasticity
Proportional limit
Mean
Mean
S.D.*
Premium Plus 174,000 5,600 2,510 Premium 170,000 6,300 1,920 Special Plus 164,000 4,300 1,530 Special 150,000 5,000 1,480 Unisil 153,000 8,800 1,620 Dentaurum 160,000 5,800 1,460 Elgiloy yellow 169,000 8,500 1,380 All units are MNm-2 (meganewtons per “S.D. = Standard deviation.
0.02%
S.D.
proof
stress
Meun(
SD.
33 63 2,610 73 2,150 13 32 11 1,710 98 1,740 129. 82 95 1,810 43 1,710 43 219 1,460 220 square meter).
0.05% proofstress Mean
S.D.
2,700 42 2,350 48 1,940 50 1,920 130 2,030 99 1,900 34 1,560 217
0.10% proof
stress
Mean
S.D.
2,830 2,520 2,160 2,180 2,270 2,120 1,670
29 23 63 117 98 18 214
tensile strength Mew
3,070 2,870 2,790 2,630 2,850 2,680 2,000
S.D.
15 3 12 77 62 5 37
To obtain some indication of the effect of long-term loading on elastic properties, 160 cm. lengths of Wilcock Special Plus and Unisil wirea were loaded for 112 days under a dead load of 30 kg. at 22O C. After this time, the effect of the stress on the original form of the wire was observed. Rerults
Tensile tests. Table I shows the average values of Young’s modulus, proportional limit, proof stress, and ultimate tensile strength determined from a minimum of six tegts on each wire. Stress relaxation tests. Stress relaxation measurements were performed on all wires subjected to tensile tests. The mean values of stress determined from duplicate tests are shown in Fig. 3. These values were found to be reproducible to within + 8MNm-“. No stress relaxation was detected in Wilcock Premium Plus, Premium, and Speoial Plus over the 3-day period, but in the other wires some stress relaxation could be detected and it would be expected from Fig. 3 that this behavior would continue for longer periods. The results of the longer-term dead-load tests are recorded in Fig, 4. Fig. 4, A shows 160 cm. lengths of Special Plus and Unisil immediately after removal from the spool on which they are supplied, and Fig. 4, B shows the two lengths after testing. Special Plus maintained its original coil size, whereas Unisil showed a significant increase in curvature of the coil shape. Both wires maintained their coil shape after loading to 30 kg. for 1 minute, so that the changes in coil shape reported above are dependent on time. Stress relaxation in Unisil is one possible explanation for these observations, but further investigation of the elastic properties of plastically deformed orthodontic wires is necessary.
The wires tested in our studies exhibit a range of values of elastic modulus, proportional limit, proof stress, and ultimate tensile strength. A similar observation has been recently reported by Masson and Ware.5 In addition, the ratio of proportional limit to ultimate tensile strength was found to vary between ap-
686
Twelftree,
Am. J. Orthod. Deoember 1977
Cocks, and Sims
SPECIAL PLUS
UNISIL
f
\
\
ltlcm ‘0 8
0A Fig.
4. Appearance
of
wires
before
test
(A) and
after
loading
at
30
kg.
for
112
days
(6).
proximately 0.55 and 0.82, supporting the conclusion of Wilkinson6 and Masson that this ratio for orthodontic wires is not constant. The parameters and data recorded in Table I may be useful in the selection of the appropriate wire for a particular application. We believe that while the ultimate tensile strength is useful for the broad classification of orthodontic wires, it would be desirable to include the proportional limit or a proof stress value in standards71 8 to provide an additional basis for comparison. The stress relaxation tests indicated that some orthodontic wires exhibit detectable amounts of relaxation over 3 days when loaded initially to 20 kg. and that the amount of stress relaxation varies from one wire to another. The clinical significance of this phenomenon has not been established. However, since the efficiency of the Begg orthodontic appliance depends on the maintenance of applied forces, without removal of the arch wire for periods of up to 3 months,’ this is a process which must be investigated. The work is being extended to include oral temperatures, different initial loads, and longer periods of time. Conclusions
1. Wires with a range of tensile properties are available for construction of a Stage 1 arch wire in the Begg orthodontic appliance. 2. Over a period of 3 days, a detectable amount of stress relaxation occurs in some orthodontic wires loaded initially to 20 kg. Discussions Adelaide, are
with D. acknowledged
R. Miller, Professor with pleasure.
of
Materials
Engineering,
University
of
REFRRENCES
1. Begg, P. R., and Kesling, P. W. B. Saunders Company. 2. Munday, M.: An analysis of 3. Skinner, E. W., and Phillips, 1967, W. B. Saunders Company. 4. Masson, R. J.: An evaluation thesis, University of Sydney, 5. Masson, R. J., and Ware, A. 4: 53-61, 1975.
C. : Begg
orthodontic
archwire, R. W.:
Begg J. Orthod. 5: 57-70, 1909. The science of dental materials,
of 0.016 inch (0.406 1969. L.: Physical properties
theory
mm.)
and
technique,
orthodontic
of orthodontic
Philadelphia,
1971,
ed. 6, Philadelphia light
wires,
wires, Aust.
M.D.&. Orthod.
J.
Tensile properties
of orthodontic
wire
687
ti. Wilkinson, J. V.: Some metallurgical aspects of orthodontic stainless steel, AM. J. ORTHOD. 48: 194-206, 1962. 7. British Standard 2056.: Rust, acid and heat resisting steel wire for springs, London, 1953, British Standards Institution. 8. Australian Standard T32.: Resilient orthodontic wires, Sydney, 1965, The Standards Association of Australia. 9. Begg, P. R., and Kesling, P. C.: The differential force method of orthodontic treatment, AM. J. ORTHOD. 71: l-39, 1977.
It is impossible That must be be of service:
to give absolute left to the judgment (1) If there is any
directions what Tooth or Teeth ought of the operator; but the following one Tooth very much out of the row,
to be pulled out. general hints may and all the others
regular, that Tooth may be removed, and the two neighboring ones brought closer together. (2) If there are two or more Teeth of the same side very irregular, (as for instance, the second incisor and cuspidatus) and it appears to be of no consequence, with respect to regularity, which of them is removed, I should recommend the extraction of the farthest back of the two, viz. the cuspidatus; because, if there should be any space, not filled up, when the other is brought into the row, it will not be so readily seen. (3) If the abovementioned two Teeth are not in the circle, but still not far out of it, and yet there is not room for both; in such a case I would recommend the extraction of the first bicuspis, although it should be perfectly in the row, because the two others will then be easily brought into the circle; and, if there is any space left, it will be so far back as not to be at all observable. (John Hunter, Surgeon Extraordinary to the King and Fellow of the Royal Society: The Natural History of the Human Teeth, ed. 2, London, 1788.)
