Fatigue characterization of nine dental amalgams

Fatiguecharacterizationof nine dental amalgams L.D. Zaxdiacb aad

S.C.Bayne

School of Dentistrv. University of Mississippi Medical Mississippi 392 16, USA (Received 21 May 1984; revised 17 August 1984)

Dental amalgams prepared from nine commercially

Center, 2500

North State Street, Jackson,

available dental amalgam alloys, including high copper ternary,

high copper blend, and a conventional alloy, were evaluated for their compressive fatigue behaviour to at least 1 O6 cycles. While the fatigue behaviour of all high copper amalgams was similar, statistically

significant differences in

cycles to failure amongst these amgalgams were detected as a function of load, with Sybraloy demonstrating greatest Keywords:

overall msistance Dental

amalgams,

amalgam

strength,

amalgam

Dental amalgam is formed by a liquid phase sintering reaction between mercury and a silver-tin containing alloy’d. Dental amalgam may exhibit significant plastic deformation in compression at low strain rates via static or dynamic creep5, 6. Concern over the creep of amalgam resulted in the addition of a static creep test to ADA specification No. 1. To date, the static creep value of amalgam has been utilized by many individuals to indicate the clinical acceptability of a dental amalgam-. The longterm mechanical performance of a material largely depends on its response to cyclical loading. However, little information has been reported on the long-term cyclical behaviour of dental amalgam”‘. Currently, the rising cost of alternative materials has promoted the use of amalgam for applications beyond the scope for which it was previously intended. In light of dental amalgam’s expanding role as a restorative material, its fatigue resistance becomes critically important. The objective of this work was to examine the fatigue response of nine dental amalgams to extensive cyclic compressive loads. MATERIALS

AND

METHODS

The product names, manufacturers, batch numbers, and amalgam alloy types used for this study are given in Table 7. Four products were included as representative of high copper ternary alloys: four products were used to represent the blend type of high copper amalgams and New True Dentalloy was included as the only conventional lathe cut amalgam. Materials were proportioned and triturated in a Caulk Vari-Mix II* according to instructions recommended by each manufacturer. Cylindrical samples 4.0 mm in diam. lLD. 0

Caulk Co., Lakeview and Clarke Avenues, Milford, DE 19963,

1965

Bunerworth

8 Co (Publishers)

the

to fatigue failure.

USA

fatigue resistance

were prepared according to the procedure outlined in ADA specification No. 1 l3 and were stored dry in an incubator at 37” f 1 “C for 7 d. Prior to testing, each sample was placed in a holding assembly attached to a BuehlerEcomet polisher and wet ground with 600 grit silicon carbide polishing paper to produce a length of approximately 6.5 mm and end faces perpendicular to the cylinder axis. After grinding, the length of each cylinder was recorded to the nearest 0.01 mm. Up to 6 samples were prepared for evaluation at any one load level. In order to establish a baseline for fatigue tests, ultimate compressive strength tests were conducted. Samples were prepared according to ADA specification No. 1. Compressive strengths were determined in air at 7 d on 3 samples of each amalgam alloy type using a MTS 812+ servohydraulic mechanical testing machine operated in the stroke control mode at a piston rate of 0.25 mm/min. Fatigue testing was performed in air on the MTS system in the load control mode at 37’ f 0.2”C using the Probit method14. Maximum fatigue load levels were successively lowered, from approximately 80% of the ultimate compressive strength at intervals of 50 or 100 kg, to a load level at which failure did not occur at 1 O6 cycles. Prior to the start of the first cycle of loading, the samples were manually preloaded to one half the difference between the maximum load and a constant 5 kg minimum load. Load was maintained within a tolerance of Z!I 0.1% of the programmed value. Cycling at 10 Hz was initiated in compression from the preload to the prescribed maximum load as a sinusoidal wave function. The number of cycles was recorded on an electric counter; the length of amalgam samples was monitored continuously, using the MTS stroke transducer, ‘MTS Systems Corporation,

Box 24012,

Minneapolis,

MN 55424.

USA

Ltd. 0142-9612/85/010049-06$03.00 Biomaterials

1985,

Vol 6 January

49

Amalgam

fatigue:

L. D. Zardiackas

Tab/e I

Materials

and methods

Dental amalgam

and S. C. Bayne

for amalgam

alloy and manufacturer

p~pamtjon Batch No.

Trituration*

Alloys: Hg ratios

speed

Trituration

time

fmg:mg) High copper ternary alloys Sybraloy Kerr Mfg. Co., Romulus, MI Tytin S.S. White, Phila.. PA Cupralloy ESP Star Dental, Valley Forge, PA Aristaloy CR Baker Dental, Cateret. NJ

10s

112978-2278

625:521

M-3

077601

600~462

M-2

032280

BOOr600

M-2

12s

0208677-3

625595

M-2

10s

OAO06

600~600

M-2

10s

0113917

600:600

M-2

12s

021480

600:650

M-2

14s

1 to777

600~600

M-2

8s

077402

6DO:600

M-2

6s

5s

High copper blend alloys Dispemalioy Johnson Et Johnson, East Windsor. NJ Cupralloy Star Dental, Valley Forge, PA Ease L.D. Caulk, Milford. DE Phasealloy Phasealloy Inc., Cajon, CA Conventional

alloy

New True Dentalloy S.S. White, Phila., PA *All t~tu~tion

done on a Cauik Van-Mix

II, LD. Caulk

Milford,

DE, U‘SA

and recorded on a chart recorder. All amalgam cylinders were cycled to failure, or the test was terminated at not less than IO6 cycles. Based upon the mean strain value at the highest load value for each amalgam the approximate stroke rates at 10 Hz were calculated. Calculations showed that these stroke rates ranged from approximately 80 mm/min to over 800 mmlmin. In an attempt to estimate the intersection of the fatigue curve with the stress axis at one cycle, mean stress at fracture was determined using 3 samples prepared as previously described and tested at stroke rate of 250 mm/min. This stroke rate was chosen since it is roughly the same order of magnitude (80 mm/ mm-800 mm/min) as that ~cu~ing during the fatigue tests at the highest load values and represents a stroke rate which is three orders of magnitude greater than the 0.25 mm/min rate outlined in ADA specification No. 1. The results of the number of cycles to failure versus the load level were analysed statistically employing two methods. Initially, fatigue data was evaluated using Halperin UH statistics14 at each comparable load level. This type of statistical approach is typically used to treat fatigue data, but is subject to more Type 1 statistical error due to the many pairings. In addition, this approach cannot treat fairly censored data for amalgams that did not fail at low load levels, either because of an insufficient number of cycles or because the endurance limit had been reached. In an attempt to keep from biasing the results, fatigue data at each load level was more conservatively evaluated using several different statistical methods where applicable.

stress, mean cycles to failure, and associated standard deviations are tabulated. Large standard deviations were associated with mean values for cycles to failure regardless of the number of samples evaluated. This is typical of all fatigue experiments and is especially true where the internal consistency of samples cannot be precisely controlled or where surfaces cannot be highly polished. During the course of cycling, all sample underwent notable plastic deformation. cylinders Barrelling occurred in a relatively uniform manner and was most prominent at the central third of the cylinder where cracks could be identified without magnification. All of the SN data is graphically displayed in F&m 1. Due to the proximi~ of the curves, standard deviations have been omitted for clarity. Individual curves were generated by visually constructing a best fit line through the data points in order to generate the general shape of the curve. All curves slowly decreased in slope as though they might be approaching an endurance limit, but none was definitively demonstrated at 10” cycles. Table 2

Ultimate

Dental amalgam alloy

compressive

strengths

Average compressive strength at 0.25 mm/min (MPa f SD)

of dental amalgam Average

compressive

strength at 250 mm/mm (MPa & SD)

High copper ternary alloys 501 i62 488 It: 53 365 f 36 347fll

Sybraloy Tytin Cupralloy ESP Aristaloy CR

499 427 430 501

f f f f

47 52 99 33

High copper blend alloys

DATA AND RESULTS Ultimate compressive strength values for all amalgams evaluated at a strike rate of 0.25 mm/min and 250 mm/ min are reported in Table 2. Each value is the average of 3 test samples. Fatigue data for all nine dental amalgams is presented in Table 3. The load values, corresponding

50

Biomaten%is

1985,

Vol6

Januaw

Dispersalloy cupratloy Ease Phasealloy Conventional

387fll 356 z!z 28 352 + 37 356 + 27

406f20 381 f32 378 + 23 344st18

326 f

370 + 40

alloy

New True Dentalloy

11

Amalgam

Table 3

Results of compressive

fatigue

fatigue:

L.D. Zardiackas

and S.C. Bayne

testing at 10 Hz Cycles to failure (mean -t sb)

Alloy code

Load level (N)

Initial stress (MPa)

No. tested

Sybraloy

(S)

(T)

387 348 309 271 232 387 309 271 232 193

Cupralloy ESP

w

5 3 3 3 4 3 4 3 3 3 6 3 5 3 2 3 3 2

19 162 140617 191 300 933 280 >1000000 3 997 37 142 315 100 391 647 >1000000 6 067 16483 101 162 111 420 >1000000 23 267 136 530 504 800

(I!Z 18 835) (+ 16 835) (k114538) (k571 173)

Tytin

4903 4413 3922 3432 2942 4903 3922 3432 2942 1961 3922 3432 2942 2452 1961 3922 2942 2452

4413 3922 3432 2942 2452 3922 3432 2942 2452 1961 3922 3432 2942 2452 1961 3922 2942 1961 981

348 309 271 232 193 309 271 232 193 155 309 271 232 193 155 309 232 155 77

(k (zk (+ (+

7300) 4323) 3581) 98 886)

2 3 3 3 3 3 3 3 3 3 3

6 743 10433 59 233 182 083 >1000000 36 110 14 380 49 200 147 810 >1000000 10 930 38 320 59 230 475 357 >1000000 1 863 12093 214 953 >1000000

(k (+ (+ (+

37 963) 2645) 3084) 50 530)

(+ (+ (xk (k

2739) 25 259) 14 935) 65 886)

3922 2942 1961 1471

309

3 307 10307 152 575 >1000000

(+ 1744) (k 1440) (+ 40 238)

Dental amalgam

alloy

High copper ternary alloys

(4

Aristaloy CR

Hugh copper

(D)

Cupralloy

(C)

Ease

(El

Phasealloy

(P)

I+ (t (k (f.

1163) 1681) 93 934) 72 878)

(k 10 072) (k 7067) (k 342 438)

(I 809) (+ 4825) (526931)

alloy

New true dentalloy

(N)

232 155 116

Results of the U, analyses are reported in Table 4 and have been referenced to the original alloy type. Each dental amalgam is ranked in order of statistically significant differences from the most to the least fatigue resistance. Since this type of statistical analysis can only test paired fatigue performance values at common load

levels, the interpretation is limited. The distinctions between alloys are ranked as: (1) significantly different, (2) not significantly different at some load levels, or (3) not significantly different at all load levels. Each of these results is reported for significance at the 0.10 level which is common for fatigue testing14. A second statistical approach was utilized to extend the analysis to those loads where failure did not occur. The data was analysed utilizing ANOVA or Student’s r-test at individual load levels where failure occurred. For load levels at which failure did not occur, the nonparametric method called the Proportional Hazards General Linear Table 4 Statistical in Region II

-:,

il--

,r,%

lo* N

Figure 1

1943) 28 870) 157 095) 157 067)

blend alloys

Dispersalloy

Conventional

271 232 193 155 309 232 193

(t (* (+ (+

Dental

amalgam

S-N

ICICLES

curves.

TO FliiLLJRE,

-lI

,-t

‘” --!,..-

analysis of fatigue curves for dental amalgam

Statistrcal method (significance)

Load levels

D@ > 0.10) D@ > 0.10)

All Some

Dental amalgam data

alloy fahgue

(S) (T) (A) (D) (5) (C) (U) (P) (N)

S>T>A=D>E=C=U>P=N S>T>A=D>E>C=U>P>N

D: Halperin U, statistics to test for the probability of srgnifrcant differences as a function of the number of load levels included.

Eiomatariais

1985,

Vol 6 January

5 1

Amalgam

fatigue:

L. D. Zerdiackes

Table 5 Statistical analysis in regions II and Ill

and S.C. Bayne

of fatigue curves for dental amalgam

Dental amalgam data

Statistical method (significance)

Load levels

A (p = 0.2723) A@=0.0121) B (Q = 0.05)

500 kg 450 kg 400 kg

S=T

B ((I = 0.05) B (a = 0.05)

350 kg 300 kg

S>T> S>T> T>

alloy fatigue

(S) IT) (A) ID) (5) (C) VJ) (P) (N)

B (a = 0.05) c @
250 200 150 100

s>

D

S>T=A=

C>U= D=E D=E=C=” E=C=“=

kg kg kg kg

P=N P=N P=N

A=D> A=D=E=C=”

T=

E=C=U>

P=N N P

A: t-test with log 10 transformation of data: B: one way analysis of variance with log 10 transformation and Student-Newman-Keuls multiple range test; C: proportional Hazards General Linear Model.

Model Procedure15 was used. Those statistical results are summarized in Table 5. This segmented statistical approach is more conservative but is often necessan/ for fatigue data. A direct comparison of individual S-N curves cannot be conducted unless a specific mathematical expression is known from which to model the type of experimental data.

DISCUSSION There are three different types of elementary S-N curves14 which typify the fatigue behaviour of materials with apparent endurance limits (Figure 2). Often a specific class of materials will fall into one of the three general types. Once assessment of the shape of the curve has been made, the number of samples necessary for subsequent testing is reduced. Differences in these fatigue curve types are most easily noted by assessing trends at the beginning, middle, and end of the curves. For this reason and for clarity in discussion, these areas have been indicated in Figure 2 as Regions I, II, and III. By comparison of the curves in Figures 1 and 2, one can see that fatigue curves for dental amalgam alloys seem to be of Type 1 or 2. To delineate the specific curve type, however, ultimate compressive strength values must be known at the same strain rate as that forfatigue cycling.

While these values are not strictly available, the stroke rate while cycling at 10 Hz has been estimated in many cases to exceed 250 mm/min. This stroke rate is three orders of magnitude greaterthan the rate of 0.25 mm/min prescribed by ADA specification No. 1, and the fatigue data at the higher load levels has been determined at a stroke rate in this range. Results of compression testing at the 250 mm/min stroke rate indicate that the fatigue curves would be most probably exhibit Type 1 behaviour and remain nearly level at high loads. However, precaution must be taken in ascribing an exact form to the curve since any fatigue testing in load control results in different strain rates at all load levels of cycling. In addition, if cycling had been carried out at a slower rate, say 2 Hz, which is probably closer to that experienced intra-orally, there is little doubt that the fatigue curves would be shifted toward lower total cycles to failure since the strain rate would be lower. It is well known that the surface finish, morphology, phase distribution, internal porosity, etc., play a major role in fatigue behaviour. Since this type of dental amalgam sample is so difficult to polish over its entire surface area and also since the purpose of the study is to examine the behaviour of ‘real amalgam’, i.e., that representative of what might be found in vivo, no attempt was made to alter the ADA specification No. 1 type samples. While more consistent results could be obtained by polishing the sample or in some way reducing voids such as by high temperature and/or high pressure condensation during sample preparation, the purpose of the study would have been negated. It has therefore been assumed that, since all samples were prepared in the same manner in the same die, variables such as surface finish and void concentration are representative. Fatigue curves for all nine dental amalgam types were comparable in Regions II and III. The slopes of the curves in Region II appear to be decaying as though they might approach a practical endurance limit somewhat above lo6 cycles. Due to the constraints of long testing times at moderate cycling rates, no samples have been evaluated to this limit. However, some information about endurance limits was calculated indirectly using normalization and pooling techniques. Failure mechanisms and limits may often be more clearly recognized when absolute stress values are normalized to relative stress values16. In this case, the fatigue stress level was divided by the ultimate compressive stress determined at a stroke rate of 0.25 mm/min and expressed as a fractional fatigue stress or stress ratio. The normalization process may be expressed mathematically as: o, a, = *0

where a, = engineering stress level at the start of the fatigue test a0 = engineering stress level for the ultimate compressive strength I

I

IO’

IO’

I

IO6

I

IO6

N (CYCLES TC FAILURE) Figure 2

52

Three types of elementary

Biomaterials

1985,

Vol6

fetigue curves.

January

a, = normalized fatigue stress from 0 to 1.0 (or stress ratio) This procedure requires some precautions. In this case the ultimate compressive strengths were not measured at precisely the same strain rate as the stroke rate at which

Amalgam

fatigue:

L.D. Zardiackas

and SC.

Bayne

which can be detected should be viewed cautiously. In this instance, the ranking should not be used to predict clinical performance until the effects of hand condensation, strain rate, corrosion, and other variables affecting fatigue have been elucidated. Because it was difficult to ascertain the exact time of failure, it was decided that failure would be defined as catastrophic fracture even though cracks may have been present both internally and externally. Examination of fracture path was thoroughly evaluated for these samples using light and scanning electron microscopy. In addition, energy dispersive X-ray analysis was performed on sample surfaces and across crack interfaces. The results of this work will be published in a subsequent paper.

Figure 3 Amalgam strengths.

fatigue data normalized

to ultimate

compresswe

fatigue tests were performed. Also, engineering stress does not document variations in plastic deformation which may occur prior to fatigue failure. Although this procedure may not be ideal, it is reasonable for generating first approximations. This approach permitted utilization of the entire pool of fatigue data to generate a master fatigue curve. The normalized fatigue data is reported in Figure 3 as a plot of relative fatigue stress versus number of cycles to failure. Each amalgam type is indicated in the figure. If the master curve was extrapolated to the range of 1 O6 to 1 O7 cycles, a practical endurance limit of approximately aR = 50% might be observed. This is consistent with values of 50-55% observed for most nonferrous metal alloys. If there were a specific strain rate for clinical situations or if a mean could be established, it would be possible to determine the compressive strength value at that strain rate and predict the endurance limit for dental amalgams solely as a function of the compressive strength. Despite this shortcoming in knowledge, some preliminary estimates of fatigue life may be calculated. A reasonable maximum for intra-oral cycling under a load might be estimated as being of the order of 1400 cycles each day. This translates into approximately 5 X 1O6 cycles of loading over 10 yrs. If the load levels were below the practical fatigue limit of a,-, = 50%, then no failure due to mechanical effects alone might be expected. However, superimposed effects of electrochemical corrosion could significantly shift the fatigue curve toward lower levels. The effect of fatigue on electrochemical corrosion and of artificial saliva in fatigue has been examined by Mueller17, ‘a. Obviously, as restorations become more complex, such as in pinretained amalgams, increased marginal and/or bulk fracture due to combined corrosion and fatigue effects would be expected. Comparison of the fatigue characteristics of individual amalgam types depends entirely on the region of interest of the fatigue curve. At low loads, all amalgam types would be expected to survive to lo6 cycles. For complex dental amalgam restorations, stress concentration would be more common; and fatigue behaviour would be more critical. At higher load, the conclusions between the two statistical approaches for ranking amalgam products were slightly different in Region II. The UH statistics in Table 4 excluded Region III data, while the second analysis presented in Table 5 included it. The pattern of incomplete distinction of alloy ranking is a hazard in fatigue analysis. The distinctions

SUMMARY

AND CONCLUSIONS

Fatigue behaviour of amalgams made from nine commercially available dental amalgam alloys were determined at load levels of approximately 40-80% of their 7 d ultimate compressive strength values. Dental amalgams in general exhibited fatigue behaviourtypical of other metallic materials. Over the range of stress levels investigated, significant differences in fatigue performance could be detected; but fatigue behaviours of all nine amalgam types were similar. At stress levels of approximately 50% of the ultimate compressive strength, substantial plastic deformation occurred. In that same range, the fatigue curves may have approached an endurance limit. Sybraloy displayed the best fatigue resistance of all amalgam alloy types investigated.

ACKNOWLEDGEMENTS The authors wish to thank Dr E.F. Meydrech and Mr T.Y. Barnes for their help in statistical analysis of the data.

REFERENCES

2 3

4

8

9 10 11

Reynolds, CL.. Warner, F.E. and Wrlsdorf, H.G.F., Amalgamation in the AgsSn-fig system. I. In situ observations. J. Appl. Phys. 1975. 46. 568-572 Wing, G. and Tyge, G., Setting reactions of spherical-particle amalgams, J. Dent. Res. 1965. 44, 1325-1333 Schoenfeld. CM. and Greener, E.H., Microstructural observations of the initial stages of amalgamatton, J. Dent. Res. 1971, 50, 35D-355 Okabe. T.. Mitchell, R.J., Butts, M.D. and Fairhurst, C.W.. A study of high copper amalgams,. Ill. SEM observations of amalgamation of high copper powders, J. Dent. Res. 1978, 57. 957982 Mahler, D.9. and van Eysden, J.. Dynamrc creep of dental amalgam, J. Dent. Res. 1969, 48. 501-508 Espevik, S. and Sorensen, S.E., Creep of dental amalgam. Stand. J. Dent. Res. 1975. 83, 245-253 Osborne, J.W.. Phillips, R.W.. Norman, R.D. and Swartz, M.L., Static creep of certain commercial amalgam alloys. J. Am. Dent. Assoc. 1974, 89. 620-622 Malhotra. M.L. and Asgar. K., Physrcal properties of dental silver-tin amalgams with high and low copper contents, J. Am. Dent. Assoc. 1978, 96, 444-450 Wilkrnson. E.G. and Haack, D.C., A study of the fatrgue characteristrcs of silver amalgam, J. Dan?. Res. 1958, 37, 136 Sutow, E.J. and Jones, D.W.. Fatrgue behavior of dental amalgam, IADR Prog. Et Abst. 1979. 58. No. 24 Sutow, E.J., Jones, D.W. and Milne, E.L. Fatigue behavror of dental amalgam at a representative chewmg frequency, IADR Prog. & Abst. 1980. 69. No. 11 1

Biomaterials

1985,

Vol 6 January

53

Amalgam

12

13

14

54

fatigue:

L. 0. Zardiackas

and S.C. Bayne

Young, F.A. and Draughn, R.A., Compressive fatigue limits of high copper and conventional amalgams, IADR Prog. ti Abst. 1981, 60, No. 680 Revised American Dental Association specification No. 1 for

15

alloy for dental amalgam, J. Am. Dent. Assoc. 1977,95. 614617 Little, R.E., Manual on Statistical Planning Analysis for Fatigue Experiment, American Society for Testing and Materials, Philadelphia, PA, 1975

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Biomatarials

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22