AD—A 035 79j ARMY INST OF DENTAL RESEARCH WASHINGTON D C F/G 6/5STRESS RELAXATION OF DENTAL AMALGAM ALLOYS, (LI )FEB 77 E F HLJGET, L B DE SIMON, R S HERTERT

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STRESS RELAXAT ION OF DENTAL ANALGAM ALLOYS

Eugene F Huget

**Laszlo B. de Simon

Robert S. Hertert

Division of Dental Materials_ _ _ _ _

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U.S. Army Institute of Dental Research

Washington, D C 20012 ~~~~

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Cornmerical materials and equipment are identified in this report to

specify the experimental procedure. Such identification does not

imply official recommendation or endorsement or that the equipment

and materials are necessarily the best available for the purpose.

* Lieutenant Colonel, Dental Corps, U.S. Army, Chief, Divisionof Dental Materials, U.S. Army Institute of Dental Research.

** Physical Science Technician, Division of Dental Materials,U.S. Army Institute of Dental Research.

*** Captain, Medical Service Corps , U.S. Army , Division of Dental

~ iterials, U.S. Array Institute of Dental Research .

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Stress Relaxation of Dental Amalgam Ailo~sy~ Manuscript ~~~~~—

________________ - 6. PERFORMING ORG. REPORT NUMBER

V 7 AU?HL~~~J 8. CONTRACT OR CHA RT NUMè ~ER(.)

Eugene F.,4iuget , -~~~~~~~~~~~~~ Laszlo 8./de Simon )

~~~ Robert S~ erte~~~~~PT , SC -..

~~. P~~P~~QRMING .QRGAP4 IZ A T IO H NAME AND AD DRESS - ~~~. PROG RAM ELEMENT. PROJECT . TASKU.~~~. Army Institute of Dental Research k’~~~ A REA A WORK UNIT NUMBERS

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Amalgam alloy ; dental restorative materials; stress relaxation; reologicalproperties and viscoelasticity.

ABSTRAC T (Continue on rev,?., aid. ii necea.a~~ aid identify by block number)

Silver—amalgam is the most commonly used dental material for the restora—

~tion of carious and damaged posterior teeth. The popularity of these uniqueklloys is based primarily on their ease of manipulation and high strengthc\aracteristics. Unfortunately , however , the longevity of an amalgam restora—tikn seldom exceeds ten years. Premature clinical failure appears to resultf to breakdown of the restorative material at the “tooth—filling” Interface.

he Division of Dental Materials, U.S. Army Institute of Dental Research ,has conducted detailed stud{c’ of the stat Lc aiti dvn~u~(~ pro~ c~rt~ es of ds~ntal —

DD ~~~~~~~~ 1473 EOITION OF I N OV 65 IS OB SOLETE UNCLASSIFIED . ISECuRITY CLASSIFICATION OF THIS PAC E (~~iCn Veto Frdor.tS)

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S~~~~~ t I r Y ~~~L~~~.,,i~ICAhtQN OF THI PAGC(Wh.’n Dotu r,~tervd.1 ____________________________________________

aicalg~.a alloys. We have found that stress relaxation behavior of these material- - r?~r~re~ we 1 with observed & lIn-ica]. longevity. Alloys that exhibit excessi’e

stre~~ r’ iaxation over the range of temperatures encountered in the oral cavityfail relatively earl y In clinical service. Conversely, alloys that show littlest r~~~;s ri’lieE in l abo ra to ry t e s t ing yield cons i s t en t ly better and longer lastingser’:ic~~-~b i l i cy .

Di~ seainatiun of precise and accura te data that charac te r ize the relaxationbehavior of dental ana lg am alloys available for us~ w i t h i n the mili tary services

/ will eenaur a~ e r e formula t ion of inf ~ r 1or prod ucts by responsible commercialsau rces. Delivery of unsa t i s fac tory alloys to the Federal Health Services dillthereb y be precluded.

~~~Short term stress relaxation of four amalgam alloys wss studied . Tes t ma—terials included three L. D. Caulk products (20th Century Micro Cut , 20th Centur~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ and Johnson & Johnson ’s Disper salloy Specimens

( were 24 h~~ o1d 4 X 8 mm ~~~I i.~~Th~~~pec [mens were compresse and held atconstant strain on development of a stress (se) of 4 ,000 psi. Subsequent stressrelaxation (Dr ) wa~ recorded for 60 sec. Data/obtained at eight nominal.temperatures between 0 and 55C. wer e

• / Over the experimental temperature range, fractional stress losses at 60 sec

( O.~ \ . for specimens made from lathe cut alloys increased from 10 to 58%.Spherical Alloy and Dispersalloy exhibited fractional stress losses

ranging from 9 to 47% and 9 to 31%, respectively. Fractional stress losseswere linear when plotted against log time. Stress relaxation for all fouralloys increased exponentially with time in accordance with the expression..0.,.

~~~~~~~~~ Coefficients for the alloys were: 20th Century Micro Cut,a 2.52 x io—~ and ~~

= 3.08 X 10—2; 20th Century Fine Cut,ct = 1.22 X 1O 5 and8 = 3.30 X io—2 ; Spherical Alloy, a 1.65 X 10~~ and~ 3.14 X 10—2 ; Dispersallo> ,a 1.06 X iO~~ and B= 2.49 X 10—2 . Correlations between coefficients ranged fri i

.976 for Dispersalloy to .995 for 20th Centure Fine Cut.

~~~~~ II t would appear that particle morphology and alloy composition affect stres~relaxation behavior of dental amalgam. Stress decay patterns enhance signifi-cantly the mechanical characterization of amalgam alloys.

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The den tal ara lga~a restoratica exhibits responses to applied

stresses which contain both viscous and elastir components. The

.;c~—called viscoela.~tic behavior of hardened silver amalgam is time I.

dependent. Oglesb; et al1 have sho~ia tha t three patterns of strain

typify the behavior of amalgam alloy : (1) Instantaneous elastic

strain; (2) retarded elastic strum (transient creep) and (3)

- -viicous strain (steady state creep). In addition to creep, a

progressive incr ease in deformation under constant load, materials

that behave viscoelastically may also show other time—dependent

characteristics. One such characteristic is stress relaxation, a

gradual decrease in stress under a constant strain. Stress relaxa-

tion results from the rearrangement of structural units. Alteration

of structural configuration leads to the development of a new state

of equilibrium .

Data obtained from stress relaxation studies have been of

value in the selection of a number of industrial materials for

load bearing applications.2

Since amalgam restorations are

repeatedly subjected to transient pressures, it would appear that

similar studies would have relevance in the mechanical characteri— ¼

~atIon of a variety of available alloys. The present investigation Lwas conducted to study the stress—relaxation behavior of four

a:ralgam alloys over the range of temperatures encountered in the

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Y~~crials and Methods

Test materials included two conv Cnt LorLal l athe—cut alloys

(:~ and B+), a ~p i~~r Lcai prodl .Lct (C s ) and a hi gh—coppe r , dis—

p~~st~d phase alloy (Lr). Amalgam mixture.-; we r e made as directed

by manufacturers ’ instructions regarding mercury—alloy portions

and trituration t Ln.-~s. irituratlon was accomplished with the use

of a mechanical device.11

The amalgamated materials were packed

.i:tto 4 mm X 8 mm steel molds by the all—mechanical procedure pre—

scribed by American Dental Association Specification No. l.~ The

condensed specimens were stored for 24 hours at 37C and 100 per—

cent relative humidity. The ends of the 24 hr—old test pieces

were surfaced plane and at right angles to their axes. The trim~ed

cylinders were conditioned for two hours in a constant temperature

~;ater bath . Then, the specimens were tested within the water bath

over a 0 to 55C temperature range by the following procedure. Each

specimen was compressed axially on a constant strain—rate testing

machine~ at a crosshead speed of 0.02 inch/ruin. When a stress

( S ) of 28 ~~/m2 was reached , descent of the crosshead was stopped .

V Subsequent relaxation of the loaded specimens allowed a measurable

* 20th Century Micro Cut , L. D. Caulk Co., Milford , DE 19963.

+ 20th Century Fine Cut , L. D. Caulk Co., Milford , DE 19963.

~ Cau lk Spherical , L. D. Caulk Co., Milford , DE 19963._~ flispersalloy , Johnson and Johnson , East Winds or , NJ 08520 .

Wig—L—Bug, Crescent Dental Mfg. Co., Lyons , IL 60534.

Instron Universal L’rsting Mach ine , Instron Corp., Canton , MA 02021. r —

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~.tress loss (D) with no chan~~ in strain. Stress was measured

at 6—second fl~terva1 over a 60 -second period . Eight test

:-~ eratures ~VJjth nominal valu~s of 0, 10, 20, 30, 38, 42 , 48 ,

and 55C were used. Six specLmens of each of the four alloys

were tested at each temperature . To compensate for inherent

relaxation of the testing machine, a 4 mm X 8 mm stainless—steel

specimen was subjected to the compression procedure before and

after the testing of three amalgam cylinders. The resulting

system—relaxation pattern was used in adjusting the experimental

data. A schematic of the compression—relaxation sequence and rele-

vant terminology are given in Figure 1.

Results

Over the experimental temperature range, relaxation curves V

for the four alloys were similar in shape to the curve shown in

Figure 1. Also, fractional stress—loss (D~IS) curves were similar

to one another in shaoe but differed in rate as temoerature varied.

Typical fractional stress—loss (Dr/S) curves, with temperature

as a variable, are shown in Figure 2. Fractional stress—loss patterns

did not vary linearly with time (Fig 2, A). However, the curves

became linear when plotted against log time (Fig 2, 8). Relaxation

rates with respect to log time were delineated by the sloDes of L .

these lines. Relative relaxation rates for the amalgam alloys are V

pre-~ented in Figure 3. Stress—relaxation rates for all test materials V

v

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rie-i exponentially with temperature.

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Curves for total stres-; loss at 60 seconds (U Is /60) aret o

g iven in Figure 4. Over th~ 0 to 55C t~’aperatu re range , s t ress

losses at 60 seconds [or the lathe-cut alloys (A and B) increased

fr om abou t 10 percent to 58 pe rcent . The spherical m a t e r i a l (C) V

and the di5persed phase produ ct (D) showed stress losses (D~

/S /60)

that ranged from 9 per c~.nt to 47 percent and from 9 percent to 31.

parcen t, respectively. Stress relaxation pa tterns of all fo ur alloys

conformed to the expression D /s ne . Coeff ic ients whicht o

describe the exponential increase in stress—loss over the 60—second

V test per iod wer e obtained by regression analysis of values for in

(D~

/S /60) versus temperature . Validity of the analysis was reflected

by r values that ranged from 0.976 for alloy D to 0.995 for alloy B.

V Coeff ic ients for the a11oy~ were: Alloy A , cx = 2 .52 X 1O~~ and

= 3.08 X io_2

; alloy B , a = 1.22 X 10~~ and 5 = 3.30 X 10 2 ;

alloy C, a = 1.65 X l0~~ and 5 = 3.14 x io 2 ; alloy D, cx = 1.06 X

and ~ = 2.49 X 10-2 .

Discussion

V The viscoelastic behavior of many materials is dependent upon

both long-duration and short—durat ion mechanisms . Some rheological

data can be ob tained onl y through relatively long (minutes to hours)

V periods of testing . Static creep values (percent change in length)

based on one— and four—hour mc’.asurernents have been reported byV

Osborne, et al.4 Also, stress re]Vaxation behavior of dental amalgam

has been observed over a duration of 100 minutes by Paddon and Wilson.5

~V V

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V ~~~~~~~~~~~~~~~~~~~~~~~~~~~ - ___________ -— —- ~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~ ~~~~~~~~~

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— - - -~~ ~~V V_ _ V . V V VVV ~~~ V V V V

T~-~e t~.st

•~~~ ; ~~ j r ~~j ~ s~~ cc~~ , I to t v ~~r , wao limited to 60 seconds

b a ’~~~ : s:i~ atory~~ or~ r~; i r e br ie f and the short—term responses

1~of s tr ~~ :~~ b: c r inl ; rc;~ ordLi .a 1flateL~ia1~ merit consideretion. rT~;o coi~:.: ~enco ot t ’:~e onset of relaxation with :laximum

aD~)1ied s’ ro~s (Fig 1) reflects an instantaneous loss of strain

energy . This , in turn , would suggest that strain—recovery of

dental amalgam after deformation by external forces of short

duratioa and of relatively low magnitude would not be complete.

It would appear , therefore, that stress relaxation is a precursor

of steady state (nonrecoverable) creep.

Although creep has been correlated with the rate at which

marginal deterioration of an amalgam restoration occurs, its mode

of influence has not been elucidated . Stress relaxation which follows

deformation induced by transient masticatory forces or thermal

expansion and contraction may provide the means by which clinical

observable opening of the tooth—amalgam margin occurs.

Summary

Stress-relaxation behavior of four amalgam alloys was studied. V

Over the experimental temperature range of 0 to 55C, conventional

lathe—cut alloys exhibited the greatest increase in fractional stress .~~.

loss, whereas a material that contained a silver—copper dispersant

showed the least.

Stress—decay patterns significantly enhance the mechanical V

charac terization of dental amalgam alloys.

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Legends for Fi~~ur~ :;

~igure 1. Stress relaxation curve . Time t is onset of relaxation0

phooe , coinciden t w It h maxim um ot ress, S . Stress at any

time t is S~ . Shaded area rc pros~ nts relaxation of testing

machine. D is numerical difference between S and S Vt - o t

af ter subs trac tion of test ing machine relaxation .

Figure 2. Typical fractional stress—loss curves with temperature

as a parameter. (A) Dr/S versus time; and (8) D

r/S

versus ln (time + 1).

Figure 3. Relative relaxation rates of dental amalgam alloys.

Curves are best—fit approximations.

Figure 4. Stress l~Vsses of dental amalgam alloys at 60 seconds.

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Rot eren

1. Oglosb y, P. L . ; i i - ~~~~ n , G . ; Rodr lgu z , M. L . ; Davenpor t , R . M~;

a-:~ SV .:~~V~~ V ; ~~t Y , W . 7.: Vi o- o o ]V a s r i r ~ioh-tv1or of Dent::l Am algam , V

~~~~ ~~~ ~~~~~~ Stds 72C:203-213 , 1968.

2. K~ ras , C. C . : Shor t Term Stress Relaxation Studies at Low Initial

Strain Rates , Br PL a~~tt c s 34:489—490, 1961.

3. GuL d e to Dcnta1 Mat~ r ials and Devices , 6th ed , Chicago: American

Dental Association, 1972—1973 , pp 168—171.

4. Osborne , J. W.; Phillips , R. W.; Norman, R. D.; and Swartz, M. L.:

Static Creep of Certain Co r.mercial Amalgam Alloys, JADA 89:620—

622 , 1974.

5. Paddon, J. M.; and Wilson , A. D.: Stress Relaxation Studies on

Amalgam, J Dent Res 54:427, 1975.

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