Stress Fatigue: BasicPrincipies and

Prosthodontic Implications

H.W. Anselm Wiskotl, DMD. MS. MSD-Unñeríity of CenevaGeneva, Switzerland

lack I. Nicbolls, PhD"Univeisity of WashingtonSeattle, Washington

Urs C. Belser, DMD'"University of GeneraGeneva, Switzerland

Clinical evidence indicates that the majority of fractures that occur inprosthodontic structures do so after a period of many years. Such failuresgenerally are not related to an episode of acute overload hut result from fatiguefailure. This paper reviews the current knowledge oí fatigue idilure and testmethods. An overview oí puhlished studies is given, and the authors suggestguidelines for future prosthodontie studies of this nature. Int I Prostbodont1995:8:105-116.

F atigue is a mode of fracture whereby a structureeventually fails after being repeatedly subjected

to loads that are so small that one application appar-ently does nothing detrimental to the component.'

The term fatigue was first proposed by Panalet in1839, a time when the industrial revolution hadstarted and rapidly moving parts beeame increasinglycommon. In those times, the main line of thoughtexplained fatigue fractures by "crystallization" of thematerial which became brittle after continued useand thus more prone to fracture. Much credit shouldbe given to researchers such as Rankine (1843),McConnell (1849], Wohler (1858), and Fairbairn(1864), who, through systematic investigation andtesting, were able to reproduce fatigue failure bycyclic loading. They also developed the concept offatigue limit and the S-N curve (Table 1 ).'

Today fatigue failure Is explained by the devel-opment of microscopic cracks in areas of stressconcentration. With continued loadings, thesecracks fuse to an ever-growing fissure that insidi-ously weakens the restoration. Catastrophic failure

'Lecturer, Division of Fixed Proslhodontii:;, School ofDentis!ry.

"Professor, Depsrlmenl of Restorative Dentistry, School ofDentistry.

•"Professor and Chairmm, Division of Fixed Prosthodontics,School of Dentistry.

Reprint requests: Or H.W.A. Wiskatt, Division of FixedProsthodontici, School of Dentistry, University of Geneva, 19,rue Bartheiémy-Menn, ¡205 Geneva, Switzerland.

results from a final loading cycle that exceeds themechanical capacity of the remaining sound por-tion of the material.

When subjected to cyclic stresses of sufficientmagnitude, almost any manufactured componentis likely to fail by fatigue. Similar processes havealso been observed in biologic structures. Militaryrecruits and athletes are especially prone to "stressfractures," as they are often referred to in the ortho-pedic literature.' Similarly, "spontaneous fractures"have also been linked to fatigue phenomena."

Table 1 Abbreviations Used1, II, tila

ö „

tfuoT

EfK

NN,nP

RsssS.s„,s„„

s.TX

(as indices) Fracture modes 1, II or IIIApplied stress (monotonie) (tensile: +,

compressive. -)Yield strengthUltimate tensile strengthApplied stress (monotonie, shear)Modulus of elasticity (tensile)Frequency of cyclic loadingStress concentration factorCritical stress concentration in mode 1Number of cycles in a (atigue testNumber of cycles at failureNumber of specimens in tesl sampleProbability of non-failureStress ratio: S^^/S.,„Applied stress (cyclic, tensile)Sample standard deviationStress amplitudeMean stress - p re stressMaximum level of applied cyclic stressMinimum level of applied cyclic stressConventional endurance limit at W cyclesApplied stress (cyclic, stiear)Sample mean

B, Number 2, 1995 105 The Intematioral louitial of Pro5tliodonrics

: Princifte and [mplíf.iuoii

Neutral fiber

Tension

Fig 1 Elemenlary leatures of crack nucleation and progression. Under compression from above, the beam tends to dislocate onthe tensile (lower) surface oí the beam. The crack appearing on the tensile side is said to open in mode / (the most frequent).Modes II and III are shear and twist modes.

Fig 2 Close up of an intact beam and the crack of Fig 1.Note concentration of force "flow-lines" around the crack tipwith increasing sharpness

Indeed, microcracks do appear in heterogeneousanisotropic iiving tissue such as hone.^ It has beenhypothesized that the damage caused by these fis-sures may act as a slimulus for hone remodeling."

The purpose of this report is to present anoverview of present knowledge on fatigue failure,its physical or ig in, methods of analysis, andprosthodontic implications.

Physical Mechanism

Inasmuch as fatigue is a peculiar mode of mate-rial ruptijre, a brief overview of elementary fracture

mechanics is indicated. For explanatory purposes, asimplified beam model of fracture is presented in Fig1. Since Hooke (1676) it has been known that forsmall loads, such a bar deforms elastically in amode that is proportional to the stress applied.Bending the beam, as in Fig 1, causes two zones todevelop: in the superior layers, the material isplaced under compression, whereas tensile stressesdevelop in the inferior zones. Because of the generalshape of the beam, the transition between zones oftension and compression is smooth and gradual. If,however, notches or grooves are machined into thesurface under tension, they act as local stress raisersand, depending on their location and shape, canincrease the stresses that develop inside the materialby several orders of magnitude (Fig 2)- Similarly, if aflaw develops, its tip acts as a stress concentrator,thereby accelerating crack propagation until even-tual failure.

Thus, the resistance of the material to crack pro-gression (ie, its fracture toughness) is an importantparameter. This property is characterized by thestress intensity factor A', which can be looselydescribed as the mathematical equivalent of thestress fringes in polarized light tests." Under load,(he beam of Fig I develops internal stresses thatlocally may he high enough to initiate a crack andcause it (o progress. In other words, K has reachedits critical ievel and is therefore equated to thefracture toughness of the material. In general termsfracture toughness is given by

K,, = \<T^-f^ (Table 1),

where af is the overall applied stress at failureand a is the length of a surface crack. The ind-x cstands for critical (ie, inducing failure)

I oí Prosthodorlii 106

5tr«5 Fatigue: Principles and Implications

Table 2 Typical K^ of Some Dental Restorative Materials

Material Composition

Modulus ofRupture

[MPa| [MPa] [MPam'^lH Igh-a Hoy steels Fe+ 0.1Gr0.5Mn

18Cr8NiSuperalloys

TitaniumAluminumDental ceramicsZirconiaSiliconsConcrete

Poly(methylmethacrylate]

Epoxy

Composites

Ni + 10Co 10W9Cr5AI 2TiTI + 6AI 4VAl + 3Mg 0.5 Mn

ZrO, + 5wt%MgOSijAION,CaO + SiOj + AI20ä

[n- ]1- H COOCH J „

[

75-200300-500500-830

7

OH

170-1600

800

800-90040-300

460-1700

1300

900-1000120-430

50-170

> 100

50-8030^03-5

4-125

0.2

O - C,H,-C - C,M, - o - CH,-CH - CH,-

58% uniaxial C in epo>;y

80-90 1.6[20'C)

0.6-1

1050 32-45

Data from engineered stnjclures are given lor comparison. Note extremely lew material constants o( concrete.'

Opening crack

Fig 3 Irwin's plastic zone correction. The metal distends ahead ct the crack tip. Therefore, the effective crack length is a„ = a +ir„isthe limiting stress factor inside the material.

refers to the fracture mode (Fig 1). Y is a correctionparameter that is required because of the finitenessof the specimen whose boundaries also alter theforce "flow lines" of Fig 2. It is a dimensionlessgeometry factor on the order of 1. Table 2 lists K,,,(T„, <7„,„ or modulus of rupture for some restorativeand other materials."

Two further considerations are relative to thebehavior of the material at the crack tip. First, mostmetals exhibit some degree of ductility and deformplastically in the zone ahead of the crack if thestresses exceed the yield strength in this area. It fol-lows that: (1) some form of crack tip blunting

occurs, thereby decreasing stress concentration,and (2) as a result of distention of the material infront of the crack, it behaves as if the crack werelonger than its physical size. Integrating this phe-nomenon into fracture mechanics is known asIrwin's plastic zone correction" and is diagramati-cally described in Tig 3 (/eftj.

Second, unlike metals, brittle materials such asceramics have little or no capacity to deform andthus decrease the concentration of stresses at a cracktip. In these instances, the Griffith crack model'" thatencompasses the shape of the crack tip is applica-ble (Fig 3, right}, in this model the highest stress is

a. Number 2,199"; 107 Tlie Inreriiaiional lournal oí Prosrhodontii

Fíitigue: Principies and Implicali

where er is the overall applied stress, a, the cracklength, and p, the radius of the crack tip, IT„,.„ is thestress inlensity that develops at the crack tip.Laboratory production and handling of ceramicsmake Griffith flaws inevitable, and since the cracktip radius can be as small as an interatomic spac-ing, stress intensification can be extreme.

As stated, fatigue failure is initiated by micro-scopic cracks that develop in areas of stress con-centration at or near the surface. The most com-mon iocal stress raisers are grain boundaries,inclusions, local intrusions and extrusions, andsudden changes in the geometric configuration ofthe surface. This initial step is termed nucleationand represents a mandatory stage of fatigue failure.It has been shown that periodically halting afatigue test and polishing a layer off the materialunder investigation could extend tbe fatigue life ofthe specimen indefinitely,"

When a fissure has reached its critical size, it willdefinitely progress at each loading cycle. This pro-cess is referred to as propagation and amounts toabout 90% of fatigue life. It is commonly dividedinto three stages. During stage 1, the fissures propa-gate in crystallographic shear mode, intragranularlyalong the slip-bands of the crystal. At this stage, therate of crack progression is of the order of a fewnanometers/cycle. In stage II propagation, the direc-tion of flaw progression has been altered and is nor-mal to the tensile stress (plane strain conditions).When the internal stresses induced by the crack tipare significantly influenced by the outer dirnensionsof the specimen, the flaw propagates under planestress conditions that are at 45 degrees to the stressaxis,'^ Catastrophic failure occurs in stage III pro-gression by intergranular cleavage,'*

Two other specific material behaviors have beenrelated to fatigue. In alloys, small and very thinsheets of metal, called slip-bands, are sometimesextruded at the surface of a fatigued crystal,"Conversely, the metal may also locally intrude intothe body, in effect leaving tiny cracks at the sur-face. If this occurs, the surface profile becomesragged and the notches in this profile can act asnuclei of fatigue cracks.

In ceramics and glasses, cracks may becomeunstable under static stress alone, in absence ofcyclic loading. This phenomenon is termed staticfatigue and is related to the presence of moisture inthe environment,'^ By chemically reacting with thesilicate network, an H-O unit generates two Si-OHterminals. Since the hydroxyl units do not bond toeach other, they leave a break in the glass or

ceramic structure. In dentistry, this mode of fs'''^'^^bas been addressed by Southan and Jorgensenand Morena et a l , " A definite weakening of thematerial strengths was shown when the specimenswere exposed to water.'" '"

Testing

Devices applicable to fatigue testing are all capa-ble of repeatedly placing a test sample under stress(5), However, they may differ considerably in thestress parameters that are applied to the structure.

These parameters are: the cycling frequency, theprestress (S4, the stress amplitude (5,,), the stressratio (R. = 5™/5„J and the algebraic value of thestress (compression, tension, alternating) (Fig 4),Complex load spectra can also be generated. Somemachines attempt to reproduce a clinical environ-ment by adding moisture and a controlled temper-ature to the test conditions. As to the number ofload cycles that should be applied to dental struc-tures, tbe following computation can be made.Assuming 3 periods of 15 minutes of chewing perday, at a chewing rate of 60 cycles per minute (1Hz), the average individual chews 2,700 times perday. This amounts to roughly 10' times per year. Ifthe half-life of a fixed partial denture is given as 20years,"' this prosthesis will have undergone 2 x 10"stress cycles. Conversely, it can be argued that notevery chewing cycle is "active" (ie, applying amaximum stress cycle to the structure). Therefore,the total of 2 X 10' chewing cycles previously cal-culated should be decreased by a factor rangingbetween 5 and 20 if a realistic value is to beobtained. For dental applications, fatigue testsshould be performed for a minimum of 10^ cycles.Also, as shown in Fig 5," functional loading ofteeth implies a multidirectional force pattern thatcomprises a compressive as well as a buccolingualcomponent. Loading a prosthodontic test structureuniaxially wi l l thus only partly reproduce, themechanical conditions of the oral environment.

Many contemporary devices are based on a ser-vohydraulic closed loop circuit under pressure thatdrives an actuator. The specimen can thus be sub-jected to bending or compression-type stresses.Further stress application can be either unidirec-tional (bend-release, compress-release) or reversed(push-pull, reversed bending). In more complexdesigns, the specimen is simultaneously subjectedto torsional forces. Since these machines are com-puter-driven, virtually any load spectrum may begenerated on the actuators. By combining severalactuators on a single specimen, extremely complexstrains can be induced.

lofPrusthiüclor)(it> 108 Volume 8,

Wiskott et al S(re;í F.iligue: Pnritinles and Implications

Applied stress [S]

-

s/

sii t

Onestress

-^ cycle ~

j A

Vy Vy s

Time

Applied stress [S\

Tension

0-

Compression

%"^\J ^T.nB,le

Time

Fig 4 Elementary stress parameters in fatigue testing. For alternating stresses, the stress ratio (Rs) is negative.

Fig 5 Functional forces onteeth (Adapted from Graf andGeering '),

5Kp

12 Kp

2Kp

A comparatively simple testing device for thetesting of material constants of wagon axles wasintroduced by Wohler.'- In its most elementarydesign, one end of a (roughly cylindrical) test sam-ple is clamped into a grip and rotated around itsmain axis. A force is applied to the protruding endand thus a reversed-bending, sinusoidal stress isinduced in the specimen. The characteristics of theapplied stress are S™. = -5™.. and 5„ = 0, This typeof apparatus has been extensively used in indus-trial tests where it is also known as an "R,R, Mooremachine."'

Fatigue tests can be based either on Nf or onthe monitoring of the fatigue process. In the for-mer situation, a specimen is loaded at a givenstress and the number of cycles to failure isrecorded. In alternate tests, changes in materialparameters {E, (T,J are evaluated as a function of Sand Nf. In this category a most significant testconsists of monitoring the progress of a fissure.Such a test requires a specific specimen design(Fig 6)," Cyclic loading of the sample is discontin-ued at periodic intervals, and the length of thecrack is recorded.

8, NLmbei2, 1995 109 The International lournal oi Prostliodontics

and Iniplicatii

Descriptive Techniques

Micrographs

inasmuch as fatigue processes are determined bya progressing fissure, in materials that present somedegree of ductility, the crack front may leave agroove in the walls of the crack at each load cycle.This develops a typical pattern of ripples that arevisible using electron microscopy and are referredto as fatigue striations. Such striations are an abso-

Growing crack

S

pi

s

Fig 6 CT (Compact-Tension) specimen.== It contains achevron formed notch and is loaded through two pins in a ten-sile machine. Cyclic loading is applied to introduce a latiguecrack.

Fig 7 Fatigue striations on a solder joint fatigued experi-mentally'' Note differences in ripple depth wtiich depends onthe orientation relative to the direction of the applied stress.

lute indication of fatigue failure and must not beconfused with beach marks on brittle structures.Figure 7 shows striations obtained during a fatiguetesl on solder joints," The depth of the striations isrelated to stress intensity, the greatest stress devel-ops when the crack progresses perpendicularly tothe main load. It has been shown that striations arerelated to stage II crack growth. Often closeinspection of the micrographs indicates the site ofcrack nucleation.

S-N Diagrams

Known interrelationships indicate that heavyloading will cause failure after a few cycles, whilethe material might sustain up to 10', 10", or an infi-nite number of cycles if the load is decreased.Such a behavior is traditionally depicted in a plotreferred to as an 5-/V diagram, endurance curve, orWohler diagram. These are drawn by plotting theapplied stress (the independent variahle) on theOrdinate and the log of the number of cycles untilfailure on the abscissa. As shown in Fig 8, twotypes of response to fatigue loading are observed.Most materials follow curve A in that a lowering ofthe stress amplitude leads to a longer hut still finitefatigue life. Curve B is typical for steels; in thisinstance, when the applied load is kept below acertain level, the material will not fail on any real-istic timescale and for all practical purposes can becycled indefinitely.

For normative purposes, S-iV diagrams are subdi-vided into three regimes:

1. Low cycle fatigue spans the range 1 to lO'cycles. On the lower end of this regime, theapplied stresses are often superior to the elasticlimit of the material, thereby causing plasticdeformation of the specimen. Because of thelow number of cycles sustained, tests restrictedto this range have only limited applicability torestorative dentistry.

2. Limited endurance fatigue in whieh the appliedstress definitely lies below the elastic limit ofthe material and spans fatigue lives between10' and 10' cycles. Tests conducted up to thisrange do approximate the lifespan of clinicalrestorations and can be regarded as conclusive.

3. tjniimited endurance encompasses tests cycledabove 10' and essentially applies to industrialstructure.^.

The largest stress amplitude that a material cansustain for an infinite number of cycles is termedfatigue limit (Fig 8). The existence of such a

The imernarional loumal of Proitliodonlîcs no

Wiskod el al Strew Fítigue: Principles anil ImplicaUi

FI9 8 General shape of S-Ndiagram. Cun/e A: Material witha fatigue limit. Curve B;Material without fatigue limit.

Stress amplitude {5^)

Endurance limitFoil g us limit

Number of cycles to failure

Applied stress

[MPa]

400

375

350

325

300

275Endurance limit

250

225

1

xcAUE

Anr

(Juts

IOC

Spe

Kt =

- " ^ • a-

;iOSteetenitization

ealling 500

- 305 N/mn-

= 400 N/mn

ational fatic

Hz, air

cimen FR Í

= 1.03

' ' •

"a 1 " "

0^ 10* 10^ 10^ 10^ 1

Number of cycles to failure

9>»

5

B

1900"C 30 min

C 2 hours

ue Rs = -1

SO 45.6 mm=

5%

»»»»»»»» 27»»»»»»»» 43

S„=255N/mm=S=12N/mm^

Fig 9 Example of S-A/diagram generated forXCIO steel (the X designates a special steel while CIO indicates 0.1% carbon), x:failures; >: run-outs (Adapted from Lieurade^.

fatigue limit has been demonstrated for steels, forwhich it can be extrapolated after 10' loadcycles.^' In most applications, however, the struc-ture's life is limited and therefore is characterizedby a (conventional) endurance limit. This value isdefined as the largest stress amplitude for which50% of the specimens wi l l sustain a predeter-mined number of load cycles. This numberdepends on the material and its functional require-

ments. Values between 10" and 10° cycles are typ-ical. A conventional endurance limit is thus a cen-sored value. It is symbolized by S™ or T.-. depend-ing on the type of stress applied (tension or shear).

An S-N diagram as generated for an industrialalloy is presented in Fig 9.'' Several aspects shouldbe considered. As shown, the lower the appliedstress, the greater the number of fatigue cycles sus-tained by the specimens. In the illustrated situa-

B, Number 2, 1995 111 The liilernational lournal of Prostliodontjcs

f aligue: Principles and I mpl italic

logl,

da/dN)

I

ÍAK

/n

jf

III

log AK

Fig 10 Crack growth [oa/úN] as a tunction of AK (ie, S„„ vsS, .). On a log scale, stage II crack growth is linear and fol-lows the Paris relation: da/dN = c{AK}".

tion, the test was conducted up to 10" cycles, atwhich time it was discontinued for those samplesthat did not fail (run-outs). The solid line describesthe central tendency at which 50% of the sampleshave tailed.

Fracture Mechanics

S-N diagrams are based on a fail or not-failapproach and therefore take little or no account ofthe parameters that determine crack propagation.They are highly specimen specific and, since theylack a mechanistic base, cannot be extended tosimilar designs or related materials research.

The K parameter can be applied to crack pro-gression under fatigue by relating the crack growthrate per cycle (da/clN) to K. Since A" is permanentlychanging with the applied load, it is defined as K„,„- K„„„ and commonly written as AA". The generalshape of such plots has been established by Parisand coworkers" and is shown in Fig 10. Threezones are apparent on this diagram. In the centerportion the data fall on a straight line and can becharacterized as

dadN -

in which c and n are material constants. Values on usually vary between 2 and 4. Figure 10 alsoshows that a threshold {K.,,y exists below which nomeasurable crack growth will occur, hi zone III,AAT has reached a magnitude great enough tocause very rapid failure.

Analysis

As shown in Fig 9, for any applied stress, theresultant fatigue lives are spread over a range ofabout one order of magnitude. The lower theapplied stress, the larger the fatigue life range- Thisamount of dispersion is considered normal infatigue testing. Indeed, contrary to earlier beliefs, itis now accepted that inasmuch as fatigue resis-tance is determined by randomly distributed inter-nal flaws, the scatter field cannot be significantlyreduced by enhancing specimen preparation pro-cedures. It follows that S-iV diagrams should beexamined in terms of chances of survival. Such anapproach is shown in Fig 11, in which a family ofcurves has been developed. They show the proba-bility of failure of a component for given stresses.In many instances, however, it may be useless togo through the tedious procedure of generating afull S-N diagram. For a specific application, theonly required information may be non-failure for aset number of cycles {N]. In this instance, a logicalstep is to determine an endurance limit (St,) and itsscatter around the 50% mean. Several techniqueshave been recommended for this type of analysis,which is based on quantal (fail or non-fail) data.''Procedures exist which allow a crude estimate of5.N using few specimens (3 < n < 10).'' For a morereliable estimate of S• and s, however, larger sam-ples are required. To this effect, the staircase tech-nique'"" is a straightforward procedure in which aseries of samples are tested in sequence,'-"

Recently advanced models of fatigue data analy-sis have been published by Conway and Sjvdahl"and by Drummond.'"'

Fatigue of Dental Structures

In a classical paper on the long-term survival ofrestorations, Schwartz et aP'' subdivided the failureof fixed partial restorations into biologic (ie, plaquerelated) and mechanical failures, of which break-age was a significant proportion. Analysis of thesedata revealed that mechanical failures occurredafter 5 to 10 years. Similar results were reported byWalton et al. ' ' Other authors reported failures interms of a mean annual rate which, depending onthe type of restoration, varied between 2.5 snç\ 1 '^

The Internal i on a I lournal ol Proíthoclontí< 112 Volume 3, Nu

WiskoH et al Slreís Fíligue: Prinriplei jnd Implications

Fig 11 Family of curvesshowing levels of probaDility offailure.

1

¿5

Dlltu

de (

Stress

am

AN\ /I\ " ' *\ / ' 10%

Number of cycles tc failure

years." It appears tbat short-term failure and acuteoverload are fairly rare and generally related tomaterial and design flaws or trauma. The majorityof mechanical failures are thus attributable to aprocess that finds its catastrophic end only aftermany years of service. Furthermore, in the author'sexperience, patients frequently indicate that break-age was not related to the chewing of hard orfibrous food. Also, Fig 12 shows the micrograph ofa clinically failed solder joint on which fatigue stri-ations are evident, A very similar view was pub-lished by Wictorin and Fredriksson,'' All thoseobservations are consistent with the concept of aslowly growing fissure under fatigue stress, struc-tural weakening of the component, and finalbreakage, it tbus stands to reason that emphasisshould also be placed on a characterization ofdental materials and structures by dynamic tests, Anon exhaustive list of publications on fatigue ofdental materials and structures is presented inTables 3 to 5,

When considering the studies cited above, itreadily appears that vastly divergent methodologieshave been applied during testing procedures andanalysis of results. In only a few reports have tech-niques been applied that are acceptable in otherareas of fatigue testing. If data from various back-grounds are to be compared, some guidelinesshould be provided that would allow normaliza-tion of the tests applied. Progress in materials andprosthodontic research thus requires a standardiza-tion of the procedural aspects of fatigue testing.

In engineering terms, a structure is consideredsafe if it can withstand three times the maximum"thinkable" stress under function. Such a predictiveapproach, however, is only possible if pertinent

Fig 12 Clinically failed solder joint. Note presence of fatiguest nations.

material constants are available to the designer.Unfortunately, these data are rare in dentistry andclinicians generally compensate for missing infor-mation by applying design criteria that stem fromlearning as well as their own "clinical experience."

Guidelines for Testing

Fatigue testing is a valuable procedure to evalu-ate dental materials, but great diversity without dis-cipline exists. For prosthodontics, to generate datawith the greatest "predictive potential", the follow-ing guidelines would prove useful:

1, Fatigue tests should be based on fracturemechanics and the K parameter. However,such an approach requires special training

ïi=B, Number 2, i 995 113 The Intemaiiorial Jojrnal of Prosthodontics

-, Fatigue' Principles and lm|

Table 3 Fatigue of Structures

(Hz)Type ofstress Analysis Findings

Outhwaite et al'°

Martinet ef a l "

Saunders"

Fissore et a l "

Kovarik et al"'

Stewart et al"'

Gjfidler et a l "

Retentiontechniques

Partial denture

Mar/land FPDs

Compositerestorations

Core materials

i m pi ant-retained

Crown retention

1

1,5

3,6

1,3

1

4

2

5-

5-

1,2.

>

9«

10=

lO"

10'

1œ

10«

10'

Reversedbending

Bend-re lease

Push-re lease

Bend-release

Bend-re lease

Bend-release

Push-release

GM

GM

SC

Gtvi

GM

%S

%S

Slot retained > pin retained

Ticonium > Vitallium = Wironiurrclasps

No difference between electronicallyetched, lost salt, Panavia EX

An increase m loroe applicationdecreases the fatigue life of Ihedentine bond

Amalgam > composite > glass-ionomet

L-stiaped beams increase instrength prostheses when thevertical wall increases in length

A smaller taper increases retention

GM: group means; %S: percentage survival: percentage of Sundetermined by cycling n samples at a preset load undl failure.

a\ curves for a given load are plotted; SC: staiicase analysis: SN: S-W cun/e. x-.

Table 4 Fatigue of Metals

Auttiors

Peyton"Wilkinson, tHaack"Earnstiaw"Bates'*Hawbolt, iVlcEntee"Sutow et aP'

Sub|eot

Gold alloysAmalgam

Co-Cr alloyCo-Cr alloyhJi-Cr alleyAmalgam

/(Hz)

12,53016.713.360

1,330

Type ofstress Analysts Findings

10'8«iœ5*10=

Zardiakeas. Baynes" Amalgam 10Wiskott et aP Solder joints 1Butson et a r Solder joints 30

Bend-releasePush-pull

Reversed bendingBend-re lea sePush-release

Push-pull

Push-releaseBend-re lease

Rotational

SNSNSNSNSNSN

SNSNGM

S,„,:344-413MPaS,,7; 96,5 MPa

S,„>: + 275 MPaS5, ,CÍ ;±551 MPa

Description of fracturepatterns. S,, increases with

increasing/Ranking of 9 brands

S.is: ± 300 MPa

GM: group means; %S; percerlage su/vwal; percentage of sun/ivaldetennined by cycling n samples af a preset load until failure.

for a given load are plotted; SC: Staircase analysis; SN; S-N cunie, x-.

which may not be available to the dentalresearcher. Therefore, procedures based onquantal (fail or not-fail) data should be usedsince they are more easy to conduct andinterpret,

2. Tests should bo brought at least to 10^ cyclesif a clinically relevant service life is to beapproximated,

3. For prosthodontic structures, a negative stressratio (back and forth) is advisable,

4. Whole 5-N diagrams are not required since thelow-cycle regime (<10' cycles) is applicableonly to temporary materials and structures,

5. The determination of a conventional endurance(S„) limit in the 10" cycles range is recommended,

6. Presently, staircase analysis does appear to bethe most straightforward procedure to deter-mine S ., if staircase analysis is applied, samplesizes >20 in the relevant stress range should beused. Initial approximation steps are worthlessin the determination of Sn.

Great cycle numbers and larger sample sizesrequire greater cycling frequencies. However,accelerated test methods may require a mathe-matical model or algorithm for converting suchdata to values applicable to clinically relevantfrequencies,""In the author's opinion, a testing procedurebased on rotational fatigue is a fast and cost-effective means to generate relevant data."

References

Nutt MC, Mefallurgy and Plasfics for Engineers, Oxford:Pergamon Press, 1976:360,

Anderson RC. Inspection of Metals, Vol II: Destructive testing.Metals Park, Ohio: ASM Infernatioral, 19B0;] 71-192.Meyer SA, Saltzmann CL, Altjright |P, Stress frariures ofthe fooL and leg. Clin Sparls Med 1 9 9 3 ; 1 2 : 3 9 5 M I iMichel MC, Guü XD, Gibson L|, McMahon TA, Hayesw e , Compressive fatifije behavior of bovine trabecularbone, J Biomech 1 993,26:453-463,

The imernölional lournai of Prosihodonti< 114 Volume 8, N'

iples and Implicilii

Table 5 Fatigue of Resins

Subject/

(Hz)Type olstress Analysis Findings

Barber" Denture resins 1.7

Johnson, Matthews" Denture resins 0.5 1.6*10'Johnson, Matthews*' Denture resins 0.5 1.6*10^

Peyton et Auto vs heat- 12.5polymerizing 37.5

resins

Bend-release

Bend-reí easeBend-re lease

Bend-release

GMGtul

SN

Vulcanite = phenolformaldehyde > vinyl resin

PEMA.PMMA mix > PMMAHeating to 100"C duringpolymerization increasesfat i que strength

S _ g 28 (ulPa

amitn

Kelly*

Kelly='

Stafford, Smith«"Draughn"

Johnston et a l "Asmussen, Jorgensen'

Stafford et a l "

Zardiakas et aPDrummond"'

uenture resins

Denture resins

Denture resins

Denture resinsComposite resins

Denture resins^ Different types

ol resinsPMMA

Resin cementsComposite resins

¿

5 7

5.7

22

5.73

1.22.350.5

McCabe et a l " Compcsite resins plaster

Saunders"

Aquilino et aPLlobell et al"'

Composite resins

Adhesive resinsPorcelam reparr

systems

3.360.36

30

—

4.3-10^

1.B-10'

10'5*10=

1.3-10«10«

>2'10«

10«4-10'

10'

5-10'

10=2« 10-

—

Bend-release

Bend-release.Reversed every

30 minutesBend-re leasePush-release

Bend-re leaseReversed bending

Bend-release

Pu II-re lea seBend-releasePush-releaseBend-releaseBend-release

Pull-releaseRotational

—

GM

GM

SNSC

GMSN

GM

SCSNSNSCSC

SCGM

Fractographs of clinicaly andexperimentally failed dentureand acrylic specimens

Heat-polymerized > cold-poly-merized resinFine beads > large particlesNotches and contamination

decrease tatigue resistanceS,„ .±30MPaCompressive limit in fatigue

/ static compressive strength= 0.64

Ranl<ing ot 10 brandsSw'-. i. 50 MPa

Raniting of 6 products

Comspan > ESPE-'E'

Authors identify twc types ctfatigue related behavicr

Intact specimen > repairedones

Flanking of 4 techniquesRanking of 12 techniques

GM: group means; %S: percentage sun/pval percentage of survival curves tor a gii/en load are plotted: SC. staircase analysis; SN: S-N idetermined by cycling n samples al a preset lead until failure.

5. Li G, Zhang S. Chen G. Radiographie and histolcgicalanalyses of stress fractures in rabbit tibias. Am | SportsMed 1985;13:285^291.

6. Currey |D. Stress concentration in bone. I Micro Sei 1962;103:111-133.

7. Broek D. Elementary Engineering Fracture Mechanics, ed4, Dordrecht: Martinus Nijhoft, 1986:350-355.

8. Ashby MF, Iones DRH. Engineering Materials 2: AnIntroduction to Microstruttures, Processing and Design.Oxford: Fergamon Press, 1 9S6:1 50-1 51.

9. Irwin GR. Fracture. In: Flügge W (ed), Handbuch derPhysik VI. Berlin: Springer, 1958:551-590.

10. Shackelford |F. Introduction to Materials Science forEngineers. New York: McMillan, 1985:318-319.

11. Weertman |, Weertman JR. Mechanical properties t, [n:Cahn RW (ed). Physical Metallurgy, ed 2. Amsterdam-London: North Holland, 1974:970-981.

12. Meyers MA, Cbawla KK. Mechanical Mefallurgy,Principles and Applications. Englewood Cliffs, Newlersey: Prentice Hall, 1985:706-709.

13. Asami K. Report from the subcommittee on microstructureand fracture. No. 1 : Stage 2 fatigue crack propagation andfractography. In: Koterazawa R, Ebara R, Nisliida S leds).Current lapanese Materials Research: Fiactograpby. London:Elsevier, 1990:1-8.

14. Forsylli P|£. Exudation of material from slip bands at thesurface of fatigued crystals of an aluminum-copper alloy.Nature 1953;1 71:1 72-173.

15. Chargles R|. Static fatigue of glass: I, ii. | AppI Phys 1958;291549-1560.

16. Southan DE, jorgensen KD. The endurance limit of dentalporcelain. Aust Detitj 1974,-19:7-11.

1 7 Sherrill CA, O'Brien W|. Transverse strengtb of aluminousand leldspatbic porcelains. | Dent Res 1974,53:683-690.

i 8 Wiederborn SM. Influence of water vapor on crack propaga-tion in soda-lime glass. | Am Ceram Soc 1967;50,407^14.

19. Myers ML, Ergle |W, Fairhurst CW, Ringle RD. Fatiguecharacteristics of a high-strength porcelain, Int JProsthodont ;994;7:253-257.

20. Leempoel PJ6, Van't Hof MA, De Haan AF|. Survival stud-ies of restorations: Criteria, methods and analyses. | OralRehabil 1 989;1 6:387-394.

21. Craf H, Geering AW. Rationale for tbe clinical applicationoi different occlusal pbilosophies. O'al Sei Reviews1977;1O:1-10.

22. Wobler A. über die Eestigkeitsversutbe mit Eisen undStahl. Z f Bauwesen 1858^6,

23. Broek D. The Practical Use of Fracture Mechanics,Dordiecht: Kluwer Academic. 1988:67-70.

ber2, 1995 115 The international lournal of Proithodoniics

; Principles and lm|)l¡catit

24. Wiíkott HWA, Nicholls II, Taggart R. Fatigue strength of a 48.Au-Pd Al luy/585 solder combinat ian. | Dert ResI991;70;I40-145. 49.

25. Brand A, Fl.ivenol |F, Grégoire R, Tournier C, Donnée?technologiques sur !a fatigue. Senlis: CETIM, 1992:45. 50.

26. Lieurade HP. La pratique des essais de fatigue. Paris; PYCEditions, 1982:101-102. 51.

27. Paris PC, Coniez MP, Anderson WE. A rational analytictheory uf faligue. Trends in Engineering 196! ;13:9-14. 52.

38. Taylor D. Fatigue Thresholds. London: Butterworths,1989:4-7. 53.

29. Little RE, lelie EH, Statistical Design of Fatigue Experiments.Londoni Applied Science, 1975:10-12,168-197. 54.

30. Finney D¡. Probit Analysis: A Statistical Treatment 01 theSigmoid Response Curve, Cambridge: CambridgeUniversity Press, 1952:236-245. 55

31. Dixon W], Mood AM. A meihod (or obtaining and analys-ing sensitivity data. I AmerStJt Assn 1948:43:109-126. 56

32. Dieter GE, Mechanical M(itallLtr|;y. New York: McGraw-Hill, 1961:446-450. 57

33. Draughn RA. Compressive fatigue limits of compositerestorative materials. | Denl Res 1979;58:1O93-IO96, 58

34. Conway IB, Sjodahl LH. Analysis and Presentation ofFatigue Data. Materials Park, Ohio: ASM Irternational,1991:89-132, 59

35. Drummond |L, MJescke KL. Weibull models for the statis-tical analysis of dental composite data: Aged in physiolog- 60.I cal media and cyclic-fatigued. Dent Mater 1991 ;7:25-29.

36. Schwartz NL, Whitsett LD, Berry TC, Stewar! |L.Unserviceable crowns and fixed partial dentures, iife span 61.and causes of loss of serviceabiiity. | Am Dent Assoc1970;81:1395-1401. 62.

37. Walton |N, Gardner FM, Agar ¡K. A survey of crownand fixed partiai denture failures: Length of service and bZ.reasons for repiacement. J Prosthet Dent 1986;56:416-421.

38. Foster LV. Failed conventional bridge work from general 64.dental practice: Clinical aspects and treatment needs oí

142 cases, Br Dent i 1990:168:199-201. 65.39. Wictorin L, Fredriksson H. Microstructuie of the solder-

casting zone in bridges of dental gold alloys. Odontol Rev 66.1976:27:1 R7-196.

40. Oulhwaite WC, Twiggs SW, Fairhurst CW, King GE. Slotsvs pins: A comparison of retention under simuiatcd chew- (¡7.ing stress. J Denl Res 1 982;6t :400-402.

41. Martinet CN, Nally |N, Barraud R, Meyer JM. Propriétés 68.mécaniques et resistance s ia fatigue de trois aliiages"steliites" utilises en prothèse dentaire. Rev Mens SuisseOdontostomat 1 9!i4;94:l 11 1-11 22. h").

42. Saunders WP. The effect of fatigue impact forces upon theretention of various designs of resin-retained bridgework.DentMater l987;3:85-89. 70.

43. Fissore B, Nichoils | I , Vuodelis RA. Load fatigue of teethrestored by dentine bonding agent and a posterior com-posite resin. J Prosthet Dent 1991 ;65:80-85. 71.

44. Kovarik RE, Breeding LC, Caughman WF. Fatigue life ofthree core materials under simulated chewing conditions, 72J Proslhet Dent 1992;68:584-590.

45. Stewart RB, Desiardins RP, Laney WR, Chao EYS. Fatiguestrength of cantilevered metal frameworks for tissue-integrated prostheses. I Prosthet Dent 1992;68:83-92.

46. Cundler A, Lockowandt P, Eihardson S. Crown retention 73.and cyclic loading [in vitro). Scand | Deni Resl993;IOI:252-256.

47. Peyton FA. Flexure fatigue studies of cast dental goldalloys. I Am Dent Assoc 1934:21:394-415.

Wilkinson EC, Haack DC. A study oí the fatigue charac-teristics of siivcv amaigam. I Dent Res 1958;37:1 36-143.Earnshaw R. Fatigue tests on a dental cobalt-chromiumalloy. Br Denl ) 1961 ;110:341-346.Bates IF. Fracture of partial dentures. Flexural fatigue of acobaU-throrrium alloy. Br Dent I !965;1 lß:5 32'537.Hawbolt EB, MacEntee Ml. Effects of fatigue on a solderedbase metai alloy.] Dent Res 1983;62:1 226-1 228.Sutow E|, Iones DW, Hall GC. The response of dentalamalgam to dynamic loading. | Dent Res 1985,64:62-66.Zardiackas LD, Bayne 5C. Fatigue characterization of ninedentai amalgams. Biomaterials 1985;fa:49-54.Butson T|, Nicholls |1, Ma T, Harper RJ. Fatigue life of pre-ceramic soldered and postceramic soldered loints. Int |Prosthodont 1 993;6:468-474.

Barber R. Preiiminary tests of some of the newer denturematerials. I Am Dem Assoc 1934;21:1969-1985.lohnson W, Matthews E. Fatigue studies on some dentalresins. Br Dent j 1949;86:252-253.

lohnson VJ, Matthews E. Further iatigue studies on somedental resins. Br Dent] 1 952;92:91-92.Peylon FA, Shiere HB, Delgado VP. Some comparisons ofself-curing and heat curing dental lesms. J Prosthet Dent1953:3:332-340.

Smith DC. The acrylic denture. Mechanical evaluation.Mid-line fracture. Br Dent | 1961 ;111:257-267.Kelly EK, Flexure fatigue resistance of heat-curing andcold-curing polymethyl methacrylate, | Am Dent AssocI967;74:1273-I276.

Kelly EK. Fatigue failure in denture base polymers. JProsthet Dent 1967;74:1 273-1 276.

Stafford GD, Smith DC. Flexural fatigue tests of some den-ture base polymers. Br Dent) 1970;127:442-445.Johnston EP, Nicholls | l . Smith DE. Flexure fatigue of 10commonly used denture base resins. J Prosthet Dent1981;46:478-483.

Asmussen E, Jorgensen KD. Fatigue strength of someresinous materials. Scand J Dent Res 1 982;90:76-79.Stafford GD, Lewis TT, Huggett R, Fatigue of testing ofdenture base polymers. J Oral Rehabil 1982:9:139-154.Zardiackas LD, Caldwell D|, Caughman WF, Lentz DL,Comer RW. Tensile fatigue of resin cements to etchedmetal and enamel. DentMater 1988:4:163-168.Drummond |L. Cyclic fatigue of composite restorativematerials. | Oral Rehabil 1989:16:509-520.McCabe |F, Carrick TE, Cbadwick RG, Walls AWG,Alternative approaches lo evaluating the fatigue charac-teristics of materials. Dent Mater 1 990;6:24-28.Saunders WP. Effect of fatigue upon [he interfacialbond strength of repaired composite. ) Dent 1990I8-158-162.

Aquilino SA, Dia!-Arnold AM, Piotrowski TJ. Tensilefatigue limits of prosthodontic adhesives i Dent Res199l;70:208-21O.

Liobell A, Nicholls II, Kois |C, Daly CH. Fatigue iife ofporcelain repair systems. Int | Prosthodont 1992,5-205-213Yukowsky W, Fulton DW, An assessment of acceleratedtesting. In: Testing for Prediction of Material Performancern Struciures and Components. ASTM STP 515Philadelphia: American Society for Testing and Materials1972:75-88.

Wiskotl HWA, Nicholls JI, Belse- UC. Fatigue resistance ofsoldered joints: A methodological study. Dent Mater1994;1O:21.5-22O.

Tiie Irternútional lournal ol ProBt 116 Volume a, Numbi