stress fatigue principles

8/9/2019 stress fatigue principles

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Stress Fatigue: BasicPrincipies and

Prosthodontic Implications

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

lack I. Nicbolls, PhDUniveisity of WashingtonSeattle, Washington

Urs C. Belser DMD'University of G e n e r aGeneva 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 kno wledge o í fatigue idilure and testmethods. A n ove rview oí pu hlished studies is given, and the authors suggestguidelines for future p rosthodontie studies of this nature. Int I Prostbodont1995:8:105-116.

F atigue is a mode of fracture whereby a structureeventually fails after being repeatedly subjectedto 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 w hen the industrial re volution 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 lim it and the S -N curve (Table ).'

Today fatigue failure Is explained by the devel-opment of microscopic cracks in areas of stress

concentra t ion. With cont inued loadings , thesecracks fuse to an ever-growing fissure that insidi-ously weakens the restoration. Catastrophic failure

'Lecturer, Division of Fixed Proslhod ontii:;, Scho ol ofDentis ry.

"Professor, Depsrlmenl of estorative Dentistry, School ofDentistry.

•"Professor and Chairmm, Division of Fixed ProsthodonticsSchool o f Dentistry.

Reprint requests: Or H.W.A. Wiskatt, D ivision of Fixed

Prosthodontici, 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 .' Sim ilarly, spontaneous fractureshave also been linked to fatigue phenomena.

Table Abbreviations Used1, II , tila

ö „tfuoT

E

fK

NN,nPRsssS.s„ ,s„„

s .TX

(as indices) F racture 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 Number 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 m ean

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: 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 I II 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,i t s phys ica l o r ig in , me thods o f ana lys i s , andprosthodontic implications.

Physical Mechanism

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

mechanics is ind icated . 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 the

surface 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 flaw 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 ma thema tical equ ivalen t of thestress fringes in pola rized light tests. Und er 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 h s reachedits critical ievel and is therefore equated to thefracture toughness of the material. In general termsfracture toughness is given by

K = \


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5tr«5 Fatigue: Principles and Implications

Table Typical K^ of Some Dental Restorative Materials

Material Composition

Modulus ofRupture

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

18Cr8NiSuperalloys

Titanium

AluminumDental ceramicsZirconiaSiliconsConcrete

Poly(methylmethacrylate]

Epoxy

Composites

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

ZrO, + 5wt%MgOSijAION,C aO 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

50 803 0 ^ 0

3-5

4 125

0.2

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

5 8 % uniaxial C in epo>;y

80-90 1.6[20 C)

0.6 1

1050 32-45Data 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 m ode (Fig 1). Y is a correctionparameter that is required because of the finitenessof the specimen whose boundaries also alter theforce flo w lines of Fig 2. It is a dimensionlessgeometry factor on the order of 1. Table 2 lists K,,, T„,


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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 t ip.Laboratory production and handling of ceramicsmake Griffith flaws inevitable, and since the crack

tip 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 show n that per io dic al ly ha l t ing afatigue test and polishing a layer off the material

under 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,'*

T wo other specific material behaviors have beenrelated to fatigue. In alloys, small and very thinsheets of metal, called slip-bands, are sometimesextruded a t the surface of a fa t igue d c rys ta l ,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-OH

terminals. 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 wate r.' '

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),Com plex load spectra can also be generated. Somemachines attempt to reproduce a clinical environ-ment by adding moisture and a con trolled 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 de nture is given as 20years, ' this prosthesis w ill have undergon e 2 x 10stress cycles. Conversely, it can be argued that notevery chew ing cycle is ac t ive ( ie , applying a

maximum stress cycle to the structure). Therefore,the total of 2 X 10' che wing cycles previously cal-culated should be decreased by a factor rangingbetween 5 and 20 if a realist ic value is to beobtained. For dental applications, fat igue testsshould be performed for a m inim um of 1 0^ cycles.Also, as shown in Fig 5, func tiona l loading ofteeth implies a multidirectional force pattern thatcomprises a compressive as well as a buccolingualcomponent. Loading a prosthodontic test structureunia xia l ly w i l l thus only pa r t ly reproduce, themechanical conditions of the oral environm ent.

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 several

actuators on a single specimen, extremely complexstrains can be induced.

lofPrusthiüclor)(it> 8 Volume 8,


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

Applied stress [S]

-

s

/

sii t

Onestress

-^ cycle ~

j

Vy sTime

Applied stress [S \

Tension

0 -

Compression

% ^\ ^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^'),

5 K p

p

2 K p

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 spe cimen. The characteristics of theapplied stress are S™. = -5™.. and 5„ = 0, This typeof apparatus has been extensively used in indus-

trial tests whe re it is also know n as an R,R, Mo oremachine . '

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 fai lure isrecorded. In alternate tests, changes in materialparameters {E (T,J are evaluated as a func tion of Sand Nf. In this category a most sign ifican t testconsists of monitoring the progress of a fissure.Such a test requires a spec ific sp ecim en d esign(Fig 6 ), Cyc lic loading of the sample is disco ntin-ued at periodic intervals, and the length of thecrack is recorded.

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and Iniplicatii

escriptive Techniques

icrographs

inasmuch as fatigue processes are determined bya progressing fissure, in m aterials that present somedegree of ductility, the crack front may leave a

groove 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

Fig 6 CT (Compact-Te nsion) 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 joi nt s, 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 are

re la ted to s tage I I c rack growth. Of ten c loseinspection of the micrographs indicates the site ofcrack nucleation.

S N iagrams

Known interrelat ionships 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 be

cycled indefinitely.For normative purposes, S-iV diagrams are subdi-

vided into three regimes:

1. Low cy cle fatigu e 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 wh ieh 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. tjn iim ited endurance encompasses tests cyc ledabove 10' and essentially applies to industrialstructure.^.

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

The imernarional loumal of Proitliodonlîcs


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Wisk od el al Strew Fítigu e: 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

AUEA n r

(Juts

IOCSpeKt =

- ^ • a-

;iOSteetenitizationealling 500- 305 N/mn-= 400 N/mnationa l faticHz, aircimen FR Í

= 1.03

' ' •

a

0^ 10* 10^ 10^ 10^ 1Number of cycles to failure

9>»

5

B

1900 C 30 minC 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 forXC IO 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 i t can be extrapolated af ter 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 will 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 applied

stress, the greater the number of fatigue cycles sus-tained by the specimens. In the illustrated situa-

B, Number 2, 1995111 The liilernational lournal of Prostliodontjcs


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f aligue: P rinciples and I mpl italic

logl

da dN

I

AK

n

log AK

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

t ion, the test was cond ucte d 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.

racture echanics

S-N diagrams are based on a fai l or not-fai lapproach and therefore take little or no account ofthe parameters that determine crack propagation.They are highly specimen specific and, since they

lack a mechanistic base, cannot be extended tosimilar designs or related materials research.Th e K parameter can be app lied 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 c om m on ly written as AA . The generalshape of such plots has been established by Parisand co w or ke rs and is show n 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 IIIAAT has reached a mag nitud e great eno ug h tocause very rapid fa ilure.

nalysis

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- Thisamo unt of d ispers ion is conside red 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 procedu re of generating afull S-N diagram. For a specific application, theonly required inform ation may be non-failure for aset number of cycles {N]. In this instance, a logicalstep is to determine an endurance limit (St,) and its

scatter 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 fe w spec imen s (3 < n < 10 ).'' For a morerelia ble estimate of Ŝ • and s, how eve r, larger sam-ples are required. To this effect, the staircase tech-ni qu e' is a straightforward procedure in wh ich 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 byWa lton et a l. ' ' Other authors reported failures in

terms of a mean annual rate which, depending onthe type of restoration, varied between 2.5 snç\ ^

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WiskoH et al Slreís Fíligue: Prin riplei jn d Implications

i 11 Fam ily of curv esshowing levels of probaDility offailure.

1

¿5

Dlu

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-tu ra l weaken ing o f the componen t , and f ina lbreakage, it tbus stands to reason that emphasisshou ld also be placed on a cha racte rization ofdental materials and structures by dyn am ic 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

thin ka ble stress under fun ctio n. Such a predictiveapproach, however, is only possible if pertinent

i 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 from

learning as we ll as their ow n clinic al expe rience.

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 po tentia l , the follow -ing guidelines w ould prove useful:

1, Fatigue tests should be based on fracture

mechanics and the parameter. However,such an approach requires special training

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Fatigue' Principles and lm|

Table 3 Fatigue of Structures

(Hz)Type ofstress Analysis Findings

Outhwaite et al'°

Martinet ef al

Saunders

Fissore et al

Kovarik et al '

Stewart et al '̂

Gjfidler et al

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«

1 0 '

Reversedbending

Bend-re lease

Push-re lease

Bend-release

Bend-re lease

Bend-release

Push-release

GM

GM

S C

Gtvi

G M

% S

% S

Slot retained > pin retained

Ticonium > Vitallium = Wironiurrclasps

No difference between electronically

etched, lost salt, Panavia EXAn 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 Fatigue of Metals

Auttiors

PeytonWilkinson, tHaackEarnstiawBates'*Hawbolt, iVlcEnteeSutow et aP'

Sub|eot

Gold alloysAmalgam

Co-Cr alloyCo-Cr alloyhJi-Cr alleyAmalgam

/(Hz)

12,53016.713.36 0

1,330

Type ofstress Analysts Findings

10 '8«iœ5 1 =

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

Bend-releasePush-pull

Reversed bendingBend-re ea sePush-release

Push-pull

Push-releaseBend-re lease

Rotational

S NS NS NS NS NS N

S NS NG M

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-.

w h i c h may not be avai lable to the den ta lresearcher. Therefore, procedures based onquantal (fail or not-fail) data should be usedsince they are more easy to c o n d u c t andinterpret,

2. Tests sho uld bo brought at least to 10^ cyclesif a c l in ica l ly re levant service l i fe is to beapproximated,

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

4. Whole 5 N diagrams are not required since thelow-cycle regime (


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iples and Implicilii

Table 5 Fatigue of Resins

Subject/

(Hz)Type olstress Analysis Findings

Barber Denture resins 1.7

Johnson, Matthe ws 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 ease

Bend-release

G MGtul

SN

Vulcanite = phenolformaldehyde > vinyl resin

PEMA.PMMA mix > PMMAHeating to 100 C during

polymerization increases

fat i que strengthS _ g 28 (ulPa

amitn

Kelly*

Kelly='

Stafford, Smith«Draughn

Johnston et alAsmussen, 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'

1 05*10=

1.3-10«10«

>2'10«

10«4-10

1 0

5-10'

10=2« 10-

—

Bend-release

Bend-release.Reversed every

30 minutesBend-re easePush-release

Bend-re easeReversed bending

Bend-release

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

Pull-releaseRotational

—

G M

G M

S NS C

G MS N

G M

S CS NS NS CS C

S CG M

Fractographs of clinicaly andexperimentally failed dentureand acrylic specimens

Heat-polymerized > cold-poly-merized resin

Fine beads > large particlesNotches and contamination

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

/ static compressive strength= 0.64

Ranl 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.

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stress fatigue principles