Atomic Energy of Canada Limited

IMPACT FRETTING OF HEAT EXCHANGER TUBES

by

P.L. KO

Chalk River Nuclear Laboratories

Chalk River, Ontario

October 1973

AECL-4653

IMPACT FRETTING OF HEATEXCHANGER TUBES

by

P . L . Ko

ABSTRACT

Impact fretting of heat exchanger tube and bafflematerials has been studied in air and in demineralisedwater, The excitation frequency, amplitude, ratioof major-axis (Ymax) to minor-axis (Xjnax) of an ellipticorbit (X-Y plot) of the unrestrained tube movement,clearance between tube and baffle and number of cycleswere varied, The ratio of Ymax to Xmax governs the amountof normal impact and sliding motion. Electrical contactresistances between the impacting materials were moni-tored and the signals correlated with weight losses.The results show that the amount of wear per millioncycles increases exponentially with excitation frequency,and increases with amplitude and diametral clearance.The amount of wear also varies with the ratio of Ymax

to Xmax (Y >. X) of the excitation amplitude; a peak isreached when the ratio is between 2 and 3. In the drycases, oxidised wear debris was found on the surfaces;the wear rate decreased with time during the earlystages of the test and then became almost constant. Inthe wet cases, the worn surfaces were shiny in appearance,and the wear rate remained almost constant throughout thetes ts .

Engineering Research BranchChalk River Nuclear Laboratories

Chalk River, OntarioOctober 1973

AECL-4653

Usure par frottement d'impact destubes d'ëehangeur thermique

par

P . L. Ko

Résumé

On a étudié, dans l'air et dans l'eau déminé-

ralisée l'usure par frottement d'impact des tubes et

des chicanes d'échangeur thermique. On a fait varier

la fréquence d'excitation, l'amplitude, le rapport entre

l'axe majeur (Y ) et l'axe mineur (X ) d'une orbitemax max

elliptique (tracé X-Y) du mouvement non restreint du

tube, l'intervalle entre le tube et la chicane et le

nombre de cycles. Le rapport entre Y et X contrôlemax max

la quantité d'impact normal et le mouvement de glissement.

On a contrôlé les résistances des contacts électriques

entre les matériaux d'impact et on a mis en corrélation

les signaux avec les pertes de poids. Les résultats

montrent que la quantité d'usure par million de cycles

augmente exponentiellement par rapport a la fréquence

d'excitation ainsi que par rapport a l'amplitude et à

l'intervalle diamétral. La quantité d'usure ^arie, parailleurs, en fonction du rapport entre Y et X

> max max

(.Ï-X) de l'amplitude d'excitation; un sommet est atteint

lorsque ce rapport est situé entre 2 et 3. Lors des

essais a sec on a observé des parcelles oxydées d'usure

sur les surfaces; le taux d'usure décroissait dans le

temps au cours des premiers stades des essais et ensuite

il devenait presque constant. Lors des essais humides,

les surfaces usées étaient d'apparence brillante et le

taux d'usure restait presque constant tout au long des

essais.L'Energie Atomique du Canada, LimitéeLaboratoires -Nucléaires de Chalk River

Chalk River, OntarioOctobre 1973

AECL-46-5 3

CONTENTS

Page.

1. INTRODUCTION 1

2. APPARATUS 4

3. INSTRUMENTATION 6

4. TEST SPECIMENS 6

5. RESULTS 6

5.1 Dry Tests 7

5.2 Wet Tests 9

6. DISCUSSION 9

7. CONCLUSION 11

8. REFERENCES 12

FIGURES 13 - 26

LIST OF FIGURES

FIGURE 1 Test Apparatus

FIGURE .2 Diagram of Vibration Generator

FIGURE 3 Curves of Contact Duration Versus Time

FIGURE 4 Curves of Total Contact Duration VersusWeight Loss

FIGURE 5 Variation in Contact Levels duringVibration

FIGURE 6 Electrical Contact Resistance VersusTime during Vibration

FIGURE 7 Curves of Weight Loss Versus ExcitationFrequency (Dry)

FIGURE 8 Curves of Weight Loss Versus Y /Xmax max

FIGURE 9 Curves of Weight Loss Versus DiametralClearance

FIGURE 10 Curve of Weight Loss Versus Time (Dry)

FIGURE 11 Curves of Wei it Loss Versus MaximumExcitation Amplitude

FTGURE 12 Photo^ioropranhs of Worn Surfaces

FIGURE 13 Curves of Weight Loss Versus ExcitationFrequency (Wet)

FIGURE 14 Curve of Weight Loss Versus Time (Wet)

IMPACT FRETTING OF HEATEXCHANGER TUBES

1. INTRODUCTION

In design and manufacturing practices, a clearancebetween the tube and tube supporting device in steamgenerators and heat exchangers is required. Vibrationof these tubes will make them susceptible to impactingand rubbing with the supporting device or the adjacenttubes causing local wear damage. The tube vibrationmay be excited by cross-flow perpendicular to the cubecentreline and/or longitudinal flow along the tube ortubes. However, the actual flows in most practicalsituations are mixed; therefore, the tube will oscillatemulti-directionally resulting in some form of combinedsliding and impact motion between tube and supportingdevice, and possibly between adjacent tubes.

Owing to the vibratory nature of these impact andrubbing motions, the damage is often considered asbeing a result of fretting; although, fretting, inits ordinary sense, is characterised by minute recipro-cating motion between the wear materials held togetherby a normal force. In the case of tube fretting, themating materials are not held together by a normalforce, rather, there is a certain clearance betweenthem. Depending on the motion of the tube, theresultant movement between the tube and its supportingdevice may resemble one or a combination of thefollowing motions: namely, normal impact only, combinednormal impact and sliding, sliding cnly due to whirlingof the tube, longitudinal sliding, and oscillatingimpact, i.e. a secondary reciprocating type rubbingmovement du .ing each contact interval. This last formof impact fretting, if it happens, may initiate fatiguecracks.

The mechanism of fretting has been discussed by Uhlig ,Waterhouse2, Halliday and Hirst3 and many others.Originally, fretting was known as fretting corrosion ;its mechanism was thought to include a chemical factor,oxidation, and a mechanical factor, welding andshearing of metal asperities. But, it has been shownthat a corrosive atmosphere is not necessary for frettingto occur and that some materials which do not oxidisedo fret". Waterhouse and others2 have suggested threemechanisms by which fretting corrosion can arise:

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1) The removal of metallic particles by grindingor by the formation of welds at the points ofcontact followed by tearing. Subsequent oxida-tion of the particles is supposed to play nopart in causing wear.

2) The removal of metal particles which subsequentlyoxidised form an abrasive powder. The abrasiveaction is then regarded as being the more severecause of wear.

3) The direct oxidation of the metal and thecontinuous removal of this oxide layer by thescraping of one surface over the other.

In a recent review of the mechanism of fretting,Hurricks5 suggests that fretting is a three stageprocess. Initially, a surface formed oxide filmprevents metallic contact; this becomes dispersed bythe oscillatory movement; adhesion, plastic deformationand metal transfer then occur. The transferred parti-cles may become oxidised and dislodged to becomediscrete wear particles; or the transferred particlesmay build into the surface forming an intermediatezone, the partially oxidized surface zone resistingfurther transfer, the fretting action then producingloose wear particles. Finally a steady state is reachedwhich is characterised by a general disintegration anddispersal of the zones affected by the early stages.In short, the three stages are adhesion and metal transfer,production of oxidised debris and finally attainment of asteady wear rate.

Damage due to fretting may vary from only a discolorationof the mating surfaces to the wearing away of largequantities of materials. The frequency, total numberof cycles, amplitude of motion, normal pressure, physicalcharacteristics of the mating materials and environmentalconditions all contribute to the results. The slipamplitude is generally regarded as one of the majorparameters influencing fretting. It has been shownthat the wear rate increases as the amplitude of slipis increased. On the other hand, there are contra-dictory results regarding the influence of normalpressure and vibration frequency on the amount of weardamage. Some workers report that the wear rate decreaseswith increasing frequency6 while others that the wearrate increases with increasing frequency7. It wouldappear that the mechanical properties of the materialstested and the amplitude of slip influence the wear

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rate-frequency relationship. Vaessen et al.7 find thatthe fretting wear between Cu-Nl-Al alloy and SAE 1045carbon steel in air is adhesive by nature and that thewear rate increases linearly with frequency andincreases with amplitude following an S-shape. Feng,Uhlig* and Wright also find that under unlubricatedconditions, wear varies with amplitude and frequencyaccording to a linear relationship. In general, manyinvestigators have observed the existence of a criticalamplitude below which very little or no fretting damagetakes place. This is explained as being due to thefact that at small amplitude of oscillation, alltangential relative motion is absorbed by elasticdeformation of the asperities. Adhesive weld formationand metal transfer can only take place if the asperitiesof the contacting surfaces are plastically sheared.Uhlig1 finds that fretting wear damage is greater indry air than in moist air. He suggests that moisturedecreases abrasive wear of the oxide debris by hydratingthe debris. On the other hand, adsorbed water alsocould be considered to form a film of lubricant betweenthe contacting surfaces over which asperities couldmove without mechanically activating the reaction betweenadsorbed oxygen and metal.

Very little is known about the mechanism of impactfretting. Davis and Read9 have investigated the effectsof rubbing action and impact action (they call thelatter chattering) separately on fretting damage ofZircaloy-2 pressure tube specimens. They find that astainless steel grid impacting on a Zr-2 tube specimencauses less damage than a Zr-2 grid impacting on a tubespecimen of the same material, although the stainlesssteel is harder than the Zr-2. They attribute this tothe fact that adhesion between stainless steel and Zr-2is more difficult. DeGee et al.1 ° have investigated thewear of sintered aluminum powder under conditions ofvibrational contact, and find that for an appreciablewear to occur, welds which may form between the surfacesmust be subjected to shear in the plane of contact andthe removal of loose wear debris must be stimulated.While normal oscillating force alone would stimulatethe removal of wear debris, it was only when bothnormal and torsional oscillations were superimposedthen the wear rate became high. They also observedthat wear increased exponentially with increasingvalues of the sinusoidally changing normal load.

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The significance of tangential force on impact frettingmay be explained also by the adhesion theory11.Courtney-Pratt and Eisner12 and Tabor13 have shownthat tangential forces serve to increase the real areaof contact between asperities. O'Connor and Johnsonhave confirmed this process and shown that in manyengineering problems the growth in real contact ismost marked in the micro-slip region. Further, aftera number of cycles of oscillating slip, due to someform of self-cleaning action, the adhesion between theasperities in the contact region is increased, resultingin further increase in true contact area.

In the present work, the influence of excitation frequency,amplitude, tube-baffle clearance and the ratio of tangen-tial to normal components of the excitation force underboth dry and wet conditions on the fretting wear ofMonel 400 tube against plain carbon steel ring wass tudied.

2. APPARATUS

The test apparatus, which consists of a vibrationgenerator, tube and baffle test specimens, transducerplatforms, and top and bottom supporting plates, isshown in Figure 1.

The apparatus was designed to provide combined impactand sliding. The magnitude and direction of theexcitation force can be easily calculated, and theoscillating unit can produce unidirectional oscillationsor multi-directional oscillations, the latter generatingsome form of sliding impact motion.

In most fretting apparatuses, vibration is introducedby a cam type mechanical drive or by an electromagnetictype device. The present vibration generator employstwo small stepping motors each driving an out-of-balance mass to provide the vibration required. Themotors, out-of-balance weights and motor housing areall built into a compact unit weighing only 1.5 lb.It can be easily attached to any tubes or bars to betes ted.

The arrangement of the motors is shown in Figure 2.The speed of the stepping motor is controlled by anactuator circuit and a frequency generator. Since thestepping motor speed is controlled by varying the inputfrequency instead o-f varying the input voltage as inthe case of a d.c. motor, its speed can be accurately

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controlled. For instance, a 16-step stepping motor willturn 22.5° (360/16) for every input pulse, thus aninput frequency of 160 cps will rotate the motor at aspeed of 10 rev/sec or 600 rpj. The stepping motorsused in this apparatus have a speed range up to 2,640rpm and can be set for 22.5 or 45 degree steppingangles, i.e. 16 stepsor 8 stepsper revolution. The twostepping motors were connected in parallel to theactuator control unit, thus ensuring a synchronizedmotor movement which is important in controlling thedirection and magnitude of the resultant excitationforce. By utilizing two out-of-balance masses rotatingin opposite directions, and by pre-setting the startingangles of these masses, vibrations of different amplitudeand direction can be generated. The out-of-balancemasses are made of lead, weighing up to 6 g. They canbe placed at radii of 0.5 inches or 0.6875 inches.

The motors are housed in two circular holes drilled outof a circular aluminum block. Voids were drilled outof this block to reduce the mass of the unit as wellas to create more surface area for dissipating heatgenerated from the motors. Tubes can be attached eitherto one end of the oscillating unit to form a cantilevertype configuration or to both ends of the unit to forma fixed-simply supported end configuration as illustratedin Figure 1.

The tube is mounted on the top plate by means of atapered plug which is made from bakelite material andis slit open longitudinally. When the plug togetherwith the tube is pressed down into the tapered holein the top plate, the tube is firmly locked in.

When a fixed-simply supported end configuration ischosen, a second tube is attached to the oscillatingunit and the other end of this lower tube is held onthe bottom plaf.e by means of two rubber '0' rings.This arrangement allows longitudinal movement of thetube assembly during vibration.

The specimen ring is press-fitted to a bakelite holderwhich IF attached to one of two mounting platforms.These platforms can be slid up and down to any designatedpositions along four columns whose ends are fixed tothe massive top and bottom plates forming a rigidstructure. When these platforms are tightened to thefour columns they also provide added reinforcement to theassembly.

- 6 -

3. INSTRUMENTATION

Two induction type displacement transducers which aremounted at right angles on the mounting platform areused for positioning the ring to ensure that it is con-centric with the tube. They are also used for monitoringthe x-y movement of the tube during vibration. Straingauges cemented longitudinally along the tube length areused to determine the vibration mode. A biaxial acceler-ometer is inserted inside the tube and positioned behindthe impact area for vibration analysis.

Since both the tube and ring are insulated from the restof the apparatus, electrical contact resistance betweenthe tube and ring can be monitored during tests. Thesecontact signals are normally recorded on magnetic tape atregular intervals throughout the test and are ciigitisedand analysed by computer.

TEST SPECIMENS

Monel 400 is a nicke1-copper alloy composed of 63-70%Ni and 25-32% Cu and Fe, Mn, Si and C in small quantities.A tube specimen one inch long weighing approximately10 g is placed on the master tube. The ring specimenis made from plain carbon steel and is 1/4 inch high.These specimens are cleaned in a boiling Alconox solutionbefore and after each test and are weighed in a micro-balance having a resolution of 1 microgram.

5. RESULTS

Two series of tests, one in air and one in demineralisedwater, have been performed to investigate the effect ofvarious parameters on impact-fretting. These parameterswere excitation frequency, excitation amplitude, ratioof maximum Y-component to maximum X-coinponent of theexcitation amplitude (Y £ X ) , clearance between theimpact pairs, and time (or number of cycles). Themajority of tests were performed in air to form a 6x6Latin-squares for the excitation frequency and vmax/ratio. The other parameters: clearance, time andamplitude were investigated individually. A smallerseries of tests was performed in a wet environment inwhich a continuous flow of demineralised water wasdirected through the clearance between the specimens.

- 7 -

5. 1 Dry Tests

i) Electrical Contact Resistance Measurements

During the majority of tests, the electrical resistancesof contacts were recorded on magnetic tape at half-hourintervals of 20 seconds each throughout the test. Theserecordings were later digitised in a computer to obtainthe total duration of contacts at various resistancelevels, namely, below 0.11 ohms, 0.11-0.6 ohms, 0.6-3.5ohms, 3.5-6.5 ohms, 6.5-20 ohms, and 20 ohms and above.The total duration of contact for each resistance levelwas then expressed as a percentage of the scanning time,i.e. 20 seconds in the present case. In general, thecontact resistances were low during the early stage ofthe test. The percentage of these low resistance contactsdecreased rapidly with time, and after about three hours,only few contacts with resistance below 0.6 ohms wererecorded. On the other hand, the higher level bandsfluctuate slightly in a decreasing pattern indicatingoccasional breakthrough of the oxidised debris. Towardsthe end of a 16 hour test, the majority of contacts wereof 20 ohms or higher. Three sets of these contactresistances versus time plots are shown in Figure 3.Figure 4 shows a series of plots of total duration ofcontacts versus weight losses. The total duration ofcontacts were obtained by integrating the curves ofFigure 3. With the exception of one low resistanceband, they appeared to have increased fairly linearlywith weight losses. The low resistance band did notvary in a consistent pattern and remained at a lowpercentage level.

In Fifure 5, the contact resistance is plotted againstthe itstantaneous tube location in polar coordinates.The geometry of the baffle hole which is circular andconcentric with the tube was later superimposed on theplot. It can be readily seen that where there is nocontact, an infinitely high resistance is indicated;while at other spots where contact is heavy a lowresistance is shown. This technique is useful forestimating the impact force, around a circumferencewhere force transducers cannot fit in. Figure 6 showsthese contact resistances displayed on a time base..It appears that during each impact there are severalcontacts with durations lasting between 0.7 and 1.5msec. However, the resistance levels of these higherfrequency contacts seem to indicate that the two sur-faces never break off completely during each impact.

- 8 -

i i ) Effect of Excitation Frequency

Figure 7 shows a series of curves of weight loss versusexcitation frequency which covers a range from 15 to 35Hz. The weight losses are expressed in units of milli-gramsper million cycles. Each curve represents one setof parameters, namely, diametral clearance, maximumexcitation amplitude (the major axis of an elliptic plotduring unrestrained vibration at the frequency tested),and the ratio of maximum Y to maximum X components ofthe excitation amplitude. A high ratio of Y/X impliesthat the motion is mainly of normal impact with verylittle sliding contact around the circumference of thebaffle hole; whereas a Y/X ratio of unity would causethe tube to whirl having little or no impact.

The results show that within the frequency rangeinvestigated, the weight loss increases exponentiallywith the excitation frequency and has the relationshipW = ke a w, where k and a are constants depending onthe ratio of Y/X, to is the excitation frequency in Hz,and W the weight loss in milligram s/106 cycles. InFigure 7 is also shown a frequency-time curve whichillustrates that when d) ->• 0 , t -+ °°; hence weight lossin a finite time would amount to '.ero.

i i i) Effect of Ratio of Maximum Y-Component to MaximumX-Component of the Unrestrained Excitation Amplitude

The results, Figure 8, show that the weight lossincreases as the Y/X ratio is decreased; a peak isreached when the ratio is between two and three, thenthe weight loss decreases slightly as the ratio isfurther decreased. The variation appears to be morepronounced at high frequencies.

iv) Effect of Tube-Baffle Clearance

Diametral clearances ranged from 0.005 inches to0.02 inches have been tested. Figure 9 shows threecurves from three sets of parameters. They all showan increase in weight loss with increasing clearanceas long as contact is maintained between the tube andthe baffle. In each curve, the excitation amplitudeis constant. The weight loss of the plai,n carbonsteel ring, although not shown here, also increaseswith increasing clearance.

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v) Effect of Time

Only one set of parameters was chosen to investigatethe change in weight loss with time. It is found thatin air the rate of weight loss decreases with time,Figure 10, until a steady state is reached, then therate of weight loss becomes more or less constant andthe weight loss increases almost linearly with time.Using the steady state wear rate of 35 yg/h at 22.5Hz, the life span of a 0.05 inche thick, 0.5 incheOD tube worn through uniformly around a ]/4 inch wide-band is estimated to be 9 years.

vi) Effect of Excitation Amplitude

The excitation amplitude is the maximum amplitude ofthe tube at the contact region during unrestrainedvibration at the test frequency. It is expressed asa multiple of the radial clearance. Although only afew tests have been made to investigate this effect,they all show an increase in the amount of wear withincreasing excitation amplitude, Figure 11.

5.2 Wet Tests

The electrical contact resistances of the wet tests donot increase with time as ia the case of dry tests;rather, they remain at a low level throughout the test,The appearance of the worn surfaces also differs fromthose of the dry tests. The surfaces of wet testspecimens have a shiny and polished appearance whereasthose of the dry tests have a dull and porousappearance as illustrated in Figure 12.

The results of the wet tests show that the amount ofwear also increases exponentially with the excitationfrequency, although the rate of increase is not asrapid as that of the dry case, Figure 13. Weightlosses at frequencies below 25 Hz are in the sameorder of magnitude as those of the dry case; however,it differs from the dry case in that it increasesfairly linearly with time,, (Figure 14).

6. DISCUSSION

The curves of Figure 3 show that the percentage of lowand medium resistance level contactrapidly with time suggesting that a

durations decreaselayer of oxidised

- 10

wear debris probably forms on the surfaces soon afterthe test has begun. At higher excitation frequencieswhich correspond to higher excitation forces,some of this oxidised wear debris is being dispersedmore readily and the decrease in low and medium levelcontact durations become less rapid. These resultsappear to be in good agreement with the weight loss-time curve of Figure 10, in which the weight loss ratedecreases with time until a steady state is reached.Evidently, the layer of oxidised wear debris growswith time during the early stage; hence both the wearrate and the percentage of contact duration of lowand medium resistance levels decrease with time.The linear variation of weight loss with total contactduration at various resistance levels, except the verylow resistance one, suggests that this technique may beuseful for estimating weight losses where the testspecimens are too heavy to be weighed in a sensitivebalance. It is interesting to note that all curves ofFigure 4 intercept the vertical axis (resistance axis)at points slightly above the zero. Perhaps, initially,for a very short duration, contacts are made withoutcausing damage to the surfaces; the action merelyremoves the surface film and prepares the surfaces forthe subsequent metal removal.

The results seem to indicate that the mechanism ofimpact fretting is similar to the one suggestedby Hurrick5 for fretting wear: the dispersion of thesurface film by the oscillating movement; then adhesion,plastic deformation and metal transfer; the wearparticles then become oxidised and form an intermediatezone; the fretting action then produces loose wearparticles; and finally reaches a steady state. Inimpact fretting, the dispersion of this intermediatezone of partially oxidised wear debris is furtherstimulated by the action of normal impact. The com-bined effect of sliding and normal components duringimpact fretting would promote the shearing of weldsformed between the surfaces, and the removal of loosewear debris. However, if there is only normalimpact, no shear occurs; on the other hand, if thereis only sliding, the removal of wear debris is hamperedand the mechanism would become that of sliding wear.The results of Figure 8 support this explanation. Theweight loss reaches a peak when the ratio of maximumY-component to maximum X-component of the excitationamplitude is decreased to approximately two (Y _>. x) •Further decrease of this ratio results in a slightdecrease of weight loss. It can be assumed that the

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weight loss would equal the weight loss due to slidingwear when the r a t i o is reduced to one.

A possible explanation for the increasing weight losswith increasing diametral clearance as shown in Figure 9is that the energy at impact which increases withdecreasing clearance for a given excitat ion amplitude isabsorbed not into the surface asper i t ies but into thebulk of the tube causing a possible change in mode shape.I t can also be visual ised that for the same unrestrainedexci ta t ion amplitude the smaller the clearance thehigher will be the ra t io of normal to sl iding components,hence lower weight loss .

From the curves of Figure 9, a ser ies of curves relating thepercentage of penetrat ion of the tube wall to theexpected l i f e span of the tube can be developed (Appendix 1)These curves provide an estimate of the hypothetical l i fespan of a worn tube from periodic inspections of the depthof wear.

7. CONCLUSION

A testing apparatus has been developed to aid in the studyof fretting wear between tubes and tube supporting devices.In this apparatus, the tube is excited by means of a com-pactly built vibration generator employing stepping motorsand out-of-balance rotating masses. This arrangement faci-l i t a t es a study of the importance of sliding component intube fretting wear.

The results have shown that the amount of wear increasesexponentially with excitation frequency following therelationship W = KeaU), and increases with excitation amplitudeand diametral clearance. It has also been found that theamount of wear varies with the ratio of Ymax to Xmax of theexcitation amplitude, Y being the larger of the two compo-nents; a peak is reached when the ratio is between 2 and 3.

In the dry cases, the wear rate is found to be affectedby the oxidised wear debris formed on the worn surfaces;the wear rate decreases with time during the early stagesof the test and then a steady s ta te is reached when thewear rate becomes almost constant. In the wet cases,the worn surfaces are shiny in appearance, and the wearrate remains almost constant throughout the tes t .

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8. REFERENCES

(1) Uhlig, H.H., Mechanism of Fretting Corrosion, J. ofApp. Mech. Trans. ASME, Vol. 21, pp.401-407, 1954.

(2) Waterhouse, R.B., Fretting Corrosion, Proc. I. Mech. E,(London), Vol. 169, p.1157, 1955.

(3) Halliday, J.S. and Hirst, W., The Fretting Corrosionof Mild Steel, Proc. of Roy. S o c , London, Vol. 236A,p.411-425, 1956.

(4) Godfrey, D. and Bailey, J.M., Early Stages of Frettingof Copper, Iron and Steel, Lubrication Engineering,10, p.155, 1954.

(5) Hurricks, P.L., The Mechanism of Fretting - A Review,Wear, Vol. 15, pp.389-409, 1970.

(6) Feng, I-Ming and Uhlig, H.H., Fretting Corrosion ofMild Steel in Air and in Nitrogen, J. App. Mech.,Vol. 21, pp.395-400, 1954.

(7) Vaessen, G.H.G., Commissaris, C.P.L. and deGee, A.W.J.,Fretting Corrosion of Cu-Ni-Al Against Plain CarbonSteel, Proc. I.M.E., Vol. 183 Pt3P, 125-128, 1968.

(8) Wright, K.H.R., Fretting Corrosion of Cast Iron, Proc.of the Conference on Lubrication and Wear, Institutionof Mech. Eng., pp. 628-634, 1957.

(9) Davis, S.M. and Read, D.T., Unpublished•Information.

(10) deGee, A.W.J., Commissaris, C.P.L. and Zaat, J.H.,The Wear of Sintered Aluminum Powder (SAP) Under Con-ditions of Vibrational Contact, Wear, Vol. 7, 535-550,1964.

(11) Bowden, F.P. and Tabor, D., The Friction and Lubricationof Solids, Oxford Press.

(12) Courtney-Pratt, J.S. and Eisner, E., The Effect of aTangential Force on the Contact of Metallic Bodied,Proc. Roy. Soc. London, A Vol. 238, pp. 529-550.

(13) Tabor, D., Junction Growth in Metallic Friction - TheRole of Combined Stresses and Surface Contamination,Proc. Roy. Soc. London, A Vol. 251, pp.378-393, 1959.

(14) O'Connor, J.J. and Johnson, K.L., The Role of SurfaceAsperities in Transmitting Tangential Forces BetweenMetals, Wear, Vol. 6, pp.118-139, 1963.

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FIGURE 1: Test Apparatus

GASKETS

OUT-OF-BALANCEWEIGHTS

ROTATING DISCS

FIGURE 2: Diagram of Vibration Generator

PERCENTAGE OF TOTAL CONTACT DURATION, %

oa

oc

ol-h

o30)O

o

O

ITO

- 16 -

3.0 —

_-2-0

j -

ht-

2 2-5 Hz

Ymax= 21

X max

inO

x;

(J !

J //

f

J_10 20 30 40

Time I50

Hour60 70

FIGURE 4: Curves of Total Contact DurationVersus Weight Loss

- 17 -

10 Hi

Electrical Contact Resistance

Tube Orbit(x-y displacement plot)

open

9 Hz

FIGURE 5:

12.5 Hz

Variation in Contact Levelsduring Vibration

- 18 -

f-5

I

V r V 1 *\ s

Beginning of test

After 16 hours

FIGURE 6: Electrical Contact ResistanceVersus Time during Vibration

- 19 -

FREQUENCY-TIME(TIME REQUIRED TOREACH 1 MILLIONCYCLES)

W - Kea(i;(ORY) _

0.1149 A0.1541 a0.1441 o0.1404 •

0.1348

100906070

60

50

40

30

20

10 -

8 *7

6

20 30 40 50EXCITATION FREQUENCY u) , Hz

60

FIGURE 7: Curves of Weight Loss Versus ExcitationFrequency (Dry)

7.0,

6.0

u»>u

- 4.0

t/jto

3.0

m 2.Of

25 HZ

" m i

FIGURE 8: Curves of Weight Loss Versus Y /Xmax max

- 21 -

o

CO

00

EXCITATIONFREQUENCY

A 30 HzB 32.8 HzC 20 Hz

Y/X

3

9

6

UNRESTRICTEDEXCITATION AMPLITUDE Y

I I I I I10 15

DIAMETRAL CLEARANCE 2C, _ i _ , n

20

1000

FIGURE 9: Curves of Weight Loss Versus Diametral Clearance

Ei -

i '\ \ .

1 0 ' _, [__. L

\\ WEIGHT LOSS = • 12 mg

1 , 1 . 1

'1=

o i 4 i a IO i; >4

•EISHI LOSS J 4 n

. 1 . 1 , 1 . ,

IE I CHI LOSS t 5.5 rag

• I • I • I . I4 ~ i i IO 12 u o 1 t 1 • 11 1? 14

TIKE t, hr TIME t, hr TIME t. ft r

FIGURE 1 0 : C u r v e of W e i g h t L o s s V e r s u s Time

- 23 -

3.0i—

2.0x:CO

ABC

EXCITATIONFREQUENCY

30 Hz20 Hz20 Hz

Y/X3

3

6

CO

o

z i.o

11.0C 1.5C

(C = RADIAL CLEARANCE)

MAX. EXCITATION AMPLITUDE (YCOMPONENT)

FIGURE 11: Curves of Weight Loss VersusMaximum Excitation Amplitude

Dry x5 xlOO

Wet xlOO

FIGURE 12: Photomicrographs of Worn Surfaces

- 25 -

10

8

uu

COCO

C9

0.8

0.6

0.4

D

(D(2)

(3)(4)

Y/X

1.69

3.0

9.0

AVERAGE

W = K e a U J

A

O

D

X - -

(WET)

K

0.2142

0.211

0.184

0.2015

a

0.0705

0.0635

0.0551

0.0646

0.2

0.110

FIGURE I 3 :

20 30 40 50

EXCITATION FREQUENCY o>, Hz

60 70

Curves of Weight Loss Versus ExcitationFrequency (Wet)

- 26 -

E

LOS

S

i-i

UJ

6.0

5.0

4.0

3.0

2.0

1.0

-

o>

22.5' mox

WET

o

/ Oo

1

Hz

= 2.i

TESTS

o /

/o

1

/

o

1 1

o /

//

1 1

10

i'lGURE 14

20 30 40 50 60 70TIME t , HOURS

Curve of Weight Loss Versus Time (Wet)

APPENDIX I

to

AECL-4653

ESTIMATE OF TUBE LIFE SPAN

The curves of Figure 9 show that the wear rate is afunction of diametral clearance. If a linear relation-ship is assumed, then it can be written as

dW— = a C + 3 (1)dt

We a l so have, assuming the tube i s worn uniformlyaround i t s circumference,

C = C + (D -D)o o

o rD = (Co+Do) - C (2)

where C and D are respectively the initial diametralclearance and Initial outside diameter of the tube,C and D are respectively the diametral clearance andoutside diameter of the tube at time t.

Weight loss W at time t = 1/4 IT (D 2-D )pe (3)

where p is the density and e the thickness of thebaffle plate.

Substitution of equations (2) and (3) to equation (1)yields

1/4 TT pe [2(C O+D Q) - 2C] dC = (a C+8) dt

(C +D ) - C 2

_°_._° dc = dt (4)aC + 3 TTpe

A-2

Integrating both sides

C +D Cg 2— In (aC+g) - - + — An (ac+3) + K. = ta a a2 fvpe

when t = 0, C =

C +D 3 C

Hence

C +D 3 a(D -D.)R+aC +3 (D -D,)R( _ £ _ ^ + } ^n [. , o 1 o j o i

a a aCQ + 3 J a

2TTpe

where D is the internal diameter of the tube, and

is the ratio of depth of wear to originalR = S.

VDithickness of the tube.

Figure 15 shows two curves obtained by applying theresults of Figure 9 to equation (5). A third curvehaving constant wear rate is also shown in the samegraph.

A - 3

lOOp

9 0 -

8 0 -am

zO 70h-

E 60

a 50

° 40UJID

^ 30ZUJ

UJQL

10

REF. CURVE CFIGURE 9

dWdt = CONSTANT

\REF. CURVE A

FIGURE 9

0 O.I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0EXPECTED LIFE SPAN OF TUBE (WORN THROUGH)

FIGURE 15 (APPENDIX I)

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