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CAVITATION, TRIBOSYSTEMS AND CAVITATION TRIBOMODELS

Octavian Bologa, Ion Crudu

“Dunărea de Jos” University, Galaţi, octavian.bologa@ugal.ro

ABSTRACTIn the first part of the work is presented the epoch-making of the phenomenon

and the chronology of its studying. Here are also presented the water’s properties:its adhesion to solid surfaces, the compressibility, the superficial tension, gasdesorbtion. It is described the way of coming to be and the development ofcavitation. It is presented the concept of tribosystems and tribomodel. There aredescribed the cavitation tribomodels built up for the Machine Design Department.

KEYWORDS: cavitation, compressibility, adhesion, superficial tension, cavitation tunnel

1. Short epoch-making

The cavitation phenomenon has been observedfor the first time at a vessel propeller in 1856 duringsome experiments made by the French GeorgeRennie. He noticed that altering the immersion of thetop of the propeller blade from 0 to 1.35m, thepressing grew proportionally. The real nature of thephenomenon wasn’t noticed by then, the researcheronly concluding that the output of the drivingmechanism grew with the depth.

The importance of cavitation phenomenon ishighlighted in 1894 when the torpedo boat Daringbuilt by Dita Thornzcraft reached the speed of 24 Ndinstead of 29 Nd, as projected, and after the propellerchanging it reached 34 Nd with the same power. Itbecame obvious the fact that the cavitationphenomenon depends on the pressure which drivesthe propeller’s disk. For studying this phenomenon,Parsons built a tight tunnel with water, in closedcircuit. The tunnel pressure is modified afternecessities and it has allows the study of thephenomenon on propeller models. In 1910 Parsonsbuilt a second cavitation tunnel, bigger and improved,in which he put, for the first time, the stroboscopicsystem of observation. The phenomenon could havebeen now photographed. The installation has beenendowed with a sight for seeing the cavitationphenomenon and with measurement devices ofpressing and for the moment of the propeller axle.There were made separated ways for propeller modelsand spiral pumps and a tank for calming the water.

In 1914 Fottinger built a hydrodynamic tunnelendowed with a workroom in which it could bereached speeds up to 56 m/s, the installed powerbeing of 200 kW.

In 1932 Shreter accomplished a tunnel provi-ded with a workroom, like a diffuser, in which itcould be reached speeds of the fluid current up to 44m/s. In Haller’s installation, built in 1933, it could bereached speeds up to 100m/s. The work room dimen-sions for hydrodynamic tunnels are bigger and bigger,fact which implies higher installed powers. Thus, thehydrodynamic tunnel built at Technological Institute,Massachusetts, between 1942-1947 has a power of300HP, the hydrodynamic tunnel of NationalUniversity, Pennsylvania built between 1947-1951hasa power of 2000HP, the hydrodynamic tunnel ofAdmiralty (Teddignton UK) built between 1951-1954has a power of 350 HP and the hydrodynamic tunnelof the National Centre of Science Marine Researches,Washington built between 1959-1962, has a power of350HP. In these cavitation tunnels propeller modelsor pump impellers are tested, existing the possibilityof correcting the cavitation number, pressingmeasurement and the cavitation noise.

Today, in the research laboratories are manyinstallations allowing the research on materialbehaviour at cavitation wear or modelling thecavitation destruction. There are used: vibrationinstallations, ultrasounds installations, hydrodynamictunnels with tight chamber, devices with rotationalimmersed disks, equipment with intermittent jet.

The first vibration installation on themagnetostriction principle was realised by Mason inUSA (pattern in 1945). The frequency of work is of20 KHz on an amplitude oscillation of 50-70µm,getting accelerations of 60,000 g.

A similar installation was realised by WheelerEast-Kilbrad Laboratory, Holland. The work frequen-cy is of 8 KHz at amplitude of 40.6µm. The NASAlab of Lewis Research Centre has a magnetostrictive

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installation for studying cavitation destruction onliquid metals at a work temperature of 8200 C degrees,with a protective atmosphere of inert gases. AtMichigan University it was built a piezoelectricinstallation with the frequency of 20 KHz. At theTechnological Institute of California, Plesset accom-plishes a magnetostrictive installation with sensingdevice provided with exponential concentrator, with awork frequency of 14.2 KHz at an oscillation ampli-tude of 50.8µm. The installation produces the socalled “rhythmic variation cavitation”, forhighlighting the corrosion effects in metal cavitation.Such installations have been built at the Institute forHydraulic Machines of Gdansk, Technological Insti-tute of Darmstadt, Technical Institute of Zurich, andElectro-Technical Faculty of Split. Tight workroomsinstallations are used in the laboratories of Technolo-gical Institute, Massachusetts, having a Venturi typeworkroom. Thus, Schroter, De Haller, Petracchi,Hamit, Salnev, Varga used these installations.

A device with immersed rotation disk was usedfor the first time by Rasmussen. Lichtman used asimilar installation and than by Wood who researchedthe material behaviour in liquid materials. Knudsen,Thiruvengardam and Symala Rao used also deviceswith rotation disks.

Devices with intermittent jet were used for thefirst time by Akeret and Haller for studying materialbehaviour to cavitation wear, in the hydraulic machi-nes construction. Similar installations to those madeby Akeret and Haller are presently equipping differentlaboratories from France, Holland, Russia, India, etc.

Hereby we enumerate in chronological orderthe contribution brought about by a few hydraulicresearchers to study the cavitation phenomenon andits close aspects.

Isaac Newton (1642-1727) discovers the jets’contraction.

Daniel Bernoulli (1700-1782) researchesexperimentally the majority of the phenomenainvolved in fluids movement.

Leonard Euler (1707-1783) explains for thefirst time the pressure effect in the liquid flow andformulates fundamental equations of the movement,introduces the concept of cavitation and the principleof centrifugal machine.

Giovani Battista Venturi (1746-1822) makestests on different shapes of nozzles.

James Bicheno Francis (1815-1892) makes thefirst researches in USA on diffusers and turbines.

Osborne Reynolds (1842-1912) describesoriginal experiences in different domains: cavitation,similitude, and pipes’ resistance.

John William Strut (Lord Reyleigh) (1842-1919) researches the hydrodynamics of the cavitatio-nal balls’ inrush, the jets’ instability, the analogy ofthe laminar flow and the dynamic similitude.

Nicolai Egorovici Jukovski (1847-1921)makes the first correct analysis of the hydraulic

hammer; he contributes to the study of the wingpropeller’s hydrodynamics.

Herman Fottinger (1877-1945) accomplishes:cavitation researches on turbines and pumps and thefirst modern hydraulic transformer.

Lewis Ferry Moody (1880-1953) studied: thetube wall of turbines. He introduces in 1922 thecavitation coefficient of hydraulic machines, generallynamed Thama’s Coefficient.

The concerning of Romanian scientists inhydromechanics are marked by presenting PhD thesisabroad (N. Enache, V. Valcovici, D. Pavel, E.Carafoli,, c. Iacob, D. Dumitrescu). In 1913 V.Valcovici has presented in his PhD thesis a study onthe movement of a liquid jet which comes out frompipe, hits an obstacle forming free lines of current (thephenomenon is known as the scheme Vlcovici –Birkhoff). The same was studied by C. Iacob, whoreferred to the flow from a pipe with solid walls in thepresence of a spherical obstacle.

In 1929 it is set up the Laboratory ofHydrodynamic Machines, Timisoara, being the firstone of its kind in our country. Here there were donethe first researches on cavitation by the professorAurel Barglazan, correspondent member of RomanianAcademy. In 1940 Aurel Barglazan presents his PhDthesis to Polytechnic School, Timisoara, with thesubject “ The Hydraulic Transformator”. It is the firstPhD thesis presented in hydraulics in our country.Presently, within LMHT, led by acad. Ioan Anton,provided with over 30 research stations, there aredeveloped researches in hydraulic turbines, pumpsand vents, wind turbines and turbo-transmissions.

The cavitation phenomenon is studied incavitation tunnels of the lab and the materialbehaviour at cavitation destruction in differentenvironmental conditions and different liquids arestudied under the guidance of the researcher A. F.Kuzman on magnetostriction installation atcompatible parameters, as required by USA standards.

Many PhD thesis were presented in cavitation. Inour country, in this domain, there were presented PhDthesis by the followings: PhD. dr. eng. M. Popoviciu(the evolution of cavitational bulls produced byelectricsparks); dr. eng. Gh. Baran (contributions tothe cavitational study and of the cavitational wear);professor dr. Eng. F. Gyulai (the study of thesecondary zones of cavitation from turbo-pumps).

2. The water properties

The state parameters and the water propertiesdetermine the development of the cavitationphenomenon. The water properties having influenceon cavitation, are:

Adhesion to solid surfaces is a phenomenon ofthe same state with cohesion manifesting through theattraction forces with the next particles, of a liquidand of a solid in contact state which is at molecular

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distances one from the other. The adhesion forcesdepend on the nature of the surface, the solid nature,liquid nature, and temperature.

The compressibility is the property of a liquid tooppose resistance when changing its volume.Numerically, the compressibility is characterised bythe cubic compressibility factor β. For a liquidvolume V, compressed with ∆ p at a constanttemperature, it comes up with ∆V/V=-β∆p . With thenote ε = 1/β (where ε is the modulus of resilience ofthe liquid), it can be written:

1 Ddp dp

dV d

ρε = = − =β ρ

The modulus of resilience and of compressibilityvaries with the temperature. For water the variation ispresented in table 1.

Superficial tension. A liquid mass is in balance ifthe superficial energy is minimum. The superficialenergy is given by the expression W A= σ where σ isa superficial tension and A is the area of the freesurface of the liquid. The value of the super-ficialtension drops with the temperature is given in table 1.

Gas absorption The liquids have the property toabsorb thegases with which come in contact. Theabsorption grows with pressure growth. In the case ofwater, at usual temperature and a normal atmospherepressure the air content is equal to 2% of its volume.

Gas desorption It is a process conversely toabsorption. It is produced when the concentration ofthe absorbed phase in a dissolved state is bigger thanthe balance concentration, adequate to the givenpressure and temperature. The desorption rises withthe temperature drop. For a given temperature there isa pressure pv for which it is a biphasic state with aninstable balance; a small increase of pressureproduces an homogeneous liquid phase, and a verysmall drop of pressure produces an homogenous gasphase. The pressure noted with pv is called vaporisingpressure, and its variation depends on the temperature,(for water, it is given in table 2).

3. The initiation of cavitation

If in certain parts of a moving liquid, thepressure drops until the value of vaporising pressureto a given temperature, it is produced the liquidvaporisation followed by the dissolved gas’desorption. Within the liquid it shows the tendency of

bubbles formation filled with vapours and gases. Theappearance and evolution of these cavitationsconstitutes a complex phenomenon called cavitation.

The formed cavitations within the moving liquid(at pressure pv) are developing and become almost –spherical. They can be changed by liquid in a regionwith higher pressure than the vaporising pressure thatis practically the pressure within the bubbles. It isnoticed than a sudden inrush of the cavitation’s wallstowards their interior. This process, called implosion,is followed by a complex physical and chemicalphenomenon, still less cleared up, but having impor-tant effects on the solid walls which enclosed themoving liquid in a certain zone. First, the implosion isfollowed by the appearance of very high pressures,because of the fast diminishing of the cavitation volu-me. If the implosion is produced near a solid wall, thepressure is made directly on the wall; if the implosionis produced at some distance of the wall, the pressureis transmitted under the form of pressure waves. It hasbeen noticed that the inrush of the bubbles isproduced without following the spherical symmetryand micro jets appear which forces the solid walls.

The mechanical effects of the implosion areextremely powerful, fact which results from that noknown material resist to cavitation. Besides themechanical phenomenon, which are prevailing,regarding the implosion, there are thermalphenomenon, due to cavitations gas compressing aswell as chemical phenomenon due to corrosivesubstances. There were noticed electrical phenomena,too. Always cavitation is followed by strong noisesand vibrations. The solid surfaces placed in theimplosions producing zones presents, after a timeinterval, phenomena under the form of contourcaverns of irregular depths.

4. Cavitation as wear

Taking into account the phenomena and theprocesses of the superficial layers and the relativemovement of the bodies in contact, Kostetki [1] andAyel [2] defined the following types of wear:adhesion wear, thermal wear, abrasive wear, oxidationwear out and fatigue wear out. This classification isregarding only the solid bodies in contact.

Table 1t [0C] 0 10 20 30

910− ε [N/m2] 1.95 2.03 2.11 2.15

1010 β [m2/N] 5.12 4.92 4.74 4.66

σ [N/m2] 0.0755 0.0741 0.0726 0.0711Table 2

t [0C] 0 10 20 30 40 60 80 100Pv/γ [m] 0.062 0.152 0.238 0.433 0.752 2.031 4.828 10.33

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When a fluid in moving restraint by solid borders,a phenomenon appears that is generally called mate-rial erosion. By erosion it is generally understood [3]the destroying of an object surface and dislocation ofmaterial due to almost mechanical loads. The aboveclassification it could be completed with a few typesof wear considered as aspects of the erosion process.

Here are highlighted three such aspects: cavita-tion wear (including the impact wear due to liquidjet), hydro-abrasive wear and gas abrasive wear. Inalmost cases the mechanical requests are under theform of multiple impacts produced by the action of aliquid on a solid surface or on solid particles (being insuspension within a liquid), on a solid surface.

In all the cases, the material behaviour dependson the loads given by the collapse of cavitationbubbles, the shock of a liquid drop or of a jet, theimpact of the solid particles being in suspension in afluid. The effects of these loads depend on theintensity of the impact and on the number of itsrepeated action. Generally, the erosion problem ofmaterials has two important aspects. A first one is theunderstanding of the baffle conditions for eachmaterial, meaning the conditions in which the impacttensions reach a final value capable to initiate erosionmeasurable after the first shock or after repeatedshocks. A second aspect is the prediction of thedestructions, qualitatively speaking, if they areovercome the baffle conditions.

Taking into account the preponderance within thesuperficial layers of the mechanical phenomena andprocesses, similar to the above-mentioned wear types,the destruction by cavitation is also a sort of wear.The cavitation wear is one of the major engineeringproblems with which the designers of hydrodynamicssystems, marine propellers, rudders, supersonicsounder, plain friction bearing, pumps, taps, coolingsystems for internal – combustion engines, momentconverter, are confronted, from the researchesregarding the lab tests and the experience intechniques of protection.

5. Industrial cavitation tribosystems

The scientific researches of the past decades,using work techniques of the most improved ones, aswell as practical observations, have shown that thetribological phenomena and processes which developin the superficial layers of the machine elements withrelative movement, have a interdisciplinary character.The systematic study of a real case presumes asynthesis of knowledge from domains such as:mechanics, physics, chemistry, physic metallurgy,thermodynamics, fluids’ mechanics, machineconstruction technology and machine dynamics.Although the problematic is very complex, theconcerning for systems’ theory applying withintribology is related first to the systematisation of dataoffered by practice. That allowed establishing some

wear types, accepted, with small differences, in theworld. It was followed too the lab modelling of thegiven data by practice for realising some researchmethodologies with general character.

In this order it comes the concerning for syste-matic studying of the machine tribological phenome-na and processes. The proposal of introducing thenotion of tribomechanics system or tribosystem ismade in [4] and the notion is standardized in DIN50320 [5]. The works of professor I. Crudu whoapproaches this subject, finalized in [6], allows a deepanalyses and a systematic study of the tribologicalprocesses not only in the case of wear phenomena butalso in industrial technological systems. Generally, atribosystem is considered to be formed by a fix triboe-lement 1, a mobile triboelement 2 (solid or liquid),and an interposing material 3 (lubricant or abrasive)and the work environment 4 (Fig. 1). Considering thetype of relative movement between the two triboele-ments 1 and 2, and the nature of the interposing ma-terial, than it may be differenced [7], [8], [10]: tribo-systems of sliding or sliding with rolling TA, tribo-systems of rolling or rolling with sliding TR, abrasivetribosystems TZ, cavitation tribosystems TC. As youcan notice in figure 1, the cavitational tribosystemscan be: hydrodynamic cavi-tation tribosystems TCh,cavitation tribosystems with impact jet Tci, vibrationcavitation tribosystems TCv. From manyexperimental data, it can be asserted that for everytype of tribosystem corresponds a given wear law.

Fig.1The type of wear that appears, is determined by

the nature of the prevalent process (mechanical,thermal, chemical) within the superficial layers of themachine elements that compound the tribosystem.The type of wear depends on the work parameters(forces, speeds, materials) as well as on the workenvironment (characterized by temperature, pressure,electro-thermal action).

In practice, the exploiting time of a tribosystem,in most of the cases, is dictated by the maximumadmissive wear of the machine elements and scarcelythis is taken on the account of mechanical action. Thedurability of a tribosystem depends on the exteriorparameters (design and exploitation ones), parametersof the superficial layer (micro geometry, metallurgicalcharacteristics) and tribosystems’ parameters (motiontype and magnitude, the thickness or the quantity ofthe disposed material, contact flow). Thecharacteristic parameters of a cavitation tribosystemare indicated in figure 2. The model of a tribosystemincludes as inputs and outputs, the parameters of the

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superficial layer Ss and tribosystem parameters Cr,and command external parameters E.

Figure 3 presentes the model for the tribosystemTC and the incubation phase. For tribomodelling, theequation that describes the evolution of thetribomodel in time, will involve, generally, theexternal parameters so that, for a pre-establishedtesting period to produce a wear law similar for thestandard one for the actual tribosystem.

6. Cavitation tribomodels

The laboratory research methods involve theartificial creation of cavitation or of some phenomena

and processes having similar effects on the naturalcavitation. The cavitation tribosystems used inlaboratory are made for studying the destructions ondifferent materials and are called cavitationtribomodels. The intensity of destruction on tribomo-dels is higher than on actual cavitation tribosystems.

For reproducing in a laboratory the cavitationdestruction based on flowing cavitation, thereare usedtribomodels presented in figure 4a., marked with TCh.

Tribosystems are studied on TCv tribomodels aspresented in figure 4c, and for destruction made intribosystems with impact liquid jet is used the Tcitribomodel as presented in figure 4b.

Cavitation germsThe temperature of work environmentThe pressure of work envornmentChemical composition of work environmentThe viscidity of work environment

Characteristic parametersof work environment

The air content of work environmentDimenssionsConstructive parametersFormCinnemetical parametersEnergetical parametersProtection systmes

Exploitationparameters

Exploitation parameters

External tensions

HardnessResilience

( )1k −σ

Mechanicalcharacteristics

; ; %r cσ σ δTensions of order IITension regulator and

deformings Tensions of order III

StructureChemical composition

Metallurgicalcharacteristics

Purity

The parameters of thesuperficial layer

Harshity

j=1….m ; where m is the number of mechanical characteristics

Incubation time( )m f t∆ =

( )m f ttδ∆ =∆MDP

Wear

( )max

mt

δ∆∆

Phisical phenomena

Tribosystems’parameters

Connective effectsChemical phenomena

Fig. 2

Fig.3

U1

U2

.

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.

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.

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.Ue

Xcj

.

.

.

.

.

.

.

.

.

.

Xk

.

.

.Xp

.

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.Xn

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Fig. 4 Fig.5

a) b)

Fig. 6

Fig.7.a Fig. 7b Fig. 8

The cavitation tribomodel with hydrodynamictube, realised in the research laboratory of MachineDesign Department, at “Dunarea de Jos” UniversityGalati is presented in figure 5. The tribomodel isdesignated to the research of cavitation phenomenonin the conditions of a hydrodynamic current. Theinstallation contains the following elements:

- electro pump with electric engine actuator of7.5 kW and the revolution speed of 2900 rot/min;centrifugal pump with 200 l/min flow and themaximum pressure of 80 meter of water column;

- main tank with a capacity of 1200 l of waterand the pressure of 2.5 daN/cm2 ; - buffer tank with capacity of 130 l (havingopenings and sight);

- workroom, with a rectangle section 10x25mm.having the forms presented in figures 3 and 4; - Venturi tube with the characteristic sectionreport of 0.3;

- straight–way valve (with spigot, valve andsluice) water and air piping.

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The hydrodynamic cavitation tribomodel withrotational disk realised in the research laboratory ofMachine Design Department, at “Dunarea de Jos”University of Galati, is presented in figure 6a.

In a tight workroom a disk is rotated and on it anumber of triboelements are placed. In front of themcavitation drivers are mounted (figure 6b) containingoval holes or cylindrical bolts fixed on the disk. Themaking tight of the room at the output of the shaft onwhich is mounted the work disk is realised by asystem of rings maintained in contact trough acompression spiral spring. The sealing rings aremanufactured of antifriction material and the springis made of bronze. The driving of the main shaft ismade via a narrow trapezoida belt transmission. Theelectric engine for this tribomodel has a power of13kW and a revolution speed of 2900rot/min; inorder to diminish the current shocks when startingthe installation it is used a triangle-star switch. Tohinder the movement of the work liquid in therotation movement of the disk, it was endowed anumber of six elements of deviation on both sides ofthe rotation disk. To visualise the phenomena in theworkroom there was placed a sight. The visualisationis made by a stroboscope. The tribomodel functionsin opened water circuit, fact that allows maintainingof a constant temperature in the workroom.

The cavitation tribomodel with liquid impact jetwith closed circuit is presented in figure 7a.

Kozirev bases the functioning of the tribomodelon the proposed model: the destruction of thetriboelement surface is made by an intermittent shockbetween them and of a liquid jet. The installationcontains four fixed tubes on a disk that is driven in arotation movement by an electro engine via acompression coupling. Each tube has three systemsof evacuating the water through adjustable nozzles.On the inferior disk there are mounted the branchesfor fixing the triboelements driven in the rotationmovement, backwards than the nozzles by a separateengine (figure 7b).

The vibration cavitation tribomodel ispresented in figure 8 and contains two importantparts: the ultrasounds generator and the ultrasonicunit. The vibration cavitation appears in theconditions of oscillations of a solid body inopposition to a liquid or a liquid in opposition to asolid border. Because of the relative oscillationmovement, the pressure varies cyclically, givingsituations when are outrun the baffle conditions forinitiating cavitation (p<pv). The ultrasound generatoremits low ultrasound frequency oscillations on 18 –22 kHz, according to the recommendations from [9].The generator is compound of the following units:sinusoidal oscillator, power amplifier, final leveller,power unit. The ultrasonic unit contains a magneticamplifier, concentrator and the tested triboelement.The active element of the ultrasonic unit is themagnetic amplifier, stack of sheets type with stacks

which have four work columns. The stacks are madeout of Ni plate 99.9% by pressing. To diminish theeddy currents, the stacks have the thickness of 0.1mm and are isolated between them by an oxide layer,obtained after a thermal treatment. In order totransmit the ultrasonic energy from the magneticamplifier to triboelement, it is used a waveguideconcentrator. This transforms oscillations of lowamplitude that appears on the radiation surface of themagnetic amplifier in oscillations of high amplitudein the assembling zone of the tribomodel.

7. Conclusions

Using the concept of tribosystem formulated byprofessor Crudu [6], the conclusions are as follows:

1. The study of the cavitation wear out can bedone on three types of cavitation tribomo-dels: hydrodynamic cavitation tribomodelsTCh, vibration cavitation tribomodels TCv,impact liquid jet tribomodels TCi.

2. Tacking into account that in the superficiallayers subdued to cavitation destructionappear similar processes and phenomena tothe wear out destruction, the cavitationdestruction can be considered a type ofwear.

3. The analysis of the cavitation wear is donestarting from the cavitation tribosystemparameters (external of the superficial layerof the tribosystem), as presented bellow.

- The constructive parameters of thecavitation tribomodels met in referenceliterature have a wide diversity, fact whichhardens the possibility of comparing theresults obtained in different laboratories.

- There is no agreement on appreciatingmaterial behaviour at cavitation destruction;the researchers’ majority uphold that themost suggestive presentation of a materialbehaviour when destructed, are kinetic bowsof erosion.

The authors proposed the further objectives:- the realisation of typical cavitation

tribomodels (TCh, TCv, TCi) allowing tostudy cavitation destruction of differentmaterials,

- the characterisation of cavitationtribomodels functioning using triboelementsof soft materials,

- obtaining the wear kinetic bows for differentvalues of external parameters of atribomodel,

- the approximation of wear kinetic bowsusing adjusting Spline functions. Althoughthe majority of the researchers use the wearsout kinetic bows, they haven’t yet succeededtheir transposable through mathematicalfunctions,

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- the investigation of possible tribomodelsparameters’ influence by applying theexperiment projection method. The majorityof the researchers analyse the singularinfluence of some parameters of cavitationtribomodels. On the basis of the facilitiesoffered by the experiment projection’smethod, is the possibility of pointing out thesimultaneous influence of many parametersand mathematical modelling of thedestruction process,

- The analysis of the cavitation destruction fora large scale of materials. Taking intoaccount that in our country, it is developinga large atomic technique, the cryogenictechnique and that in shipbuilding, it isseriously considering the replacement of thedeficit materials for the propellers, it isnecessary a thoroughgoing study of nativematerials from the point of view ofresistance to cavitation destruction.

REFERENCES

1. Kosteski B.I.- Incercarea metalelor la uzură. IDT,1957

2. Ayel J. – Les diferentes formes tribologique d’usure de surfacemetallique. Publ. Inst. Franc. Petrole, Coll. Colloq. EtSemin.,p.91,1979

3. Crudu I., Bologa O. - Cavitaţia un tip de uzură. Simp. Tehn.,Gheorgheni, 1979

4. Czichos H. – Tribology. Elsevier Scientific Publishing, NewYork, 1980

5. *** - ANSI/ASTM Norm G 40 – 77, Standard method ofvibratory cavitation erosion

6. Crudu I. – On the concept of tribo-System and aTribomodelling Criteria. Euro-trib 85, Lyon

7. Crudu I – Incercarea metalelor la uzură. In “ Incercareamaterialelor” vol.1. Ed. Tehnică, Bucureşti, 1982

8. Crudu I ., Bologa O. – Asupra posibilităţii modelăriiîncercărilor de uzură pe calculator. Construcţia de Maşini,Nr.11, Bucureşti, 1980

9.*** - DIN 50320 Vershleiss. Dezember,1979,Berlin, Beuth-Verlag.

10.Bologa O. – Contribuţii la studiul uzurii în tribosistemele decavitaţie. Teză de doctorat. Galaţi,1986

11.Yamaguchi, A., Kazama, T. , Wang, X.: Evaluation ofErosion-Resisting Properties of Plastics and Metals UsingCavitating Jet Apparatus, IFPE 2002 Hydraulic FluidsSymposium, (2002).

12.Yamaguchi, A., Kazama, T., Inoue, K. , Onoue, J.:Comparison of Cavitation Erosion Test Results BetweenVibratory and Cavitating Jet Methods, International Journalof Fluid Power, 2-1 (2001), 25/30.

13.Kazama, T. ,Yamaguchi, A.: Effects of Configuration ofNozzles, Outlets of Nozzles and Specimens on Erosion due toImpingement of Cavitating Jet, Proc. 48th NationalConference on Fluid Power, (2000), 263/272.