Essay Example on Cavitation erosion of hydraulic machines









As the speed of hydraulic machines increases these machines are more prone to cavitation erosion that reduces the machine performance and the lifetime Cavitation erosion is defined as the formation and subsequent collapse of cavities or bubbles that contain vapor or mixture of vapor and gas within a liquid 1 The collapsing of the bubble will produce highly localized large amplitude shock waves and or microjets that hit the solid surface in contact with the liquid Accumulation of this loading due to repeated collapses causes local surface damage The early features of the damage are pit formation and surface deformation These features rapidly develop and lead to a loss in the surface material This process of development is still controversial or ambiguous A vibratory cavitation erosion test is widely used in studying the erosion mechanisms 2 and factors influencing the erosion damage such as liquid properties test conditions It is well recognized that the rate of erosion is time dependent where this rate increases from trivial values reaches a maximum then decreases and levels off to a steady value 3 Therefore cavitation erosion rate of a material can be divided into four stages according to ASTM standards G 32 1 an incubation period an acceleration period a steady state period and attenuation period 

The initial period i e the incubation period has gained extensive researches because of the onset of distinctive features of the cavitation erosion process and because these features are isolated from one another making it easier to identify the corrosion mechanism These features are plastic deformation pit formation surface hardening and the spread of cracks which control material removal often occurs in successive periods Based on examination of eroded surface topography and morphology of dislodged particles from different materials cavitated in different liquids Ahmed et al 4 7 Abouel kasem et al 8 11 and Saleh et al 12 have found that the plastic deformation development and the surface hardening lead to the fatigue failure And that fatigue failure is the mechanism prevailing in the removal of material in cavitation erosion The formation of a pit its characteristics its role in the development of erosion has been the focus of researchers e g 13 17 because it is used as a measure to predict the erosion intensity 18 21 However pit formation is generally recognized to be a complex process The pit features such as the shape depth size and appearance will depend on the nature of the forces generating that pit which depend on the method used to generate the cavitation 20 In vibratory erosion tests however pit formation remains controversial among many investigators 17 22 24 partially because the test conditions and the mechanical properties of test materials and their corrosion resistance are somewhat different from each other The cavitation pits generated by vibratory erosion test and their effects on erosion progression for SUS304 stainless steel brass 60 40 and pure aluminum Al 99 999 and Al 99 92 are clarified by Abouel Kasem et al 14 In this study two distinctive kinds of pits formed on the specimen surface have been observed one by micro jet impact and the other by shockwave blow

They added that stepwise erosion of the feature of microjet pits showed that the sizes and shapes of microjet pits did not change at all and such pits scarcely played an important role in the developing erosion When a ductile material exposed to cavitation it is locally deformed plastically The plastic deformation is in the form of persistent slip bands PSBs PSB is one of the earliest manifestation of fatigue The favorite sites for initiation cracks are the interface of PSBs with the matrix With increasing the exposure to the cavitation the cracks are propagated along the PSBs giving rise to the falling of particles Then the cavitation damage grow through the joining of the local damage events which resulted in removal of large areas a few hundred microns in size However the progress of the damage process is still vague In the present study the stepwise erosion combined with relocation SEM was used to pursue the cavitation damage growth processes in considerable detail Based upon the SEM images the joining of the damage sites was analyzed As well the material removal processes were investigated 2 Experimental set up Cavitation erosion experiments were carried out using vibratory apparatus according to ASTM standard G32 06 22 The schematic diagram of test apparatus is shown in Fig 1 The device consists of a quartz transducer oscillating at a resonant frequency of 20 kHz and amplitude of 50 µm attached to an exponential shaped prop generally termed the horn which amplifies the amplitude of vibration The Test specimens were cut from single round rod of austenitic stainless steel SUS 304 to ensure metallurgical uniformity

The specimens were in form of flat surfaced discs of 15 9 mm in diameter and 11 mm in height with threaded sections for attachment to the cavitation vibratory horn The initial roughness of the samples is recognized to be of great importance in the development of damage features such as the creation of pits 4 the facilitation of crack initiation 5 the development of fracture and the increased rate of linear particle removal 5 The samples were polished with grade 3000 dry emery paper Samples were submerged 1000 ml open glass beaker containing 700 ml of tap water Since the test liquid temperature significantly affects the degree of cavitation erosion the test water temperature in the beaker was kept constant using the temperature controlled water bath as shown in Fig 1 The test water temperature was 22 1 C Two specimens were tested at successive intervals of 5 10 15 45 min Such short experimental time i e elapsed time is within 5 min was scheduled to reduce other damage effect on the surface such as corrosion

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