Cavitation erosion characteristics of poly(methyl methacrylate) in a rotating disk device

Experimental results pertaining to the initiation, dynamics and mechanism of cavitation erosion on poly(methyl methacrylate) specimens tested in a rotating disk device are described in detail. Erosion normally starts at the location nearest to the center of rotation (CR). As the exposure time to cavitation increases, additional erosion areas or sites appear away from the CR and secondary erosion (induced by eroded pits) spreads upstream and merges with the main pit. The microcracks increase in density towards the end of the incubation period and transform into macrocracks in most cases. A study of light optical photographs and scanning electron micrographs of the eroded area shows that material particles are removed from the network of cracks because of crack joining and pits indicate particle debris. Optical degradation (loss of transmittance) is observed to be greater on the back of the specimen than on the front.     

Cavitation erosion in a dilute emulsion

The cavitation erosion of mild steel, brass, and pure iron in emulsions made with distilled water and 1.0% NaCl solution was investigated using a vibratory apparatus operating at 20 kHz and 15 μm amplitude. The emulsion reduced the steady-state erosion rate of mild steel in distilled and salt water to about 80% and 30%, respectively, of the values for no emulsion, but had no effect on brass in distilled water. Changes in the microstructures of the eroded surfaces and in the pH of the solution are presented and discussed in relation to the erosion curves.     

Cavitation erosion incubation period

To evaluate the “incubation period” (IP) stage of cavitation erosion, short-duration vibratory horn tests in tap water were made on soft aluminum alloy (aluminum alloy 1100-O) and also on a much more resistant alloy (316 stainless steel). Curves of weight loss versus time, and corresponding scanning electron microscopy photomicrographs taken during the IP, are presented and discussed. The effects of horn amplitude and temperature are investigated for “open-beaker” tests. The IP for 316 stainless steel is found to be about 500 times that for aluminum alloy 1100-O for the same amplitude and temperature. This ratio can be predicted almost exactly by applying an assumed relation between MDPR_max and IP, i.e. MDPR_max^?1 = k(IP)ⁿ.Fatigue cracks and individual-blow craters were found for 316 stainless steel but only individual craters were found for aluminum alloy 1100-O, although their ductilities are approximately equal. It is found that the IP based on the eroded area only, IP_erod, is much less than the conventional IP (based on the total specimen area) if IP is based on the attainment of a given mean depth of erosion MDP.Relations between the eventual erosion rate MDPR_max and the IP are considered. It is found that IP data can often be used to predict eventual MDPR_max values according to the relation MDPR_max^?1 ∝ (IP)ⁿ where n ≈ 0.93 and n ≈ 0.95 for our vibratory and Venturi data respectively. However, different values for n have been reported in the literature. By assuming a “characteristic” erosion-time curve the time of occurrence of MDPR_max can also be estimated.It is verified that only bubble collapse stresses are important in the vibratory horn test, although specimens are vibrated under very high accelerations.     

Cavitation erosion behaviour of niobium

Cavitation erosion behaviour of niobium was investigated by means of a 20 kHz ultrasonic vibrator at peak-to-peak amplitude of 50 μm, aiming to determine the niobium potential as a material for the manufacturing of hydraulic machine components. The study was emphasized for the three first cavitation stages of the cumulative erosion–time curve. The modification of the niobium surface morphology as a function of the testing time in the incubation, acceleration, and maximum erosion rate stages was verified by SEM analysis. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bar. The study was performed for niobium samples in both the cold-worked and annealed conditions. Samples of CA-6NM martensitic stainless steel, a typical material utilized for hydraulic turbines manufacturing, were also analysed for comparison purpose. Annealing treatment of niobium decreases its hardness and increases its ductility, leading to an increase of the incubation period when compared with the cold-worked niobium. Cavitation erosion failure mechanism in niobium occurs in a sequence of events comprising the work-hardening effect and the fracture of debris allied to the effect of fatigue and microcracks formation. Finally, annealed niobium presents similar incubation period but worse behaviour in the maximum erosion rate stage than CA-6NM steel.     

Cavitation erosion size scale effects

Size scaling in cavitation erosion is a major problem confronting the design engineers of modern high speed machinery. An overview and erosion data analysis presented in this paper indicate that the size scale exponent n in the relation erosion rate ∞ (size or diameter)ⁿ can vary from 1.7 to 4.9 depending on the type of device used. There is, however, a general agreement as to the values of n if the correlations are made with constant cavitation number.     

Cavitation erosion of plasma-sprayed NiTi coatings

Ni-Ti protective layers are under development to increase the cavitation resistance of materials such as stainless steel. For the present work, layers were obtained by vacuum plasma spraying (VPS) from both elemental powder mixtures and pre-alloyed powder. Characterization was done with respect to phase formation and impurity content. Cavitation erosion resistance was examined using a standard ultrasonic cavitation test. In the coating from the pre-alloyed powder, a higher resistance due to the lower presence of non-shape memory phases was observed. The cavitation erosion resistance of pre-alloyed plasma-sprayed layers is in good agreement with the one of pore-free NiTi samples evaluated in this study and others described in the literature.     

Cavitation erosion of metals and alloys

Several high purity metals (aluminium, cobalt, chromium, copper, iron, magnesium, molybdenum, nickel, tungsten and zinc) and some commercial copper-, iron-, nickel- and cobalt-base alloys were investigated by weight loss measurements and by surface analysis with scanning electron microscopy and X-ray photoelectron spectroscopy. The results lead to the conclusion that the cavitation erosion resistance (CER) is decisively determined by the binding energy and the crystal structure of the base metal. The ability of alloys made of base metals with a high CER to deform and to transform allotropically is the second important factor.     

Cavitation erosion and rolling contact fatigue

A cavitation apparatus has been used to evaluate the fatigue-resistance of lubricants, and a good correlation is found between cavitation and laboratory fatigue tests. Both fatigue pitting and cavitation pitting appear to be due primarily to hydrogen penetration, which is aggravated by reactive gas atmospheres, chemically-active lubricants or additives and especially by water.Cavitation tests differ from laboratory fatigue tests in that anti-friction additives such as oleic acid are found to be harmful and not helpful. In this respect, cavitation may be a better predictor of fatigue performance in actual service, where sub-surface fatigue predominates.     

Cavitation erosion behaviour of Zr-based bulk metallic glasses

The cavitation erosion (CE) behaviour of four Zr-based bulk metallic glasses (BMGs) of general composition Zr–Al–CuNi(Co) was investigated by means of an ultrasonic vibration device and compared with that of S30431 austenitic stainless steel. The results show that the CE resistance of Zr-based BMGs, in terms of incubation time and maximum rate of erosion, is approximately 10 times higher than that of S30431 austenitic stainless steel. It was also found that a small addition of Co (3 at.%) decreases significantly the maximum rate of erosion of the Zr-based BMG investigated. Initial hardness of the Zr-based BMGs was found to be in a good correlation with their maximum cavitation erosion rates.     

Cavitation erosion resistance of stellite alloy weld overlays

Stellite alloys have excellent cavitation erosion resistance and are often used for liquid machinery, but the erosion properties of various stellite alloys have not been evaluated by a standard method. In this study, we evaluate the erosion resistance for various stellite alloy weld overlays of ST6 and ST21 in a vibrating method and in a cavitating liquid jet method. The grain size of the Co matrix affects the cavitation erosion resistance of stellite alloy weld overlays of ST6. The erosion rate of the maximum rate stage of stellite weld overlay alloys of ST6-1, ST6-2 and ST6-3 were found to be about 1/13 to 1/7 times that of SUS304. Moreover, we clarified the cavitation erosion mechanism of SUS304 and ST6 by scanning electron microscopy. Furthermore, by comparing the erosion behavior in a cavitating liquid jet method with that in a vibratory method, it was found that the erosion rate of the cavitating jet method and the vibratory method have a good correlation.     

转轴疲劳强度的可靠性设计

提出了采用复合应力法建立转轴的疲劳强度可靠性设计的理论,该方法与安全系数法相比计算简便,易于转轴工程可靠性设计与应用。     

A study on the effect of surface orientation on erosion wear of flat specimens moving in a solid-liquid suspension

Wear characteristics of a ductile material, namely, brass have been investigated by orienting the flat specimens at different orientations relative to the velocity direction in a pot tester containing a solid-liquid suspension. The erosion behavior has been studied at various orientation angles, defined as the angle between the tangent to the plane surface and its velocity. Results at different orientation angles show that the wear at any orientation angle increases with increase in velocity and particle size but decreases with increase in solid concentration. It is also seen that the wear at various operating conditions increases with increase in the orientation angle till 30° attaining the maximum value and then decreases up to 90°. It is further observed that the maximum wear is around 3-4.5 times higher than the surface wear measured at 90° orientation angle.     

Cavitation erosion of NiAl-bronze layers generated by friction surfacing

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material.     

Using nonlinear springs to reduce the whirling of a rotating shaft

Vibrations in rotating machinery cause many problems such as fatigue of the rotating components, excessive noise, or transmission of vibration to the supporting structure. A major source of this vibration is out-of-balance forces and this paper proposes that the rotor response is reduced by suspending the machine on nonlinear springs. In the field of vibration isolation, nonlinear mounts have been proposed which have the same static stiffness as an equivalent linear support, i.e. load bearing capability, but at the same time offer a low dynamic stiffness, i.e. a lower natural frequency. Thus the isolator is effective over an increased frequency range. These mounts are known in the literature as high-static-low-dynamic-stiffness (HSLDS) mechanisms. In this paper, the rotor is suspended on a hardening HSLDS spring to considerably reduce the critical speeds to values far away from the operating speed. The advantages of the nonlinear supports are demonstrated using a simple two degree of freedom rotating machine model consisting of a rigid disk, and shafts, bearings and supports that are flexible but have negligible mass. Following a linear analysis to highlight the benefits of a low dynamic stiffness, an approximate analytical solution of the nonlinear equation of motion is presented. A comparison between the linear and nonlinear response shows the effectiveness of the nonlinear supports. Finally, the problems that occur if the nonlinearity is too strong are highlighted.     

Inverse theory of elasticity problem of mounting a disk on a rotating shaft

A theoretical analysis of the determination of the tightness of mounting an elastic disk on a rotating shaft ensuring the absence of stress concentration on the inner rim of the disk, i.e., constant normal circular (optimal) stress, is performed on the basis of the equal-strength principle. The value of optimal constant stress is determined upon solution of the optimization problem. A closed system of algebraic equations providing the possibility of solving the optimal design problem is constructed. The found mounting tightness ensures an increase in disk bearing capacity.     

Cavitation erosion of Ti–Ni base shape memory alloys

Cavitation erosion tests were carried out by using Ti–Ni base SMAs (shape memory alloys) with the addition of third elements (Co, Fe, V and Cu) to Ti–Ni SMA. The erosion resistance and its mechanism are discussed. Erosion resistance of Ti–Ni base SMAs is about 1/4 to 1/2 as compared with that of Ti–Ni SMA, but it is about 6–16 times higher than that of SUS304. It was found that the erosion resistance of the martensite phase is superior to that of the austenite phase. We conclude that the erosion resistance of Ti–Ni base SMAs mainly depends on the defect density and the removal rate at the eroded area.     

Cavitation erosion of aluminum matrix sintered composite with AlN dispersoids

The cavitation erosion resistance of P/M aluminum alloy-sintered composite with AlN dispersoids, prepared via the in situ synthesis and the conventional premixing process, was evaluated by using magnetostrictive-vibration type equipment. In situ synthesized AlN particles were effective for the improvement of the erosion resistance of the composite because of their good bonding with the aluminum matrix. The additive AlN by the premixing process were easily detached from the specimen surface due to the insufficient coherence with the matrix, and caused the poor resistance. The cavitation resistance also depended on the porosity of the sintered composite. The continuously opened pores accelerated the wear phenomena by the cavitation due to the high-pressure attack on the primary particle boundaries of sintered materials in the collapse of the bubbles.     

Cavitation erosion of aluminum considering bubble collapse,pulse height spectra and cavitation erosion efficiency

Water erosion data on 1100-0 aluminum specimens obtained using a cavitating venturi are compared with bubble collapse pulse height spectra measured using a microtransducer. The data are resolved into erosion power and acoustic power. The former is defined in terms of the power applied to the eroded material to cause the observed pitting and volume loss. The ratio between these power quantities is termed the cavitation erosion efficiency η incav and is found to be essentially constant for the range of tests, being approximately 1.4 × 10^?6. The acoustic power which is easily measured can then be used to estimate the eventual material volume erosion rates, i.e. the mean depth of penetration (MDPR), with much greater accuracy than is otherwise possible. The MDPR is measured directly from the weight loss and is calculated from individual pit counts on damaged surfaces. The effects of the degree of cavitation (the extent of the cavitation cloud or the cavitation number) and the throat velocity on the MDPR is examined. An overall velocity damage exponent of n = 4.75 is found.     

Cavitation enhancement of silt erosion—An envisaged micro model

Cavitation and silt erosion often co-exist causing severe damage. The effect that the silt erosion is worsened by the presence of cavitation is here referred as cavitation enhancement of silt erosion. Its mechanism is still not fully understood despite the effort made in past decades. In this article, the need for a micro model is firstly demonstrated by reviewing the phenomenon from the viewpoint of fluid–particle interaction. Then, the postulated model is presented from the viewpoint of a driving force. The potential of this model in furthering our understanding is also demonstrated by re-visiting Sato's experimental study on synergetic erosion. Further development of this model through experimental and numerical simulations is thus recommended.     

Dynamic stability of a rotating shaft embedded in an isotropic winkler-type foundation

The dynamic instability of a rotating shaft subjected to axial periodic forces and embedded in an isotropic, Winkler-type elastic foundation is studied by the finite element technique. The equations of motion for such a rotating shaft element are formulated using deformation shape functions developed from the Timoshenko beam theory. The effects of translational and rotatory inertia, gyroscopic moments, bending and shear deformation are included in the mathematical model. The numerical results show that the elastic foundation can shift the regions of dynamic instability away from the dynamic load factor axis and thus reduces the sizes of these regions, whereas the effect of gryoscopic moments is to shift the boundaries of the regions of dynamic instability outwardly and, therefore, increases the sizes of these regions.