Tsunenori Okada

Relation between impact load and the damage produced by cavitation bubble collapse

Abstract In order to study the relation between impact load by collapsing cavitation bubbles and erosion damage, a new pressure detector which can measure impact loads and erosion damage (size of indents or pits and volume loss) simultaneously was developed. The detector consisting of a test specimen with a diameter of 3 mm and a piezoelectric ceramic disk was used for venturi tests and vibratory tests. Impact loads by collapsing bubbles were observed directly in a venturi tunnel and as piled-up pulses on an alternate wave of hydrodynamic pressure produced by a vibrating disk in a vibratory device. Peaks of waves were held and measured before arrival of the reset pulses with an interval of 100 μ. The distribution of impact loads was computed after A/D conversion of the peak hold waves. Impact load occurring in an early stage in the venturi facility and vibratory device were compared with indent size which were observed with a microscope on the surfaces of pure aluminum, pure copper and austenitic stainless steel (SUS304). A linear relation was obtained between impact load and the area of indents in both the venturi and vibratory tests. There exists a linear relation between the dynamic hardness under impact loads due to cavitation bubble collapse and critical impact loads for fcc metals, but bcc metals deviate from this relation. The plastic energy required to form a pit of aluminum and copper under impact load is about one order larger than that under static load. The energy based on impact loads was compared with the volume loss of the specimen. It is concluded that a linear relation was obtained between the accumulated impact energy and the volume loss irrespective of test conditions such as type of apparatus, flow velocity for venturi test and the distance between a vibrating disk and specimen for vibratory test.

Quantitative Evaluation of Cavitation Erosion

In order to evaluate the quantitative cavitation-erosion resistance of materials, a pressure-detector-installed specimen was developed, which can measure both the impact load produced by cavitation bubble collapse and the volume loss simultaneously. Test specimens (pressure-detection rod) used were nine kinds of metals and were exposed to vibratory cavitation. A linear relation was obtained for all materials between the accumulated impact energy ΣFi 2 calculated from the distribution of impact loads and the volume loss, independent of test conditions. Impact energy accumulated during the incubation period and the energy for a unit material removal in steady-state period were obtained from the relation. These values are very important concerning quantitative erosion resistance evaluation. That is, when the distribution of impact loads is acquired for different cavitation conditions, the volume loss can be estimated. This idea was applied to the venturi cavitation erosion. The experimental results for venturi test corresponds well with the prediction using these impact energy values. It was concluded that the quantitative impact energy values of materials can be determined independent of the apparatus and the test condition by using the newly developed pressure-detector-installed specimen.

A study of cavitation bubble collapse pressures and erosion part 1: A method for measurement of collapse pressures

Abstract A method for measuring the distribution of the pressures at collapse in caviation bubbles, and a data acquisition system, were studied in a magnetostrictive vibratory test. Impact loads (collapse pressures) were measured by our specially developed method using a pressure detector with a piezoelectric ceramic. The distribution curves for the impact loads were compared with those for the erosion pit sizes occurring at an early stage on the surfaces of aluminium, copper and mild steel. Assuming that an individual pit is formed by a single pulse from a collapsing bubble, the impact loads necessary to form a pit 4 μm in diameter were found to be 9.1 N, 9.7 N and 13.7 N for aluminium, copper and mild steel respectively. The number of impact loads larger than that required for the initiation of an erosion pit was found to be very small.

A study of cavitation bubble collapse pressures and erosion part 2: Estimation of erosion from the distribution of bubble collapse pressures

Abstract Cavitation damage was studied both by the measurement of the collapse pressures of cavitation bubbles (impact loads), using a specially developed method, and by means of cavitation erosion tests on various metals in a vibratory facility. The magnitude of impact loads produced by the collapse of bubbles fluctuates over a wide range and there is a threshold value for the impact load which can contribute to the fatigue fracture of a surface. This value is considered to correspond to the fatigue limit of the material. Assuming the S - N curve for the fatigue in the eroded surface, Miners law of impact loads is followed for the incubation period and the reciprocal of the volume loss rate during the stable period, regardless of the cavitation conditions and the materials.

An application of bubble collapse pulse height spectra to venturi cavitation erosion of 1100-o aluminum

Abstract Venturi cavitation erosion tests were performed and correlated with bubble collapse pulse height spectra measured by a microtransducer. The effects of the throat velocity and the cavitation number σ (referred to the downstream pressure and throat velocity) on the erosion rate (MDPR) were studied. The velocity damage exponent was 4.11 for 0.62 ⩽ σ ⩽ 0.80, while the MDPR is almost independent of velocity for σ = 0.85. The MDPR decreases with increased σ for 0.62 ⩽ σ ⩽ 0.85. The data were reduced to “acoustic power” (from pulse height spectra) and “erosion power” (the ultimate resilience multiplied by the MDPR). A near-linear relationship was found between these. Their reciprocal ratio η cav ≈ 7 × 10 −11 . For σ = 0.62, the data deviated from the others, possibly because of the work hardening of the eroded surface.

A fundamental study of cavitation erosion using a magnesium oxide single crystal (intensity and distribution of bubble collapse impact loads)

A magnesium oxide single crystal has simple characteristics in regard to a slip system and reveals clear dislocation etch pits on the surface or on the cross-section. In this study, a magnesium oxide single crystal was mounted as a detection surface of impact loads on a piezoelectric pressure detector, and was exposed to vibratory cavitation. As a result, dislocation-etch-pit pattern was observed on a magnesium oxide specimen, and this crystal provided visualization of the traces of cavitation bubble collapse impact loads, i.e. how the impact loads act on the surface. It was clarified that the spatial distribution of etch pit patterns corresponds well to the frequency distribution of bubble collapse impact loads obtained from the pressure detector. Therefore, the possibility was established that cavitation damage can be evaluated from mechanical viewpoints by using this system.

Effect of temperature on the cavitation erosion of cast iron

Abstract Vibratory cavitation erosion tests of gray cast iron, together with tests of tool steel and 316 stainless steel for comparison, were performed at various water temperatures and horn amplitudes under a suppression pressure of 1 bar. The erosion processes for cast iron under the highest temperatures used (200 and 230 °F, i.e. 93 and 110 °C) are similar to those at room temperature. For each of the materials tested, the maximum weight loss rate increases, shows a peak and then decreases with increasing temperature. However, the maximum damage temperature for cast iron decreases with amplitude, i.e. 200, 170 and 160 °F (93, 77 and 71 °C) for double-horn amplitudes of 1.0 × 10 −3 , 1.38 × 10 −3 and 1.78 × 10 −3 in (25.4, 35.1 and 45.2 μm). The peak for tool steel and 316 stainless steel occurs at 160 °F (71 °C) regardless of amplitude. Liquid temperature effects for cast iron erosion were explained by considering the interrelation between corrosive action and mechanical action due to cavitation bubble collapse.

Effects of plating on cavitation erosion

Abstract To study the cavitation erosion behaviour of interfaces and base metals used for plating, magnetostrictive erosion tests were carried out in a 3% NaCl solution using a structural carbon steel specimen plated thinly with zinc, tin, chromium or nickel. When erosive attack extends to the interface between the plating and the base metal, a chromium or nickel plating, which is more noble than the base metal, forms a rough surface because of the large difference in hardness and electrochemical corrosion between the plating and the base metal, even though it shows high erosion resistance in itself. Thus the protective effects of a thin and hard plating are lost immediately. However, a zinc plating or a tin plating which is underplated with copper becomes effective because of the electrochemical reactions of the plating layer that remains on the surface outside the eroded region.

Effects of hard chromium plating on cavitation erosion

Abstract The cavitation erosion of hard chromium plating on steel was studied by vibratory erosion tests. The mass loss as a function of the exposure time curves is divided into three stages: the first stage involves erosion of the chromium itself; the base metal below the interface begins to be damaged in the second stage; the third stage occurs when large particles of the plating layer are removed. Mass loss rates in each stage are greater in a 3 wt.% NaCl solution than in ion-exchanged water because of intercrystalline fracture. The damage in each stage is also affected by the plating thickness. The plating life increases with plating thickness in ion-exchanged water. In 3 wt.% NaCl solution, however, the life is lower and does not increase with platings more than about 65μm thick.

Influence of cavitation erosion on corrosion fatigue and the effect of surface coatings on resistance

Abstract The fracture of S35C test pieces under cavitation erosion in 3% salt water occurs in the area of corrosion as in the case of corrosion fatigue without cavitation erosion. However, erosion fatigue strength decreases more than corrosion fatigue strength owing to the formation of a macrogalvanic cell between the erosion and corrosion areas. When the surface of the test piece is coated with either a less noble or a more noble metal than that of the matrix, its fatigue strength is recovered. The effects of different materials, test liquids and distances between the disc and test piece are also considered.