S. M. Ahmed

Scanning electron microscopy observation on the incubation period of vibratory cavitation erosion

Abstract The behaviour of the vibratory cavitation erosion on typical machinery material, SUS 304 stainless steel, is investigated using a scanning electron microscope and a profilometer. The tests are carried out under conditions of uniform cavitation nuclei size distribution. It is observed that the polishing Unes act as weak points for the formation of pits. The diameter and the shape of these pits do not change with the test time (t

Fatigue failure of SUS 304 caused by vibratory cavitation erosion

Abstract The mechanism of cavitation erosion is investigated in detail for SUS 304 stainless steel, a typical erosion-resistant material. Systematic observations of eroded surfaces and dislodged particles show that the predominant failure mode in cavitation erosion is fatigue. Characteristic features of such failures include (1) packets of slip bands, (2) cracks that nucleate at regions of high stress (strain) concentration, (3) cracks that propagate first at an inclination to the surface and then parallel to the surface, and (4) fatigue striations and ‘tire tracks’ on the rupture surfaces. These last two features are the most indicative of a fatigue process. It is also deduced that massive voids accompanying pressure shock-rings are the most erosive kind of cavitation.

Investigation of the temperature effects on induced impact pressure and cavitation erosion

Abstract The effects of temperature on the impact pressure, the erosion of pure aluminum (Al-99.999) and the cavitation aspects developed by vibratory cavitation erosion are investigated. It was found that the impact pressure strongly depends on temperature, as well as on the radial distance. The pressure is higher for lower temperature especially in the central region. The level of deformation differs with the temperature, and it is higher for lower temperature. The characteristic features of the eroded surfaces reveal that the predominant erosion mechanism of pure aluminum is fatigue, irrespective of temperature. It is also observed that the number of bubbles is larger for higher temperature.

Investigation Into Cavitation Erosion Pits

Cavitation erosion pits and their effects on erosion progression were investigated in detail for SUS 304 stainless steel, α + β brass (60/40), and pure aluminum (Al-99.999 and Al-99.92) by means of vibratory erosion. Two kinds of erosion pits were found on the specimen surfaces, one by microjet impact and the other by shockwave blow. Systematic observations of the feature of microjet-pits with the testing time showed that the sizes and shapes of microjet-pits did not change at all and such pits scarcely played an important role in developing the erosion. Moreover, the feature morphology of eroded surfaces, and dislodged particles and their large sizes revealed that microjet-pits had a limited effect on erosion and that the predominant failure was a fatigue process.

Design and Performance of Slurry Erosion Tester

A slurry whirling arm erosion test ring was constructed and a series of erosion tests andpost-erosion analysis were carried out using a paint erosion indication technique. Thepattern of the paint removal presented a highly visual and accelerated map for theerosion process and its behavior. Also, the erosion rate of paint removal was investigatedunder a number of erosion variables. It was observed that the rebounding of the erodentparticles from the sample surface play an important role in developing erosion for thistester. The erosion pattern showed that the effect of the rebound particles depends on theimpact velocity and impingement angle. It was also observed that the erosion behavior ofpaint as a function of impingement angle, impact velocity, and erosion time was similarto that reported in literature for engineering materials. The slurry whirling arm erosiontester seems to be promising for simulating the slurry process in real cases.

Cavitation Erosion Mechanism Based on Analysis of Erosion Particles

The mechanism of cavitation erosion is investigated in detail through observations of the removed particles from pure aluminum (Al-99.999). The particles removed during the incubation stage have distinctive characteristics compared to those removed in the subsequent stages. These characteristics are distinguished by lamellae structure, foldness, waviness, large size as well as one of the particle surfaces appeared to be part of the original face of the specimen. Therefore, these characteristics can be used as a monitor for early detection of cavitation erosion in closed systems. Systematic observations of disloged particles and eroded surface morphology show that the dominant failure mode in cavitation erosion is fatigue.

Investigation of Temperature Effects on Cavitation Erosion Behavior Based on Analysis of Erosion Particles

The effect of temperature on the wear particles produced by vibratory cavitation erosion tests on Al-99.92 in distilled water was analyzed. Scanning electron microscope images of wear particles were obtained, forming a database for further analysis. This study showed that the variation of average particle size with temperature was very much similar to the variation of weight loss with temperature. The average particle size was maximum at 40°C. It was also observed that the average particle size was time dependent. The particles morphology features revealed that the predominant erosion mechanism was fatigue failure, irrespective of the temperature.

Quantitative Analysis of Cavitation Erosion Particle Morphology in Dilute Emulsions

Wear particles produced by vibratory cavitation erosion tests on 1017 carbon steels in water and oil-in-water (o/w) emulsions were analyzed. Scanning electron microscope (SEM) images of wear particles were acquired, forming a database for further analysis. The particle morphology features were first clarified. Next, the size parameters (size, area, and perimeter) arul shape factors (elongation and roundness) were determined for each test liquid, using image analysis software. The size parameters of the removed particles were higher in water than in o/w emulsions. While the shape factors could not significantly discriminate between the particles produced in water and o/w emulsions. The size distribution was in a wide range for water than that for o/w emulsions. The cavitation erosion mechanism is fatigue failure for water and o/w emulsions.

Characteristics of Cavitation Erosion Using Image Processing Techniques

This paper deals with the analysis of eroded surfaces obtained from cavitation-erosion experiments on stainless steel in water and oil-in-water (o/w) emulsions using image processing. Two analysis techniques that are very promising in this respect are the wavelet decomposition transform and fractals. These can be used to extract parameters that characterize the cavitation intensity in a similar manner to that of the mean depth of erosion (MDP). The extracted parameters are the wavelet energy and entropy as well as the fractal dimension. Both of the image feature parameters and the MDP decrease with adding oil to water. Also, it was found that the variation of image feature parameters versus concentration of oil-in-water emulsions has a general trend that does not depend on magnification factor. The cavitation erosion behavior and mechanism for water and o/w emulsions were analyzed and it was found that the predominant failure mode was fatigue for water and o/w emulsions. The results show that a corrosive effect appears at 5 and 10 wt% o/w emulsions.

Fractal Characterization of Slurry Eroded Surfaces at Different Impact Angles

In the present work, the topographical images of slurry erosion surfaces at different impact angles were quantified using fractal analysis. The study showed that the variation of fractal value of slope of linearized power spectral density with the impact angle is largely similar to the relationship between the erosion rate and the impact angle. Both the fractal value and erosion rate were maximum at 45 deg and 90 deg for ductile and brittle materials, respectively. It was found also that the variation of fractal values versus the impact angle has a general trend that does not depend on magnification factor. The fractal features to the eroded surfaces along different directions showed high directionality at oblique impact angle and were symmetrical at normal impact.