Microjet Impact Research Articles

Modeling of cavitation peening: Jet, bubble growth and collapse, micro-jet and residual stresses

Surface treatment methods like shot peening are used to introduce compressive residual stresses in metals and alloys. The presence of compressive stresses prevent the initiation and growth of cracks and hence improve the fatigue life of mechanical parts. Cavitation peening is a similar surface treatment process which acts by the generation of cavitating bubbles in front of the workpiece surface. The modeling of this process is challenging because of the complexity of cavitation phenomenon. The main difficulty is the prediction of the mechanical loading at the surface of the material. One of the key process parameters is the pressure within the reservoir where cavitation is generated. A strategy based on the modeling of a single cavitating bubble is proposed. Spherical and aspherical collapse of bubbles close to a solid surface are studied first analytically and then numerically. These two sources of mechanical loading are compared and it is shown that the pressure pulse impact due to the collapse of a spherical bubble near a wall is much greater than the one resulting from a micro-jet impact. A comparison with experimental data is made and it is found that the magnitude of the residual stresses within the treated material produced by the collapse of a single bubble matches. However the impact of one pressure wave is not sufficient to represent the entire process because of the low compressive depth generated. Finally, a macroscopic cavitation model has been derived by associating the numerical simulation of the cavitating jet ahead of the nozzle and the localization of the cavitation zone. A good overall agreement is found between the residual stress profile calculated with the proposed model and the experimental data.

Analysis of the effect of impact of near-wall acoustic bubble collapse micro-jet on Al 1060

The bubble collapse near a wall will generate strong micro-jet in a liquid environment under ultrasonic field. To explore the effect of the impact of near-wall acoustic bubble collapse micro-jet on an aluminum 1060 sheet, the cavitation threshold formula and micro-jet velocity formula were first proposed. Then the Johnson-Cook rate correlation material constitutive model was considered, and a three-dimensional fluid-solid coupling model of micro-jet impact on a wall was established and analyzed. Finally, to validate the model, ultrasonic cavitation test and inversion analysis based on the theory of spherical indentation test were conducted. The results show that cavitation occurs significantly in the liquid under ultrasonic field, as the applied ultrasonic pressure amplitude is much larger than liquid cavitation threshold. Micro pits appear on the material surface under the impact of micro-jet. Pit depth is determined by both micro-jet velocity and micro-jet diameter, and increases with their increase. Pit diameter is mainly related to the micro-jet diameter and dp/dj≈0.95–1.2, while pit’s diameter-to-depth ratio is mainly negatively correlated with the micro-jet velocity. Wall pressure distribution is mostly symmetric and its maximum appears on the edge of micro-jet impingement. Obviously, the greater the micro-jet velocity is, the greater the wall pressure is. Micro pits formed after the impact of micro-jet on aluminum 1060 surface were assessed by ultrasonic cavitation test. Inversion analysis results indicate that equivalent stress, equivalent strain of the pit and impact strength, and velocity of the micro-jet are closely related with pit’s diameter-to-depth ratio. For the pit’s diameter-to-depth ratio of 16–68, the corresponding micro-jet velocity calculated is 310–370m/s.

Incubation pit analysis and calculation of the hydrodynamic impact pressure from the implosion of an acoustic cavitation bubble

An experimental study to evaluate cavitation bubble dynamics is conducted. The aim is to predict the magnitude and statistical distribution of hydrodynamic impact pressure generated from the implosion of various individual acoustic cavitation bubbles near to a rigid boundary, considering geometrical features of the pitted area.A steel sample was subjected to cavitation impacts by an ultrasonic transducer with a 5mm diameter probe. The pitted surface was then examined using high-precision 3D optical interferometer techniques. Only the incubation period where surface is plastically deformed without material loss is taken into account. The exposure time was adjusted in the range of 3–60s to avoid pit overlapping and a special procedure for pit analysis and characterisation was then followed. Moreover, a high-speed camera device was deployed to capture the implosion mechanisms of cavitation bubbles near to the surface.The geometrical characteristics of single incubation pits as well as pit clusters were studied and their deformation patterns were compared. Consequently, a reverse engineering approach was applied in order the hydrodynamic impact pressure from the implosion of an individual cavitation bubble to be determined. The characteristic parameters of the cavitation implosion process such as hydrodynamic impact pressure and liquid micro-jet impact velocity as well as the hydrodynamic severity of the cavitation impacts were quantified. It was found that the length of the hypotenuse of the orthographic projections from the center of the pit, which basically represents the deformed area of the pit, increases with the hydrodynamic impact aggressiveness in a linear rate. Majority of the hydrodynamic impacts were in the range of 0.4–1GPa while the corresponding micro-jet velocities were found to be in the range of 200–700m/s. Outcomes of this study, contribute to further understanding the cavitation intensity from the implosion of acoustically generated bubbles and could certainly represent a significant step towards developing more accurate cavitation models.

Crystal formation and growth mechanism of inorganic nanomaterials in sonochemical syntheses

A clear understanding of the nucleation, growth, coarsening, and aggregation processes of nanomaterials is necessary to enable the preparation of highly controlled nanostructures. Among wet chemical synthetic methods, ultrasound-assisted preparation has become an important tool in material science. The formation and crystal growth mechanism under ultrasound is special compared with other wet chemical synthetic routes. In this review, we discussed the chemical and physical effect of ultrasound and summarized the ultrasonic effect on crystallization. The sonolysis of water and the cavitation-induced microjet impact and shockwave are the two key factors in the sonochemical formation of inorganic nanomaterials. The ultrasonic-assisted Ostwald ripening and oriented attachment processes have been reviewed for the possible crystal growth mechanisms in the fabrication of inorganic nanostructures.

Crystal formation and growth mechanism of inorganic nanomaterials in sonochemical syntheses

A clear understanding of the nucleation, growth, coarsening, and aggregation processes of nanomaterials is necessary to enable the preparation of highly controlled nanostructures. Among wet chemical synthetic methods, ultrasound-assisted preparation has become an important tool in material science. The formation and crystal growth mechanism under ultrasound is special compared with other wet chemical synthetic routes. In this review, we discussed the chemical and physical effect of ultrasound and summarized the ultrasonic effect on crystallization. The sonolysis of water and the cavitation-induced microjet impact and shockwave are the two key factors in the sonochemical formation of inorganic nanomaterials. The ultrasonic-assisted Ostwald ripening and oriented attachment processes have been reviewed for the possible crystal growth mechanisms in the fabrication of inorganic nanostructures.

Effect of Escalating Versus Fixed Voltage Treatment on Stone Comminution and Renal Injury During Extracorporeal Shock Wave Lithotripsy: A Prospective Randomized Trial

ESWL is a minimally invasive, efficacious therapy for most renal stones. However, an optimal voltage treatment protocol ensuring effective stone comminution while minimizing tissue injury is not well established. We performed a prospective, randomized trial of the stone-free rate and renoprotective effect of an escalating vs a fixed voltage treatment strategy during ESWL. Between February 2006 and June 2008 we enrolled 45 patients undergoing ESWL for a renal stone in this institutional review board approved trial. A Dornier DoLi 50 lithotriptor was used. Patients were randomized to receive the escalating strategy of 500 shocks at 14k V, 1,000 at 16 kV and 1,000 at 18 kV or the fixed strategy of 2,500 shocks at 18 kV. Abdominal x-ray was done to determine the stone-free rate at 1 month. Voided urine was analyzed for beta2-microglobulin and microalbumin before, immediately after and 1 week after ESWL to evaluate renal damage. Median patient age was 48 years. Median stone size was 8 mm. Of patients in the escalating group 81% were stone-free vs 48% in the fixed group (p <0.03). There was a significant difference between microalbumin and beta2-microglobulin 1 week after the procedure (p = 0.046 vs 0.045). There was trend toward a difference in microalbumin and beta2-microglobulin immediately after the procedure (p = 0.17 and 0.25, respectively). This prospective, randomized study shows that an escalating voltage treatment strategy produces better stone comminution than a fixed strategy. The study suggests that there may be a protective effect against damage caused by ESWL with an escalating treatment strategy.

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.

実在気体効果が非球形気泡の崩壊に及ぼす影響(S25-3 気泡力学に関する基礎解析とその応用(3),S25 気泡力学に関する基礎解析とその応用)

The numerical method developed by Yasuda and Takahira has been improved to consider the real gas effect on the motion of a nonspherical collapsing bubble. The van der Waals equation of state is used for the internal gas in dealing with the real gas effect. The heat transfer of the internal gas and the toroidal bubble dynamics after microjet impact are taken into account in the method. The present method is applied to the collapse of a bubble near a rigid boundary in an oscillatory pressure field. The collapse of a toroidal bubble has been simulated successfully. It is shown that the real gas effect does not appear clearly because of the moderate bubble collapse.

321 水銀マイクロジェットによる衝撃損傷評価(OS 材料・構造の動的特性(2))

Pitting damage due to ca vital ion micro-bubble collapse is one of critical issue to realize a MW-scate mercury target for spallation neutron source. Micro-impact analysis was carried out to discuss the pit formation by mercury micro-jet impact. It was confirmed from the comparison between analytical and experimental results that D(depth)/R(radius) of pits is a useful parameter to estimate the impact velocity and surface hardening treatment is effective to suppress the pitting damage.

Interactions of Inertial Cavitation Bubbles with Stratum Corneum Lipid Bilayers during Low-Frequency Sonophoresis

Interactions of acoustic cavitation bubbles with biological tissues play an important role in biomedical applications of ultrasound. Acoustic cavitation plays a particularly important role in enhancing transdermal transport of macromolecules, thereby offering a noninvasive mode of drug delivery (sonophoresis). Ultrasound-enhanced transdermal transport is mediated by inertial cavitation, where collapses of cavitation bubbles microscopically disrupt the lipid bilayers of the stratum corneum. In this study, we describe a theoretical analysis of the interactions of cavitation bubbles with the stratum corneum lipid bilayers. Three modes of bubble-stratum corneum interactions including shock wave emission, microjet penetration into the stratum corneum, and impact of microjet on the stratum corneum are considered. By relating the mechanical effects of these events on the stratum corneum structure, the relationship between the number of cavitation events and collapse pressures with experimentally measured increase in skin permeability was established. Theoretical predictions were compared to experimentally measured parameters of cavitation events.

Diagnosis of Oxide Films by Cavitation Micro-Jet Impact

Oxide films form readily when aluminum alloy castings are melted and/or poured. They could be primary or secondary types of oxide films. The former type is inherited from the ingot and has existed in aluminum alloy casting for a long period of time. During the filling of the mold cavity, the free unstable surface of the molten metal causes a secondary oxide film to exist on the aluminum alloy castings. The oxide films are usually rich in oxygen and are difficult to observe by optical micrographs. This paper presents a simple but powerful method to observe the shape and size of oxide films on the aluminum matrix. During ultrasonic-vibration treatment, cavitation bubbles nucleated, grew and collapsed, generating micro-jets on the surface of sample. The water micro-jets then had an impact on the oxide film initiating micro-cracks. The cracks grew or became linked together which caused fractures in the oxide film. Small or tiny oxide particles detached from the oxide film to erode the surface of the treated sample. This eroded surface would show as a foggy mark via visual or optical observation. A series of photographs were made and are shown to illustrate the cavitation erosion process of oxide film on the surface of an aluminum sample. In addition, the presented method was shown to be useful in the diagnosis of oxide films that formed on aluminum and magnesium alloys, including the ingots, casting or wrought products.

Numerical Analysis of the Dynamics of Toroidal Bubbles Considering the Heat Transfer of Internal Gas

This paper describes a new numerical method for computing the motion of a toroidal bubble that is formed after a liquid microjet threads the bubble surface. We used the boundary element method combined with the finite volume method to calculate the toroidal bubble dynamics with consideration of the heat transfer inside the bubble. The unstructured triangular grids were used to analyze the internal field. We investigated the bubble collapse near a plane rigid wall. The results show that the initial bubble radius affects the motion of the toroidal bubble, the temperature fields inside the bubble and the pressure fields outside the bubble. When the initial radius is small, the internal thermal boundary layer becomes thick and the bubble collapse is accelerated. This violent collapse induces an extremely high-pressure region near the point of liquid microjet impact. The pressure at a rigid wall, therefore, becomes locally very high when the tiny bubble collapses near the wall.

Phase transformation in austempered ductile iron by microjet impact

Specimens of ductile irons austempered at 320 °C and 360 °C for 2 h were conducted with ultrasonic vibration treatment. Microstructures were characterized using optical (OM) and scanning electron microscopy (SEM) and X-ray diffraction. Vickers hardness was also measured to assess the variation of hardness affected by the microstructure. Experimental results show that specimens after cavitation erosion progressively developed phase transformation induced by plastic deformation. Some stringer-type austenite was found to precipitate carbides, while some island-like austenite was transformed into martensite. If the microstructure led to deplete carbon within the island-like austenite, this austenite readily transformed to martensite due to stress induced by microjet impact. If austenite was rich in Mn, the transformation from austenite to martensite would be retarded. The observations then indicate the coexistence of martensite and precipitated ferrite particles within the colony of island-like austenite. With increased cumulative ultrasonic time, microhardness values were enhanced both in areas near graphite and in intercellular regions. In addition, the elastic strain-energy density estimated from a microjet impact was lower than the theoretical value of homogeneous nucleation of martensite nucleus. Clearly, the transformed martensite was induced by heterogeneous nucleation.

RELAXATION OF MICRO INDENTATIONS IN CALCIUM OXALATE URINARY STONES

We define energy requirements for stone micro indentation as a quantifiable event equivalent to in vivo energy delivery and investigate the change in indentation characteristics with time. The 7 stones extracted from 7 patients were cut, embedded in resin and polished. Multiple micro indentations were performed on each stone section using a diamond Vickers micro indentor with a 500x light microscope and video system. The resulting indentations were observed by optical and scanning electron microscopy as a function of time. Organic matrix content was determined by dissolving stones in ethylenediaminetetraacetic acid solution. The energy requirement for stone indentation varies among stones (median range 43.6 to 109.9 kg/mm2) and at different locations in the same stone. Indentations relaxed by 10 to 70% during the first 2 weeks after indentation. Stones with a high organic matrix content were ductile and the phenomenon of indentation relaxation was pronounced. Brittle, low matrix stones relaxed to a lesser extent. The relaxation phenomena may have a practical implementation when considering repeat shock wave lithotripsy. A significant fraction of the energy invested in a stone which did not cause fracture or critical cracks is lost within 1 to 2 weeks after the procedure, particularly in elastic stones with a high organic matrix content. We suggest that the preferred interval for repeat shock wave lithotripsy be less than 2 weeks.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. II: Application in extracorporeal shock wave lithotripsy

To better understand the mechanism of stone fragmentation during extracorporeal shock wave lithotripsy (ESWL), the model developed in Part I [P. Zhong and C.J. Chuong, J. Acoust. Soc. Am. 94, 19-28 (1993)] is applied to study cavitation microjet impingement and its resultant shock wave propagation in renal calculi. Impact pressure at the stone boundary and stress, strain at the propagating shock fronts in the stone were calculated for typical ESWL loading conditions. At the anterior surface of the stone, the jet induced compressive stress can vary from 0.82 approximately 4 times that of the water hammer pressure depending on the contact angles; whereas the jet-induced shear stress can achieve its maximum, with a magnitude of 30% approximately 54% of the water hammer pressure, near the detachment of the longitudinal (or P) wave in the solid. Comparison of model predictions with material failure strengths of renal calculi suggests that jet impact can lead to stone surface erosion by combined compressive and shear loadings at the jet impacting surface, and spalling failure by tensile forces at the distal surface of the stone. Comparing responses from four different stone types suggests that cystine is the most difficult stone to fragment in ESWL, as observed from clinical experience.

Propagation of shock waves in elastic solids caused by cavitation microjet impact. I: Theoretical formulation

To understand the physical process of the impingement of cavitation microjet and the resultant shock wave propagation in an elastic solid, a theoretical model using geometrical acoustics was developed. Shock waves induced in both the jet head (water) and the solid were analyzed during a tri-supersonic impact configuration when the contact edge between the jet head and the elastic boundary expands faster than the longitudinal wave speed in the solid. Impact pressure at the boundary was solved using continuity conditions along the boundary normal. Reflection and refraction of shock waves from a solid-water interface were also included in the model. With this model, the impact pressure at the solid boundary and the stress, strain as well as velocity discontinuities at the propagating shock fronts were calculated. A comparison with results from previous studies shows that this model provides a more complete and general solution for the jet impact problem.

A study on impulsive pressure of a cavitation bubble. At the 2nd collapse-reexpansion stage.

The impulsive pressure is detected by the local pressure sensor, used in the previous report, at the 2nd collapse-reexpansion stage of a spark-induced cavitation bubble near a solid boundary. At the same time a flash photograph of the cavitation bubble is taken, and it is revealed that a microjet might be generated at the 2nd collapse-reexpansion stage of the cavitation bubble and the impulsive pressure of the microjet is about 1-2MPa, when the cavitation bubble is generated at 7-8 mm from the solid boundary. It is also revealed that the time lag between the microjet impact to the solid boundary and the shock wave generation is about 40-60 μs.

Polymer effects on microjet impact and cavitation erosion

THERE is a growing interest in using high molecular weight polymers as additives in various fluid-engineering systems. These activities are mainly stimulated by the anomalous drag reducing characteristics some high polymers exhibit in turbulent flows1. Early experiments also showed that polymeric additives suppress flow-generated cavitations2,3. An interesting question may therefore be raised as to the effects of high molecular weight polymers on cavitation erosion. We report here attempts made to examine such effects on static vibration-generated cavitation and flow cavitation.

Damage Due to Cavitation and Sub-Cooled Boiling Bubble Collapse

The possibility of the applicability of spherical symmetry to cavitation and highly sub-cooled bubble collapse is considered in the light of present photographic and theoretical evidence, and it is concluded that such symmetry is unlikely in situations of engineering importance. Rather an asymmetry which generates a high-velocity microjet is a more likely mode of collapse. The present evidence relative to the importance of microjet impact as opposed to the classical spherical shock-wave model for cavitation damage is examined and some new experimental evidence presented. It is concluded that the microjet model is most likely of predominant importance in cavitation damage. Some estimates for the pertinent parameters of such microjets are presented.