Ultrasonics 82 (2018) 322–326

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Ultrasonics

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Short communication

Peening the tip of a notch using ultrasonic cavitation

https://doi.org/10.1016/j.ultras.2017.09.0040041-624X/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: shjung@dankook.ac.kr (S. Jung).

Sunghwan Jung a,⇑, Murugesan Prabhu b, Hyungyil Lee b

aDept. of Mechanical Engineering, Dankook University, Yongin-si, Gyeonggi-do, Republic of KoreabDept. of Mechanical Engineering, Sogang University, Seoul 121-742, Republic of Korea

a r t i c l e i n f o

Article history:Received 9 May 2017Received in revised form 2 August 2017Accepted 5 September 2017Available online 6 September 2017

Keywords:PeeningUltrasonicCavitationNotchTip

a b s t r a c t

A peening technique using ultrasonic cavitation was proposed to peen the tip of deep notches. The work-ing theory of the present peening technique for the notch tip was described and numerically demon-strated. An experiment using a deep notch shape and an ultrasonic loading with a frequency of 20 kHzachieved noticeable compressive residual stresses at the notch tip.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Notches introduced in mechanical components substantiallydegrade structural durability due to the stress concentration whenthe components are subject to tension. To improve the durability,the tensile stress concentration at the tip needs to be mitigated.Shot peening, which is to impact the surface with shot streamsand produce a compressive residual stress layer, has been usedto relieve the tensile stress concentration. However, for deep andnarrow notches, shot peening becomes less suitable to peen thetips of the notches due to the difficult geometries, which are chal-lenging to access.

To address the challenges to peen deep notches, we propose ashot-less peening approach using ultrasonic cavitation. Whenwater is ultrasonically excited, cavities form and grow. When thecavities reach a volume where energy can be no longer absorbed,they collapse violently generating shockwaves. Early works usingthe shock waves demonstrated compressive residual stresses onflat metal surfaces or increases of the fatigue limit [1–3], whichimplies that ultrasonic cavitation is able to provide surface treat-ment like shot peening. The present approach makes use of theshock waves in producing a compressive residual stress layer atthe notch tip.

In this work, we presented the working theory to enable theproposed approach using ultrasonic cavitation to peen the tip ofdeep notches. A finite element analysis and a peening experiment

were carried out to verify the working theory. Also, the residualstresses achieved at the notch tip produced from the experimentwere measured and discussed.

2. Working theory

The converging geometry of notch is the key feature to achieveintensive cavitation leading to generate compressive stresses at thenotch tip. As shown in Fig. 1, the notch shape tends to focus theapplied ultrasound as the rigidity of the sidewalls prevents ultra-sonic energy from being transmitted from water to the steel partat the interface. As the ultrasound is focused at the tip, the ultra-sonic intensity reaches the maximum at the tip of the notch, inten-sifying ultrasonic cavitation as illustrated in Fig. 1. Thus, thepeening effect by cavitation is significantly enhanced at the tip asdesired.

The degree of the intensity concentration at the tip scales withseverity of the notch; with the notch angle and the notch tip radiusdecreasing, the intensity concentration is scaled up. Thus, it can beclaimed that, based on the scalability of the intensity concentrationeffect, the ultrasonic cavitation peening approach becomes morepreferable to peen the tips of deep notches which are fairly chal-lenging to reach.

In the present work, a finite element simulation and an experi-ment were carried out to verify the theory using the configurationof a peening system shown in Fig. 2. The present notch was intro-duced with the depth of 28 mm and the opening angle of 30�. Thesides were covered to further confine the ultrasonic filed. Thevibration probe was directed to the notch tip.

Fig. 1. Notch peening using ultrasonic cavitation. Ultrasound is focused in the zone of the notch tip inducing cavitation, which, in turn, produces the compressive stresses atthe metal surface.

Fig. 2. Schematic of the present peening system. Steel covers are added on the side to enhance the intensity concentration.

S. Jung et al. / Ultrasonics 82 (2018) 322–326 323

3. Acoustic finite element analysis

The present finite element simulation aims to verify the validityof the assumption that the rigidity of the steel sidewall affords tofully constrain the ultrasonic field leading to concentration ofultrasonic intensity at the tip. The dimensions of the notch in thesimulation model were set as given in Fig. 2. The properties of ele-ments comprising the steel part and the water part were given inTable 1. The water part of the simulation model adopted the 8-node linear brick acoustic elements from Abaqus while the steelpart used the 8-node linear solid continuum elements [4]. At theinterfaces between water and solid, the nodes of the steel partwere coupled with the corresponding nodes of the water part. Tosimulate vibration to ultrasonically excite the water, the vibrationloading with the amplitude of 35 lm was set to a frequency of20 kHz. The simulation was run on steady state dynamic analysis.

Table 1Material constants set for the finite element analysis.

Material constants Steel Water

Density (kg m�3) 7860 1000Young’s modulus (GPa) 210 –Bulk modulus (GPa) – 2.2Acoustic impedance (kg m�2 s�1) 47.2 � 106 1.45 � 106

Fig. 3. Alternating pressure amplitude contours. The quarter symmetry was used inthe simulation.

324 S. Jung et al. / Ultrasonics 82 (2018) 322–326

The alternating pressure amplitude shown in Fig. 3 demon-strates that the intensity i.e. the pressure amplitude continues toelevate and reaches the maximum at the tip as predicted in theworking theory. This proves that the steel sidewalls, whose acous-tic impedance is far greater than that of water as given in Table 1,are fully rigid to be able to constrain the ultrasonic field leading tothe concentration of the pressure amplitude.

It should be noted that the pressure presented at the notch tipfrom the simulation is sensitive to the dimensions of the elementsnear the tip so that the pressure will continue to increase with themesh more refined at the tip, in other words, since the tip radius ofthe notch is not given in the present model, the pressure fieldwould come to be singular at the notch tip by refining the mesh.Thus, for the result to be more accurate, a proper tip radius needsto be properly implemented. However, the present finite elementsimulation only aims to collectively understand the pressure distri-bution inside the notch so that a full evaluation of the individualaccuracies of the pressure values near the notch tip were not con-ducted here.

4. Experimental procedure and results

An experiment was conducted to measure residual stresses atthe notch tip which was produced by the cavitation resulting fromthe concentrated ultrasonic field in the notch. The experimentalset-up was shown in Fig. 4. A SPCC steel strip was assembled onthe side of the working sample as shown in Fig. 4, which was tocollect the stress by the cavitation and be removed after the exper-iment for the ex-situ stress measurement by the X-ray diffraction(XRD) method to be made; with the notch angle, the notch tip sur-face is only partially visible so that direct XRD measurements onthe notch tip surface are not allowed. Underneath the assembledstrip, an elastic material was added to ensure that a full contactof the strip with the counter part is made. Once the strip wasimplemented, the front side was covered by the steel plate. Thenotch sample was fully immersed in water at the temperature of20 �C. The vibration probe connected with an ultrasonic processor(Vibra-cell VCX750, Sonics) was chosen with the diameter of

Fig. 4. Experimental set-up (The container filled with water is not present to achieve theshows the strip assembled on the side to collect the residual stresses produced by cavitameasurements.

25 mm, which well covers the opening area of the notch. The probewas vertically set and separate from the top surface of the sampleby 2 mm. The tip of the probe was partially immersed in water tocontact and excite water. The probe vibrated at a frequency of20 kHz. The vibration amplitude of the tip was set to 35 lm. Vibra-tion using the vibration probe was carried out for 3 min.

After the vibration was complete, the strip was removed fromthe hole and the stresses generated by the vibration loading weremeasured using the X-ray diffraction method (Rigaku Miniflex X-ray Diffractometer) on the exposed side surface. Each X-ray diffrac-tion measurement involved a slender irradiated area of 2 mm wideby 10 mm. The slender irradiated area was oriented in parallel withthe notch tip line, which moved away from the notch tip by anincrement of 2 mm to capture the stress variation over the imme-diate area of the notch tip.

The stress values from the measurements revealed that appre-ciable compressive residual stresses were locally produced nearand at the notch tip (Fig. 5). The maximum compressive residualstress was found at the notch tip, as expected. The residual stressrapidly decreased with distance away from the notch tip. The localstress field indicates that intensive cavitation was locally producedat the tip as a result of the pressure concentration shown in Fig. 3.

It is possible that the residual stress was further raised by thegeometric discontinuity associated with the notch shape, whichwas, however, not counted in the ex-situ measurement [5]. Theex-situ stress measurement was carried on the surface of the stripafter it was removed from the notch configuration. Thus, whenthey were measured, the residual stresses especially at the tipcould be less than the actual ones by the stress concentration fac-tor associated with the geometric discontinuity.

5. Visualization of cavitation

In order for the concentration of ultrasonic intensity to be visu-ally confirmed, the images of cavitation inside the notch wererecorded during the process. The steel cover was replaced with aglass cover, which is transparent to visualize cavitation producedinside the notch. From the recorded image shown in Fig. 6,

clear image. Instead the water is represented symbolically). The cross section viewtion at the notch tip. The strip was removed after the experiment for the XRD stress

Fig. 5. Residual stress vs X (as defined in the inset).

Fig. 6. Cavitation at the notch tip. The steel cover was replaced with a glass cover for visualization of the cavitation. In the right image taken during the vibration loading, alocal brightness appeared at the notch tip clearly indicating that intensive cavitation was locally produced. Refer to the supporting material for the video footage from whichthe present still images are extracted.

S. Jung et al. / Ultrasonics 82 (2018) 322–326 325

intensive cavitation was observed near the tip during the processas expected from the pressure concentration. Fig. 6 indicates thatthe residual stress field shown in Fig. 5 was built up as far as thelocal cavitation appeared.

Video 1.

Also, from the recorded images, migration of cavities wasinspected. As well as the notch tip, cavities formed directly under-neath the vibration probe and subsequently displaced by circula-tion flows known as acoustic streaming. However, no prominentcontribution from the migration to the formation of the cavitycluster at the notch tip was observed. Thus, this observationdemonstrates that the intensive cavitation at the tip mainlyresulted from the pressure concentration, not from collection ofthe cavities which were displaced from elsewhere.

6. Discussion and conclusion

We successfully peened the tip of the deep notch using ultra-sonic cavitation. Appreciable compressive residual stresses wereachieved in the area near the notch tip by the ultrasonic vibrationloading. The locally concentrated stress field at the notch tip indi-cates that intensive cavitation was produced at the notch tip fromthe pressure concentration, which is induced by the converginggeometry of the notch shape. It is also visually confirmed thatthe intensive cavitation at the tip mainly resulted from the pres-sure concentration, not from collection of the cavities which weredisplaced from elsewhere. The concentration feature offers thepotential to repair deep notches or flaws which are challengingto reach.

326 S. Jung et al. / Ultrasonics 82 (2018) 322–326

An extensive parametric study will be carried out to assess effi-cacy of the present approach for various notch dimensions includ-ing notch tip radius and opening angle. In addition to residualstress, strain hardening and surface roughing will be investigated.The vibrator’s loading amplitude, shape and dimensions of theultrasonic tip, duration time, and, types and conditions of cavitat-ing liquids will be optimized to further improve the residual stres-ses at the notch tips. Models of cavitation and jet flows induce bythe vibrator tip will be integrated in the numerical simulation toimprove the accuracy of the results.

In addition, the ultrasonic cavitation combined with the notchconfiguration can be useful in the applications of nano powderspreparation and mechanical alloying of powder materials. Earlyworks [6,7] reported that ultrasonic cavitation can play to increasethe reaction rates of the processes. Thus, as the chemical effect ofultrasonic cavitation was proven, the notch configuration can serveas a platform able to significantly enhance the activity by providingan amplified ultrasonic intensity.

Acknowledgments

This research was supported by the National Research Founda-tion of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1A02937009) & the Ministry of Science, ICT & Future Plan-ning (NRF-2016 M2B2A9A 02945208).

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[3] M.R. Sriraman, R. Vasudevan, Influence of ultrasonic cavitation on surfaceresidual stresses in AISI stainless steel, J. Mater. Sci. 33 (1998) 2899–2904.

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