Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro Investigation of sandstone erosion by continuous and pulsed water jets Piush Raj a , Sergej Hloch b,c , Rupam Tripathi a , Madhulika Srivastava a, ⁎ , Akash Nag a , Dagmar Klichová b , Jiří Klich b , Monika Hromasová d , Miroslav Muller d , Linda Miloslav d , Somnath Chattopadhyaya a , Pavel Adamcik e a Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India b Faculty of Manufacturing Technologies Technical University of Kosice with a seat in Prešov, Slovakia c Institute of Geonics of the CAS, v.v.i. Ostrava, Poruba, Czech Republic d Czech University of Life Sciences Prague, Czech Republic e Technicka Diagnostika, spol.s r.o., Presov, Slovakia ARTICLE INFO Keywords: Continuous water jet Pulsed water jet Erosion ABSTRACT The study compares the sandstone erosion manifestations (depth, width, and volume of the removed material) caused by continuous and pulsed water jets (WJs) with a frequency f = 20 kHz and 40 kHz. The WJ head follows an inclined trajectory at an angle of 45° to the horizontal surface. The starting point of the head was positioned 5 mm above the sandstone surface. The traverse speed of the head was varied from v = 0.25 mm/s to 1 mm/s. The structural changes and plastic deformation caused by the mechanical effect of the continuous and pulsed WJs were analysed using SEM and optical profilometer. The results showed that an optimal standoff distance was responsible for the maximum erosion effect under both continuous and pulsed regimes. The PWJ at f= 40 kHz showed better erosion performance in terms of depth (6.8 mm), width (6.83 mm) and volume removed (27.31 mm 3 ) as compared to PWJ at f = 20 kHz and CWJ. The microstructural topography at varying standoff distance (z = 10 mm to 45 mm) showed erosion features such as micro-pits, cracks, perforated holes responsible for different erosion stages. On-line monitoring via means of acoustic emission was measured to evaluate the erosion phenomenon on the sandstone surface during the impact of the PWJ and CWJ. The correlation between the acoustic emission results and the erosion parameters showed the progression of the erosion process under different technological conditions. 1. Introduction The erosion phenomenon is observed naturally in the form of weathering of rocks and soil by wind, air, and water [1]. Erosive property of water is utilized for disintegration of material during water jet machining [2]. Water jet (WJ) technology has been used to achieve selective non-thermal material removal. Using a WJ, the desired re- moval of a material is achieved by the solid-liquid interaction. The WJ erosion effect has been utilized in mining [3], chemical, and coal in- dustries for various applications such as cleaning and layer removal [4]. Although rock erosion phenomena are continuously observed in mining industries and issues related to them are still not completely under- stood. The conventional rock disintegration tools require a high force, causing wear of the tools; therefore, existing methods are considered as non-viable from environmental and techno-economical perspectives [5]. For elimination of the above mentioned disadvantages, WJ-assisted methods are adopted for rock erosion as they utilize natural forces for causing erosion of the surface. Methods using WJs for rock mining have increased its safety and production scale [6]. Continuous water jet (CWJ) and abrasive water jet (AWJ) are used in the industry for soft rock and hard rock disintegration, respectively. Because of the growing demand for hard rock disintegration, some scientists and engineers are trying to utilise the physical phenomenon of hammer effect [7] gen- erated due to water droplet for rock disintegration. This phenomenon led to development of pulsating WJ (PWJ). PWJ differs from CWJ in terms of its periodic action on the material in the form of droplet clusters. At present, prototype PWJs with 20 kHz and 40 kHz fre- quencies has been developed in Institute of Geonics v.v.i for experi- mental purpose. The advantage of a PWJ over a CWJ is reduction of working pressure below 100 MPa, which is achieved by an additional equipment ultrasonic sonotrode [8]. It allows eroding of rocks by im- pact of a series of discrete-intensity pressure transients [5]. The https://doi.org/10.1016/j.jmapro.2019.04.035 Received 14 September 2018; Received in revised form 18 April 2019; Accepted 29 April 2019 Abbreviations: CWJ, continuous water jet; PWJ, pulsed water jet; WJ, water jet; AWJ, abrasive water jet; AE, acoustic emission ⁎ Corresponding author at: Department of Mechanical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India. E-mail address: madhulikamech3003@gmail.com (M. Srivastava). Journal of Manufacturing Processes 42 (2019) 121–130 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. T