ABRASIVE PARTICLE FRACTURE IN ABRASIVE WATERJET MACHINING

Transcription

1 OBRÓBKA MATERIA ÓW- Stan obecny i kierunki rozwoju OM 2000 ABRASIVE PARTICLE FRACTURE IN ABRASIVE WATERJET MACHINING Ashraf HASSAN * and Jan KOSMOL ** ABSTRACT In abrasive waterjet machining (AWJM), the abrasive particle impinges the workpiece surface at high supersonic speed. As a result, high compressive impact stresses are developed in the impact area. Then these stresses quickly propagate inside the body of the abrasive particle leading to its fracture. This paper presents a dynamic simulation of the abrasive particle fracture using the powerful tool of the finite element method (FEM) in order to predict the behaviour of the particle during interaction with the workpiece surface. The new model takes into account AWJ dynamic loading conditions. The main objective is to develop an FE model which would enable to simulate AWJ performane without any cutting experiments. Deformations and stresses occurring in the abrasive particle material as a result of AWJ impact could be obtained. The results show that the abrasive particle is subject to severe fracture, due to high compressive flow stresses, as a result of AWJ impact and also indicate a good agreement with fragmented abrasive particles which are experimentally obtained in the literature. 1. INTRODUCTION Abrasive waterjet machining (AWJM) is a new and promising machining process which has been successfully applied in different sectors of the modern engineering production. At present, a completely successful predictive model is not possible, because the process involves many complexities. The common drawback of the previous AWJ mathematical and experimental models is that the precise representation of the constitutive behavior of the workpiece material, under AWJ dynamic loading conditions, was ignored. A survey about these models could be found in [1]. Also, these models fail to accurately represent the actual erosion occurring in AWJM. They either oversimplified modeling, by assuming linear elastic material models, or idealized the physical event, by assuming static analysis. Due to its powerful modeling capabilities, the finite element method (FEM) has been applied by the authors in modeling of AWJ [2-4]..The ability to predict the full AWJ behavior using these models was not possible. The common drawback is that these models ignore the micro dynamic behavior of the process, i.e on the abrasive particle level, which is very influential on the process performance. The

2 Ashraf I. HASSAN, Jan KOSMOL interaction period, which a typical individual abrasive particle takes to impact the workpiece material, is usually from 1 to 2 m, depending on its speed. This interaction was not described before in previous work. A new approach of non linear dynamic modeling of AWJ using virtual FEM has been recently developed by the authors [1,5]. This approach consists of tracing the abrasive particle from its early exit from the mixing tube nozzle to its reflection from the surface after interaction with the material at small time intervals, e.g s. This method has proved to be very rewarding in explaining the mechanism of material removal and the overall behavior of the process. Using this method, the depth of AWJ kerf together with its associated flow stresses are now obtainable. In this model, the flow stress was chosen equal to the theoretical fracture strength i.e. (E/14) for mild steel. The abrasive particle fracture during impact on the workpiece surface plays an important role in the process and adds to its complexities. Garnet abrasive is commonly used due to its effectiveness, flowability, availability and reasonable cost. The objective of this paper is to study the effect of AWJ impact on the abrasive particle fracture. Deformations and stresses occurring in the abrasive material in the vicinity of the cutting interface as a result of the erosion impact by AWJ could be obtained.. In the present model, the dynamic progress of the abrasive-workpiece interaction will be traced at small time increments. 2. FINITE ELEMENT MODEL In the present work, the following assumption is relevant: hydrostatic loading plays a secondary role in the cutting process [6]. Figure 1 shows the finite element mesh adopted in the present study. Due to the symmetry of geometry, we only need to model and analyze a quarter model of both the abrasive and workpiece, which greatly reduces the computational requirements. A Garnet abrasive particle is modeled using 16 twenty nodded abrasive particle workpiece Fig. 1 Finite element model of AWJM solid elements with Young s modulus of 248 E3 MPa, Poisson s ratio of 0.27 and density of 4325 Kg/m 3. A linear elastic model is chosen for the abrasive particle material. The dimensions of each

3 Abrasive Particle Fracture in Abrasive Waterjet Machining (AWJM) element are 0.1 mm * 0.1 mm * 0.1 mm. On the other hand, the workpiece is modeled by a three dimensional finite element model and is divided into 20 nodded higher order nonlinear solid elements. A total number of 250 elements were used to model the workpiece with size of each element of 0.1 mm * 0.1 mm * 0.1 mm. The overall workpiece dimensions are 0.5 mm * 1 mm * 0.5 mm. The workpiece material data includes mild steel with Young s modulus of 207 E3 MPa, Yield strength of 207 MPa, and Poisson s ratio of 0.3 and Strain hardening modulus of 34.5 E3 MPa. The constitutive model used for the workpiece is chosen as Von Mises elastoplastic isotropic hardening with linear strain hardening. The boundary conditions include a fixed support of the workpiece from the bottom as it is fixed on the table of the AWJ machine. The abrasive particle is allowed to move freely downwards perpendicular to the workpiece surface. Preliminary investigations were carried out on several models and a compromise between the excessive CPU time and accuracy of results was considered. 3D nine contact elements are added between the abrasive particle and the workpiece that allow for complete interaction including transfer of momentum between the abrasive particle and the workpiece. For abrasive particle speed of 100 m/s, a time step of 0.02 s was chosen corresponding to a distance traveled by the abrasive particle of 2 m/step. For abrasive particle speed of 250 m/s, a time step of 0.01 s was chosen corresponding to a distance traveled by the abrasive particle of 4 m/step. The model was then analyzed on a Pentium II PC workstation and deformations and stresses in the abrasive material were obtained using ALGOR Accupak/VE nonlinear dynamic stress analysis and event simulation, Version 12 WIN. 3. RESULTS AND DISCUSSION As clearly seen from Fig. 2, The bottom layer of the abrasive particle is severely compressed, up to 8 m, as it impinges the workpiece surface, Fig. 2(a). This is due to the build up of high compressive flow stresses that are responsible for the fracture of this layer as will be shown later when we consider flow stresses. The maximum of these deformations are set in the far corner opposite to the center of the impact site. This is because the abrasive particle, upon impacting the workpiece surface, tries to turn around the center of the impact site to the other side to avoid the workpiece material resistance. Afterwards, the abrasive particle is penetrating into the workpiece material making the AWJ kerf. At the beginning, penetration is more in the center of the impact site than in the corner side. As the workpiece surface becomes work hardened, the penetration rate in the center of the impact site slows down, while the corner side of the abrasive particle penetrates faster in the workpiece material, Fig. 2 (b). Hence, the most compressed layer in the abrasive particle shifts to the center of the impact site. By that time, the abrasive particle is being reflected from the workpiece surface due to the large loss of its momentum because of the resistance by which the workpiece material exerted on it. In Fig. 2 (c ), the center of the abrasive particle is retracting itself quickly from the workpiece material while the corner side of the particle away from the center of the impact site is still penetrating deeply in the workpiece material. The same procedure of deformation development is observed for lower speed abrasives (Va), e.g. 100 m/s. First the abrasive particle impinges the workpiece at the center of the impact site, then it tries to rotate around this center, compressing that layer which is opposite to the center of the impact site, Fig. 3 (a). Then it finds resistance at the center more than at the corner side, Fig. 3 (b). The contact duration at slower speeds, as the present case, is very short. It is clear from comparing Fig. 3 and Fig. 4 that as the abrasive speed increases, its deformation and its ability to fracture also increase. The analysis of the results presented in Fig. 3 indicates that there is no erosion for abrasive particle velocities

4 Ashraf I. HASSAN, Jan KOSMOL below 100 m/s. Lower abrasive speeds have no significant effect on the depth of cut. This is supported by Hashish [7] who estimated a threshold impact velocity obtained for mild (a) t = 0.4 s (b) t = 0.73 s (c) t = 1.3 s Fig. 2 Development of abrasive particle deformation with time as a result of its impact on the workpiece surface Va = 250 m/s (workpiece is removed for clarity)

5 Abrasive Particle Fracture in Abrasive Waterjet Machining (AWJM) steel to be 90 m/s. This is because, as seen from Fig. 3, they rapidly reflect from the workpiece surface and do not sufficiently interact with it to produce the necessary erosion. (a) t = 1 s (b) t = 1.18 s (c ) t = 1.2 s Fig. 3 Development of abrasive particle deformation with time as a result of its impact on the workpiece surface V a = 100 m/s (workpiece is removed for clarity) The behavior of the abrasive material under the action of AWJ impact could be further explained by considering stresses with the aid of both Fig. 4 and Fig. 5. As soon as the particle impinges the workpiece surface, stress concentrations are set up as a result of deformation. The maximum stress

6 Ashraf I. HASSAN, Jan KOSMOL concentration occurs in the center of the contact area. Referring to Fig. 4, as soon as the abrasive particle completely comes into contact with the workpiece material Fig. 4 (a), compressive flow stresses rapidly develop across the whole bottom area of impact. The maximum of these stresses lies opposite to (a) t = 0.4 s (b) t = 0.73 s (c) t = 1.3 s Fig. 4 Development of Von Mises stresses in the abrasive particle as a result of AWJ impact (Va = 250 m/s)

7 Abrasive Particle Fracture in Abrasive Waterjet Machining (AWJM) (a) t = 1 s (b) t = 1.18 s (c) t = 1.2 s Fig. 5 Development of Von Mises stresses as a result of AWJ impact (Va=100 m/s) the center of the impact site. This is because the abrasive particle tries to turn around the center of the impact site to the other side as it is traveling free in space. Hashish [6] found that the value of the dynamic flow stress must consider the small size effect as AWJ impact is very localized. The values of the generated compressive stresses exceed the theoretical fracture strength of the garnet abrasive

8 Ashraf I. HASSAN, Jan KOSMOL material. This suggests that the lower impact layer of the particle has been cracked and probably separated from the particle. Then compressive stresses rapidly propagate upward into the body of the abrasive particle. Examining the values of stresses in the upper portion of the abrasive, clearly shows that the rest of the abrasive particle is not subject to fracture and continues its role in deforming the workpiece material to produce AWJ kerf. Maximum compressive flow stresses rapidly increase following the initial impact up to 0.48 s where these stresses decrease again, Fig. 4(b), where the location of the maximum compressive stresses shifts towards the center of the impact site. At this point, the abrasive particle is retracting itself from the interaction site upwards as it loses its momentum. Flow stresses then increase sharply until the time when it totally leaves the workpiece material, Fig. 4 (c), making permanent fracture in the body of the abrasive particle. It is shown that the maximum of these stresses shifts again to the outermost corer of the abrasive particle. The same procedure of compressive stresses development occurs for lower abrasive particle speeds, e.g. 100 m/s, as seen in Fig. 5. Impact at the center of the impact site, then rotation around it so that compressive stresses increase at the corner opposite to the impact center. Two important notices are observed here; stress values are far less than for 250 m/s due to the lower momentum, and value of these stresses do not reach the level necessary to cause the abrasive particle material to fracture except some small regions in the corner of the particle. 4. ACKNOWLEDGMENTS The authors would like to express their gratitude to the Polish State Committee for Scientific Research (KBN) for its financial support of this work. 5. CONCLUSIONS The fracture of abrasive particles during dynamic interaction with the workpiece material in abrasive waterjet machining (AWJM) has been simulated using the finite element method. The bottom layer of the abrasive particle is severely compressed as it impinges the workpiece surface. This is due to the build up of high compressive flow stresses that are responsible for the fracture of this layer. The mechanism of fracture is discussed in this paper. 6. REFERENCES [1] Hassan, A.I., Kosmol, J.: A Finite Element Model for Abrasive Waterjet Machining (AWJM), proceedings of the conference XXXVIV Sympozjon Modelowanie w mechanice, Wis³a, Feb [2] Hassan, A.I., Kosmol, J. A preliminary finite element model of waterjet machining (WJM). Proceedings of the Conference: III Forum prac badawczych Kszta³towanie czesci maszyn przez usuwanie materia³u", Koszalin, 1-3 July1998. [3] Hassan, A.I., Kosmol, J.: Simulation of abrasive waterjet machining (AWJM), Proceedings of the Conference: XXXVIII Sympozjon Modelowanie w mechanice, Gliwice, Poland, 8-12 February 1999, p.81. [4] Hassan, A.I., Kosmol, J.: A New Model for Abrasive Waterjet Machining (AWJM). Proceedings of the International Conference on Water Jet Machining, Cracow, Poland, November, 6 th, 1998.

9 Abrasive Particle Fracture in Abrasive Waterjet Machining (AWJM) [5] Hassan, A.I., Kosmol, J.: Analysis of stresses in abrasive waterjet machining (AWJM), To be published in the Proceedings of the Conference: VI Konferencja Naukowo-Techniczny EC (Electromachining), Bydgoszcz, Poland, May [6] Hashish M. A Modeling Study of Metal Cutting with Abrasive Waterjets, Transactions of the ASME, Journal of Engineering Materials and Technology, Vol. 106, No. 1, pp. 88, [7] Hashish M. A Model for Abrasive-Waterjet (AWJ) Machining, Transactions of the ASME, Journal of Engineering Materials and Technology, Vol. 111, pp. 154, Apr STRESZCZENIE * ) mgr inz. Ashraf HASSAN, Department of Machine Construction, Technical University of Silesia, Gliwice. ** ) Prof. Dr hab. inz. Jan KOSMOL, Department of Machine Construction, Technical University of Silesia, Gliwice.