Finite Element Analysis to Estimate the Mechanical Behavior of a Tripod Used in Emergency Situations

Transcription

1 Finite Element Analysis to Estimate the Mechanical Behavior of a Tripod Used in Emergency Situations Moldan Dorin Ioan Department of Manufacturing Engineering Technical University of Cluj-Napoca, Cluj-Napoca, Romania, Dorin.Moldan@tcm.utcluj.ro Abstract: This article aims to analyze and improve the mechanical properties of a tripod used in emergency situations for saving people and animals from dangerous environments: cliffs, wells, canals, fountains etc, by finite element method. This device must be tested thoroughly and conducted in such a way that its operation is safe, because it is used in an area where human life is involved. Specific boundary conditions were assigned to the tripod and several simulations were performed in different cases of foot positioning taking into account its axis of symmetry (the symmetry axis actuation force support) and different working environments during the various interventions. The software Ansys 14 Finite Element Analysis program has been used in order to determine several mechanical characteristics for seven cases of positioning. These analyzes were performed for three types of materials, namely the 2014 Carbon fiber, Al 2014 and Al 6082 ). Keywords Finite Element Analysis, FEA, tripod, Ansys, deformation, strain, stress. 1. Introduction The complex tripod is an innovative device that saves people engaged in extremely critical situations, such as of persons / animals from different hazardous environments. With this device, firefighters can respond more quickly and effectively than with other accessories, which saves time, and minimize the effort involved for saving the victim [1]. This device was analyzed with the finite element method. Finite Element Analysis (FEA) is a set of methods used for solving large analytical probems to obtain an accurate solution of the problem. FEA is a multidisciplinary modern method, which is often used for the study of contacts in order to determine the most important parameters. The objective of these analyses was considered for the description of mechanical behavior of the tripod for different positions of interest. The simulations performed with ANSYS 14 FEA program had as main objective the determination of: - Total deformations; - Axial deflections; - Von Mises equivalent deformations; - Equivalent von Mises stress; - Energy deformation [2], [3], [4], [5]. Razvan Păcurar Department of Manufacturing Engineering Technical University of Cluj-Napoca, Romania Cluj- Napoca, Romania, Razvan.Pacurar@tcm.utcluj.ro 2. Case studies considered for the finite element analyses The complex tripod for rescuing people is a very important device that should be part of the military fire intervention equipment. The device consists in two main parts: connecting subassembly and subassembly support, the last being more stressed because it consists the three telescopic legs. During the interventions, sections are likely to be deformed due to high compressive forces. [6], [7], [8]. The geometry's tripod was modeled using Creo Parametric software, 3D model of which is saved in the format ".stp" to be recognized later by finite element analysis software ANSYS 14. To achieve a more precise analysis, after the introduction of the 3D model of the tripod in the program, there was added a sferical and a cylindrical articulation on each of the tripod's legs. The legs of the tripod were designed in such a way to allow its rotation in the plane perpendicular onto the plane of the axis of symmetry on where the force is being applied, thus allowing its positioning in the three operating positions, at 30º, 60º and 90º, a tripod at 0º being considered for the storage position (closed). Each leg of the tripod can be locked in three positions (30º, 60º and 90º) and by taking into account the type of intervention that may be combined together in order leading to seven cases of positioning, as can be observed in Figure 2.1: Case 1 - All tripod legs are positioned at 30º; Case 2 - Two legs of the tripod are positioned at 30º and one at 60º; Case 3 - Two legs of the tripod are positioned at 30º and one at 90º; Case 4 - A leg of the tripod is positioned at 30º and the other two are positioned at 60º; Case 5 - A leg of the tripod is positioned at 30º, one is positioned at 60º and one is positioned at 90º; Case 6 - All tripod legs are positioned at 60º; Case 7 - Two legs of the tripod are positioned at 60º and one is positioned at 90º; 174

2 Fig.3.1. The force F applied In Figure 3.2 it is presented how the 3D model of tripod was meshed in tethraedrical elements and nodes. Fig The mesh of the tripod Fig.2.1 Different possible positions of the tripod legs 3. Finite element analyses to improve the mechanical properties of the tripod and obtained results In order to perform the simulations for the positions presented in Figure 2.1, the receptacle has been applied to a single load (a force of 30,000 N), in the upper part of the holding body, as presented in Figure 3.1.The base of each foot was provided by a fixed support for better stability and fixation. It is necessary to make as many simulations, considering different extreme situations, which are given by the different positions of the legs on irregular surfaces. Finite element analysis programs provides the possibility to determine extreme values by pressing the Max / Min, in certain points of analized structure [9]. The force F applied, decomposes the tripod legs in three components R1, R2, R3 and its values is changed in concordance with the angle between the axis of symmetry and the tripod legs. In the finite element analysis library program, the mechanical properties of the three materials were selected (Al2014, Al6082 and carbon fiber), this types of materials being considered as alternatives for manufacturing the tripod. After completing and solving the analyses there were chosed the best results in terms of mechanical behavior. In the following, there will be presented the finite element analyses method that has been used in the case of positioning 5, in which a leg of the tripod is considered as being positioned at 30º, the other leg is considered to be positioned at 60º and the last one as being positioned at 90º as compared to the axis of symmetry, this case being considered as the most challenging (unfavourable). The results of numerical analysis performed revealed that, by following the application of force as presented in Figure 3.1, the maximum total deformations occur in the middle section 2 of the foot positioned at 60º as compared the axis of symmetry, as it can be observed in the images presented in Figure 3.3. The maximum displacement is red colored and has a value of 21,931mm for Al 6082, and the blue color represents the minimum values of movements. Fig Total deformations of the tripod made from a)al 2014, b)al 6082, c) carbon fiber 175

3 As it is possible to observe in Figure 3.3, the tripod made from Al 2014 material has the lowest estimated deformations (20,964mm), such as being the most advantageous case study analyzed. Analyzing the results presented in Figure 3.4 it can be observed that the maximum axial strain was at the top of the tripod (in connecting subassembly) having a value of 0,844mm (in the case of tripod made from Al 6082 material). deformation energy had a value of 163,37 mj in the case of the tripod considered as being made from A material. Fig Energy deformation of the tripod made from a)al 2014, b)al 6082, c) carbon fiber. Fig Axial deformation of the tripod made from a)al 2014, b)al6082, c) carbon fiber. The estimated von Mises equivalent deformations presented in Figure 3.5 where almost equal for each material considered for the analyses, therefore being neglected because of the small values obtained within the made analyses. The maximum value of deformations resulted in the subassembly case joining of the section 2 with section 3. Fig. 3.5 Von Mises equivalent deformations of the tripod made from a)al 2014, b)al 6082, c) carbon fiber. The equivalent von Mises maximum stress distribution on the tripod was observed in several areas, the maximum value being reached in the top area of the tripod (1496 MPa), the minimum value being determined at the joining sections 2 and 3, as could be observed presented in Figure 3.6 [10]. Fig Equivalent von Mises stress of the tripod made by a)al 2014, b)al 6082, c) carbon fiber. In Figure 3.7 it can be observed that the deformation energy had the maximum value on the outside area of the section 3 and the minimum value was determined at the joining section 1 with section 2. Therefore, by comparing the obtained results, it can be concluded that the maximum The obtained results revealed that the analyses made using the ANSYS program were favorable, the tripod presenting a good mechanical behaviour in the case of the force stressing the tripod on axial direction, for all three types of materials that were considered. However, the material not to be recommended would be A due to its maximum values of the deformation energy. The recommended material for manufacturing the tripod would be carbon fibre due to the minimum values of the deformation energy. In the same way the analysis were performed for the other six cases that were considered for the analysis, the obtained results being presented in Table 3.1 and Figure 3.8. In graphic presented in Figure 3.8 it was represented the total distribution of deformations, the axial deformations, the equivalent von Mises deformations, the equivalent von Mises stresses and the deformation energy in concordance with the positions of the tripod analyzed cases, taking into account the three proposed materials for manufacturing the tripod. The obtained average values for positioning cases (1, 2, 3, 4, 5, 6 and 7) were compared with those of the 5 th positioning case because in this case there were obtained the highest values. As it can be observed in Table 3.1, the average values obtained for the 5 th positioning case were: 21,4 mm (total deformation); 0,82 mm (axial deformation); 0,7 mm (equivalent von Mises deformation); 1510,5 MPa (equivalent von Mises stress) and 159,3 mj (deformation energy). For the first case of positioning it can be observed the decreasing of the total deformations with 29,4%, decreasing of axial deformation with 45%, decreasing of equivalent von Mises deformation with 114%, decreasing of equivalent von Mises stress with 34% and decreasing of deformation energy with 30,46%, as compared with case number 5. Comparing the average values of mechanical characteristics obtained in the case number 5, the second (2) case resulted as having the lower values, so in this case the total deformation was decreased with 486%, axial deformation with 820%, the equivalent von Mises deformation with 700%, the equivalent von Mises tensions with 5,46% and the deformation energy with 6,93%. 176

4 Table 3.1. Mechanical characteristics obtained within the finite element analyses that were made in ANSYS Possible cases The recommended material Total deformations (mm) Axial deflections (mm) Von Mises equivalent deformations (mm) Equivalent von Mises stress (MPa) Energy deformation (mj) Case 1 Al ,705 0,017 6, ,21 5, Al ,745 0,018 5, ,205 5,34 F.C. 0,735 0,019 6, ,082 5,31 Case 2 Al ,617 0,001 0, ,21 23,073 30º-30º-60º Al ,414 0,001 0, ,5 22,918 F.C. 4,414 0,001 0, ,5 22,918 Case 3 Al ,824 1,056 0, ,08 36,498 30º-30º-90º Al ,294 1,11 0, ,91 38,677 F.C. 9,92 1,08 0, ,08 37,448 Case 4 Al ,608 0,008 0, ,03 39,54 60º-60º-30º Al ,015 0,008 0, ,72 41,579 F.C. 8,793 0,008 0, ,64 40,651 Case 5 Al ,964 0,805 0, ,4 155,85 30º-60º-90º Al ,931 0,844 0, ,37 F.C. 21,316 0,816 0, ,75 Case 6 Al ,435 0,239 0,001 70,778 10,433 60º-60º-60º Al ,517 0,252 0,001 70,743 11,038 F.C. 1,499 0,264 0,001 71,836 10,986 Case 7 Al ,932 2,005 0, ,6 40,603 60º-60º-90º Al ,458 2,112 0, ,45 42,671 F.C. 11,834 2,124 0, ,7 40,791 Fig Distribution of the values obtained within all cases analyzed 177

5 It was also possible to observe the decreasing of values for the mechanical characteristics in case number 4. The total deformation has been decreased with 2,4%, axial deformation with 102,5%, the equivalent von Mises deformation with 350%, the equivalent von Mises stress with 2,72% and the deformation energy with 3,9%, as compared to the results obtained in case number 5 that has been analyzed. It should be mentioned that even in case number 6, the average values of the analyzed characteristics have been decreased with 14,46%, 3,25%, 700%, 21,24% and 14,7% as compared to the results obtained in case number 5 that has been analyzed. In case number 7, it was possible to observe an increasing of axial deformation with 2,5%, the other characteristics (total deformation, the equivalent von Mises deformation, the equivalent von Mises stress and deformation energy) being decreased with 1,87%, 233%, 2,19% and 3,8% as compared to the results obtained in case number 5 that has been analyzed. 4. CONCLUSIONS As a result of the made analyses for each case and type of material it was found that the tripod can resist to the applied force in axial direction and can be used in maximum security conditions without the risk of possible difficulties concerning its requirements and operations. As a result of the analyses performed it can be observed that the equivalent von Mises stress had the highest value (maximum value for each type of material) of 1510 MPa in the case number 5, when the tripod have been positioned at 30º in the case of the first leg, the other leg being considered positioned at 60º and the third at 90º, as compared to the axis of symmetry. Basically this was the case with the highest values obtained for all the mechanical characteristics that were considered, therefore this case being considered the most unfavorable by the point of mechanical behavior. The most favorable situation of positioning the tripod in which the values were minimal resulted as being the case number 1, when all three legs of the tripod are positioned at 30º. In conclusion after analyzing the results of simulations and by comparing the obtained results it was possible to state and recommend for manufacturing the tripod from A material due to the fact that this type of material has the proper mechanical characteristics required and imposed by this type of device. 5. Acknowledgment This paper is supported by the Human Resources Development Programme POSDRU/159/1.5/S/ financed by the European Social Fund and by the Romanian Government. 6. References [1] Moldan D. The Plane Statics of the human rescue tripod, Acta Tehnica Napocensis, Vol 57, Nr 1, Editura UTPres, Cluj-Napoca, 2014, ISBN , [2] G.R. Liu and S.S. Quek, The Finite Element Method (Second Edition). A Practical Course, 2014, pages 1-11, ISBN: [3] Kent L. Lawrence - ANSYS Workbench Tutorial Release 14, Editura Schroff Development Corporation, 2012, ISBN [4] Huei-Huang Lee, Finite Element Simulations with ANSYS Workbench 14, Editura SDC Publications, 2012, ISBN [5] Kent Lawrence-Ansys Tutorial Release 12.1, Editura SDC Publications, 2010, ISBN [6] Daisuke Mishima, Development of a Pneumatically Controlled Expandable Arm for Rescue Searches in Tight Spaces, The International Journal of Robotics Research January 2006 vol. 25 no ; [7] Stefano Carpin, High Fidelity Tools for Rescue Robotics: Results and Perspectives, Volume 4020, 2006, pp , ISBN , editura Springer Berlin Heidelberg [8] Wolf, A., et al, A Mobile Hyper Redundant Mechanism for Search and Rescue Tasks, International Conference on Intelligent Robots and Systems, Las Vegas, Nevada, October 2003, /03, pp ; [9] Ciabo Renzo, People rescue device, Patent US A, 1982; [10] Cherecheș A. Analiza static a tensiunilor și deformațiilor într-o adăpătoare utilizată în zootehnie, 178