Design of a Composite Vibration Fixture for Testing Fuel Tank for Combat Vehicle Application

Transcription

1 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 Design of a Composite Vibration Fixture for Testing Fuel Tank for Combat Vehicle Application Hafeezur Rahman A a, Sivakumar P b, Balaguru V c, Rajesh Kumar J c a Scientist, CVRDE, DRDO, Avadi, Chennai-54 b Director, CVRDE, DRDO, Avadi, Chennai-54 c Addl Director, CVRDE, DRDO, Avadi, Chennai-54 rahmanhafiz@rediffmail.com, /2085 Abstract Combat vehicles operate in extreme cross country terrains, which impose severe vibration loads on the systems. To certify such systems combatworthy, random vibration test in the frequency range of 20 to 2000 Hz is mandatory as per MIL standards. However, designing a low weight vibration fixture for the test unit with such high natural frequency to avoid resonance is challenging. Whereas, only limited information is available on the design of composite vibration fixtures, abundant literature is available on the design of such fixtures with metallic alloys such as aluminium and magnesium. This paper tries to address both these objectives through a case study, wherein a composite vibration fixture made of glass & carbon fibre with epoxy resin for testing a combat vehicle fuel tank is presented. The complete design evolution and iteration methodology is first explained along with constraints and challenges. It is then followed by modal analysis of the FE model to determine mode shapes and natural frequency. Finally, it is observed that by careful selection of materials, layup, stack up and configuration, it is possible to achieve fixture natural frequency beyond 2000 Hz. This FE study was generated, executed and post-processed using ANSYS Workbench with ACP-Pre and ACP-Post solvers. Keywords: combat vehicles, vibration, fixture, resonance, composite, glass fibre, mode shapes Introduction A vibration fixture in simple terms is a transition piece between the test unit and the shaker table. In some cases, it is possible to fix the test unit directly to the shaker table provided the hole pitches match. However, in majority of the cases it is seldom possible to fix the test unit directly, which necessitates the use of a fixture. Such a fixture to be designed for the test unit should conform not only to the test unit interfacing but also to other factors such as certification requirements. Every system that is fitted to combat vehicles such as main battle tank (MBT)operate in a rigorous field environment that induce vibration loads on the systems. There are two sources for these vibration namely terrain induced and platform induced. Whereas, the sources for terrain induced vibration are undulations and vertical obstacles, the sources for platform induced vibration include engine, transmission, tracks and gun fire induced vibrations. Therefore, to certify any system combat worthy, the system will have to withstand these vibration induced excitations. 1

2 To do so, random vibration testing of military systems as per MIL-STD-810G or AECTP400 to STANAG 4370 is a proven certification method. The minimum integrity test profile that is followed as per these standards is shown in Fig.1 below. Fig.1. Random Vibration Test Profile [1] From the random vibration test profile it is clear that the frequencies of interest lie in the range between 20 Hz and 2000 Hz. It can also be inferred that the test unit will be subjected to this frequency range and hence the vibration fixture should have a natural or resonant frequency above or below this range which presents two interesting design problems. In case, the fixture resonant frequency is to be below 20 Hz, then it has to very heavy with low stiffness. As a result, the acceleration level possible with the vibration exciter reduces, since it is inversely proportional to the total mass it has to drive as proposed by Klee BJ et al [2]. On the other hand, if high fixture resonant frequency is required then it has to be light with high stiffness. Whereas, in a very heavy fixture it is difficult to contain all ten resonant frequencies within 20 Hz, the same is possible in a light weight fixture as the first mode itself falls beyond 2000 Hz. Designing such a light-weight vibration test fixture with high resonant frequency depends on two factors namely material and geometry. Whereas literature is abundant with design of such fixtures using isotropic materials such as aluminum and magnesium [3], such an investigation with composite materials such as glass fibre and carbon fibre has very few references. This paper serves this objective through a case study, wherein the design of a vibration fixture made of composite materials namely S glass and carbon fibre in epoxy resin for a combat vehicle fuel tank is discussed and design verified using modal analysis. Test unit specification The test unit for which the vibration test fixture is to be designed is one of the multiple fuel tanks of the fuel system that is located within the engine compartment of the structure as shown in Fig.2 below. This test unit is fabricated using stainless steel AISI 304L alloy and is inbuilt with baffles to prevent surging and to increase structural rigidity. It is mounted on three mounts along with two metallic strips 2

3 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 connected to the vertical wall of the structure. The total weight of this tank along with fuel is roughly 125 kg and hence the vibration fixture designed should not only simulate these mountings but also have enough rigidity to handle the body load of the test unit. Fig.2. Test unit - Stainless Steel fuel tank Design methodology for vibration test fixture The detailed design methodology for the composite vibration test fixture is shown in Fig.3 below. The first and foremost in the design process is the modeling of the base plate upon which the fixture is built or modeled. As composite laminate/stack ups are to be created upon this, the general design method is to model the base plate as a surface with matching holes of the shaker table which however has a great drawback. A surface has negligible thickness, and hence the selection of the geometry face or the four corners of the fixture base is inadequate which results in rigid body modes with infinite time period or near zero frequencies [4]. As rigid body modes are superfluous and should not be present, a thin solid surface of 1 mm thickness (Al alloy) is initially created for the base upon which the laminate is modeled. Once the base plate is modeled as discussed above, holes (10 mm diameter) matching the diameter and pitch as that of the shaker table are created as B&K exciters are considered for this study [5]. In addition, to save mass and thereby increase the resonant frequency, additional 40 mm diameter holes are created inbetween the mounting hole pitches in subsequent iterations. Also the structural steel mounts used for mounting the test unit (fuel tank) are modeled and assembled with the base plate. Further to simulate actual mounting conditions i.e. two metallic strips attached to the structure side plate, a vertical angle or a T section is created at the end of the base plate. To complete the model and render it fit for analysis the final stage of gusset modelling is carried out. These gussets which connect the base plate & t section provide added stiffness to the structure and hence for this analysis they are modeled as planes for building up subsequent stack ups/laminates as shown in Fig.4 below. 3

4 Fig.3. Flowchart for design iteration process for composite vibration fixture Fig.4. Parts of the initial vibration fixture model Once the modeling of the assembly is complete, it is exported as shells and solids for modal analysis using ANSYS Workbench. Prior to meshing and analysis the composite material selection and setup are carried out such as ply selection, orientation, stack up, laminate build up and applying it over the named surfaces which are discussed in detail the sections given below. 4

5 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 Selection of composite materials for fixture design Although different types of composite materials are available including some exotic materials, this study will confine itself to glass fibre and carbon composite in epoxy resin. With respect to glass fibre the two candidate choices that are available are E glass & S glass fibre in epoxy resin whereas for carbon fibre woven & unidirectional (UD) are available. To determine the right choice of fibre a comparison of their properties are given in Table.1 below. Properties Epoxy E Glass UD Epoxy S Glass UD Epoxy Carbon Woven 395 GPa Prepreg Epoxy Carbon UD 395 GPa Prepreg Density in g/cc Youngs Modulus E x /E y /E z in MPa Poissons ratio v x /v y /v x Shear Modulus G x /G y /G z in MPa Tensile strength X/Y/Z in MPa Shear strength X/Y/Z in MPa Table.1. Comparison of properties of glass fibre and carbon fibre in epoxy resin [6] From the above data it is observed that the density, poissons ratio, shear modulus and shear strength is same for both E & S glass. Whereas, the tensile strength for S glass is superior to that of E glass, the Youngs modulus for S glass is superior to E glass only in x axis as compared to the other two orthogonal axes. Since UD tapes can be oriented to obtain directional advantage in properties, S glass is selected as the candidate choice for the base plate of the fixture. With respect to the epoxy carbon fibre, it is observed that the woven fabric is lighter and has superior shear modulus and shear strength as compared to the UD tape. However, the Youngs modulus value (E x )and tensile strength for UD is roughly twice than that of woven fabric. In addition, as directional properties are required to be exploited with the materials, UD with roughly twice higher E x can be oriented in the direction of maximum strength and a material saving of upto 20-30% can be obtained [7], which would offset the weight gain due to density difference between woven and UD tape. 5

6 Creation of composite stack up for analysis The first step in the creation of composite stack up is the creation of individual ply or lamina. For this analysis two stack ups one each for S glass and carbon UD are created with an initial assumed thickness of 0.5 mm. Once thickness is assumed and ply created, the next step is the creation of a stack up for which ply orientation is required. In this regard two methods namely symmetric and anti-symmetric stack ups are available. As anti-symmetric stack ups have a non-zero bending extension coupling which during fabrication induces thermal forces during curing & subsequent cooling [8], a symmetric orientation stack up is selected for this analysis. For this symmetric stack up, the orientation selected is [0/+45/-45/90/-45/+45/0] as is commonly used for UD stack ups [9]. With this, the overall thickness of each stack up is 3.5 mm and hence the overall laminate to be designed is in multiples of this thickness. The overall property of S glass and carbon UD in polar coordinates are shown in Fig. 5 & 6 below. Fig.5. Polar plot of Epoxy S Glass UD Stack up Fig.6. Polar plot of Epoxy Carbon UD Stack up Based on the above stack ups the laminate engineering constants are derived as shown in Table.2 below. From this data it is observed that although the laminate stiffness E 1 has reduced by more than half, the laminate stiffness E2 has increased more than twice as compared to the ply property. In addition, the flexural laminate shear stiffness and laminate shear stiffness have increased multifold indicating the efficacy of the stack up process. However, by increasing the thickness beyond 0.5 mm it is 6

7 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 observed that the out of plane shear G 23 & G 31 further decreases whereas all other constants remain unchanged. Hence, a ply thickness of 0.5 mm is considered optimum for the laminate and the analysis is carried out with this thickness. Laminate Engg Constants in MPa Epoxy S Glass UD Epoxy Carbon UD Flexural laminate shear stiffness Flexural laminate stiffness E Flexural laminate stiffness E Laminate shear stiffness Laminate stiffness E Laminate stiffness E Out of plane shear G e e-12 Out of plane shear G e e-11 Table.2. Laminate Engg constants for S glass & Carbon UD Modal analysis of vibration test fixture With all the inputs as discussed above, a modal analysis is carried out with the model. A critical aspect to be considered during the analysis is the type of element to be used for the analysis. In this regard three different choices are available namely SHELL91, SHELL99 & SHELL181[10]. Although SHELL91 (8 Node element) has non-linear and large strain capabilities it can be used only to model thick laminates with upto a maximum of 100 different layers. As the number of layers may reach just beyond 100 and there is also a possibility of using thin laminates in selected sections during the analysis,shell91 is discarded. Whereas, SHEL99 (8 Node element) does not have non-linear capabilities, SHELL181 (4 Node element) has both linear & nonlinear capabilities along with large strain capabilities. Similarly, SHELL99 is used for structural shell model whereas SHELL181 is used for thin to thick shell structures with both allowing upto 250 layers. As the requirement for this analysis range from thin to thick shells along with linear and nonlinear capabilities, SHELL181 is selected for this analysis. Using SHELL181 the meshing is carried out and the mesh quality ensured by adopting a skewness ratio of less than Once meshing is complete the first iteration-modal analysis is carried out with a simple angle fixture with two gussets one each on the front and rear. As the resonant frequency as shown in Fig.7 is Hz which is well below 20 Hz the next series of iteration is carried out by increasing the laminate thickness in the gusset and the base. By doing so, during the third iteration the frequency obtained is Hz as shown in Fig.8 below by increasing the laminate thickness in the t section from 18 to 24 mm. As only a marginal increase in modal frequency is observed, the design of the fixture is changed from a single L angle to inverse T configuration with three right angled gussets and during this seventh iteration the frequency obtained is Hz as seen in Fig.9 below. It is observed that as increasing the number of gussets increases the stiffness of the fixture, the number of gussets is increased from three to four and the frequency obtained is Hz as seen in Fig.10. The similar approach 7

8 of adding number of gussets behind the T section is continued and the results obtained is summarized in Fig.11 shown below. Fig.7. First Iteration-Modal analysis Hz Fig.8. Third Iteration-Modal analysis Hz Fig.9. Seventh Iteration-Modal analysis Hz 8

9 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 Fig.10. Ninth Iteration-Modal analysis Hz Fig.11. Number of iterations vs. natural frequency obtained for fixture Final design of composite vibration test fixture At the end of 22 nd iteration, the modal frequency obtained for the fixture is Hz which is beyond the test requirement of 2000 Hz. This finalised configuration arrived for the fixture for this iteration is shown in Fig.12 below. It is observed that the maximum stiffness is obtained when the RH side of the T section is designed as a complete solid with multiple composite stack up rather than gussets. In addition, it is also observed that with 9 gussets on the RH side the natural frequency of the fixture saturates at around 750 Hz. Beyond this, adding more number of gussets does not increase the natural frequency and hence a solid section with composite stack up/laminate seems to be best approach. However, at 1654 Hz a second saturation is 9

10 observed and hence to attain the design objective, five 120 mm diameter holes were created along with a taper reinforcement on the frontal T section. Both these modifications simultaneously reduce mass and increase the stiffness which increase the natural frequency of the fixture from 1654 Hz to Hz. Fig.12. Final design of composite vibration test fixture The first ten modal frequencies observed for this fixture are Hz, 2035 Hz, Hz, Hz, Hz, Hz, Hz, Hz, Hz and Hz respectively. The first six mode shapes are shown in Fig.13 to 15 below. Fig.13. First Mode (2008 Hz) -Stretching & Second Mode (2035 Hz)-Twisting Fig.14. Third Mode (2183 Hz) - Bending & Fourth Mode (2426 Hz) - Twisting 10

11 International NAFEMS Conference on Engineering Analysis, Modeling, Simulation and 3D-Printing (NAFEMS-3D) 2016 Fig.15. Fifth Mode (2456 Hz) - Twisting & Sixth Mode (2723 Hz) - Stretching From this finalised design, the following observations are made i. The first and second natural frequency of the fixture is Hz and 2035 Hz respectively which although are beyond the requirement of 2000 Hz. This implies that dynamic coupling between the test unit and the fixture may not result during the random vibration test [11]. Conversely even if a failure results in the test unit it may not be due to resonance between the test unit and fixture. ii. The final weight of the fixture observed is kg whereas the weight for a similar fixture made of aluminium and magnesium alloy would weight kg and kg respectively. This fixture weight of kg is only 1.5 times more than the combined weight of fuel tank and fuel, which facilitates the acceleration levels required for the test. iii. The CG coordinates for this vibration fixture are mm (X), mm (Y) and mm (Z) which suggest that the fixture is symmetrical about the vertical axis. In addition, this almost coincides with the shaker table axis which is desirable to avoid offset loads. iv. There are some difficulty in realising this design namely manufacturing aspects such as generating the 40 mm, 120 mm & 10 mm diameter holes on the fixture. To avoid regular machining process such as drilling which would induce delamination, only specialised process such as laser drilling have to be adopted which is expensive. Conclusion The design iteration methodology evolved for designing a composite vibration fixture with epoxy S glass UD & epoxy carbon UD prepreg has been presented. To achieve the desired fixture natural frequency of beyond 2000 Hz, the approach is to increase stiffness and reduce mass. Although the selected materials offer low weight and high stiffness, to obtain the desired modal frequency a design with gussets is necessary. However, as the number of gussets approach nine in the design, a saturation in modal frequency is observed. So, to overcome this problem a solid section in the RH portion of the T section is proposed. 11

12 This approach along with the desired ply thickness of 0.5 mm, symmetrical ply orientation [0/+45/-45/90/-45/+45/0] and creation of holes namely 120 mm at critical sections ensure that the first natural frequency is Hz. Although some practical difficulties ensue during the manufacturing of this fixture, they can be avoided using some specialised process such as laser drilling. In addition, the overall weight of the fixture is kg which is almost 46% and 21% lesser in weight than that of an equivalent fixture made of aluminium and magnesium alloys. Thus, by designing a vibration fixture with composite materials for the fuel tank not only saves weight but aid in achieving a natural frequency beyond 2000 Hz which serves the objective of this investigation. References [1] MIL-STD-810G, Environmental Engineering Considerations and Laboratory Tests, US MOD, 31 October [2] Klee BJ, David Kimball, Wayne Tustin, Vibration and Shock Test Fixture Design, Tustin Institute of Technology, California, Library of Congress, USA, [3] Phani Sowjanya G, Divakara Rao P, Udaya Kiran C, "Finite Element Analysis of Vibration Fixture Made of Aluminium and Magnesium Alloys", IJLTET, Vol 2, Issue 1, Jan [4] Huei-Huang Lee, Finite Element Simulations with ANSYS Workbench 16, SDC Publishers, USA, Sept [5] Bruel & Kjaer, Fixtures for B&K Exciters, Denmark, Oct [6] ANSYS Workbench 15 Online Help, Engineering Data Composites [7] Gurit. Guide To Composites [Internet] Available from (Accessed: 30 July 2016) [8] Ever J Barbero, Introduction to Composite Materials Design, CRC Press, USA, July 2010 [9] Campbell FC, Structural Composite Materials, ASM International, USA, 2010 [10] Ever J Barbero, Finite Element Analysis of Composite Materials, CRC Press, USA, Aug 2007 [11] Sound & Vibration. Why You Can't Ignore Those Vibration Fixture Resonances [Internet] Available from (Accessed:30 July 2016) 12