Dynamic FEA modelling of ISO tank containers

Journal of Materials Processing Technology 124 (2002) 126–132

Dynamic FEA modelling of ISO tank containers Stephen Tiernana,*, Martin Fahyb a

Tallaght Institute of Technology, Tallaght, Dublin 24, Ireland b Dundalk Institute of Technology, Dundalk, Louth, Ireland

Abstract ISO tank containers are used to transport bulk liquids by road, rail and sea. The tank itself is designed using ASME regulations, while the frame designs have evolved through experience. The primary area of concern is the attachment of the tank to the frame as this does not lend itself to analysis using traditional mechanics. The design objective is to reduce the overall weight of the container, thereby allowing greater payloads, as the total laden weight of the container is controlled. Traditionally containers have been built and then tested statically, but there is now a growing emphasis on the dynamic behaviour of the tank, particularly with the ISO introduction of a compulsory dynamic impact test. This paper discusses the finite element modelling of the tank container, statically and dynamically, for road and rail conditions. The static conditions are modelled using the implicit solver in ANSYS 5.6, and the dynamic conditions are modelled using the ANSYS pre-processor and LSDYNA explicit solver. Using the FEA results a new modular tank and frame was designed and built. This tank was then tested in France according to Llyods certification standards. The results from these tests were used to validate and refine the computer model. The paper will discuss the following aspects of the project:
Static modelling of the tank and support structure and verification of the model using, static load data gathered from prototype testing.
Crash test methods and mathematical modelling.
Dynamic FEA modelling to simulate a standard ISO crash test and verification of the model by comparison with SNCF crash test results obtained from the prototype.
Design modifications to improve the tank performance during impact. # 2002 Elsevier Science B.V. All rights reserved. Keywords: ISO tank containers; Dynamic impact test; FEA

1. Introduction This project followed on from an initial project, which involved the static modelling of an ISO tank container and validation of that model using test results. The tank designed during this project subsequently underwent a dynamic collision test which it failed, this resulted in the need for the tank and frame to be modelled under dynamic conditions to determine a method of stiffening the structure without incurring a major weight penalty. 1.1. Project background ISO tank containers are used to transport bulk liquids by road, rail and sea. They are designed to transport large volumes (25,000 l) of liquids door to door in a safe and efficient manner. Tanks may also include integral heating,

* Corresponding author. E-mail address: [email protected] (S. Tiernan).

inert gas blanketing and pressurisation. All tanks are insulated. Tanks are manufactured from carbon steel and stainless steel. The principle design requirements of the tanks are safety and weight. The lighter the tank the more load it may carry, as the total laden weight is fixed by state regulations (Fig. 1).

2. Tank design and modelling Following an initial project to statically model an ISO tank container a new modular tank design was developed. This tank was modelled in ANSYS, the complete tank was modelled as the loading was not necessarily symmetrical. The entire model was drawn as areas and meshed with shell elements (Fig. 2). The thickness of the shell, plates and beams were specified using the real constants section of the Shell 163 element [1]. Shell 163 is the dynamic equivalent to Shell 63. An assembly of flat shell elements can produce a good approximation of a curved shell surface provided that each flat element does not extend over more than a 158 arc.

0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 1 9 6 - 6

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2.1. Shell 163 Shell 163 is a four-noded element with both bending and membrane capabilities. Both in-plane and normal loads are permitted. The element has 12 degrees of freedom at each node: translations, accelerations, and velocities in the nodal x-, y-, and z-directions and rotations about the nodal x-, y-, and z-axes. This element is used in explicit dynamic analyses only (Fig. 3) [2].

3. Static modelling and testing Fig. 1. ISO tank container.

Fig. 2. Meshed model with shell and beam elements.

Initial work was carried out by modelling the tank under static conditions, i.e. ignoring inertia and damping effects, such as those caused by time-varying loads. ISO static tests were modelled and then the tank was tested in accordance the ISO tests, Table 1 gives a comparison of the test results. The tests are detailed in ISO 1496-3:1995(E), Sections 6.2– 6.13. Testing was carried out in the Centre National D’Essais De Conteneurs, Tergnier, France. The deflections were generally within 10–15% of actual measurements taken during testing (see Table 1). The primary areas of interest are the vertical deflections during the stacking test (test no. 1). The stacking test involved vertical loads of 850 kN on each corner post, deflections of 2–3 mm were recorded and these compare well with the FEA results. The highest stress levels are on the posts themselves towards the top corner casting. Generally these stresses were between 280 and 330 N/mm2. The test results for this and many other static tests correlate very well with the FEA model therefore it is assumed

Fig. 3. New modular design.

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Table 1 ISO tank container static test results comparison Test no. Test

SNCF results (mm)

FEA results (mm)

1 2 3 4 5 6 7

2–3 0.5–1 0–2.5 4 5 15 15

2.3–3.2 22–0.9 1–1.9 3.8 5.9 15.4 20

Stacking test Lifting: top corner fittings Lifting: bottom corner fittings External restraint Transverse rigidity Longitudinal rigidity Crash test

that the model is valid and performs in a similar fashion to the actual tank.

4. Dynamic test and simulation The dynamic test was carried out immediately after the static tests, this was before the dynamic FEA was carried out. The tank failed the test as the permanent deformation recorded was 5 mm and the test standards allow a maximum of 3 mm. This failure instigated the dynamic FEA investigation as a method of stiffening the tank was required which would have a minimum weight penalty.

static, transient, and harmonic loads. The time scale of the loading is such that the inertia or damping effects are considered to be important. 4.2. ISO dynamic test description The container is subjected to a 5 g crash test as per ISO 1496.3. The test is carried out in practice by allowing a bogie with a fixed mass to roll at 4.5 m/s into another bogie, which has the tank and frame, attached to it. Accelerometers on the frame of the tank container measure the severity of the impact (Fig. 4). 4.3. Model description A simplified two degree of freedom system [3] is adequate to determine the effects of the relationships between the relative masses, relative stiffness, and relative natural frequencies of the two subsystems (see Fig. 5). The equations of motion for the tank and the rail car are presented in Eqs. (1) and (2):  M2

      d2 x 2 dx2 dx1 þ C  þ K2 ðx2  x1 Þ ¼ 0 2 2 dt dt dt (1)

      d2 x 1 dx1 dU þ K1 ðx1  UÞ þ C  1 dt dt2 dt     dx1 dx2 þ C2  þ K2 ðx1  x2 Þ ¼ 0 dt dt 

4.1. Definition of transient dynamic analysis

M1

Transient dynamic analysis (sometimes called time-history analysis) is a technique used to determine the dynamic response of a structure under the action of any general timedependent loads. This type of analysis can be used to determine the time-varying displacements, strains, stresses, and forces in a structure as it responds to any combination of

(2)

where M is the mass, C the equivalent damping coefficient, K1 the equivalent stiffness, x the absolute displacement, U the relative displacement between the rail car and the ground

Fig. 4. Collision test setup.

Fig. 5. Two degree of freedom system for rail impact test.

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Fig. 6. Typical test results.

link and subscript 1 refers to the rail car and 2 to the tank container. Typical values for the longitudinal stiffness of the rail car are in the range of 450–10,000 kN/m. The value used was 1000 kN/m [4], based on manufacture’s data. A damping coefficient of 0.1 was used, although this is slightly conservative. Thus, the rail car is considered to be extremely stiff and the damper to behave linearly. The model input is a step velocity change. Surge effects due to sloshing of the liquid in the tank were ignored. The impact velocity was 4.5 m/s. 4.4. SNCF test results The acceleration recorded during the test was 4.41 g and the tank/frame deformation was recorded as 5 mm. This deformation was as a result of the tank moving within the frame. On inspection of the frame the maximum deflection found was 15 mm on the bottom end plate (see Fig. 7). Typical test data is presented in Fig. 6, the data from the accelerometers is reduced to a spectral (frequency) representation, which is more useful in describing the shock [5]. The natural frequency range used was 0.5–250 Hz. 4.5. FEA model The tank is subjected to an implicit/explicit analysis. It is first solved as a static analysis with gravity and increased shell density to simulate a full tank of liquid, i.e. the implicit solution. The tank is then subjected to a 5 g acceleration for 0.1 s in a dynamic or explicit simulation. The implicit or gravity solution is superimposed on to this solution as an initial condition. As no fracture of material occurred during the physical test a bilinear material model was used; with a tangent modulus of 2 GN/m2, and a yield strength of 550 MN/mm2. An inherent material stiffness–damping ratio of 10% was assumed, as recommended by Sturk [3]. The model was archived and input files were used to run the simulation. The deformation was tracked by selecting nodes in the critical region, i.e. midway across the bottom of the

end hexagonal plate, the corner post and the side wall of the vessel, these nodes are referred to as HEX, SHELL and WEB, respectively (see Fig. 7). 4.6. Material model The bilinear kinematic hardening (BKIN) [1] material model was used for the non-linear analysis [6]. It is a ‘rateindependent plasticity’ model. BKIN may be used for materials that obey von Mises yield criteria (which includes most ductile metals). The material behaviour is described by a bilinear stress (total)–strain curve starting at the origin and with positive stress and strain values. The initial slope of the curve is taken as the elastic modulus of the material. At the specified yield stress (C1), the curve continues along the second slope defined by the tangent modulus, C2 (having the same units as the elastic modulus). The tangent modulus can neither be less than zero nor greater than the elastic modulus. 4.7. FEA results Transient dynamic solutions take considerable amounts of time to solve. The results file can be visually inspected as video file in the post-processor. It stores data for the entire model but due to limits on memory cannot store as much information as the history file. The history file stores data for particular nodes but much more frequently than the results file. This information is viewed in the time-history postprocessor. As we already know the location of the area with the largest deformation it is possible to choose a node in this location and plot its deformation versus time. Data was stored for three nodes, HEX, SHELL and WEB as described in Section 4.5 (see Fig. 8). Permanent deformation was recorded after the impact test on the rear hexagonal end plate (see Fig. 7) and on the frame uprights. The FEA simulation, using the time-history plot of the deformation on the hexagonal plate, shows initial deformation of 21 mm but recovery to 15 mm, this correlates well with the test results. During the test the tank moved forward 5 mm in the frame. The FEA plot shows the movement of the

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Fig. 7. Tank after crash test and 15 mm deformation of end plate.

tank within the frame to be 20 mm, initially, recovering to 6 mm. The corner post (taking measurements at its point of attachment to the side diagonal support) moved forward 15 mm recovering to 5 mm. These results are consistent with the SNCF test, and thus it is assumed that the model behaves as the tank did during the collision test (Fig. 9). The aim of this project is to investigate methods of stiffening the tank as the maximum deformation, between the tank and frame, allowed by the ISO standard is 3 mm. Therefore, the tank results described above failed the test. Two options were considered: 1. Including stiffeners on the uprights and on either side of the cut out on the bottom of the tank frame connection.

These would join the hexagonal plate to the bottom cross-member, thus the cross-member would act as a torsion spring to resist the movement of the hexagonal plate. This would have a minimal weight penalty. 2. Increasing the thickness of the hexagonal plates attaching the skirts to the frame at either end (see Fig. 3). A number of FEA simulations were run on each option, the following were the results for the first option:
Stiffeners from 5 mm thickness to 15 mm were tried but the maximum reduction in permanent deformation was approximately 5 mm (Fig. 10).
The webs attached to the corner post had a slight but not significant effect in reducing its forward motion.

Fig. 8. Deformation of node on bottom end plate during crash test simulation.

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Fig. 9. Deformation and stress induced during crash test simulation.

The final solution proposed using the 8 mm hexagonal plates and using 8 mm stiffeners at the bottom cut out of the tank. The FEA prediction for permanent deformation with this arrangement was less than 1 mm (Fig. 11).

5. Conclusion

Fig. 10. Deformation with stiffeners.

The second option was then considered using a number of different hexagonal plate thicknesses the optimum was found to be 8 mm, this reduced the permanent deformation to approximately 1.5 mm.

Fig. 11. Deformation with 8 mm hexagonal plates.

Initially the tank and container was modelled using static linear FEA simulation. This resulted in a new modular, and weight efficient, design of tank and frame. This tank and frame subsequently failed a dynamic collision test, a permanent deformation of 5 mm was recorded between the tank and frame. This failure instigated this project as a method of stiffening the tank was required which would have a minimal weight penalty. The dynamic simulation described in this paper found that the optimum way of reducing this deformation was by increasing the thickness of the end hexagonal plates from 5 to 8 mm and adding stiffeners at either side of the skirt cut out. It is predicted that with this arrangement the movement of the tank within the frame will be less than 1 mm. The ISO standard allows for a maximum permanent deformation of 3 mm. The FEA model developed during this project will also be used for the following purposes:
To allow modifications to be made to the tank for certification purposes without the need to repeat the ISO tests. (These are both expensive and time consuming.)
To convince potential customers of the tank’s strength versus low tare weight characteristics.
The FEA modelling of the tank container has led to a better understanding of the tank structure and its inservice behaviour. The new modular design is as effective in static situations as previous heavier designs. The understanding gained allows intelligent, effective and safe designs to be implement without the need for large scale building and testing of prototypes.

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References [1] ANSYS 5.6 Manuals, ANSYS SAS IP Inc., 1999. [2] LSDYNA Manuals, Livermore Software Technology Corp., 1998. [3] M. Sturk, Development of a Tank Container Impact Test Standard (TP13127E), Centre for Surface Transportation Technology (NCR), 1997.

[4] Mott, Robert, Machine Elements in Mechanical Design, 2nd Edition, Prentice-Hall, Englewood Cliffs, NJ, 1999. [5] M. Omer, Tank Container Impact Standard Phase 11 Report, Centre for Surface Transportation Technology (NCR), 1998. [6] M.J. Fagan, Finite Element Analysis: Theory and Practice, Longman, New York, 1992.