Validated Simulation of a Sandwich Roof in a Crash Load Case
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MAN verifies and validates the crash properties of a sandwich bus roof in a dynamic overturn. The company has thus demonstrated that simulation methods are suitable for assessing the crash resistance of functionally integrated lightweight components.
Approval of Crash Simulation
The constantly growing requirements regarding comfort, safety and environmental compatibility are a driving force behind the development of vehicles. This also applies to commercial vehicles, in particular buses for passenger transport. The development of new technical solutions should also meet the manufacturer’s economic objectives. The following paper describes the development and application of a very large functionally integrated lightweight construction element using the example of a bus roof in sandwich construction. As part of the body-in-white structure of a bus, the component must also meet the technical requirements with regard to crash resistance and fatigue strength. Weight considerations are also examined in more detail. The motivation for the validation and verification of the sandwich roof is to obtain approval for a computer simulation procedure in relation to Annex 9 of Regulation No. 66 , Amendment Series 02, “Buses and coaches with regard to the crash resistance of their bodywork.”
Lightweight Construction of Buses
Buses with a self-supporting body-in-white structure were built for the first time in the 1950s. This means that the separation between car body and chassis structure has been eliminated and the entire body is sufficiently stable and torsionally rigid as one unit. This design has established itself above all with complete bus manufacturers.
The skeleton, as the supporting structure of a bus is also called, is built using the differential construction method. Individual parts are joined according to the principle of bar and framework construction by easily controllable joining techniques such as MAG welding.
This requires little effort and the costs for manufacturing a bus skeleton are therefore comparatively low. This design also allows a weight-optimized design. With the help of modern simulation methods, the weight could be further reduced over time by optimizing, for example, the flow of force or the use of materials, although the requirements regarding passive safety (crash), comfort (equipment) and environmental aspects (e.g. exhaust emission standards) on the body have steadily increased. In order to further reduce weight, it is also necessary to consider the choice of materials. In our example, the composite design approach was chosen.
In addition to its purely structural functions, a bus roof also has other functions such as protection against environmental influences such as heat, cold or water. In addition, it serves as a carrier for various interior and exterior paneling parts as well as functional components, which in the previous design were taken over from many different components such as roof paneling, insulation sections, brackets or interior ceiling.
By using a sandwich component, many of these functions can be integrated and in some cases even improved, such as insulation due to the elimination of cold bridges. Above all, however, by reducing the number of parts, avoiding duplication of material and function and optimizing the geometric design, further potential for weight reduction can be realized. Thus, a functionally integrated sandwich roof module can be approximately 25 % lighter than a conventional steel-frame skeleton.
A functionally integrated sandwich roof module can be 25 % lighter than a conventional steel-frame skeleton.
Roof Module in Sandwich Construction
▸ top layer outside and inside: aluminum material, visible surfaces coated with a final lacquer finish
▸ core foam: extruded plastic foam
▸ edge profiles left and right: aluminum material
▸ empty conduits (inlays): aluminum material
▸ composite adhesive: two-component (2C) adhesive.
The sandwich panels are manufactured by vacuum pressing. The individual elements are placed in a vacuum mold one after the other and glued together. After all components for the sandwich roof have been positioned on the vacuum table, the sandwich panel is covered with a rubberized tarp. The air below the tarp is extracted with vacuum pumps, and the tarp wraps around the raw part of the sandwich roof. The roof module is now pressed. The requisite pressing force of approximately 8 t is generated by the ambient air pressure. Depending on the choice of adhesive, the roof module blank rests under the tarp in the pressed state until the adhesive hardens.
The sandwich panel is removed from the vacuum table after the two-component adhesive has hardened. In a further process step, all cut-outs such as air vents, inlet and outlet openings of the air conditioning system, escape and roof hatches, as well as holes for pipes or antennas are removed from the sandwich panel according to configuration and equipment for a roof module. Mechanical processing takes place on a CNC portal milling machine. All cut-outs and bores are milled out in one operation.
For the approval procedure of a bus type an experimental or a numerical procedure can be used. If the numerical method is chosen, the mathematical model must be certified in a validation procedure . As a validation procedure for the certification of the mathematical model, a comparison was made here between a practical test and a computer simulation. For the practical test, a “practical prototype” was defined and established as the test object.
The test object was displayed in a city bus segment. The segment included the front skeleton structure of a typical city bus, a front axle segment and a further window section, so that the segment represented almost half a standard city bus. The functionally integrated lightweight roof was installed on top of the entire segment. The roof contained all component cutouts that can be found in a typical city bus. Windows and seats were not considered as structure- reinforcing components. However, the segment was equipped with additional mass to demonstrate a representative weight in comparison to the full vehicle and to generate plastic deformations in all structural components.
The computer simulation was and will be carried out with the LS-Dyna  software. The expected results were handed over to the technical service before the practical examination. The technical service was present during the practical test, so that the first comparison with the computer simulation could take place directly at the test object on site after the test was carried out. Subsequently, a validation report was prepared by the technical service and MAN received the approval of the technical service for the numerical approval procedure. Thus, the mathematical model is validated and certified. All other bus types can undergo the approval procedure with the help of virtual testing by computer simulation. This eliminates the need for a real test for each individual bus type.
Crash Load Case ECE-R66
Aim of the Validation Procedure
The dynamic numerical method has already been certified and validated on a welded steel structure in the past . The aim was to use the same process for steel structures, alternative lightweight structures and alternative joining processes.
All other bus types can undergo the approval procedure with the help of virtual testing.
The practical test was carried out according to the current state of the art. The segment was tilted from an 800 mm high platform onto a dry concrete surface. The platform was lifted slowly by a mobile crane. The mobile crane was driven constantly and slowly so that a dynamic influence could be excluded.
The test was documented using five high-speed cameras. In combination with 2-D target tracking points, the rotational speed at the time of impact, the deformation in the skeleton and the movement over the test period could be precisely traced and compared with the computer simulation. The maximum deformations of the individual wall columns in the direction of the survival space could be determined using cable extension sensors. The triggers on each side wall column on the overturn side gave a detailed assessment of the falling, deformation and slipping properties of the skeleton on the concrete floor.
Process Chain up to Certification
determination of material characteristics
tests on the composite
quasistatic segment test
various dynamic overthrow tests
validation attempt: dynamic overthrow.
Simulation and Practical Testing
Almost all areas of the framework with plastic deformations correspond to the computer simulation.
The photometric evaluation in combination with 2-D target tracking points showed a deviation of 0.1° in the tilt angle between the practical test and the virtual environment. The rotation speed also shows a minimum deviation of 0.01 rad/s. Thus, the mass of the test person, the center of gravity and also the kinetic energy were identical in the practical test as well as in the numerical method.
In the direction of movement x, the curves of the real and the virtual evaluation diverge, since the friction coefficient in the practical test is not ideally equal over the length of the segment. In the area of the A-pillar, the segment is inhibited in its slipping process and thus the slide properties in the area of the D-pillar are increased, Figure 8 (bottom).
As explained, lightweight construction does not necessarily have to be “purchased.” By using a sandwich roof, both weight and manufacturing costs can be significantly reduced. This is achieved by selecting materials in accordance with requirements, integrating functions and pre-assembly coordinated with the production planning.
An important milestone for the series launch is that it is demonstrably possible to obtain homologation according to ECE-R66 0.2 using a numerical method. The computer simulation procedure regarding the crash resistance of the body of buses and coaches with a lightweight roof was accepted according to the ECE-R 66.02 regulation and approved by the technical service. This means that all future bus types from MAN Truck & Bus can be certified using the numerical approval procedure.
The integration, pre-assembly and structural design of the sandwich roof were already developed using simulation methods in the concept phase and confirmed in practical tests. In future, this concept can also be extended to other vehicle series and derivatives.
- Directive 2007/46/EC of the European Parliament and of the Council of 5 September 2007 stablishing a framework for the approval of motor vehicles and their trailers, and of systems, components and separate technical units intended for such vehicles (Framework Directive). September 05, 2007Google Scholar
- LS-DYNA. Livermore Software Technology Corporation (LSTC). 2013Google Scholar
- Regulation No 66 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of large passenger vehicles with regard to the strength of their superstructure. March 2011Google Scholar
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