Structural Modelling of a Monocoque Closed Integral Sedan Car Body

This The car body structure is a highly redundant structure with more than one load path and the sharing of the load is a function of the component relative stiffness. In static stress analysis, only the loads that cause elastic strains and stresses in the structure are studied in such a way that there is no yielding of the material or joint failure . Practical experience suggests that a vehicle structure which is strong enough to endure the maximum dynamic loads, which occur sporadically, will also have sufficient resistance against repeated dynamic forces at less than their average value (fatigue loading). Then the key factor in the design analysis process is the determination of The Dynamic Load Coefficient’ which can be used in the static analysis. [1] The structural stress analysis of the car body is carried out by the discussion of: 1The concept of the load-bearing structure. 2The determination of the input loads. 3Implementation of different theories for the static loading of the structure and the strength of the body and its joints. The first part of the structural stress analysis is the study of the load-bearing structure. The general description of the car body structural components under consideration in this paper and its three subassemblies are discussed followed by the descriptions of the structural model idealization.


I. INTRODUCTION
The main objectives in car structural design are to obtain a minimum weight design which makes the best use of material by arranging for each member to support as near as possible its maximum load potential, to make the structure direct and continuous by providing an unbroken path from point of application to point of reaction, to minimize weight; by spreading concentrated loads as much as possible; which are efficiently distributed to react applied load with the minimum redundancy, to avoid buckling of the thin sheet materials in monocoque structure type, and to provide passenger protection in accidents. [1] Although the modern passenger car, due to aerodynamic and styling requirements has surfaces with high curvature, the structure behind these surfaces can be approximated to components or subassemblies that can be represented as plane surfaces. [3] The-Integral (monocoque) structure is that type of structure where the all bodywork, except the door and hatches, are load carrying elements, so obtaining lighter integral body/chassis structure with less Hysteresis effect in the unloading curve than the ladder frame due to slippage between the body & chassis at the mounting points obtaining a more lighter and stronger car, as shown in Fig 1 [4] Modeling process, which is considered as the major part in structural stress analysis, is obtained by representing the physical car body as an idealized structural model valid for applications of different suitable structural analysis theories to ensure the continuity for the load path through the structure, the integrity of the structure, and that the structure is capable of transferring different types of loads within allowable stress range of the material with a factor of safety.
The idealized representative structural model of the principal internal structural load carrying members is obtained so as to give a simple and yet accurate representative idealization of a generic real sedan car body structure [2]. This idealization is then applied on a real existing saloon car "Nasr DOGAN 1.6 liter " to obtain its structural model. The crosssectional properties and gravimetric analysis need to be specified in the model as a part of the idealization of vehicle body structural model and to be used in the stress analysis that depends mainly on these cross-sectional properties.
II. METHOD OF APPROACH The vehicle body structure is considered to consist of three subassemblies in series (Frontal part, Central (passenger compartment) part, and Rear part). Both the front-end and the rear-end assemblies should have relative lower stiffness than the central part to form the crumple zones in case of collision, [1], [5], [6] Maintaining the Integrity of the Specifications The template is used to format your paper and style the text. All margins, column widths, line spaces, and text fonts are prescribed; please do not alter them. You may note peculiarities. For example, the head margin in this template measures proportionately more than is customary. This measurement and others are deliberate, using specifications that anticipate your paper as one part of the entire proceedings, and not as an independent document. Please do not revise any of the current designations. The front-end assemblies made up from: the two front side wheel panel assemblies which are connected to the front bulkhead and enclosed by the front cowl (or dash panel) at the rear and at the front by the radiator panel assembly, while the engine cross member assembly is a transverse beam that transfers the engine's weight to the longitudinal members of the front-end panel assembly. It is attached to the main floor panel at the toe panel, and by two lower supporting forks.
The central part (passenger compartment) comprises of the main floor panel assembly, side frame, and roof panel. The bulkhead towards the front and rear complete the box-like form. The curved shape of the roof panel prevents to be in a lozenge (going out of alignment in a diamond shape). Side frame assemblies reinforce the floor pan along the sill sections which consist of the A-post, the BC-post, the D-post, and therear quarter section The rear-end assembly consists of the rear wheel arch integrated with quarter section, the rear seat heel board (rear bulkhead) forming a closed torsion box with boot floor and rear panel assembly.
The following are the steps of the idealized representative structural model for the principal internal structural load carrying members, which give a simple yet accurate representative idealization of a generic real sedan passenger car body structure [7].

-The idealized structure is a combination of Frameworks
(such as the edges of larger openings as Doors, Windows) and Shell constructions (which are combinations of Beam sections and Skin panels, in which the skin plays a major part in carrying loads and the beams to take up the concentrated loads) that must be represented by Thin-Walled Shell Structures in the final design stage, and by a simplified one during the first stage as Structural Surfaces. 2 -The side frame is a flat stiff frame, then one shear panel is sufficient to take up the reaction at the upper front and rear bulkheads as shown in Fig.2 [8] 8-There are several areas that are 'closed-boxlike' structure that will be connected to the side frame which will provide additional torsional stiffness to the overall structure, namely: 1-the engine and luggage compartments, and 2-cowl and footwell assembly (the region enclosed by the front bulkhead, cowl, lower A-pillar, and floor) as shown in Fig.7.
[6] Eqn (1) Thus it's very important to have sub-assemblies with correct load-path design and sufficient stiffness and the connections between them are structurally sound to obtain a well tensional stiffness structure. 11 -The upper rail of the frontal part is positioned to run directly to the side frame to minimize or bypassing the torsion on the parcel shelf (the front cowl) as shown in Fig.9. [6] 12 -Determine which elements of the structure are to be included in the analysis for overall strength and which for local strength and identify all the load-bearing structure elements of the car body before the analysis is made, and neglect the set of elements which transfer up to 10% of loads as a limited accuracy required for design calculations. 15-Three-Dimensional representative structural model idealized representative structural model of the principal internal structural load carrying members is obtained so as to give a simple idealization of a generic real sedan car body structure. This idealization is then applied on a real existing saloon car "Nasr DOGAN 1.6 liter" to obtain its structural model Fig. 11. 16- Fig .12 shows the "Nasr DOGAN 1.6 liter" car body structural model, implementing different loading cases imposed on the car structure and applying different stress analysis theories using different program packages.  Investigating the effect of changing the type of fixations (strained DOF) of the structural model on the obtained results on the above three models by changing the two rear contact points to be with roller support with one DOF constraints instead of simply supported contact points.
Performing structural optimization process; to get the optimal construction under different constraints (strength, deflections, and stiffnesses) that is capable of carrying the required loads and meets all specified requirements criteria with a minimum component weight for both bending and torsion cases; by using the suitable optimization method, defining the structural model properties in terms of parameters from which design variables may be selected, selecting the design variables from beam and shell thicknesses, and specifying the state variables that serves constraints to the design. Comparing the total car torsional stiffness; calculated automatically by a batch file throughout the optimization analysis process; with the more generalized formula for closed body structure deduced by C.J.Cooke to validate the structural analysis process. [9] III. DISCUSSION OF THE RESULTS Effect of type of fixation and the degree of meshing on the obtained results The comparison of the obtained results from different meshing stages showed a convergence of the deflection components at all nodes as going through from coarse to fine mesh stages. There are some local areas in which the deflections at the corresponding nodes, especially the vertical components, have high values namely: the main floor assembly, B-pillar beam, and the roof assembly. It is noticed that all these nodes are in the passenger compartment because there is no intermediate vertical shear panels. So, making a fine mesh at these areas will be reflected on the results and get a clear picture on their behavior.
Optimization results A comparison was made for the best feasible set of the optimization data files; that satisfy all specified constraints within allowable limits of Design Variables (DVs) and producing the minimum weight; obtained from both the Torsion and the Bending Cases. Then selection of the DVs that satisfy both the Bending and Torsion cases, which converge to the nearest integers that could be used as a sheet metal thickness, and running the program to obtain the model with the best selection of DVs that are reliable at the worst operating conditions and also satisfy all stiffness constraints. Results of the sensitivity analysis of the optimization processes are shown in Table 1. The table shows a comparison between the starting, best optimization in bending and torsion, and the manual combined optimization model for the most governing DVs.

IV. CONCLUSIONS
The structural analysis was performed, on a real existing sedan closed integral monocoque passenger car, by using dynamic load factors to represent the dynamic loading conditions imposed on the structural model to be nearly as the reality. Results revealed that the torsion loading case has higher internal forces and stresses in some members of the load paths than the corresponding bending loading case. The comparison between the theoretical and the calculated torsional stiffness of the overall car body structure in the optimization process for the combined torsion loading cases are almost consistent and enhanced through the optimization process, as shown in Fig. 14.
Although both the bending and torsion best optimization results give smaller DVs values, they take values near the starting in the final manual selection because of constraints on "DVs" imposed on them as follows: 1 -Beam thickness# 3 (engine cross-member) because of bunching and fatigue imposed on it during operation. 2 -Shell thickness# 4 (Floor panel assembly) because of having sufficient stiffness for the payload and other point-loads acting perpendicular on it, which are corrugated at some local areas.
The torsion optimization analysis results are affected by an increase in 'Beam thickness' which include the following beams (Front wheel panel  The comparison between the total car volume, which is a function of its weight reveals that a decay in its values through the optimization process. Fig. 15. Shows a comparison between the total car volume in both Torsion and Bending optimization processes.
Fig .14.Comparisons between the calculated and theoretical total torsional stiffness variation during optimization looping, Torsion Case. Fig .15.Comparison between the total car volume variation during optimizationlooping in both Bending and Torsion cases.