Optimization of Bus Body Structure

Simulation Driven Innovation 1

Optimization of Bus Body Structure Using OptiStruct

Harmeek Singh Engineer, R&D

Swaraj Mazda Ltd.

Village Asron, Ropar,

Distt Nawanshahar-144 533, Punjab (India)

[email protected]

ABSTRACT

Established in 1983, the Swaraj Mazda Ltd is presently manufacturing vehicles for goods

and passenger applications. Over the years it has built up a wide product portfolio covering regular

as well as niche segment needs. Swaraj Mazda Vehicle population today stands at over 115000.

The Swaraj Mazda Bus Body plant is set up by the Sumitomo Corporation, Japan to

manufacture the Luxury Buses powered by the Isuzu Engine.

The Bus is designed for the Indian Transportation Industry focused on the Serviceability

and Crashworthiness. This 39 Seater bus has the Gross Vehicle Weight as 11.9T having wheelbase

5400mm, overall length 1.2mtr and is powered by powerful ISUZU Euro IV Engine.

Sustainable profit-making through customer satisfaction in the form of better product and

service remain the main concerns of any manufacturing Industry. Increasing market competition

and demand of Quality Product has narrowed the margins of the manufacturers.

Topology and Topography refinements in the design maintaining the level of

Crashworthiness and meeting all the design standards can help in producing an ideal product.

Efforts have been made using the FEA with Hypermesh for Pre-Processing, Optistruct as Solver

and HyperView for Post-Processing to study the Material and Geometric requirements in the Bus

Body Structure & Chassis Frame based on the Boundary Conditions.

A significant amount of material was saved using small Geometric changes and by reducing

the thickness of the sections, maintaining the same strength and clearing all safety standards of the

vehicle.

BACKGROUND

High Performance, Better Quality and Passenger Safety Standards in the CV Industry have

made the job of the designer tough, Market Competition and Increasing Cost of raw material has

further made it tougher. Designing the Tubular Structure of the Bus with appropriate available

sections and thicknesses at the correct location in the structure to handle the forces and moments

within the Stress Range becomes a challenge for the team.

Different methods of Optimization (i.e. Shape Optimization and Size Optimization) are

used to reduce the overall mass of the structure maintaining the same level of crashworthiness in


the shell. It is very difficult to identify the concerned members in a cage of approx 500 tubes to

remove, to relocate, to change the section or to reduce the thickness manually with very less scope

of error. The physical testing of the bus is seen rarely as the time and huge amount of cost is

involved along this, the time of development cycle is increased which causes delays.

So, the need of such kind of tool that optimizes our product at the Concept Level itself.

PRE-PROCESSING

The FE-Modeling is done in the HyperMesh on the Middle Surface of the Tubular Sections

using various Elements maintaining the basic Quality Checks of the Model. The model is made

free from Penetrations and Interferences. The normals and Free Edges are also checked.

Glasses, Doors and other components have been considered as the lump loads and are modeled

using the RBE2 and Mass Elements. The bolts are modeled with multiple RBE2 elements with 2

layer washers.

Elements used in Modeling:

1. Shell Elements (Quad & Tria Elements) for the Tubular Geometry

organized in 75 Component Collectors.

2. RBE2 Elements for depicting the Welding between the Tubes.

3. Mass Elements for the Concentrated Loads in the Bus.

Material & Properties is assigned to the model using the cards defined in the Software. The

Overall Weight and Center of Gravity of the FE-Model is brought in consonance with the CAD

Modal to correctly define the Model.

The model is constrained using SPC elements at the appropriate locations and the model is set

for analyzing the Structure for Bending in the Static Load Conditions.


OPTIMIZATION

Size and Shape Optimization are the methods where Variables can be assigned to

perturbation vectors, which control the shape of the model. Variables can also be assigned to

properties, which control the thickness, area, moments of inertia, stiffness, and non-structural mass

of elements in the model.

In size optimization, the properties of structural elements such as shell thickness, beam

cross-sectional properties, spring stiffness, and mass are modified to solve the optimization

problem.

In shape optimization, the outer boundary of the structure is modified to solve the

optimization problem. Using finite element models, the shape is defined by the grid point locations.

Hence, shape modifications change those locations.

The model was setup for the Shape Optimization (with 3 variants of sections

20X30X2mm, 20X40X2mm, 30X30X2mm) using the load-case for Bending by constraining the

Maximum Stress and minimizing the Mass of the Structure with Shapes as the variables. The

analysis was run using Optistruct as the Solver.

The next setup for Size Optimization was run by freezing the sections by the output from

the last solution. In this the variable was the Thickness (1.2mm lower bound and 2.0mm as upper

bound) with stress as the constraint and again minimizing the mass of the structure and the Solution

was run using the Optistruct as the Solver with the same load-case.


RESULTS Post-Processing was done using the HyperView and the results were obtained were as:

STRESS PLOTS

Original Model Structure Weight: 1390Kg

Optimized Mode Structure Weight: 1315Kg


DISPLACEMENT PLOTS

Original Model Structure Weight: 1390Kg

Optimized Mode Structure Weight: 1315Kg


ORIGINAL MODEL OPTIMIZED MODEL DIFFERENCE

MASS 1390 Kg 1315 Kg -5.4%

STRESS

(Von-Mises) 325 MPa 321 MPa 1.2%

DISPLACEMENT 31.6mm 31.9mm 0.1%

The overall weight of the Structure is reduced by 5.4% by making the following alterations:

1. The Cross Sections of the certain key members is to be reduced to (40mmX20mm) from

(40mmX40mm).

2. The thickness of some members in the Floor and Roof Area is reduced to 1.6mm from

2mm.

CONCLUSION

The optimization of the design was carried out using the Shape and Size optimization with Section

Shape and Thickness as variables and with the objective to minimize the mass. The Optimization

tool, Optistruct has proved to be useful tool in Optimizing the Complex Tubular Bus Structure with

reduction in Stresses. The Optistruct has also given the freedom to alter the cross section,

thickness, their Locations and orientations of the members used in the Bus Structure and validate

the results.

NEXT STEPS….

The Structure will be further verified based on the load-case for Rollover Analysis of the Bus

Structure, which can reduce some more weight and confirms the Crashworthiness of the structure.

Documents

Optimization of Bus Body Structure