design optimization of a roll cage of a sae baja car

ME 511 FINITE ELEMENT ANALYSIS Fall 2015

Design optimization of Roll cage of formula type SAE vehicle

Project report Instructor Name: Dr. K. Nematollahi

Students: Satyajeet Udavant

Abhishek Khoje Surendra Patil

PURDUE SCHOOL OF ENGINEERING

ME 511 FEA Fall 2015 Satyajeet Udavant

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Objective

The objective of the project is to optimize the roll cage of a formula-style

vehicle by considering weight as a parameter. The roll cage is tubular frame which

has a circular cross section. The project consists of modeling the roll cage in CAD

software considering the rules mentioned in the rulebook for FSAE 2015. The solid

model is then analyzed by modal, static and frontal impact at certain speed. Two

different materials will be used during the analyses and the results will be

compared.

The analysis results from this project will help in optimizing other systems which

require high mechanical properties with low weight.

Introduction

The purpose of the roll cage is to provide a minimal three‐dimensional space

surrounding the driver ensuring his safety under all circumstances. The main

objective of the design process was to design a compact, sporty robust and

manufacturing friendly roll cage taking into consideration the FSAE Rule Book. The

cage is required to be designed and fabricated to prevent any failure of the cages’

integrity.

An optimized roll cage design is one which helps in achieving the best performance

parameters. Weight of a race car plays an important role in achieving the latter.

Finding a balance between the strength and weight of the roll cage is the

underlying reason of the optimization process.

Finite element analysis helps in the optimization process by discretization of the

entire model into smaller elements and then analyzing their behavior under

different loading conditions.


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The finite element method for optimization includes the modeling of the roll cage

structure in a CAD software considering standard parameters. This model is then

imported into the FEA environment by using the ANSYS Workbench software. The

ANSYS software virtually assigns different material with different thickness to the

tubular frame structure. The structure was further meshed and then subjected to

different loading conditions and results were obtained. The results thus obtained

were compared and the best material with an optimum thickness for the tubes

was determined.

Background

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 time-dependent loads. You can use this type of analysis to

determine the time-varying displacements, strains, stresses, and forces in a

structure as it responds to any combination of static, transient, and harmonic

loads. The time scale of the loading is such that the inertia is considered to be

important.

Model Details

The roll cage is a tubular frame of a uniform cross section. The roll cage has been

designed around a 95 percentile male sitting in an optimized and comfortable

driving position. The chassis consists of the front bulkhead for the pedal box and

front suspension components, a cockpit for the driver and the driving controls, and

an engine bay for the engine, powertrain and rear suspension components.

For impact analysis, velocity of the vehicle was assumed to be 60 km/hr and the

impact time was considered to be 0.4 sec. The force was calculated from the mass


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and the deceleration after impact. The force was calculated to be 18,348 N. The

mass of the vehicle is considered to be 244 kg (Roll cage: 64 kg Driver: 80 kg

Engine/powertrain: 75 kg and Mountings: 15 kg)

Materials used

Properties AISI 1018 Mild Carbon Steel

AISI 4130 Chromoly

Outer Diameter (mm) 25.4 25.4

Thickness (mm) 3 2.3

Density (kg/m3) 7870 7870

Ultimate tensile strength (MPa)

440 670

Yield tensile strength (MPa)

370 435

Poisson’s Ratio 0.29 0.29


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Procedure

The roll cage was modeled on a CAD software. This model was imported into

ANSYS workbench. The geometry was discretized with a medium size mesh. The

material properties for the roll cage were uploaded and were assigned to the

geometry. The following analyses were done:

Static Analysis:

Static analysis was an initial step for the later transient analysis done for the

impact study. Static analysis was done by fixing the rear end or the rear

frame of the rollcage and by applying the driver weight, engine/powertrain

weight and the mounting weights at their respective positions in the rollcage.

The calculated force was applied to the front faces of the front bulkhead.

Gravity was applied to the whole frame structure. The static analysis was run

and results were obtained.

Modal Analysis:

Modal analysis was performed as a prerequisite for the transient analysis.

The analysis was performed by fixing the rear end of the rollcage frame

structure and six modes were obtained.

Transient analysis:

Transient analysis was done for the impact study of the rollcage. The

conditions used for the transient analysis were similar to the static analysis

done earlier and deceleration factor which was computed from the velocity

and impact time factor was included for the analysis. Hence the inertia effect

was considered for this analysis. The transient analysis was run and the

deformation and the stresses were obtained.


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Results and Discussions

1. Static Analysis

AISI 1018 (Mild Carbon Steel)

Equivalent Stress

Total Deformation


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Factor of Safety

Material second

AISI 4130 (Chromoly)

Equivalent Stress


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Total Deformation

Factor of Safety


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Modal Analysis

AISI 1018 (Mild Carbon Steel)

Mode Frequency (Hz)

1 22.438

2 29.95

3 44.977

4 56.75

5 70.138

6 93.252


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Mode Frequency (Hz)

1 22.036

2 29.893

3 44.261

4 55.9

5 68.638

6 92.76


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Transient Analysis

AISI 1018

Equivalent Stress

Total Deformation


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Factor of Safety

Material second


Equivalent Stress


13

Total Deformation

Factor of Safety


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Discussions

Material Properties:

Properties AISI 1018 Mild Carbon Steel

AISI 4130 Chromoly

Outer Diameter (mm) 25.4 25.4

Thickness (mm) 3 2.3

Density (kg/m3) 7870 7870

Ultimate tensile strength (MPa)

440 670

Yield tensile strength (MPa)

370 435

Poisson’s Ratio 0.29 0.29

Comparison:

AISI 1018 Mild Carbon Steel

AISI 4130 Chromoly

Equivalent Stress (MPa) 144.58 215.79

Total Deformation (mm) 1.0381 1.918

FOS 1.6599 2.0158

Wall thickness (mm) 3 2.3

Conclusion

Increased crash worthiness of the roll cage model. As shown in above table of comparison I

was successful in reducing the diameter thereby reducing the overall weight of the roll cage

and increasing the stress handling capacity.

design optimization of a roll cage of a sae baja car

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