Static and dynamic analysis of space frame

STATIC AND DYNAMIC ANALYSIS OF SPACE FRAME Anand Krishnan.M , Jithin Pallath P School of Mechanical and building sciences, VIT University, Vellore - 632014, Tamil Nadu, India

ABSTRACT : This study is based on space frame used for formula SAE. In this study we focus to build a frame, which is stiff and safe for the driver. Design model was prepared using anthropometric parameters of tallest driver (95th percentile male), SAE rules book and design knowledge. Static and dynamic analysis is done to validate model. One of the dynamic characteristics we focus in our study is the natural frequency .Different frequency modes were analysed and found considerable factor of safety as required.

Keywords - Space frame, Modal Analysis I.

INTRODUCTION

Space frame are widely used by sports cars and in competitions like formula SAE and in mini Baja design competition Since the car have to perform in very high acceleration, braking, handling and safety aspects, each part of the frame should have design for good stiffness, strength and durability. A space-frame is constructed from an arrangement of small, simple members which make up a larger frame. A space-frame is analogous to a truss style bridge which is made up of small members in a triangular pattern which are always in pure compression or tension. Light weight is a primary goal for all components in a race car as a lower weight requires less force to accelerate by the same amount. So given the same force, a lighter car will accelerate quicker. Stiffness is also a desirable property for a race car chassis to have. These properties can be well in cooperated in space frame. The global vibrational characteristic of a vehicle is related to both its stiffness and mass distribution. The frequencies of the global bending and torsional vibration modes are commonly used as benchmarks for vehicle structural performance. Bending and torsion stiffness influence the vibrational behaviour of the structure, particularly its first natural frequency. The mode shapes of the car chassis at certain natural frequencies are very important to determine the mounting point of the components like engine, suspension, transmission and more. Therefore it is important to include the dynamic effect in designing the chassis. This paper deals with the design, static and modal analysis, carried out using ANSYS Workbench.

II.

CHASSIS LOADING

Frame is considered as a fabricated structural assembly that supports all functional systems on vehicle. The assembly may be of a single welded structure, multiple welded structures or a combination of various composites and welded structures. Depending upon application of load and their direction of application, chassis is deformed in respective manner pointed as follows:  Longitudinal Torsion  Vertical Bending  Lateral Bending  Horizontal Lozenging

III.

METHODOLOGY

The model is made in ANSYS using the key point and given pipe cross-section having outer diameter 25 mm and thickness 3mm. The material for pipe we selected was stainless steel with yield strength 200MPa.Meshing and analysis done on ANSYS. The vehicle load considered is 300 Kg. Detail brake up is show below in Table 1 (Mass round up to 300 kg to includes mass of wishbones (front and rear), petrol tank , radiator etc ).

Table 1 Vehicle load consideration

Components

Mass (Kg)

Driver

80

Engine

70

Drive-train

20

Steering

10

Battery

3

Chassis

54

TOTAL

237

Table 2 Material Properties

Material

Stainless steel

Tensile Yield Strength Compressive Yield Strength Tensile Ultimate Strength Density

2.5X10^8 2.5X10^8 4.6X10^8 7850 kg/m3

Table 3 Boundry Condition Applied

Sl. No. 1 2 3 4

Test

Boundary condition

Force Moments

Static torsional

Clamp- rear suspension mounts

Clockwise Moment at bulkhead side

Front impact Static Vertical bending Modal analysis

Clamp- rear suspension mounts

Force applied towards rear

Clamp- front and rear suspension mounts

Uniformly distributed load

Clamp- front and rear suspension mounts

Figure 1Meshing of space frame

IV.

RESULTS AND DISCUSSIONS

Model analysis Model analysis of the frame is critical since it tells us whether the frame is in resonance at any instant. In this, Chassis was clamped at both rear and front due to suspension construction with no external load applied. The model were analysed for first six natural frequencies. Taking only the structural mass to account for analysis. Maximum translational displacement of 437.85 mm was noticed for 33.994 Hz frequency. Of the infinite modes of vibration that exist on the frame structure, only the lowest frequencies are of interest. The lower modes of vibration maximize the kinetic energy and maximize the strain energy, while the high modes act in an opposite manner. The frequency of the vibration due to road undulation varies up to 25Hz. The results of the modal analysis shows that all frequency are above 26Hz .From this we can infer that the chassis is safe from resonance and as the load is added when sub system is assembled.

Table .4. Shows frequency for different modes

Figure 2 Displacement versus mode graph

Table 5. Showing various displacements for corresponding frequency

Fig 3. First mode of frequency

Fig .4.Second mode of frequency

Fig .5.Third mode of frequency

Fig .7.Fifth mode of frequency

Fig .6.Fourth mode of frequency

Fig .8. Sixth mode of frequency

Torsional Rigidity Torsional rigidity test is one of the most important tests which validates/rejects the chassis structure. If the chassis is rigid, the car will responds only to the spring, damper and antiroll bar change. In this case, chassis is assumed to act as a cantilever with one end fixed and other end free and subjected to torque of 500 Nm about its longitudinal axis as shown in Figure 9. A chassis should be able to resist angular deformation and resultant shear stresses. Again clamping is shown by the blue colour and clockwise torque is shown in yellow colour. Maximum combined stress of 1.9485e+007 N/m2 was observed with maximum direct stress of 3.8212e-008 N/m2 at few points as shown in red colour. Maximum translational displacement of 1.7126e-003 m was noted in front bulkhead supports and lowers side impact (act as a cantilever with one end fixed and other end free and subjected to torque of 500 Nm about its longitudinal axis as shown in Figure 9. A chassis should be able to resist angular deformation and resultant shear stresses. Again clamping is shown by the blue colour and clockwise torque is shown in yellow colour.) members. Almost all other areas were found to be safe with approximately no or minimum stress and displacement

Fig 9 Torsional rigidity on total deformation

Fig 10 Torsional rigidity on direct stress

Front Impact An acceleration of 1.5g is considered for the frontal impact. We fix the rear suspension mount and load of 4500N is applied at the front bulk head. A total stress of 8.35X10^5 N/m is developed in the chassis ,which is much less than the yield strength of the material. Total maximum deformation happens is 1.6mm,which is acceptable

Fig11 Total deformation on front impact

V.

Fig12 Direct stress on front impact

CONCLUSION

A space frame model was designed and analysed  From modal analysis at lower frequencies (below 33.994 Hz), no considerable stress was found and chassis was assumed to be safe with considerable factor of safety.  Static vertical bending test maximum deflection was observed in the centre of driver cabin floor.  Torsional rigidity test maximum translational displacement was noted in front bulkhead supports and lowers side impact members other parts are found safe.  At front impact test deformation happens is 1.6mm,which is acceptable . Since the model and static results shows that all the deformation and stress are well within the acceptable limit .The space frame that designed can be used without any further modification.

REFERENCES [1]

William F.Milliken and Douglas L.Milliken , “Race Car Vehicle Dynamics”.

[2]

Reid F.Allen ,Design and Optimization of FSAE car chassis and suspension,MIT 2009

[3]

Formula SAE rule 2013