smc 4133 automotive structures design for body bending body bending strength requirement: to locate...

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

Body bending strength requirement:• To locate and retain the vehicle subsystem in the correct position• Does not fail under static/dynamic loading conditions

Shear loads & moments can be identified from the S-BM diagrams

All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.



Severe bending conditions can be occurred due to dynamic loading andjacking/towing

A factor of 2-g loading is typically used to represent dynamic condition

These two extreme conditions might cause the structure to fail




The H Point Bending Test is used toapproximate bending moment envelope

It can be 1 or 2 point loads applied atthe seating location

The H Point Bending Test:Body is supported at the suspension attachmentsThe loads are increased incrementallyand the deflections are recorded until itreachs permanent deformation




Bending stiffnessCan be measured from the load-deflection curveThe reason is to cater for body vibration so that it can achieve the feeling ofsolidnessThe desired bending frequency is from 22-25 HzAssume that the structure as a uniform beam; the primary bending frequency is

M = wL/g

Now, with a single static load at its center span, the bending resonant frequency is

Simply supported

l

L




Typical values of body strength and stiffness for a mid size vehicle are;

6680 N without permanent deformation7000 kN/m



DESIGN FOR BODY BENDINGLoad Path AnalysisoTo meet body strength requirement, the structure must be carefully designedoOnly end and shear loads are allowed in the structural surface modeloThe applied load will represent the bending strength requirementoEach surface must be capable of reacting the loads without excessivepermanent deformation

Example 1



DESIGN FOR BODY BENDINGAnalysis of Body Bending Stiffness

• Focus on the side frame due to its significant contribution on bending stiffness• The model consists of beams, rigid plates and pin connections• Applied load acts at the center of rocker/end of B-pillar• Approximation of the stiffness is made using finite element method




• Example of the analysis is given below with the initial guess for beam section size• The result shows that the total bending stiffness is 2088 kN/m• Only 30% of the target value (7000 kN/m)• Change the beams section size/shape. BUT which BEAM?

Example 2



DESIGN FOR BODY BENDINGFinite element analysis



Importance of joint flexibility

• Previous analysis assumed the beamswere rigidly connected

• In reality, when two or more thin-walled beams are joined, localized deformationmay occur

• Thus, it has the effect of a flexible jointand this can be represented by rotationalspring

• The rotational stiffness can be determinedby taking ratio of moment over rotationalangle




Joint Efficiency

•To check whether the joint stiffness is a very stiff or very flexible•It can be define as the ratio of the combined stiffness of the beam-joint to the stiffness of the beam alone

Example 3

The steel rocker beam has section size of h = 100mm, w = 50mm, t = 1mmL = 1000mm. Compute the joint efficiency for Hinge pillar to rocker joint.




Solution

I = 4.15E+5 mm^4K = 0.2E+6 Nm/rad from diagramE = 207 GPa

Joint efficiency, f= 1/(1 + (2x207000x4.15E5/1000x0.2E6))

= 0.537

The joint reduces ½ of the beam alone



DESIGN FOR BODY BENDINGExample 4

Consider Example 2 with reasonable joint stiffness to three of the joints.Re-run FEA.

It is found that the deflection is increased and hence, reduce the bendingstiffness to 1735 kN/m; closer to the test data.

However, the value is far from the target value. HOW to achieve it? Which beams or joints to adjust?




Strain energy and stiffness-As the beams deform under load application, strain energy is stored. -The strain energy can be determined as a function of the end moments on the beam

- The highest fraction of strain energy will improve stiffness of a structural system



DESIGN FOR BODY BENDINGExample 5Consider the seat mount system consisting of a beam connected by aFlexible joint to a rocker. The system does not meet the stiffness requirement,.Which element needs to be changed: the beam or the joint?

Solution

SE beam = 200xM^2/(6x1E10) = 3.33E-9M^2

SE joint = M^2/(2x2E8) = 2.5E-9M^2

The beam stiffness has a larger effect on overall system stiffness


smc 4133 automotive structures design for body bending body bending strength requirement: to locate...

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