design for body bending - universiti teknologi malaysiaarahim/l4-body bending...

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



� Severe bending conditions can be occurred due to dynamic loading and

jacking/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 to

approximate bending moment envelope

� It can be 1 or 2 point loads applied at

the seating location

The H Point Bending Test:

� Body is supported at the suspension

attachments

� The loads are increased incrementally

and the deflections are recorded until it

reachs permanent deformation



Bending stiffness

� Can be measured from the load-deflection curve

� The reason is to cater for body vibration so that it can achieve the feeling of

solidness

� The desired bending frequency is from 22-25 Hz

� Assume 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 deformation

� 7000 kN/m



Load Path Analysis

o To meet body strength requirement, the structure must be carefully

designed

o Only end and shear loads are allowed in the structural surface model

o The applied load will represent the bending strength requirement

o Each surface must be capable of reacting the loads without excessive

permanent deformation

Example 1



Analysis 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



Finite element analysis



Importance of joint flexibility

• Previous analysis assumed the beams

were rigidly connected

• In reality, when two or more thin-walled

beams are joined, localized deformation

may occur

• Thus, it has the effect of a flexible joint

and this can be represented by rotational

spring

• The rotational stiffness can be determined

by taking ratio of moment over rotational

angle



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 = 1mm

L = 1000mm. Compute the joint efficiency for Hinge pillar to rocker joint.



Solution

I = 4.15E+5 mm^4

K = 0.2E+6 Nm/rad from diagram

E = 207 GPa

Joint efficiency, f

= 1/(1 + (2x207000x4.15E5/1000x0.2E6))

= 0.537

The joint reduces ½ of the beam alone



Example 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 bending

stiffness 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



Example 5

Consider the seat mount system consisting of a beam connected by a

Flexible 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

design for body bending - universiti teknologi malaysiaarahim/l4-body bending...

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