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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

� 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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

� 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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

� 6680 N without permanent deformation

� 7000 kN/m

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

• 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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

Finite element analysis

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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.

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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?

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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

SMC 4133 AUTOMOTIVE STRUCTURES

DESIGN FOR BODY BENDING

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