smc 4133 automotive structures design for body bending body bending strength requirement: to locate...
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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.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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 deformation7000 kN/m
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDINGFinite element analysis
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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 = 1mmL = 1000mm. Compute the joint efficiency for Hinge pillar to rocker joint.
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY BENDING
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
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?
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
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
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.