USPS BATTERY TRAY ASSEMBLY FEA

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USPS BATTERY TRAY ASSEMBLY FEA

Project Report

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Background

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• Based on the review of Phase 3 report with A123, we were asked to incorporate a different methodology.• The new methodology consisted of applying initial velocities along with the full pulse at the mounting points.

Global Composite Solutions 4

Methodology/Assumptions

• The mounts were assumed to be rigid i.e. they hold their form during impact.

• The velocities were applied to the entire structure.• The sandwich is modeled as 3 layers with Balsa core encapsulated between two layers of RTM.• Material was assumed isotropic. The average properties used for analysis

were taken from the data provided by A123.• The other assumptions related to bonding are same as they were in the

previous work.• The NCAP, Side 50th and Side 5th pulse were evaluated as these were the

most severe pulses based on the velocities.• For the remaining pulses there was no information on whether the barrier

moves or the car moves and they were at significantly lower velocities (Thus these were not evaluated).


Baseline


NCAP Baseline

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The RTM Cover is showing a peak tensile stress of 235 MPa at the corners.

The RTM Base is showing a peak stress of (206MPa, -173MPa). The regions with stress above 140 MPa are shown in red.

Signed VonMises Stress plot


The steel strap is showing a peak stress of (276MPa, -236MPa). The regions with stress above 274 MPa are shown in red.

NCAP Baseline Signed VonMises Stress plot


NCAP Baseline Signed VonMises Stress plot

Bolt forces are within the threshold of 8100 N.


Side 5th - Baseline

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The RTM cover is showing a peak tensile stress of 32 MPa.

The RTM Base is showing a global stress of 80 MPa. The areas exceeding 140 MPa would be shown in red.



The steel strap is showing a peak stress of (270MPa, -248MPa). The areas above 274 MPa would only be in red.

Side 5th - Baseline Signed VonMises Stress plot


Side 5th - Baseline

Bolt forces for two bolts exceed the threshold of 8100 N.


Side 50th - Baseline

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The RTM cover is showing a peak tensile stress of 32 MPa.

The RTM Base is showing a global stress of 80 MPa. The areas exceeding 140 MPa would be shown in red.


The steel strap is showing a global stress of 300 MPa. The areas above 274 MPa would only be in red.

Side 50th - Baseline Signed VonMises Stress plot



Side 50th - Baseline


Observations

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• The bolt forces exceed the threshold of 8100 in the Side 5th pulse.• Stresses for the RTM base are higher than threshold of 140 MPa for

Frontal NCAP. These stresses are localized at couple of regions only.• Stresses on the cover are higher than threshold of 140 MPa for Frontal

NCAP in two corners only.• Stresses in other places are within the allowable limits.


Design Modifications

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• In order to improve the load bearing capacity of the floor i.e. to withstand the large forces seen in two bolts, the thickness of the base RTM was increased to 4 mm from baseline thickness of 2 mm.

• The thickened results are corresponding to this increased base thickness to 4 mm.

• Henceforth, all the thickened designs would refer to the RTM layer with the thickness of 4 mm.

• Other pulses would also be run with this thickened floor to evaluate the implication of this change on the composite structure.

RTM: 2 mmBalsa: 6 mmRTM: 2 mm

RTM: 4 mmBalsa: 6 mmRTM: 4 mm

Baseline Design Modified Design: Thickened Base


Modified Design & Recommendation


NCAP-Thickened

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The RTM cover is showing a peak tensile stress of 234 MPa. The regions with stress above 150 MPa are shown in red.

The RTM Base is showing a peak stress of (111MPa, -82MPa)


The cooling pads is showing a peak stress of 134 MPa at the corners.

The steel strap is showing a peak stress of (276MPa, -236MPa). The regions with stress above 274 MPa are shown in red.

NCAP-Thickened Signed VonMises Stress plot


NCAP - Thickened



Side 5th – Thickened

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The stresses in all the components are within permissible limits for the Side 5th pulse with thickened base.




Bolt forces for all bolts are within the threshold of 8100 N.



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The RTM base is showing a peak tensile stress of 100 MPa. The areas above 140 MPa would be in red.

The steel strap is showing a peak stress of (250MPa, -159MPa)




Bolt forces for all bolts are within the threshold of 8100 N.


Bolt Force Comparison

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Beam ID NCAP NCAP-thk side-50th side-50th-thk side5th side5th-thk

136498 1454 1424 487 410 1217 1312136516 2680 2633 1091 828 807 986136545 1143 1613 496 586 1170 1585136561 6548 6118 2831 2800 1570 1715136568 1415 1823 1031 1546 3110 4169136586 2205 2418 2099 2160 4548 5122136604 1081 1027 484 776 5551 4865136621 2335 2280 1341 1562 4490 3644136645 2116 2201 758 799 5424 5655136663 2143 2167 1124 1233 6914 5285136695 2035 2631 669 765 6810 5169136712 567 979 1664 2223 7156 5329136721 3051 2837 3019 2759 6138 4794136753 1768 2239 887 1228 6761 5143136763 4973 5095 675 1489 9064 6762136779 3652 3653 1988 2016 8894 6524136798 4821 4718 1442 1406 7159 5750136815 4031 3870 508 552 8077 6715136833 2053 2449 1159 1729 5502 5014

136851 2561 2534 1291 1868 6057 6411


Bolt Force Plot

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1364

98

1365

45

1365

68

1366

04

1366

45

1366

95

1367

21

1367

63

1367

98

1368

330

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

NCAPNCAP-thkside-50thside-50th-thkside5thside5th-thk

Baseline Thickness

Base Thickened

Bolt forces are within the threshold of 8100 N for thickened base.

thk – Base Thickened


Observations:

1) The bolt forces for 2 bolts exceed the threshold force with the baseline thickness for the Side 5th pulse.

2) The stresses are within permissible levels for the base of the assembly.

3) The cover shows couple of spots with high stresses and solution to lower those stresses is part of the recommendations.

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4) The stresses in the steel straps are within the permissible levels. Yielding is observed in the steel straps.

Phase IV Observations

Update of 12/15/2009


Recommendations

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1. The battery tray using sandwich construction carrying a massive weight of battery stacks needs higher stiffness. This can be achieved by:

A. Use of more glass content in the composite skin by improving the process or using vacuum or using infusion.

B. Use of higher density/compressive strength core.

C. Use alternate material/processes for the skin.

D. Improving adhesion between the core and the skin. Change to PLIOGRIP 7400 series adhesives which are resistant to exposure to elevated temperatures, moisture, fuel, most solvents and chemicals. An established application pattern should be developed. Additional test data would be needed.

2. To remove the high stress local spots, we recommend that isolation pads be added to where the steel straps are mounted to the vehicle.

3. The battery tray holes design for bolts need improved design. This can be achieved by:

A. Modeling the holes as clearance holes.

B. Do not have raw balsa exposed in the thru-plane direction at the holes.

C. Use a compressive sleeve top arrest loading of composite skin.

D. Compressive sleeve should use an isolator grommet to avoid transferring the vibration. This should address the skin stresses seen in the high stress areas.


Recommendations

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3. The corner high stresses of 234 MPa on the cover could be addressed using local patches of bidirectional fabric of E-Glass or Carbon Fiber along with continuous fiber mat as shown below. These solutions would improve the threshold stress value to 300 MPa.

Local patches of 5-in circles at the locations highlighted above

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USPS BATTERY TRAY ASSEMBLY FEA

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