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STATUS OF THE COMPOSITE UNDERBODY COMPONENT AND ASSEMBLY STRUCTURAL TEST-ANALYSIS CORRELATION

Hannes Fuchs and Eric Gillund

Multimatic Engineering

Abstract

Computer aided engineering-based design methodologies have been utilized throughout

the Automotive Composites Consortium Focal Project 4 to assess the vehicle level structural

stiffness and impact performance of the composite underbody design proposals, and to

estimate the potential mass reduction for several candidate material scenarios.

To increase confidence in the vehicle level CAE model predictions and to better understand

the effect of material and manufacturing variability, prototype molded underbody components

were fabricated, and subsequently built into underbody assemblies to assess their structural

performance. Non-destructive component and assembly tests were devised to assess the

general static and modal performance of the underbody component, and a quasi-static

destructive test of a built-up underbody assembly was developed to simulate the deformation

and loading observed in the worst case vehicle impact design load case.

The paper will discuss the preparation and fabrication of the built-up test assemblies, the

structural stiffness and modal performance testing of trimmed underbody molded components

and assemblies, and the destructive testing of assemblies. The predicted performance was

investigated for two composite thickness assumptions to account for the additional thickness

observed in the prototype components. Predictions were then compared to the measured test

results to understand the status of correlation between the response of idealized components

and the as-molded prototype test components.

A comparison of the non-destructive stiffness and modal test results to the predictions

indicated that the stiffness and modal response were reasonable. The destructive underbody

test was developed to better represent the physical composite and metallic components. The

destructive underbody test was limited by buckling of the longitudinal rail. The results correlated

well with predictions up until rail buckling occurred, after which significant local damage was

introduced into the underbody. The fiberglass SMC fabric composite material and the weld bond

joints were found to very robust in terms of their ability to tolerate large local deformations

without separation.

Additional correlation studies and modeling refinements were underway at the time of this

writing and these results will be reported at a future date.

Background

The Automotive Composites Consortium Focal Project 4 (ACC FP4) is a joint program

between GM, Ford, and Chrysler to develop structural automotive components from composite

materials. Part of this project is to develop a structural composite underbody capable of

carrying crash loads as has been previously reported in [1-4].

A compression molded multi-layered fiberglass fabric reinforced SMC composite [5-9] was

selected as the material and process system [10-11] to best meet the performance and cost

objectives of the program. The process of weld bonding was selected as the means to join the

composite underbody to the steel body-in-white (BIW) structure [12-14].

A schematic of the fabric SMC composite underbody structure design and the integration

into the donor BIW via weld bonding is illustrated in Figure 1. The underbody was designed as

single piece molded component to replace 14 major stamped components and partially replace

4 minor stamped components. The underbody assembly was designed to be weld bonded to

the vehicle BIW to compatible with conventional body assembly.

(a) Donor vehicle

BIW and composite

underbody

(b) Weld bonded

joints

Fiberglass

fabric SMC

composite

(a) Donor vehicle

BIW and composite

underbody

(b) Weld bonded

joints

Fiberglass

fabric SMC

composite

Figure 1: Composite underbody design overview

CAE-based design methodologies were utilized throughout the program to assess the

structural performance of the underbody design concept at the vehicle level and to determine

the potential mass reduction. A net mass saving of ~11.3kg (26%) was predicted for the

composite underbody design based on the following structural load cases [1]:

EuroNCAP/IIHS 40 mph frontal offset deformable barrier (ODB)

NCAP 35 mph full frontal impact

FMVSS 214 33.5 mph side impact

FMVSS 301 50 mph rear offset impact

BIW static torsion and bending stiffness

BIW modal response

Several physical tests, including material coupon and sub-element tests, were conducted

throughout the program to determine material properties, validate test-analysis correlation, and

to develop modeling strategies [5-9, 11-13]. In order to evaluate the underbody materials and

processing, and to evaluate the performance for the actual underbody components, a prototype

component mold tool was build and prototype components were fabricated by the ACC team [1,

10].

A test plan was developed to conduct non-destructive component and assembly tests, and

also destructive tests of the assembly to simulate the impact performance of the underbody in a

vehicle environment.

The build of the test components and assemblies, the subsequent testing, and the

comparison to analysis predictions conducted to date are discussed in the following. As some of

the test and analysis result interpretations were being completed at the time of this writing, these

are presented as “work-in-progress”.

Test Assembly Component Build

As-molded underbody components were supplied by the ACC to the Multimatic Technical

Centre for component and assembly preparation and build, subsequent testing, and test-

analysis correlation. The underbody components were trimmed according to the 3-D CAD

design using a 5-axis laser to demonstrate the feasibility of this potential production trimming

methodology. The as-molded components, trimmed component design, laser trim fixturing, and

trimming detail are illustrated in Figure 2.

As will be described in a subsequent section, trimmed components were tested using non-

destructive tests only, and trimmed components were built into test assemblies for subsequent

assembly level non-destructive and destructive tests.

X~1647mm

Y~1470mm

Z~445mm

Component fixture / laser trimming Trimmed components

As-molded

components

Laser trimming detail

X~1647mm

Y~1470mm

Z~445mm

X~1647mm

Y~1470mm

Z~445mm

Component fixture / laser trimming Trimmed components

As-molded

components

As-molded

components

Laser trimming detail

Figure 2: Component trimming

During the molding of the prototype underbody components, it was found that the

components were thicker than the design intent [10]. This was verified during the trimming and

preparation of the seven underbody components. The mass summary in Figure 3 indicates that

the mass of the trimmed components varied between 28.2kg and 30.0kg, with a coefficient of

variation of (COV) of 2.3%. The average mass of all prototype underbodies was found to be

5.8kg, or nearly 20%, heavier than the design intent mass of 23.6kg.

Trimmed Underbody Mass

30.029.5

30.0

28.6

29.5

28.2

29.5

20.0

22.0

24.0

26.0

28.0

30.0

32.0

121510-5 121510-3 121510-6 121410-1 121510-4 120910-4 121510-8

Underbody ID

Ma

ss

[k

g]

Average

29.3kg

Design

23.6kg

Difference 5.8kg (19.6%)

COV = 2.3%


30.029.5

30.0

28.6

29.5

28.2

29.5

20.0

22.0

24.0

26.0

28.0

30.0

32.0

121510-5 121510-3 121510-6 121410-1 121510-4 120910-4 121510-8

Underbody ID

Ma

ss

[k

g]

Average

29.3kg

Design

23.6kg



30.029.5

30.0

28.6

29.5

28.2

29.5

20.0

22.0

24.0

26.0

28.0

30.0

32.0

121510-5 121510-3 121510-6 121410-1 121510-4 120910-4 121510-8

Underbody ID

Ma

ss

[k

g]

Average

29.3kg

Design

23.6kg



30.029.5

30.0

28.6

29.5

28.2

29.5

20.0

22.0

24.0

26.0

28.0

30.0

32.0

121510-5 121510-3 121510-6 121410-1 121510-4 120910-4 121510-8

Underbody ID

Ma

ss

[k

g]

Average

29.3kg

Design

23.6kg


COV = 2.3%

Figure 3 : Design vs. as-molded underbody component mass

The thickness of all seven underbody components was measured at each trimmed hole

location to gain insight into the thickness distribution relative to the design thickness. The image

on the left in Figure 4 shows the design thickness based on the 0.45mm ply thickness. The

image on the right in Figure 4 indicates the average deviation of all seven components relative

to the design thickness, showing that the average floor thickness was greater than the design

thickness nearly everywhere. The maximum average thickness deviation was found to be

~2.7mm thicker (38%) at the thickest area at the rear of the tunnel (locations T08-T11). The

maximum percentage increase relative to the design intent was ~47% (+2.4mm at locations T02

& T03) at the front of the tunnel.

Several reasons for increased underbody mass and thickness are being investigated by the

ACC team, however it seems plausible that the higher than expected areal weight of the

compounded fabric SMC material contributed to the increased thickness due to the increased

resin content.

Test Assembly Build

A test assembly was designed to simulate the worst case ODB design load case. The

objective was to create a simplified test to induce deformation and loadings similar to those

observed in the full vehicle impact simulation. To achieve a reasonable test configuration, the

test was designed around carry-over components from a donor vehicle BIW. The final test

assembly design is shown in Figure 5 along with the original donor vehicle components.

The test assembly was built-up from modified production BIW sub-components, a trimmed

underbody molding, and custom components including all of the weld bond doublers. The

molded composite underbody components were obtained from the ACC, the BIW components

were provided by GM, and the remaining components were fabricated. All of the components

were sequentially positioned in a custom designed assembly fixture according to a build

process, and then joined via adhesive bonding spot- and MIG welding. An automotive grade

crash-toughened epoxy adhesive was used to bond all components, with 0.75mm diameter

glass beads used to maintain the minimum bond gap. The adhesive was cured according to the

manufacturer’s specifications using a large walk-in oven. The various BIW components and the

assembly fixture used to build the assembly are depicted in Figure 6. The multi-step process

that was used to build each test assembly is illustrated in Figure 7.

Figure 4: Design vs. measured average thickness deviation of as-molded components (average of 7 components)

2125mm

1650mm

600mm

Original donor

vehicle components

Test assembly

design

Cross-member

attachments

2125mm

1650mm

600mm

2125mm

1650mm

600mm

Original donor

vehicle components

Test assembly

design

Cross-member

attachments

Figure 5: Test assembly

Figure 6: Test assembly build components

Figure 7: Test assembly build procedure

Test and Analysis

A total of six assemblies were built out of the seven composite underbody components that

were trimmed. The test matrix for all component and assembly tests is shown in Table 1. Note

that assembly 121510-6 was not tested to allow it to be used for display, and component

121510-8 was non-destructively tested to provide an additional test data point.

Table 1 : Test matrix

Stiffness Modal Stiffness Modal Destructive

120910-4 Assembly X X X X #1

121410-1 Assembly X X - - #3

121510-3 Assembly X X - - #4

121510-4 Assembly X X X X #5

121510-5 Assembly X X - - #2

121510-6 Assembly X X - - -

121510-8 Component X X - - -

AssemblyComponentUnderbody

Component /

Assembly

A finite element analysis (FEA) model was developed for the design intent layup. The layup

model included all design intent ply overlaps specified for the layup of each component which

resulted in the definition 315 individual laminates. A version of the FE model was created to

conduct linear elastic stiffness analyses and to evaluate modal performance of the underbody

using MSC NASTRAN [15]. Another version of the FE model was created to analyze the

nonlinear response in the destructive ODB test using LS-DYNA [16]. Additional thickness and

layup variations were also investigated. Adhesive bonding was modeled in each case using

solid elements, while the sheet metal and composite components were modeled using shell

elements. The composite and adhesive material properties and modeling strategies were based

on properties used in previous studies [11, 12].

Thickness Bounding Cases for Analysis

Ideally, the measured component thickness and actual layup definitions would we used

predict the response of the underbody. However, as seen in Figure 4, the thickness deviation of

the underbody varied throughout the component. An additional complication is that the laminate

thickness is associated with the fiber volume of the laminate, which affects the local laminate

stiffness and strength. Thus, material properties may even vary in the same laminate if the

thickness varies.

As a first step in accounting for the observed differences between the as-molded underbody

thickness and the design intent thickness, two bounding cases were considered to compare

with the test results. The bounding cases are illustrated in Figure 8, where Trial 57 represents

the design intent thickness and layup, and Trial 58 represents an assumed 33% average

increase in ply thickness from 0.45mm per ply to 0.60mm per ply. Note that the ply overlap

regions can be identified by the vertical and horizontal lines in Figure 8. Due to the prototype

nature of the test components, it is recognized that the actual as-molded thickness, layup,

overlap locations, and effective stiffness and strength properties may vary from those assumed

in Figure 8, and may influence the component stiffness, modal, and strength performance.

Trial 57 – design thickness Trial 58 – increased ply thickness

Max=5.4mm

Min=1.8mm

Max=7.2mm

Min=2.4mm

Trial 57 – design thickness Trial 58 – increased ply thickness

Max=5.4mm

Min=1.8mm

Max=7.2mm

Min=2.4mm

Figure 8: Analysis model thickness assumptions for bounding cases

Component and Assembly Stiffness Performance

Test Setup

Both molded components and built-up test assemblies were quasi-statically loaded in

bending and torsion to measure the static stiffness values. Displacements were measured at

various locations using string pots for the purpose of calculating the required stiffness values.

The physical test setup for the components and assemblies is shown in Figure 9. The

associated FE model setup is shown in Figure 10.

The physical test setup consisted of a stiff beam at the forward and aft ends of the

underbody. The forward beam was mounted to a pivot in the center of the beam, while the aft

beam was fixed. The test components were mounted upside-down in the fixture to facilitate

loading, with the component ends attached to the stiff beams via spherical joints. For the

component tests, loading plates were bolted to the composite component to distribute the

loading. For the assembly tests, load was introduced at attachment points to the front of the

longitudinal rails and sides of the rear rails.

In the case of torsion, a load was applied to the outboard end of the forward beam to induce

a rotation at the front of the floor. For the case of bending, the forward pivot was locked to inhibit

rotation, and a vertical load was applied to the loading bar at the center of the floor to induce

global bending. In each case, the average static torsion and bending stiffness was calculated

from the input force and the resulting rotation or bending displacement, respectively. Each

component and assembly was tested three times to obtain an average value for the torsion and

bending stiffness.

Figure 9: Component and assembly stiffness test setup

(a) Component test setup (b) Assembly test setup(a) Component test setup (b) Assembly test setup

Figure 10: Component and assembly stiffness FEA model setup

Results

The static component mass and stiffness results are summarized in Table 2. The mass and

stiffness results are averaged for all tests, and the results were seen to be consistent between

tests with a COV of less than 10% for all data, and a maximum individual test deviation of 16%

relative to the average. The components that were subsequently tested in assembly are

indicated in the table.

Table 2 : Component mass, bending, and torsion stiffness results

120910-4* 121410-1 121510-3 121510-4* 121510-5 121510-6 121510-8Test

AverageCOV

Mass [kg] 28.2 28.6 29.5 29.5 30.0 30.0 29.5 29.3 2.3%

Bending [N/mm] 2341 2016 2279 2005 2510 2001 2002 2165 9.7%

Torsion [N-m/deg] 66.2 55.1 64.4 57.2 62.6 63.0 63.1 61.6 6.5%

*Also tested in assembly

Component Test

The average test results from Table 2 are compared to the bounding case FEA predictions

in Table 3. Is can be seen that the average test results fall in between the predicted bounding

case results indicating that the bounding results are reasonable given the variation in

component thickness.

Table 3: Comparison between average component stiffness test results and bounding case predictions

FEA Tr57%Diff vs.

Test Avg.FEA Tr58

%Diff vs.

Test Avg.

Mass [kg] 29.3 23.6 -20% 30.6 4%

Bending [N/mm] 2165 1904 -12% 2658 23%

Torsion [N-m/deg] 61.6 49.8 -19% 70.1 14%

Increased ThicknessDesign ThicknessTest

Average

The static assembly stiffness test results are summarized in Table 4. The stiffness results

were seen to be consistent with an individual test difference of less than 6% relative to the

average for both tests. Predicted stiffness results for the assembly were not available at the

time of this writing.

Table 4: Assembly bending and torsion stiffness results

120910-4 121510-4Test

AverageCOV

Bending [N/mm] 975 1029 1002 3.8%

Torsion [N-m/deg] 564 629 597 7.7%

Assembly Test

Component and Assembly Modal Performance

Test Setup

The modal performance was evaluated for the seven molded components and the two built-

up test assemblies using “free-free” boundary conditions. The basic test setup for the

components and assemblies is shown in Figure 11. In the test, the component or assembly was

suspended from a stiff frame using bungee cords. The component or assembly was then

randomly excited in the normal “Z-direction” using a shaker and triaxial accelerometers were

used to map out the mode shapes and the associated frequencies. The component bending

and torsion FEA model used to predict the component stiffness response were the same as the

models shown in Figure 10 but with free-free boundary conditions applied (i.e. without any

fixturing). The measured mode shapes were visualized and compared to the predicted mode

shapes for comparison.

Figure 11: Assembly and component modal test setup

Results

The predicted bounding results are provided for reference as the test-analysis correlation

was in process at the time of this writing.

The primary bending and torsion mode results are summarize in Table 5 for the molded

components. The modal results were generally consistent with the exception of the torsion

response of sample 121510-3, which exhibited a higher frequency value (12.4 Hz) than all other

samples. The mode shape for this sample appeared to be different than the other samples,

possibly explaining the difference. The components that were subsequently tested in assembly

are indicated in the table.

Table 5: Component bending and torsion modal results

Component Test

120910-4* 121410-1 121510-3 121510-4* 121510-5 121510-6 121510-8Test

AverageCOV

Mass [kg] 28.2 28.6 29.5 29.5 30.0 30.0 29.5 29.3 2.3%

Bending [Hz] 23.5 24.2 24.8 24.0 23.5 24.8 23.7 24.1 2.4%

Torsion [Hz] 8.8 9.5 12.4 7.8 9.6 8.2 7.7 9.2 17.8%

*Also tested in assembly


in Table 6. The associated first bending and torsion mode shapes for the “design thickness”

bounding case (Trial 57) are shown in Figure 12, where the shapes in the figure represent the

deformed shapes at the maximum positive and negative amplitudes. The predicted mode

shapes were observed to be very similar to the measured mode shapes (not shown). The

average test results for bending are closer to the “increased thickness” Trial 58 results, while the

torsion results lie closer to the “design thickness” Trial 57 results.

Table 6: Comparison between average component modal test results and bounding case predictions

FEA Tr57%Diff vs.

Test Avg.FEA Tr58

%Diff vs.

Test Avg.

Mass [kg] 29.3 23.6 -20% 30.6 4%

Bending [Hz] 24.1 19.2 -20% 21.8 -9%

Torsion [Hz] 9.2 9.1 -1% 10.1 10%

Design Thickness Increased ThicknessTest

Average

The modal component FEA analyses were found to be sensitive to thickness distribution,

which not only would affect the local stiffness, but also the local mass distribution. Given the

differences between the actual components and the bounding assumptions, the mode shapes

and frequency results for the 1st bending and torsion frequencies were found to be reasonable,

with the predicted frequencies within 10% of the measured values for the case of increased

thickness (Trial 58).

Figure 12: Predicted component 1st torsion and bending mode shapes (Trial 57)

The modal test results for two assemblies are summarized in Table 7. The test results were

very consistent with the magnitude of the assembly bending frequency results being similar to

the component test results, and the magnitude of the torsion frequencies nearly twice that of the

component tests.

Table 7: Assembly bending, and torsion modal results

120910-4 121510-4Test

AverageCOV

Bending [Hz] 23.6 23.4 23.5 0.4%

Torsion [Hz] 16.3 16.9 16.6 2.7%

Assembly Test


in Table 8. The associated predicted first bending and torsion modes for the “design thickness”

bounding case (Trial 57) are shown in Figure 13. The predicted mode shapes were observed to

be very similar to the measured mode shapes (not shown). The average bending test results

are higher than the predicted results, while the average torsion test results are lower than the

predicted results.

Figure 13: Predicted assembly 1st torsion and bending mode shapes (Trial 63)

Table 8: Comparison between average assembly modal test results and bounding case predictions

FEA Tr63%Diff vs.

Test Avg.FEA Tr64

%Diff vs.

Test Avg.

Bending 23.5 20.0 -15% 20.7 -12%

Torsion 16.6 19.3 16% 20.5 23%

Test

Average

Design Thickness Increased Thickness

ODB Destructive Testing

Test Setup

A test setup was developed to subject the underbody to a simulated worst-case loading in

the vehicle environment. The setup was designed to apply deformations similar to those that

are predicted to occur during a full vehicle ODB impact loading.

As can be seen in Figure 14, the underbody was clamped at the rear side edges and the

rear of the floor while loads were applied simultaneously via hydraulic cylinders to the driver side

front rail and to the front of the transmission housing. The rear side attachments were made by

welding sleeves into the rocker sections, and then bolting through these into a 25mm thick steel

plate that was attached to bed plate. The rear of the floor was bolted to stiff box beam which

was attached to the bed plate.

The loading was applied quasi-statically using load control at a ratio of 2.375:1 and at a

specified initial loading angle, with the displacement of the loaded end of the driver side rail

constrained by a pivot link pinned on both ends. Additional details of the setup can be seen in

the photos shown in Figure 15. The loads were applied at specified angles through spherical

joints. The transmission housing was only attached to a bushed cross-member that was bolted

to the composite floor (see Figure 5). Therefore, the motion of the transmission was controlled

by the initial transmission load application direction, the rotation defined by the cross-member

bushings, and the eventual contact between the transmission housing and the tunnel at higher

loads. Loads were measured at the load application points with load cells and displacements

were taken from the stroke of the hydraulic actuators. Displacements and surface strains were

measured at specific locations in some tests to monitor the response during loading.

Results

A summary of the measured rail and transmission force vs. displacement responses

obtained from all five ODB tests are shown in Figure 16. The limiting factor is noted for each

test, either premature rail buckling or failure of the cast aluminum cross-member.

For test #1 (120910-4), the maximum input load was limited by the local buckling of the

loaded rail at the front of the floor. Additional analytical test development and correlation was

conducted after test #1 to improve subsequent tests and to maximize the load input capacity

into the underbody. During this development, it was found that the analysis model did not

adequately represent the connection and contact between the longitudinal rails and the internal

foam core. As a result, the model predicted a higher rail buckling force than was observed in the

test. To help compensate for the lower rail buckling load, the load application angles were

adjusted to minimize the lateral component of the rail load input.

Based on test #1, the revised test setup was used for tests #2 (121510-5) and #3 (121410-

1) with the result that the maximum input load was limited by the failure of the cast aluminum

transmission cross-member. For tests #4 (121510-3) and #5 (121510-4), which have been re-

plotted in Figure 17 for clarity, the cast aluminum transmission cross-member was reinforced

with a 25mm thick steel plate (shown in Figure 15), which enabled a higher underbody load

input due to suppression of the cross-member failure mode. Both tests #4 and #5 were very

similar in terms of overall response and maximum load. A small difference can be seen in the

slope of the load-displacement response in Figure 17, where it appears that test #5 is stiffer

above ~18kN. This difference has been attributed to the motion of the transmission housing and

the timing of the contact with the tunnel.

Figure 14: ODB destructive test setup

Figure 15 : Rail and transmission loading details

As noted previously, another key factor already identified as having an influence on the

response of the underbody was the observed differences between the design and the

measured as-molded thickness. Areas with variations in thickness will affect the local

component stiffness and strength, which may further alter the load path and failure modes.

Analytical investigations have shown that the failure locations and failure modes can change as

a result of variations in the local part thickness, stiffness, and strength. Studies were ongoing at

the time of this writing.

Figure 18 shows several video frames that depict the failure sequence in test #4. Figure (a)

illustrates that there is no damage during the initial elastic loading of the underbody. Figure (b)

shows the appearance of local composite damage at several locations as a result of the

initiation of rail buckling at the left side of the dash panel. Figure (c) shows significant upward

rotation of the rail leads to significant local composite damage near the rail attachment and also

in the tunnel area. Figure (d) indicates that significant cracking around the tunnel occurs near

the end of the test when the rail rotation is at a maximum. However, as can be seen in the post

test images shown in Figure 19, net section failure was not observed in the composite despite

the significant cracking near the rail attachment and around the tunnel. Additionally, it was

observed that the weld bond doublers performed well in terms of maintaining joint integrity

between the steel and composites components.

Figure 20 compares several predicted results to the measured rail load vs. displacement

response for tests #4 and #5. The predicted results represent three different material

assumptions as indicated in the figure. Trial 1378 and 1386 are identical other than that the

composite material stiffness and strength values are reduced for Trial 1386 based on coupon

tests conducted from in-situ tension and compression samples that were taken from test #1.

Trial 1385 is based on increased material thickness values measured from test #5 and also

uses material stiffness and strength properties that take into account the local material

thickness from samples taken from test #1. All predicted results represent the measured

loading curves well up until rail buckling occurs, with the predicted responses starting to deviate

from one another above approximately 45kN based on the various material assumptions.

Figure 16: Load-displacement response for all destructive tests

Figure 17: Load-displacement response for destructive tests #4 and #5

Figure 18: Destructive test #4 (121510-3)

Significant local

damage due to

rail buckling

Local damage

around tunnel

Significant local

damage due to

rail buckling

Local damage

around tunnel

Figure 19: Observed post-test damage for destructive test #4 (121510-3)

Figure 20: Comparison between measured and predicted force-displacement response

Summary

A composite underbody was designed to perform within a donor vehicle BIW environment

based on a CAE approach. Prototype composite underbody components were molded and test

components were fabricated and subjected to both non-destructive component and assembly

tests, and destructive assembly tests. The destructive test was devised to subject the

underbody test assembly to the worst case design load case. A test assembly was designed

and fabricated from composite underbody components and modified production BIW

components. It was confirmed that the thickness and mass of the underbody components

provided by the ACC was greater than the assumed design values.

FEA predictions were compared to test results to understand the status of correlation to the

as-molded components. Bounding studies indicated that the measured non-destructive

component and assembly stiffness and modal response were reasonable. The destructive

underbody test was developed to better represent the physical components and as-molded

material thickness values. The predicted force vs. deflection response of the destructive

underbody test was found to give a good representation of the tests up until rail buckling

occurred.

Overall, it was observed that the load carrying capacity of the as-molded composite

underbodies exceeded the load carrying capability of the adjacent metallic components.

Further, it was found that the fiberglass SMC fabric composite was very robust and was able to

tolerate large local deformations by cracking but without separating. Lastly, it was observed that

the weld bond doublers performed well in that the integrity of the composite-steel joints was

maintained.

All underbody testing was completed with correlation efforts ongoing at the time of this

writing. Therefore, final correlation results will be reported at a later date.

Acknowledgments

The authors would like to thank the ACC Underbody team for their valuable inputs, and in

particular Libby Berger (General Motors), Bhavesh Shah for facilitating the BIW components

(General Motors), Chuck Knakal (USCAR), and Dan Houston (Ford Motor Company).

This material is based upon work supported by the Department of Energy National Energy

Technology Laboratory under Award Numbers DE-FC26-02OR22910 and DE-EE0003583. This

report was prepared as an account of work sponsored by an agency of the United States

Government. Neither the United States Government nor any agency thereof, nor any of their

employees, makes any warranty, express or implied, or assumes any legal liability or

responsibility for the accuracy, completeness, or usefulness of any information, apparatus,

product, or process disclosed, or represents that its use would not infringe privately owned

rights. Reference herein to any specific commercial product, process, or service by trade name,

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement,

recommendation, or favoring by the United States Government or any agency thereof. The

views and opinions of authors expressed herein do not necessarily state or reflect those of the

United States Government or any agency thereof.

References

1. Berger, L., “Program Summary of the ACC Automotive Composites Underbody,” Society of

Plastics Engineers Automotive Composites Conference and Exhibition, Troy, MI, Sept. 13-15,

2011.

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status of the composite underbody component and …€¦ · status of the composite underbody...

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