June 2001

NASA/TM-2001-211023

ARL-TR-2539

Dynamic Finite Element Predictions for Mars

Sample Return Cellular Impact Test #4

Edwin L. Fasanella

U. S. Army Research Laboratory

Vehicle Technology Directorate

Langley Research Center, Hampton, Virginia

Marcus D. Billings

The George Washington University, Joint Institute for Advancement of Flight Sciences

Hampton, Virginia
The NASA STI Program Office ... in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key

part in helping NASA maintain this important

role.

The NASA STI Program Office is operated by

Langley Research Center, the lead center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These

results are published by NASA in the NASA STI

Report Series, which includes the following

report types:

· TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive

data or theoretical analysis. Includes

compilations of significant scientific and

technical data and information deemed to

be of continuing reference value. NASA

counterpart of peer-reviewed formal

professional papers, but having less

stringent limitations on manuscript length

and extent of graphic presentations.

· TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary

or of specialized interest, e.g., quick release

reports, working papers, and

bibliographies that contain minimal

annotation. Does not contain extensive

analysis.

· CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

· CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or co-sponsored by

NASA.

· SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

· TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results ... even providing videos.

For more information about the NASA STI

Program Office, see the following:

· Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

· E-mail your question via the Internet to

help@sti.nasa.gov

· Fax your question to the NASA STI Help

Desk at (301) 621-0134

· Phone the NASA STI Help Desk at

(301) 621-0390

· Write to:

 NASA STI Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076-1320
National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23681-2199

June 2001

NASA/TM-2001-211023

ARL-TR-2539

Dynamic Finite Element Predictions for Mars

Sample Return Cellular Impact Test #4

Edwin L. Fasanella

U. S. Army Research Laboratory

Vehicle Technology Directorate

Langley Research Center, Hampton, Virginia

Marcus D. Billings

The George Washington University, Joint Institute for Advancement of Flight Sciences

Hampton, Virginia
Available from:

NASA Center for AeroSpace Information (CASI) National Technical Information Service (NTIS)

7121 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161-2171

(301) 621-0390 (703) 605-6000

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not constitute an

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics

and Space Administration or the U.S. Army.
1

Abstract

The nonlinear, transient dynamic finite element code, MSC.Dytran, was used to simulate

an impact test of an energy absorbing cellular structure. This pre-test simulation was

performed to aid in the design of an energy absorbing concept for a highly reliable

passive Earth Entry Vehicle (EEV) that will directly impact the Earth without a

parachute. In addition, a goal of the simulation was to bound the acceleration pulse

produced at impact and transmitted to the simulated space cargo container. EEV’s are

designed to return materials from asteroids, comets, or planets for laboratory analysis on

Earth. The EEV concept uses an energy absorbing cellular structure designed to contain

and limit the acceleration of space exploration samples during Earth impact. The

spherical shaped cellular structure is composed of solid hexagonal and pentagonal foamfilled cells with hybrid graphite-epoxy/Kevlar cell walls. Space samples fit inside a

smaller sphere at the center of the EEV’s cellular structure. The material models and

failure criteria in the finite element model were varied to determine their effect on the

resulting acceleration pulse. Pre-test analytical predictions using MSC.Dytran were

compared with the test results obtained from cellular impact test #4 using a bungee

accelerator located at the NASA Langley Research Center’s Impact Dynamics Research

Facility. The material model used to represent the foam and the proper failure criteria for

the cell walls were critical in predicting the impact loads of the cellular structure. It was

determined that a FOAM1 model for the foam and a 20% failure strain criteria for the cell

walls gave an accurate prediction of the acceleration pulse for cellular impact test #4.

Introduction

The Earth Entry Vehicle (EEV) concept is designed to return materials from asteroids,

comets, or planets to Earth. One concept for a Mars Sample Return EEV developed at

NASA Langley Research Center, and illustrated in Figure 1, is a circular aeroshell

structure approximately one meter in diameter with an energy absorbing impact sphere in

the center of the vehicle. The impact sphere is constructed of three-dimensional cells

with hybrid graphite-epoxy/Kevlar walls, each cell filled with a low-density energy

absorbing carbon foam. This simple, highly reliable, and cost-effective EEV is designed

to withstand a terminal velocity land impact without a parachute1. The nominal impact

surface for an EEV would be soft clay soil2. Design criteria for the EEV concept require
2

Figure 1 – Schematic of candidate Mars Sample Return Earth Entry Vehicle.

that sample containment be assured with high levels of reliability. Thus, an energy

absorbing impact sphere has been designed to limit the acceleration of the Mars samples

and to provide a high level of containment in case Earth impact should occur on a rigid

surface outside the soft clay target.

Figure 2 – Bottom view of cellular structure showing shape and geometry of the cells.

The current concept of the energy absorbing impact sphere is a composite cellular

structure made with energy absorbing materials that limits the acceleration of the space

samples and ensure containment. The cellular structure, shown in Figure 2, is composed

of cells that are filled with energy absorbing foam and enclosed with hybrid composite

cell walls. Energy absorbing materials include carbon foam, Kevlar, graphite, and hybrid
3

Kevlar-graphite composites. Rock, soil, and atmospheric samples may be kept within the

cellular structure in a sample container designated the Orbiting Sample (OS), as indicated

in Figures 1 and 3.

The nonlinear finite element program MSC.Dytran3 was used to predict the acceleration

pulse and deformations for impact test #4 of an energy absorbing cellular structure. The

required reliability and containment assurance criteria are being addressed, in part, by

performing nonlinear dynamic finite element simulations of the impact of the EEV’s

cellular structure onto a rigid surface. A goal of this pre-test simulation was to predict

the upper and lower acceleration limits of the cellular structure for impact onto a rigid

concrete surface. Consequently, various failure criteria for the webs of the cellular

structure, and different constitutive models were used for the foam inside each cell. Each

model created for the analysis will be described. The final model that was considered the

most accurate was compared with test results. The impact conditions for test #4 were

approximately a 40 m/s impact onto concrete using the bungee accelerator facility at the

NASA Langley Impact Dynamics Research Facility4

 (IDRF).

Instrumentation consisted of one low-g accelerometer, which was integrated to obtain the

impact velocity, an accelerometer inside the OS provided by the Jet Propulsion

Laboratory (JPL), an accelerometer on the top of the OS, and an accelerometer on the

impact test plate. All accelerometers were oriented to measure vertical accelerations.

Refer to Figure 3 for the diagram of the impact test specimen.

Pretest Model Predictions

MSC.Dytran models have been developed to predict the impact response of drop test

article #4. The test specimen, which is shown in Figure 3, consists of a hemispheric

cellular structure, an impact test plate on top of the hemispheric cellular structure that

represents the top half of the sphere, and a simulated OS from JPL. The OS is designed

to contain Mars soil and rock samples. Metric units were used in this analysis; i.e.,

meters, kilograms, and seconds (MKS).

Note that carbon foam was used in the cellular structure because it is an excellent energy

absorbing material with near constant crush stress. It is also well suited to space

applications since it is relatively inert and temperature insensitive. Two different

material models were used to represent the carbon foam. First a DYMAT24 model was

used. The DYMAT24 material model was used to represent the polyurethane foam that

had filled the cells in the earlier, less refined drop test specimens. DYMAT24 is a

general elastic-plastic material model with a non-zero Poisson ratio. The second model

used to represent the carbon foam was a FOAM1 material model, which is a specialized

foam material model with a Poisson ratio equal to zero. The input for the FOAM1 model

was a stress-strain table with a linear elastic region, a plastic (crush) region, and a rapidly

rising stress region after 90 % strain to represent compaction. Intuitively, carbon foam is
4

 expected to have a Poisson ratio near zero since the foam is a very brittle material with

low shear strength. Strain-based failure criteria were used for the web (cell walls)

material inside the cellular structure. Several cases were executed including no failure,

10%, 20%, and 25% plastic failure strain. For the case without web failure, the peak

predicted acceleration is maximum as the structure is the strongest and the permanent

deformation of the cellular structure is the smallest. Analytical results for 10% plastic

failure strain allowed portions of the web to fail early, allow more permanent

deformation, and would produce the lower-bound peak acceleration (if enough crush

distance is available to prevent “bottoming-out”). For a 20 – 25% web plastic failure

strain, one would expect to obtain an acceleration pulse between the two extremes.

Figure 3 – Cellular structure impact test specimen for drop test #4.

MSC.Dytran Case I – Carbon Foam Represented by DYMAT24

The model for cellular impact test #4 contained 7667 grid points and 8720 elements.

Shell elements are used to represent the graphite/epoxy/Kevlar web, the inner and outer

hemispherical skins of the cellular-structure, and the OS titanium container. Solid

elements are used to represent carbon foam, the OS conforming foam, and the impact test

plate at the top of the structure. The finite element model is illustrated in Figure 4. The

cell-wall (web) material was modeled to be elastic-plastic, but was not allowed to fail.

OS

Cellular

Structure CV

Impact Test Plate
5

Figure 4 – Model of OS, cellular structure, and impact plate for cellular impact test #4.

For simplicity, the OS sample container was considered a rigid body. The material

properties for the OS model were taken from the JPL LS-Dyna model. Two contact

surfaces were required in the model: 1) contact between the OS and the cellular structure

and 2) contact between the combined OS /cellular structure with the impact surface. The

total mass of the model is assumed to be approximately 14 kg. The OS mass (foam +

cannister) as received from JPL is 2.374 kg, the containment vessel (CV) mass is 0.1334

kg, and the metallic OS cannister is assumed to weigh about 1.76 kg. The CV is

additional protective material (Kevlar fabric) that is between the cellular structure and the

OS. The mass breakdown is shown in the Table I below:

Test

Plate

OS

Impact

Surface

Cellular Structure
6

Table Ia. – Density, Volume, and Mass of Cellular Structure Components

Component Density (kg/m3) Volume (m3) Mass (kg)

Carbon foam 48.0 6.5e-3 0.317

Webs 1539.0 9.4e-4 1.448

Sphere outer skin 1379.0 7.8e-5 0.108

Sphere inner skin 1550. 6.3e-5 0.097

OS foam 460.0 1.3e-3 0.605

 CV 0.1334

OS metal parts

without foam

Rigid Ixx, Iyy, Izz

About .004 kgm2

1.760

Impact plate 4750. 2.0e-3 9.5

TOTAL 14.0

Table Ib. – Material Models of Cellular Structure Components

Material Effective E

(N/m3)

Poisson Ratio Yield (N/m3) Material model

Carbon foam 1.379e8 0.3 1.379e6 DYMAT24

Webs 18 layers

(Hybrid

Graphite-Epoxy

/Kevlar)

1.379e10 0.3 1.379e8 DYMAT24

Kevlar 6.895e9 0.3 1.034e8 DYMAT24

Graphite-epoxy 4.55e10 0.3 5.79e8 DYMAT24

OS foam 4.38e8 0.334 1.54e7 DYMAT24

Impact plate 2.72e10 0.3 2.68e8 DYMAT24

The model was run on a single processor of a Sun Ultra Enterprise 450 workstation and

required approximately 1 hour of CPU time. The predicted acceleration of the OS is

shown in Figure 5. The solid line is the MSC.Dytran OS model output acceleration at

50,000 samples per second. The broken line was obtained by filtering the MSC.Dytran

output with a 1000 Hz low-pass filter. The filtered response curve shows that the

acceleration rapidly climbs to 2500 g’s in 0.5 ms and increases to a maximum of slightly
7

over 4000 g’s at 1.5 ms. The pulse duration is 0.002 seconds (2 ms). Since failure of the

webs was not allowed, this response can be assumed to be the upper limit acceleration

pulse.

The deformed plots of the model are illustrated in Figure 6 for 1 ms and 2 ms. Maximum

crushing of 3.7 cm (1.47 in.) occurs at 1.5 ms.

-4000

-2000

0

2000

4000

6000

8000

0.0 0.0010 0.0020 0.0030 0.0040

Dytran - No filter

Dytran - 1000 Hz filter

Acceleration, g's

Time, s

Figure 5 - Predicted OS acceleration for DYMAT24 carbon foam, 40 m/s impact.

No failure of cell-wall material. This curve is the upper limit acceleration pulse.
8

a. Time = 0.001

b. Time = 0.002

Figure 6 - Deformed plots of the model with color-coded displacement.

The initial model was modified to capture the actual tearing and crushing of the lower

hybrid cellular structure. The OS acceleration for the model with a failure strain of 0.10

is shown in Figure 7. The dashed line shows the MSC.Dytran data filtered with a 1000

Hz low-pass filter. With the failure strain applied, elements are deleted when the strain

exceeds 0.10. Thus, the initial acceleration is lowered and a plateau forms around 2500

g’s. However, a secondary stiffening occurs which briefly causes the acceleration

to approach 3500 g’s for a short amount of time. It is not certain that the model
9

deformation pattern correctly predicts the actual test article for these large deformations

due to the complexity of the energy absorbing mechanisms. In addition to foam crushing,

which is relatively simple, the web energy absorbing mechanisms include crushing,

delamination, complex bending, tearing, etc. The present model does not include

delamination or tearing behavior and retains too much elastic energy. Some studies were

performed to simulate tearing using BJOIN elements that break under a specified failure

condition. However, numerical problems developed and the BJOIN elements were

removed for this simulation study.

-4000

-2000

0

2000

4000

6000

8000

0.0 0.0010 0.0020 0.0030 0.0040

Dytran - No filter

Dytran - 1000 Hz filter

Accelerations, g's

Time, s

Figure 7 – Predicted OS acceleration for 40 m/s impact. Cell-wall failure strain is 10 %.

The model was also run with a 25 % plastic failure strain. A comparison of

displacements of the OS for no failure, 10 %, and 25 % web failure strains is shown in

Figure 8. As would be expected, the largest displacement for a 25% plastic failure strain

corresponds to the lowest acceleration pulse. The velocity data for the same three cases

are shown in Figure 9. Note that the information from Figure 9 predicts a rebound
10

velocity of about 10 m/s. This high rebound velocity indicates that a lot of elastic energy

was stored, and would predict that the structure would rebound about 4 meters. It is very

unlikely that the structure will rebound such a distance. Consequently, it is expected that

the acceleration response will over-predict the actual experimental results for this model.

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.0 0.0010 0.0020 0.0030 0.0040

OS Disp - 0% fail

OS Disp - 25% fail

OS Disp - 10% fail

Rigid OS Disp, m

Time, s

Figure 8 – Predicted OS displacement for models with cell-wall plastic failure strain of 0,

10 %, and 25%.
11

-50

-40

-30

-20

-10

0

10

20

0.0 0.0010 0.0020 0.0030 0.0040

OS Vel - 0% fail

OS Vel - 25% fail

OS Vel - 10% fail

Rigid OS Vel, m/s

Time, s

Figure 9 – Predicted OS velocity for models with plastic failure strain of 0, 10 %,

and 25%.

The predicted acceleration for the impact plate location is shown in Figures 10 and 11 for

the model with no failure. This acceleration has a high vibration content, and the

underlying impact pulse is not apparent until the data is filtered. The impact plate

acceleration is slightly higher than the predicted OS acceleration.
12

-15000

-10000

-5000

0

5000

10000

15000

0.0 0.0010 0.0020 0.0030 0.0040

Dytran - No filter

Dytran - 1000 Hz Filter

Acceleration, g's

Time, s

Figure 10 – Predicted acceleration on impact plate at top of cellular structure without web

failure.
13

-15000

-10000

-5000

0

5000

10000

15000

0.0 0.0010 0.0020 0.0030 0.0040

Dytran - No filter

Dytran - 1000 Hz filter

Accelerations, g's

Time, s

Figure 11 – Predicted acceleration of impact plate at top of cellular structure for 10%

plastic failure strain.

Case II. Carbon Foam Material Represented with FOAM1

The models in Case I consistently showed too much rebound velocity. Also, Poisson’s

ratio was set to 0.3 for the DYMAT24 material card used to represent carbon foam as low

values of Poisson’s ratio give undesirable results. Thus, a more realistic foam material

model was needed to represent the carbon foam. The FOAM1 material model in

MSC.Dytran is recommended for large amounts of crushing for materials such as foams

with a near zero Poisson ratio. Thus, the model was modified to incorporate FOAM1 to

represent the carbon foam. No other changes were made to the original model. The use

of the FOAM1 material required input of the bulk modulus and a table of the crush stress

versus “crush displacement.” To prevent negative volume, a very large compaction stress

was included for crush greater than 90 percent.
14

The predicted vertical acceleration response of the OS using the FOAM1 material model

is shown in Figure 12, assuming no failure of the cell walls. The solid line is the

predicted MSC.Dytran acceleration output for the OS at 50,000 samples per second. The

broken line was obtained by filtering the predicted results with a 1000 Hz low-pass filter.

The filtered response curve shows that the acceleration rapidly climbs to 3000 g’s in less

than 1.0 ms and increases to a maximum of slightly over 3200 g’s at 1.5 ms. The pulse

duration is 2.6 ms.

-2000

-1000

0

1000

2000

3000

4000

5000

6000

FOAM1 - no filter

1000 Hz filter

Accel, g

 0 0.0010 0.0020 0.0030 0.0040

Time, s

Figure 12 - OS acceleration for FOAM1 representation of carbon foam, no failure of cellwall material.

It is informative to compare the OS acceleration of the model without cell wall failure

with the FOAM1 representation of carbon foam (Figure 12) with the same model using

DYMAT24 to represent the carbon foam (Figure 5). Note that the filtered acceleration

peak drops from slightly over 4000 g’s for the DYMAT24 representation to slightly over

3000 g’s for the FOAM1 representation. The deformation of the lower portion of the

cellular structure is shown in Figure 13 for times of 1, 2, and 3 ms. At the time of 3 ms,

the hemisphere is rebounding. Again, compare the deformed shape in Figure 13 with the
15

deformation in Figure 6. Note that the deformed shape of the bottom of the cellular

structure in Figure 6 is more complex than for that shown in Figure 13 where the FOAM1

model is used. The non-zero Poisson ratio material model for the carbon foam may be

responsible for the more complex folding and bulging observed in Figure 6.

 Time = 0.0 Time = 1 ms

 Time = 2 ms Time = 3 ms

Figure 13 – Four deformed shape plots of the cellular structure for time equal 0 to 3 ms.

Note that there is minimal bulging of the bottom of the hemisphere.

In Figure 14, the predicted acceleration response of the OS is shown using the FOAM1

material model to represent the carbon foam with a 20% plastic failure strain for the web

elements. The cellular structure deformation predicted for the 20% web strain failure is

at least 90 % of the allowable stroke. Consequently, a 10% failure strain model was not

run as the cellular structure would be too weak with insufficient stroking distance

available to stop the OS without bottoming out. From this new model, the minimum

acceleration peak is now expected to be within a 2000 – 2500 g range, and with a pulse

duration of about 3.5 ms.
16

-6000

-4000

-2000

0

2000

4000

6000

0 0.001 0.002 0.003 0.004

DYTRAN - 1000 Hz filter

DYTRAN - unfiltered

Accel, g

Time, s

Figure 14 – Predicted OS acceleration for a 20% cell-wall failure strain criteria and a

FOAM1 carbon foam material model..

Since the FOAM1 model produced better deformation results and had less rebound

velocity, the last two predictions shown in Figures 12 and 14 should bound the impact

test #4 experimental data. Consequently, it is expected that the peak acceleration input to

the OS should range from 2500 g’s to slightly over 3000 g’s. Since the 3000-g value is

highly conservative with the no web failure criteria, the 2500-g value is more likely to be

the best prediction for the acceleration response.
17

Post-Test Comparisons

Two drop tests were performed, designated cellular impact test 4a and 4b, respectively by

the test engineer. The results from drop test 4a will not be used due to problems with the

data acquisition system and with the manufacture of the cellular impact model itself.

The Two Body Interaction

The containment vessel (CV) provides the coupling between the OS and the cellular

structure. The measured accelerations at the top of the OS and on the top surface of the

impact test plate for test 4b (from now on referred to as test #4) are shown in Figure 15.

Note that there is a time delay of approximately 0.0005 s from the beginning of the

plate/cellular structure acceleration to the initiation of the OS acceleration. This delay is

due to the highly nonlinear CV material, which fills the gap between the OS and the

cellular structure. In addition, the relatively large spikes at the end of the traces of the

impact test plate and the OS are due to impact of the OS with the impact test plate.

Consequently, to evaluate the actual dynamic response of the cellular structure and to

remove the interaction between the two bodies, the following equation was developed,

F(t) = ACSMCS + AOSMOS = MTOTALASYS

Where F(t) is the crush force of the cellular structure, ACS is the measured acceleration of

the combined plate and cellular structure, AOS is the measured acceleration of the OS, and

ASYS is the system acceleration (or the acceleration at the cg of the system).

Solving for the system acceleration:

ASYS = (ACSMCS + AOSMOS) / MTOTAL
18

For the equation to apply, it was assumed that the impact test plate is rigidly attached to

the cellular structure and that the mass of the crushed portion of the sphere is small

compared with the total mass. By using the system acceleration, the peaks due to the two

separate bodies are eliminated (see Figure 15), and thus a better representation of the

behavior of the cellular structure for a “perfect CV coupling” is obtained. Consequently,

the CV was incorporated in the OS model by coupling the OS to the cellular structure

with equivalent nodes. This approach represented a slight change from the original

model.

-1000

0

1000

2000

3000

4000

5000

0 0.001 0.002 0.003 0.004

System

Plate/CS

OS

Acceleration, g's

Figure 15 – Measured accelerations for cellular structure drop test of the OS, plate/CS,

and the calculated system acceleration.

The results from the FOAM1 model shown in Figure 14, which was expected to be the

best pre-test model, predicted a peak system acceleration of approximately 2,600 g’s,

which occurred at approximately 2.5 ms into the impact simulation. This result compares

well with the measured system peak system acceleration of 2,700 g’s. Overall, the

simulation accurately predicted the shape, magnitude, and duration of the measured

system acceleration pulse, as shown in Figure 16 when the analysis is plotted with the

experimental system acceleration. Note that the OS and the cellular structure were

coupled together without a CV material for this analysis.
19

-1000

-500

0

500

1000

1500

2000

2500

3000

0 0.001 0.002 0.003 0.004

Test Data - System

Analysis

Acceleration, g's

Time, s

Figure 16 - Comparison of numerical and measured system acceleration for cellular drop

test #4.

The total crush stroke of the finite element simulation was 0.063 m, or approximately

90% of the available crush distance for the cellular structure. The crush of this test

specimen was difficult to measure post-test, as the deformed cell walls sprung back, and

the outer skin fold lines were not as defined as in the previous tests. However, the crush

was estimated to be approximately 90%. A deformed plot of the finite element model

showing the maximum stroke is shown in Figure 17. A photograph of the cellular

structure after the impact is shown in Figure 18.

Figure 17 – Deformed plot from finite element simulation showing max crush at 3 ms.
20

Figure 18 – Photograph of cellular structure cross section, post test.

Concluding Remarks

The nonlinear, transient dynamic finite element code, MSC.Dytran, was used to model

the acceleration pulse for the impact test of an energy absorbing cellular structure. This

pre-test simulation was performed to aid in developing an energy absorbing concept for a

highly reliable, passive Earth Entry Vehicle (EEV) designed to impact the Earth’s surface

without a parachute. In addition, a goal of the simulation was to determine an upper and

lower bound for the acceleration pulse produced at impact and transmitted to the

simulated space cargo container. The material model used to represent the foam and the

proper failure criteria for the cell walls were critical in predicting the impact loads of the

cellular structure. The first material model for the foam (DYMAT24) did not properly

account for the near-zero Poisson ratio of the carbon foam material. Also, since the finite

element results have extremely high frequency oscillations superimposed on the

acceleration pulse, low-pass digital filtering is required to extract the fundamental

acceleration pulse. Pre-test predictions to determine the maximum acceleration pulse

were based on no failure for the cellular structure webs. This assumption gave a

maximum pulse acceleration of approximately 3000 g’s using the FOAM1 model for the

carbon foam. The minimum expected pretest acceleration was based on a failure strain

slightly less than 20%. The minimum pulse acceleration based on this model was

predicted to be approximately 2000 g’s.

It was determined that a FOAM1 model for the foam and a 20% failure strain criteria for

the cell walls correlated best with the acceleration pulse for drop test #4. When the

cellular structure and OS are modeled separately, the acceleration of each component is

complicated due to a two-body interaction. To make meaningful comparisons between
21

analysis and test, a mass-weighted system acceleration of the cellular structure and OS

was used. This comparison allowed the acceleration response of the impact test specimen

to be determined for a “near perfect” coupling between the OS and cellular structure.

The predicted peak system acceleration using 20 % failure strain for the webs and a

FOAM1 model for the carbon foam is approximately 2,600 g’s. This predicted value

compares well with the measured system peak system acceleration of 2,700 g’s. Overall,

the simulation accurately predicted the shape, magnitude, and duration of the measured

system acceleration pulse and has proved to be an extremely valuable design tool.

References

1. Mitcheltree, R., Braun, R., Hughes, S., Simonsen, L., “Earth Entry Vehicle for Mars

Sample Return,” IAF Paper 00-Q.3.04, Oct. 2000.

2. Fasanella, E. L., Jones, Y. T., Knight, Jr., N. F., and Kellas, S., “Low Velocity EarthPenetration Test and Analysis”, AIAA paper 2001-1288, 42nd Structures, Structural

Dynamics, and Materials Conference, Seattle, WA, April 2001.

3. MSC.Software Corporation, “MSC.Dytran Version 4.7 User’s Manual,” Volumes 1

and 2, Los Angeles, California, 1999.

4. Vaughan, V. L. and Alfaro-Bou, E., “Impact Dynamics Research Facility for FullScale Aircraft Crash Testing,” NASA TN D-8179, April 1976.
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data

sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other

aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and

Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188),

Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

June 2001

3. REPORT TYPE AND DATES COVERED

Technical Memorandum

4. TITLE AND SUBTITLE

Dynamic Finite Element Predictions for Mars Sample Return Cellular

Impact Test #4

5. FUNDING NUMBERS

WU 728-50-10-01

6. AUTHOR(S)

Edwin L. Fasanella

Marcus D. Billings

 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center U.S. Army Research Laboratory

Hampton, VA 23681-2199 Vehicle Technology Directorate

 NASA Langley Research Center

 Hampton, VA 23681-2199

8. PERFORMING ORGANIZATION

REPORT NUMBER

 L-18087

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 and

U.S. Army Research Laboratory

Adelphi, MD 20783-1145

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA/TM-2001-211023

ARL-TR-2539

11. SUPPLEMENTARY NOTES

Fasanella: Vehicle Technology Directorate, ARL, Langley Research Center, Hampton, VA.; Billings: JIAFS,

The George Washington University

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 05 Distribution: Standard

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The nonlinear, transient dynamic finite element code, MSC.Dytran, was used to simulate an impact test of an

energy absorbing Earth Entry Vehicle (EEV) that will impact without a parachute. EEV’s are designed to

return materials from asteroids, comets, or planets for laboratory analysis on Earth. The EEV concept uses an

energy absorbing cellular structure designed to contain and limit the acceleration of space exploration samples

during Earth impact. The spherical shaped cellular structure is composed of solid hexagonal and pentagonal

foam-filled cells with hybrid graphite-epoxy/Kevlar cell walls. Space samples fit inside a smaller sphere at the

center of the EEV’s cellular structure. Pre-test analytical predictions were compared with the test results from a

bungee accelerator. The model used to represent the foam and the proper failure criteria for the cell walls were

critical in predicting the impact loads of the cellular structure. It was determined that a FOAM1 model for the

foam and a 20% failure strain criteria for the cell walls gave an accurate prediction of the acceleration pulse for

cellular impact. 

14. SUBJECT TERMS

Mars Sample Return Program, Earth Entry Vehicle, Mars Exploration, Impact

15. NUMBER OF PAGES

26

Dynamics, Space Exploration Samples, Soil Impact, Penetrometers 16. PRICE CODE

A03

17. SEC U RITY CL ASSIF IC AT ION 

O F REPO R T

Unclassified

18. SEC U RITY CL ASSIF IC AT ION 

O F TH IS PA GE

Unclassified

19. SECURITY CLASSIFICATION

 OF ABSTRACT

Unclassified

20. LIMITATION

 OF ABSTRACT

 UL

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z-39-18

298-102