Capturing the Energy Absorbing Mechanisms of Composite Structures under Crash Loading

397 Pages·2014·12.65 MB·English

by  Bonnie Wade

Capturing the Energy Absorbing
Mechanisms of Composite

Structures under Crash Loading

Bonnie Wade
A dissertation submitted in partial
fulfillment of the requirements for

the degree of Doctor of Philosophy

University of Washington
2014

Reading Committee:
Paolo Feraboli, Chair
Kuen Lin
Larry Ilcewicz

Program Authorized to Offer Degree:
Aeronautics & Astronautics Engineering
© Copyright 2014
Bonnie Wade

i
University of Washington
Abstract
Capturing the Energy Absorbing Mechanisms of Composite Structures under Crash Loading
Bonnie Wade
Chair of Supervisory Committee:
Professor Paolo Feraboli
Department of Aeronautics and Astronautics Engineering
As fiber reinforced composite material systems become increasingly utilized in primary aircraft
and automotive structures, the need to understand their contribution to the crashworthiness of the
structure is of great interest to meet safety certification requirements. The energy absorbing
behavior of a composite structure, however, is not easily predicted due to the great complexity of
the failure mechanisms that occur within the material. Challenges arise both in the experimental
characterization and in the numerical modeling of the material/structure combination. At
present, there is no standardized test method to characterize the energy absorbing capability of
composite materials to aide crashworthy structural design. In addition, although many
commercial finite element analysis codes exist and offer a means to simulate composite failure
initiation and propagation, these models are still under development and refinement. As more
metallic structures are replaced by composite structures, the need for both experimental
guidelines to characterize the energy absorbing capability of a composite structure, as well as
guidelines for using numerical tools to simulate composite materials in crash conditions has
become a critical matter.
This body of research addresses both the experimental characterization of the energy absorption
mechanisms occurring in composite materials during crushing, as well as the numerical
ii
simulation of composite materials undergoing crushing. In the experimental investigation, the
specific energy absorption (SEA) of a composite material system is measured using a variety of
test element geometries, such as corrugated plates and tubes. Results from several crush
experiments reveal that SEA is not a constant material property for laminated composites, and
varies significantly with the geometry of the test specimen used. The variation of SEA measured
for a single material system requires that crush test data must be generated for a range of
different test geometries in order to define the range of its energy absorption capability. Further
investigation from the crush tests has led to the development of a direct link between geometric
features of the crush specimen and its resulting SEA. Through micrographic analysis, distinct
failure modes are shown to be guided by the geometry of the specimen, and subsequently are
shown to directly influence energy absorption. A new relationship between geometry, failure
mode, and SEA has been developed. This relationship has allowed for the reduction of the
element-level crush testing requirement to characterize the composite material energy absorption
capability.
In the numerical investigation, the LS-DYNA composite material model MAT54 is selected for
its suitability to model composite materials beyond failure determination, as required by crush
simulation, and its capability to remain within the scope of ultimately using this model for large-
scale crash simulation. As a result of this research, this model has been thoroughly investigated
in depth for its capacity to simulate composite materials in crush, and results from several
simulations of the element-level crush experiments are presented. A modeling strategy has been
developed to use MAT54 for crush simulation which involves using the experimental data
collected from the coupon- and element-level crush tests to directly calibrate the crush damage
parameter in MAT54 such that it may be used in higher-level simulations. In addition, the
iii
source code of the material model is modified to improve upon its capability. The modifications
include improving the elastic definition such that the elastic response to multi-axial load cases
can be accurately portrayed simultaneously in each element, which is a capability not present in
other composite material models. Modifications made to the failure determination and post-
failure model have newly emphasized the post-failure stress degradation scheme rather than the
failure criterion which is traditionally considered the most important composite material model
definition for crush simulation. The modification efforts have also validated the use of the
MAT54 failure criterion and post-failure model for crash modeling when its capabilities and
limitations are well understood, and for this reason guidelines for using MAT54 for composite
crush simulation are presented.
This research has effectively (a) developed and demonstrated a procedure that defines a set of
experimental crush results that characterize the energy absorption capability of a composite
material system, (b) used the experimental results in the development and refinement of a
composite material model for crush simulation, (c) explored modifying the material model to
improve its use in crush modeling, and (d) provided experimental and modeling guidelines for
composite structures under crush at the element-level in the scope of the Building Block
Approach.

iv
Acknowledgements
I would like to express my sincere gratitude to the Federal Aviation Administration (FAA) and
The Boeing Company, for providing technical guidance and for supporting me financially
throughout the time of my doctorate. To Dr. Larry Ilcewicz (FAA), who granted my invaluable
exposure to aircraft certification in collaboration with industry leaders, and who has always been
deeply supportive of my scholarly efforts and personal wellbeing; to Dr. Mostafa Rassaian
(Boeing Research & Technology), who generously shared his expertise in crash simulation,
provided teaching and guidance, and who was an invaluable liaison with the analysis community
without which I could not have completed my research, nor my dissertation; and to Allan
Abramowitz (FAA) who provided guidance and important feedback in my experimental work.
My graduate studies and research was conducted in the Automobili Lamborghini Advanced
Composite Structures Laboratory (ACSL) which provided the ultimately unique setting and
opportunity to explore the very cutting edge of composite technologies. My research simply
would not have been possible without the ACSL, and I am deeply thankful to have received the
generous support of Maurizio Reggiani, Luciano DeOto and Attilio Masini (Automobili
Lamborghini S.p.A). È stato davvero un piacere avere l’opportunità di lavorare e imparare dai
miei colleghi italiani. Grazie mille.
Finally, I will forever be thankful and humbled to have received the guidance and support of my
advisor, Dr. Paolo Feraboli. Under his guidance, I have grown intellectually, professionally, and
personally in such a manner I could have never anticipated. I owe the completion of this body of
research to his boundless support, leadership, and encouragement. His passion has inspired me
deeply and I am truly honored and privileged to have been his student at the ACSL.
v
Table of Contents
Abstract ........................................................................................................................................... ii
Acknowledgements ......................................................................................................................... v
Table of Contents ........................................................................................................................... vi
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................. x
Abbreviations ............................................................................................................................ xxvii
Notations .................................................................................................................................. xxviii
Introduction ..................................................................................................................................... 1
Literature Review............................................................................................................................ 4
Experimental characterization of composite energy absorption ................................................. 4
Composite material models for crash simulation ...................................................................... 22
Crash simulation of composite structures ................................................................................. 29
Experimental ................................................................................................................................. 41
Flat CFRP coupon crush tests ................................................................................................... 44
Corrugated CFRP element crush tests ....................................................................................... 46
Tubular CFRP element crush tests ............................................................................................ 55
Discussion of results.................................................................................................................. 66
Experimental conclusions ......................................................................................................... 70
Analysis......................................................................................................................................... 72
vi
MAT54 single element studies .................................................................................................. 74
Simulation of unidirectional tape sinusoid crush specimen .................................................... 117
Simulation of fabric sinusoid crush specimen......................................................................... 153
Simulation of other fabric crush specimens ............................................................................ 180
Modifying MAT54 .................................................................................................................. 198
Guideline for using MAT54 in crush analysis ........................................................................ 234
Major Contributions of Research ................................................................................................ 253
General Conclusions ................................................................................................................... 255
References ................................................................................................................................... 258
Appendix A: Additional experimental data ................................................................................ 274
Appendix B: LS-DYNA theory manual entry for MAT22 & MAT54 ....................................... 284
Appendix C: LS-DYNA material model MAT54 ...................................................................... 289
Appendix D: Additional crush element simulation results ......................................................... 299
Appendix E: MAT54 source code & modifications ................................................................... 311
Appendix F: Modified MAT54 User’s Manual Entry ................................................................ 359
Appendix G: Keyword input file for fabric sinusoid crush ........................................................ 365
Appendix H: Disclaimer on work performed ............................................................................. 368

vii
List of Tables
Table 1. Comparison of SEA of carbon and glass composite tubes against steel and aluminum
tubes, from Carruthers et al. [16]. ................................................................................................. 20
Table 2. Material properties provided by the AGATE Design Allowables for T700GF 12k/2510
unidirectional (UD) tape [82] and T700SC 12k/2510 plain weave (PW) fabric [81]. ................. 42
Table 3. Experimental load and SEA results from the sinusoid crush elements. ......................... 54
Table 4. Experimental load and SEA results from the tubular crush elements. ........................... 64
Table 5. SEA results from each of the nine geometries crush tested. ........................................... 65
Table 6. Degree of curvature, φ, values for each of the nine geometries crush tested. ............... 67
Table 7. Select MAT54 user-defined input definitions and required experimental data .............. 75
Table 8. Expected baseline strength, failure strain, and output energy values for the AGATE UD
material system. ............................................................................................................................ 81
Table 9. Parametric test matrix for the MAT54 [0] lay-up, baseline and parametric values. .... 83
12
Table 10. Parametric test matrix for the MAT54 [90] lay-up, baseline and parametric values. 92
12
Table 11. Energy output values from ideal DFAILM simulations in Figure 52.......................... 96
Table 12. Expected and simulated peak stress, strain, and energy values for the baseline
[(0/90)] fabric single element with error values. ........................................................................ 99
8f
Table 13. Expected and simulated peak stress, strain, and energy values for the baseline (0/90)
3s
cross-ply single element with error values.................................................................................. 102
Table 14. Parametric test matrix for the MAT54 model of the fabric material, baseline and
parametric values. ....................................................................................................................... 104
Table 15. Parametric test matrix for the MAT54 UD cross-ply lay-up, baseline and parametric
values. ......................................................................................................................................... 107
viii
Table 16. MAT54 baseline model input deck for the unidirectional material model.
(strikethrough parameters are not used). ..................................................................................... 120
Table 17. Summary of the parametric studies performed on the UD material model (units not
shown for clarity). ....................................................................................................................... 127
Table 18. Summary of the numeric baselines for the unidirectional tape sinusoid crush element.
..................................................................................................................................................... 151
Table 19. Summary of the parametric studies performed on the fabric material model (units not
shown for clarity). ....................................................................................................................... 164
Table 20. Summary of the modeling parameters necessary to change for each crush element
geometry in order to match the experimental results, and the resulting error between simulation
and experiment. ........................................................................................................................... 194
Table 21. Original and modified material input parameters used for the UD material definition.
..................................................................................................................................................... 203
Table 22. New modified MAT54 user input parameters added for Wolfe’s strain energy failure
criterion. ...................................................................................................................................... 214
Table 23. Measured strain-energy component data for the UD and PW fabric material systems.
..................................................................................................................................................... 275
Table 24. MAT54 user-defined input definitions and required experimental data..................... 291
Table 25. Contents of major operations performed in MAT54 .................................................. 318
Table 26. Summary of new user input parameters introduced for the post-failure degradation
options in the modified MAT54. ................................................................................................ 347

ix