BLAST-TOLERANT COMPOSITE STRUCTURE FOR ROTORCRAFT
By: Mark Robeson, Clark Andrews, and Lisa Chiu
Military rotorcraft structures may be subjected to high-energy ballistic events in combat situations. Ballistic threats characterized by an advancing shockwave created by an explosive blast, alone and in combination with high-speed fragmentation, typically result in significant structural damage for both metallic and composite structures. To address this vulnerability, the U.S. Army’s Aviation Development Directorate (ADD) and The Boeing Company have collaborated to develop and integrate specifically designed composite sandwich components into large primary aircraft structures, which have shown good success at mitigating catastrophic damage caused by explosive ballistic events .
To address small-volume, enclosed aircraft structure, ADD and Boeing collaborated to develop subscale composite sandwich panels, which were shown to mitigate advancing shockwaves from representative explosive ballistic events at close proximity . This damage-mitigating sandwich structure has been developed by engineering the core with tuned characteristics covering (1) compression and tension strength, (2) density, and (3) compression load-deflection curves with progressive failure mechanisms. Now, successfully integrating the shockwave mitigating sandwich structural concepts into small-volume primary structure, via full-scale test articles, is shown to improve composite structure ballistic/ blast tolerance.
The full-scale test article configuration (shown in Figure 1) was designed to be representative of small-volume primary aircraft structure. The assembly consists of a metallic spar, carbon/ epoxy spar box, and fastened doublers that reinforce holes in the spar box. The spar box is made of carbon/epoxy face sheets with crushable core engineered with specific characteristics (strength, density, and yield behavior) to mitigate the explosive blast shockwave. Assembly of the structure is accomplished with riveted fasteners. Test articles (shown in Figure 2) were fabricated with varying spar box laminate definitions, metallic spar configurations, and hole doubler configurations, but with the same core configuration.
FULL-SCALE BALLISTIC TESTING
Ballistic testing was conducted at the ADD range (in Fort Eustis, VA) to evaluate the structural response of the full-scale test article when subjected to the pressure environment created by the specified high-energy (explosive) threat. Data gathered during the full-scale test included high-speed video and photography. The objectives of the test were as follows:
- Evaluate the damage tolerance and blast-mitigating effects of the candidate structures against the ballistic threat.
- Collect performance data (via observed damage, high-speed video, and photography) for analytical model correlation.
Two rounds of ballistic testing were conducted, with the first used to correlate with the analysis. Using data and observations from the first round, the analysis was modified to better represent the pressure environment created from the ballistic threat and improve correlation with the damage observed. The second round of testing (shown in Figure 3) was then executed to demonstrate the accuracy of the improved analysis.
The overpressure environment on the interior of the structure, resulting from the explosive event, was modeled analytically by Boeing (Mesa, AZ) using two approaches: an empirical approach and an Arbitrary Lagrangian Eulerian (ALE) approach. ALE combines both Lagrangian (where each individual node of the computational mesh follows the associated material particle) and Eulerian (where the computational mesh is fixed in space and the continuum moves with respect to the grid) formulations, such that the computational mesh may be fixed in space or move with the continuum. Both analysis methods were executed with the finite element (FE) code LS-DYNA.
The empirical explosive approach defines the pressure environment using curves that are fit to test data. The structure of interest is modeled using the Lagrangian FE formulation, and a unique pressure-time history is defined for every element subjected to the blast. The time histories are defined by peak pressure, pressure pulse duration, and pulse arrival time, which were derived from empirical test data and depended on the charge characteristics and distance between the element and the charge. The explosive source is defined by the user, including location, mass, round casing (if applicable), and shape. The pressure-time-history for an element is calculated as a function of distance from the defined explosive source and user inputs.
For the present ALE approach, the explosive material and air domain are modeled using an ALE formulation while structural components are modeled using a purely Lagrangian formulation. The interaction between the ALE domain and the Lagrangian surfaces is simulated with definitions of Fluid Structure Interaction (FSI). The pressure environment and resulting pressure-time history due to the explosive materials are a function of the resulting FSI.
Due to the complexity of the ALE approach, it is significantly costlier (up to 40 times the simulation clock time) than the empirical model. However, comparison of the approaches resulted in sufficient confidence in the empirical approach for it to be used for correlation with test results. More detail on the analytical models can be found in Andrews et al. .
BLAST RESULTS AND CORRELATION
As stated previously, the first round of ballistic testing was used to correlate and improve the analysis. The pre-test damage predictions for the second round of testing were generated with the improved empirical analysis, and they agreed favorably with the damage observed (as shown in Figure 4). High strains, predicted between the aft spar holes and along the inner face sheet of the composite spar box, indicated a risk of moderate cracking. Some cracking at these locations was observed in test. Significant damage to the metallic spar was not predicted and was not evident in test. The only notable damage to the metallic spar was due to cracking between shrapnel penetrations.
RESIDUAL STRENGTH TEST
Following the ballistic testing, residual strength testing was conducted at the ADD Structures Laboratory (in Fort Eustis, VA) to evaluate the post-ballistic capability of the full-scale test articles. The test setup is shown in Figure 5. Each of the two test articles was fastened to laboratory structure, and test loads were applied by a single actuator to simulate flight loading. Data acquisition included load, strain, and displacement. Strain was measured by four axial strain gauges, two on either side of the test article as shown, to verify the applied loads. Displacement was measured at the upper corners in the lateral direction by string pots on the right-hand side of the test article.
First, cyclic testing was conducted at load levels ranging from level flight to enhanced limit design loads. Several test blocks of progressively increasing magnitude were defined from a baseline level flight to an enhanced limit design load, corresponding to a full unrestricted flight envelope. The full test schedule provided 6,600 cycles of representative flight. After successful completion of the cyclic test, a static test was executed, including static test to failure, in which failure loads exceeded ultimate design loads.
RESIDUAL STRENGTH ANALYSIS
Residual strength FE analysis was conducted on the full-scale test article. Maximum and minimum principal strains are shown in Figure 6 for the composite spar box at design limit loading. Extreme compression strains were predicted to be relatively low. Failure was predicted when the local peak strain at the damage hole reached the materials ultimate compression strain. Running loads were extracted from the analysis for the bays in the metallic spar, assuming crack propagation between shrapnel holes. A panel buckling analysis was also conducted for each of the remaining bays. All pre-test predictions, including failure modes in the composite spar box and metallic spar, predicted no failure during cyclic testing. More detail on the analysis can be found in Andrews et al. .
RESIDUAL STRENGTH RESULTS AND CORRELATION
The full-scale test articles were subjected to the cyclic loads without failure or observable damage growth. With cyclic loading complete, the full-scale test articles were statically loaded to failure (as shown in Figure 7). For the measured test loads (Table 1), yield load is where strain response first deviated from linear, and is due to either a failure in the composite or yielding or failure of the metallic spar. The first specimen initially indicated yield behavior at 155% of design limit load when a crack in the composite spar box grew between the spar box holes. Ultimately, failure occurred when the composite spar box separated from the metallic spar along the rivet line at 209% of design limit load. For the second specimen, the initial yield behavior occurred at 172% of design limit load, when cracks in the metallic spar began to grow, and quickly resulted in ultimate failure at 191% of design limit load, when the metallic spar buckled.
The analytically predicted failure (yield) loads correlate well with those measured in the testing (Table 1). For Test Article 1, composite spar box yielding at 155% of design limit load was within 20% of predicted yield at 167% design limit load. For Test Article 2, initial buckling and crack propagation in the metallic spar at 172% of design limit load was within 10% of predicted buckling yield at 163% design limit load. The specimens generally hold more load than predicted.
Residual strength test loads are summarized in Figure 8, with the ballistically damaged test articles loaded cyclically to 105% of an enhanced design limit load and loaded statically to 209% and 191% of enhanced design limit loads before failure. These cyclic and static strength results demonstrate the exceptional durability and damage tolerance of this design.
A specifically engineered structural core solution was developed and integrated into a full-scale test article, representative of small-volume rotorcraft structure, for evaluation of ballistic/blast tolerance and residual strength. FE analysis predicted resulting damage and residual strength of small-volume aircraft structure when subjected to a ballistic event. After two rounds of ballistic testing, post-test correlation demonstrated that empirical analysis is suitable to accurately predict structural response and damage modes for ballistic events of this magnitude. Specifically, high strains predicted in the composite face sheets were shown to correlate to cracks resulting from the extreme overpressures. The test articles were then subjected to residual strength testing. Both specimens exhibited cyclic, yield, and ultimate strengths in excess of design requirements, clearly demonstrating the capability of the engineered structural core solution to mitigate the damage due to the ballistic event while contributing to the strength of the candidate rotorcraft structure.
ABOUT THE AUTHORS
Dr. Mark Robeson is an aerospace engineer with the U.S. Army Aviation Development Directorate. With more than 27 years of research and development experience, he currently serves as a U.S. Army Aviation Missile Research Development and Engineering Center Experimental Developer (designated technical expert), with expertise in aviation structures and vulnerability reduction. He is also active as a structures and materials technical expert in the Joint Aircraft Survivability Program’s Vulnerability Analysis and Reduction Subgroup. Dr. Robeson has a B.S. in mechanical engineering and an M.S. and Ph.D. in engineering mechanics from Old Dominion University.
Mr. Clark Andrews is a mechanical engineer with The Boeing Company. He has 7 years of industry experience, including a multidisciplinary focus on the development of ballistically tolerant structures, aircraft configuration design, and, most recently, production support of drive systems. He holds a bachelor’s and a master’s degree in mechanical engineering from Northern Arizona University and Arizona State University, respectively.
Ms. Lisa Chiu has worked as a structural analyst for The Boeing Company for 10 years, focusing on analysis, simulation, and testing of composite structures. Currently, she serves as the principal investigator for several research and development programs focusing on survivability, durability, and damage tolerance. She is a Boeing designated expert in ballistic structural analysis and ballistic test execution for composite structure. Ms. Chiu holds a B.S. and M.S. in mechanical engineering from the University of Massachusetts Lowell
This research was partially funded by the Government under Agreement No. W911W6-11-2-0014. The authors thank the U.S. Army’s ADD and the Boeing AH-64 Modernization Office for their financial and technical support of this research.
References  Chiu, L., D. McCarthy, C. Andrews, A. Harris, and M. Robeson. “Durable and Damage Tolerant Composites Aft Fuselage Technology.” Proceedings, AHS 71st Annual Forum, Virginia Beach, VA, May 2015.  Chiu, L., D. McCarthy, and M. Robeson. “Blast Attenuating Aircraft Structure,” Proceedings, AHS 69th Annual Forum, Phoenix, AZ, May 2013.  Andrews, C., L. Chiu, and M. Robeson. “Analysis and Test of Composite Rotorcraft Structure Subject to Ballistic Events,” Proceedings, AHS 73rd Annual Forum, Fort Worth, TX, May 2017.