Computational Modeling and Performance Evaluation ...

Proceedings of the ASME 2010 International Mechanical Engineering Congress & Exposition IMECE2010 Proceedings of IMECE2010 November 12-18, 2010, Vancouver, British Columbia, Canada 2010 ASME International Mechanical Engineering Congress and Exposition November 12-18, 2010, Vancouver, British Columbia

IMECE2010-40579 IMECE2010- COMPUTATIONAL MODELING AND PERFORMANCE EVALUATION OF A DAX-FOAM AIRCRAFT SEAT CUSHION UTLIZING HIGH LOADING RATE DYANMIC CHARACTERISTICS

Prasannakumar S. Bhonge Mechanical Engineering Wichita State University Wichita, Kansas, USA

Chandrashekhar K. Thorbole Occupant Protection The Engineering Institute Farmington, AR, USA

ABSTRACT The aircraft seat dynamic performance standards as per CFR 14 FAR Part 23, and 25 requires the seat to demonstrate crashworthy performance as evaluated using two tests namely Test-I and Test-II conditions. Test-I dynamic test includes a combined vertical and longitudinal dynamic load to demonstrate the compliance of lumbar load requirement for a Hybrid II or an FAA Hybrid III Anthropomorphic Test Device (ATD). The purpose of this test is to evaluate the means by which the lumbar spine of the occupant in an impact landing can be reduced. This test requirement is mandatory with every change in the seat design or the cushion geometry. Experimental full-scale crash testing is expensive and time-consuming event when required to demonstrate the compliance issue. A validated computational technique in contrast provides an opportunity for the cost effective and fast certification process. This study mainly focuses on the characteristics of DAX foams, typically used as aircraft seat cushions, as obtained both at quasi-static loading rate and at high loading rate. Nonlinear finite element models of the DAX foam are developed based on the experimental test data from laboratory test results conducted at different loading rates. These cushion models are validated against sled test results to demonstrate the validity of the finite element models. The results are compared for these computational sled test simulations with each seat cushion as obtained using quasi-static and high-loading rate characteristics. The result demonstrates a better correlation of the simulation data with the full scale crash test data for the DAX foam when high loading rate data is utilized instead of quasi-static data in the dynamic finite element models. These models can be utilized in

Hamid M. Lankarani Mechanical Engineering Wichita State University Wichita, Kansas, USA

the initial design of the aircraft seats, and thus reducing the cost and time of a full-scale sled test program.

INTRODUCTION The purpose of test-1 condition of 14 CFR Part 23 [1], 25 [2], 27 [3], 29 [4] is to evaluate the occupant lumbar spine load during crash landing scenario. This test condition requires that lumbar load should be less than 1500 lb load. This also provides an opportunity to identify best suited design to attenuate the lumbar load on the occupant. This test requirement is mandatory with every change in the seat design or modifications in existing bottom cushion, as the bottom cushion is the most crucial subsystem in the primary structure of the seat load path that is transferred from the ground to the passenger. The aircraft seat cushions are also required to be replaced periodically due to wear and use in the normal service. Thorbole, et al. [5] has demonstrated the affect of temperature on the dynamic characteristic of the seat cushion. In case of non availability of an identical cushion, the seat manufacturer needs to be conducted a full scale dynamic seat test to satisfy test condition 14 CFR XX.562 (b) (1) [1, 2] which requires specific acceleration, velocity and rise time. A proper selection of cushion characteristic plays a major role in absorbing vertical energy and attenuating load transferred to the occupant. The Finite Element Analysis (FEA) is an integral part of an aircraft seat design process. The goal of the FEA in product development is not limited to design but could also be used to substantiate the certification process or possibly replacing the certification tests. Advisory Circular (AC) 20-146 [6] demonstrates the methodology for dynamic seat “Certification by Analysis” for use in Parts 23, 25, 27 and 29 airplanes and rotorcraft. This study mainly focuses on the comparison of the dynamic

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Copyright © 2010 by ASME

The test fixture and methodology for the cushion component tests were adapted from the Indentation Force Deflection (IFD) method described in ASTM D3574-03 [11]. A rectangular DAX Cushion (18.5in X 19in and 4in) was utilized for the test purpose.

performance of the DAX foam when its characteristic is obtained both at quasi-static loading rate and at high loading rate typically 30 in/sec.

MODEL OVERVIEW For this study, a Finite Element Model of a relatively simple seat was developed using Altair Hyper works meshing program [7] and LSTC LSDYNA explicit solver [8]. The Seat structure, DAX foam cushion and Hybrid II dummy model were modeled on 60 deg pitched sled. A 50th percentile hybrid II dummy was used to simulate the ATD developed by First Technology Safety System FTSS [9]. Stress strain properties of DAX foam for quasi-static and 30 in/sec loading rates were quantified in the laboratory to be used for the FE model. The computational model results were validated against sled test result to demonstrate the validity of the finite element model [10]. The simulation of test 1 condition was modeled with each set of data as obtained for different loading rate from the laboratory test. The results were compared for these computational sled test simulations with each seat cushion as obtained using quasi-static and high-loading rate characteristics (30 in/sec).

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Deflection in inches

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Fig. 2: Loading Curve (Displacement Vs Time). Three samples of DAX 26 and 55 were tested at 30 in/sec loading rates. The Figure 2 shows the load profile applied during the test to achieve 90% strain (in compression). Average load–deflection characteristics were selected to derive nominal stress-strain curves and used for the proposed methodology. Quasi static foam properties were studied from the AGATE report, C-GEN-3433B-1 (REV N/C) [12] and Seat Cushion Replacement Program the FAA report, DOT/FAA/AR-00-xx, 2003 [13].

COMPONENT TEST The component tests were performed at the Wichita State University Composites Laboratory using a 220-kip MTS load frame equipped with a 110-kip servo-hydraulic actuator and instrumented with two load cells, a conventional 150-kip strain gage type load cell and a 10kip piezoelectric load cell as shown in Figure 1.

Load Deflection Curves Following results for DAX foam were quantified from the high speed test. Figure 3 and 4 shows load-deflection curves for foam model DAX55 and 26 respectively. 4500 4000

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Fig. 1: MTS Servo Machine. Fig 3: Load Deflection Curve for DAX 55 Foam (high loading rate).

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CASE STUDY

1600

The intention of this study was achieved by using a sled test carried out on an aircraft side facing passenger seat. The test was a Test-1 condition combined vertical / longitudinal velocity change dynamic test condition also known as down test to evaluate occupant lumbar load. This test was used to compare and validate the computational model. Figure 7 shows pre-test setup for the test and FEA model in which the seat was mounted aft of a cabinetry and the entire system was mounted on 600 pitched fixture. The seat cushions were placed on the seat bottom and seat back using Velcro during the sled test. The ATD was positioned in the normal condition using a three point restraint system. For the sled test, all required data channel and its corresponding filter channel class for the test conditions were verified as per the Society of Automotive Engineers (SAE), Inc "Instrumentation for Impact Test” [14]. FE modeling of the seat structure is described elsewhere [15]. A 50th percentile hybrid II dummy was used to simulate the ATD and was developed by FTSS. The foam was modeled using hexahedron solid elements and the nominal stress-strain properties were provided from the 30 in/sec high speed test results as discussed earlier. FE dummy was positioned in the normal condition using the pre-test installation still photos. Figure 8 shows the sled acceleration pulse used for the model as compared to the actual sled pulse. During the analysis time step, energy ratio and energy balance were verified to ensure accuracy of the results.

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Fig. 4: Load Deflection Curve for DAX 26 Foam (high loading rate). Figure 5 and 6 shows static load-deflection curves for foam model DAX 55 and 26 respectively.

Deflection (inch)

Fig. 5: Quasi-static load deflection curve for DAX 55 foam [13].

Fig. 7: Test and DFEA Setup for Case Study 1 Subsequent to the FE analysis, occupant kinematics, energy ratio and overall percentage mass increase were verified using global energy files for accuracy purpose and plotted as shown in Figure 9 and 10 and were observed within the limit [15].

Deflection (inch)

Fig. 6: Quasi-static load deflection curve for DAX 26 foam [13].

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were also verified and found within in the limit. Figure 11 shows overall internal energy and hourglassing energy in the system. Hourglass energy was less than 10% of the total internal energy [15].

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Fig. 8: Comparison of Test and DFEA Sled Pulse. Time, Sec

Fig. 11: Internal energy and hourglass energy in the system

RESULTS AND DISCUSSION Using foam characteristics at 30 in/sec Once the accuracy in the model was confirmed and good confidence was developed, interface loads, belt loads and lumbar loads were verified. Close observation were made on the maximum reaction load of the seat and compared with the physical test results as shown in Figures 12.

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Fig. 9: Energy ratio

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Fig. 10: % Mass increase Test

In this case, energy due to the sled acceleration was transferred into the kinetic energy, Internal energy, hourglass energy and sliding energy of the system and hence the energy equation could be written as

DFEA

Fig. 12: Comparison of Test and DFEA Interface load Similarly, lumbar load and lumbar moment were verified and compared with the test results and plotted as shown in Figures 13 and 14.

Total energy = Internal Energy + Kinetic Energy + Hourglass Energy + Sliding Energy. Hour-glassing energies in the individual components

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Fig. 15: Interface load

Fig. 13: Comparison of Test and FEA Lumbar load 500

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Fig. 16: Comparison of Test and FEA Lumbar load

Fig. 14: Comparison of Test and FEA Lumbar moment

Using foam loading

0.15 Time, Sec

The same FEA model was again used to analyze the seat response using quasi static foam stress strain characteristics as shown in Figure 5 and 6. The required load deflection data for DAX foam cushion was referred from the FAA report “Seat Cushion Replacement Program” [13]. During the analysis process, energy balance, mass increase and hourglassing were reviewed. Once the accuracy in the model was ensured at the end interface loads and lumbar load were verified. Maximum reaction from the FEA result was noted and compared with the physical test results as shown in Figures 15. Similarly lumbar load and moment were compared with the test results and plotted as shown in Figure 16 and 17. The maximum lumbar load was 1996 lbs which was 880 lbs off than the test lumbar load 1116 lbs. Similarly interface load and lumbar moment are way off as compared to the test results. Table 1 shows Result comparison chart for high speed and static foam characteristics FE results with test the results. The metric comparison is shown in the Annex-A.

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Fig. 17: Comparison of Test and FEA Lumbar moment AC 20-146 [5] demonstrates the methodology for dynamic seat and also demands correlation between simulation and the sled test be within +- 10% limit values. In second case, results using foam characteristics of quasi-static loading, though the lumbar load is positive (conservative) compare to the test results, its way off the AC 20-146 criteria of +- 10% and the same time compliance criteria of 14 CFR XX.562 which is more than 1500 lbs. In such scenario the

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validation and hence the analysis is invalid. Table 1: Result comparison chart for high speed and static foam characteristics FE results with test results

3. Title 14 code of Federal Regulations Part 27, "Airworthiness Standards: Normal Rotorcrafts," 1988.

CONCLUSION This study mainly focuses on the characteristics of DAX foams, typically used as aircraft seat cushions, as obtained both at quasi-static loading rate and at high loading rate. Nonlinear finite element models of the DAX foam are developed based on the experimental test data from laboratory test results conducted at different loading rates. These cushion models are validated against sled test results to demonstrate the validity of the finite element models. The results are compared for these computational sled test simulations with each seat cushion as obtained using quasi-static and high-loading rate characteristics. The result demonstrates a better correlation of the simulation data for the DAX foam, when high loading rate data is utilized instead of quasi-static data. These models can be utilized in the initial design cycle of the aircraft seats, and thus reducing the cost and time of a full-scale sled test program.

4. Title 14 code of Federal Regulations Part 29, "Airworthiness Standards: Transport Rotorcrafts," 1988. 5. Thorbole C.K, Lankarani H.M and Costello Tom.,”Temperature Effect on the Dynamic Characteristic of the Aircraft Seat Cushion”. IMECE2009-12164 6. Federal Aviation Administration, Advisory Circular, 20-146 “Methodology for dynamic seat certification by analysis for use in parts 23, 25, 27, 29 Airplanes and Rotorcrafts,” 2003. 7. ALTAIR, Altair Engineering Inc., www.altair.com. 8. LS-DYNA, Livermore Software Corporation, http://www.lstc.com.

REFERENCES 1. Title 14 code of Federal Regulations, Part 23, "Airworthiness Standards: General Aviation Aircrafts," 1988.

Technology

9. First Technology Safety System Inc. www.ftss.com 10. Bhonge P.S. “A Methodology of seat certification by dynamic finite element analysis,” PhD dissertation, Mechanical Engineering Dept., Wichita State University, Wichita, KS, 2008.

2. Title 14 code of Federal Regulations, Part 25, "Airworthiness Standards: Transport Airplanes," 1988.

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11. Standard Test Methods for Flexible Cellular Materials, - Slab, Bonded, and Molded Urethane Foams, Standard D3574-03, American Society for Testing Material, 2003. 12. “Development of a Seat Cushion Replacement Component Test Method for Dynamically Certified Seat,” AGATE report, C-GEN-3433B-1 (Rev N/C). 13. “Seat Cushion Replacement Program,” Draft by Lankarani H. M., 2003. 14. Society of Automotive Engineers, Inc "Instrumentation for Impact Test," SAE J211/1, SAE Recommended Practice, Rev. March 1995. 15. Bhonge, P.S., and Lankarani, H.M., “Finite Element Modeling Strategies for Aircraft Seats,” SAE – Wichita Aviation Technology Conference, August 2008.

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ANNEX-A Table 1: Metric result comparison chart for high speed and static foam characteristics FE results with test results

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