Expandable Foam Impact Attenuation for Small Parafoil Payload Packages

Transcription

1 Courtesy of Julie Arnold and Paula Echeverri. Used with permission Final Report Expandable Foam Impact Attenuation for Small Parafoil Payload Packages Julie Arnold and Paula Echeverri November 25 th, 2003 Advisor: Christian Anderson 1

2 Background and Motivation Impact attenuation for small payloads delivered by UAV s Current mechanisms: Airbags/ Parachute Retraction Paper Honeycomb Alternative: Expanding Foam Impact Attenuation (EFIA) 2

3 Hypotheses 1. 75% less pre-deployment volume and crush thickness efficiency loss of no more than 30%. 2. Increase in cost of no more than 50% and a decrease in reliability of no more than 10%. Objective Assess the ability of an EFIA to protect a payload having a 50g-impact shock limit from a 15 ft/s* vertical descent rate. Compare the pre-deployment volume, crush efficiency, cost and reliability of the EFIA against paper honeycomb. Success Criteria Evaluate the aforementioned metrics for an EFIA and for paper honeycomb to an accuracy such that the hypotheses can be assessed. *Impact velocity was 13.5 ft/s during drop tests 3

4 Hypotheses Assessment Confirmed crush thickness efficiency loss of no more than 30% Refuted decrease in reliability of no more than 10% Uncertain assessment of 75% less predeployment volume 4

5 Experimental Overview and Methods Choice of Material and Payload Characteristics Crush Efficiency Deployment Mechanism Cushion Thickness and Area Safety Margin Deployment Reliability Volume Cost LEGEND Path to primary hypothesis 5 Path to secondary hypothesis

6 Crush Efficiency Test Matrices Static Dynamic: Same only for ½ samples 6

7 Test Matrices (Cont.) Safety Margin Material: Honeycomb Material: Expanding Foam Trial Number Maximum Trial Number Maximum Acceleration Acceleration Expanding Foam Deployment Reliability Pre-Deployment Volume Trial Number 1-25 Successful Deployment Material Honeycomb Expanding Foam Volume 7

8 Error Mitigation Sampled at high data rates on accelerometer (5000Hz) and high-speed camera (2000Hz) Calibrated high speed camera before each session Confirmed impact velocity after each drop (13.5 +/- 0.3 ft/s) 8

9 Hypothesis Assessment Results Efficiency Pre-Deployment Volume Reliability Hypothesis At least 70% At most 25% At least 90% Results 83.8% (- 11.1) 27% (- 5.4, + 7.5) 83% (95% confidence) EF parameters as compared to HC 100% Hypothesis 80% 60% 40% % difference EF 20% 0% Efficiency Pre-Deployment Volume Material Reliability 9

10 Analysis of Crush Efficiency HC Crush Efficiency Honeycomb Crush Efficiency Sample Thickness (in) Static Test Samples Dynamic Tests Average Linear (Static Test Samples) - Final cushion thickness of 2.7 inches requires three 1- inch pieces of honeycomb stacked together. - Used efficiency for 1 honeycomb EF Crush Efficiency Expanding Foam Crush Efficiency Sample Thickness (in) Samples Lower Bounds Upper Bounds Linear (Samples) Linear (Lower Bounds) Linear (Upper Bounds) Used lower bound to extrapolate efficiency to final cushion thickness of 2.5 inches. 10

11 Analysis of Safety Margin Tests Results Paper Honeycomb Tests show two dominant G-loading modes: Buckling (first peak) Crushing (~plateau) 100 S a m p le F ilte re d H C S a fe ty M a rg in T ria l 80 Acceleration (G) Time (s) 11

12 Analysis of Safety Margin Tests Results Paper Honeycomb (Cont.) Peak Distribution Curves Crush: G-load peak predicted by theory Buckling: G-load peak during drop-tests 7 6 Honeycomb Distribution of Secondary Peak Accelerations Mean Peak Acceleration: 59.0 G Standard Deviation: 8.0 G Honeycomb Distribution of Primary Peak Accelerations Mean Peak Acceleration: 83.6 G Standard Deviation: 20.6 G 5 Number of Trials Number of Trials Seconday Peak Acceleration (G) Primary Peak Acceleration (G)

13 Analysis of Safety Margin Tests Results Paper Honeycomb (Cont.) Acceleration (G) Sample HC Safety Margin Trial Raw Data Filtered Data Time (s) σ crush Issue: High Frequency Vibrations Solution: Data filtered above 580Hz Motivation: X crush

14 Comparison to Previous Theory: Analysis of EF Crush Stress Previous Model: Assumes constant crush stress 2 ν τ cushion = 2 gg η Aσ dynamic = mgg t Findings: Crush stress is not constant Crush Stress (psi) Sample 1" EF Static Test Trial Displacement (in) Actual Crush Modelled Behavior y = 4.077x Linear (Modelled Behavior) New Model Model crush stress as a linear function, solve ODE Aσ && x + dynamic ka m x = = 0 mx && 14

15 Analysis of Safety Margin Tests Results Expanding Foam 7 6 Experiment Results Expanding Foam Distribution of Peak Accelerations Mean Peak Acceleration: 62.2 G Standard Deviation: 6.7 G Errors in Model Number of Trials = µ=21 G calculation error when determining parameters + µ=159 G modeling error of stress vs. displacement slope Peak Accleration (G) Corrected Model Prediction µ=65 G 15

16 Volume: Analysis of Volume and Reliability Results Use modified models and the results of the Safety Margin Tests to calculate new Area and Cushion Thickness. Honeycomb Pre-Deployment Volume = Area * Cushion Thickness Expanding Foam Post-Deployment Volume = Area * Cushion Thickness, then scale to find pre-deployment volume Expansion Reliability: Use sample size and number of failures to determine Reliability and Confidence through Larson s Binomial Distribution Graph 16

17 Sources of Error Honeycomb Expanding Foam Sources of Error Crush Efficiency 7.4 % 4.2% - Instron Machine Error - Estimating Crush Displacement Volume 11% 3.7% - Impact Velocity - Acceleration Measurement 17

18 Conclusion: Assessment of Hypothesis Confirmed crush thickness efficiency loss of no more than 30% Refuted decrease in reliability of no more than 10% Uncertain assessment of 75% less pre-deployment volume EF parameters as compared to HC 100% Hypothesis 80% 60% 40% % difference EF 20% 0% Efficiency Pre-Deployment Volume Material Reliability 18

19 Conclusion: Summary of Principal Findings 1) The pre-deployment volume of expanding foam is at most 34.5% the pre-deployment volume of paper honeycomb, under given test conditions. 2) The crush efficiency of expanding foam is at least 72.7% of the crush efficiency of paper honeycomb, for given test conditions 3) The expanding foam is 83% reliable (95% confidence) to expand to expected volume within 30 seconds. 4) Honeycomb crush efficiency is a function of cushion thickness. 5) Honeycomb buckling results in peak acceleration during impact. 6) Expanding Foam crush stress is not constant. Instead behavior during crush can be characterized using a linear approximation of crush stress. 19

20 Conclusion: Summary of Principal Findings (Cont.) 7) For a given crush stress, payload mass, and impact velocity: Honeycomb volume as a function of maximum acceleration during impact is constant. Expanding foam volume as a function of maximum acceleration during impact is linear. Pre-Deployment Volume of Attenuation Material vs. Maximum Acceleration During Impact Volume (in^3) Expanding Foam Honeycomb Maximum Acceleration (G) 20

21 Recommendations for Future Research Experimentally verify and improve the corrected models of material behavior during impact Characterize payload resonance for HC attenuation Expand experiment scope: - lowest pre-deployment volume impact attenuation material may depend on touch-down conditions Design reliable, low-volume, low-weight mechanism to expand foam onboard 21

22 Acknowledgements Christian Anderson Technical Staff 16.62X Faculty Questions? 22

23 Sample EF Safety Margin Test Trial (Raw Data) Acceleration (G) Time (s) 23