Honeycomb & Inflatable Protectors Test Report

Introduction

Goals and Process

• Motivation: Following recent discussions about back protectors, we invested time in conducting complementary tests and sharing the results with the community. We consider this work valuable for R&D, but we are happy to share it to spread experience, hoping it would participate to overall pilot safety improvements.
• Goals:
  • Characterize the impact absorption behavior of Honeycomb structures under controlled vertical drop tests, with emphasis on low-altitude impacts and the influence of material thickness.
  • Compare the honeycomb protectors and inflatable protectors responses.
  • Identify distinct mechanical response regimes (buckling plateau vs. final compression) and relate them to measurable criteria such as maximum acceleration, SIC, and DRI.
• Experimental approach:
  • Perform repeated drop tests with a fixed mass onto Koroyd blocks / inflatable protector of various thicknesses and drop heights, while recording acceleration.
  • For more accuracy and repeability, the protector is laid on the ground without harness and the honeycomb have a parallelepiped shape.
  • We focus here on honeycomb and inflatable protectors because we see them as two ends of the impact-response spectrum: honeycomb behaves as the most constant-force option, while inflatable protection behaves the closest to a spring. Moussebag as exemple are in between.
  • Particular attention was paid to ensuring that the pre-drop internal pressure of the inflatable protector was consistent between drops, using the same electric pump and the same inflation protocol.
• Metrics evaluated:
  • Peak acceleration (g), often called :max_g
  • SIC and DRI injury criteria
  • Duration above critical acceleration thresholds (EN values: 20g and 38g)
  • Absorbed momentum
  • Visualisation in the Eiban graph
• Comparative analysis:
  • Compare results across different drop heights.
  • Compare results across different honeycomb thicknesses.
  • Compare protected impacts with bare-ass drop reference test.
  • Compare results across two different technologies: honeycomb, inflatable.
• Note:
  • This test session uses Koroyd blocks as a honeycomb structure, a particular case of protection with a fixed compression force response.
  • The Interpretation paragraphs reflect our own current interpretation and may evolve with new evidence.
  • We acknowledge that one limitation of this test session is the use of simple shapes, whereas real harness protectors are often composed of multiple materials with more complex geometry. However,we can still extract some generic behaviors that remain valid.
  • All graphs can be clicked to access to their enlarge interactive version.

  Computation Details (click to unfold)

  • g₀ = 9.80665
  • Mass = 52 kg

  DRI Parameters

  • Applied on the full curve (filtered)
  • omega_n (ω_n) = 52.9
  • zeta (ζ) = 0.224

  SIC Parameters

  • Applied on the full curve (filtered)
  • Time window: 1-100 ms
  • Exponent p = 2.5

  Filtering Parameters

  • 4th-order Butterworth low-pass filter with zero-phase (forward-backward) filtering
  • Cutoff frequency from SAE J211 / ISO 6487: F_c = CFC X 2.08 (CFC 75 → 156 Hz)
  • Equivalent to standard CFC 75 acceleration filtering

  Filtering note:

  The curves shown below are filtered to offer better visual clarity as they overlap each other.

  For illustration, this is the drop for 878mm raw and filtered:

  

  Figure 1 - Drop at 878mm showing raw and filtered acceleration (click for interactive chart).

Tests

Honeycomb Drop Height Comparison

Protocol

• The dummy is dropped naked on 2 blocks of Koroyd: length: 275 mm, width: 75mm, thickness: 75mm.
• The pair of blocks are held in place with a thin piece of cardboard, untouched by the dummy
• 2 pairs of blocks were used, one receiving multiple drops. Here is a table showing the drop details:

Figure 3 - Drop test setup showing Koroyd blocks.

Drop Details Table

Acceleration Curves by Height

Figure 4 - Drop tests at different heights showing buckling and compression areas (click for interactive chart).

Analysis: Two Distinct Behaviors

There are 2 different behaviors observed:

• Buckling area: Offering a constant force, independently of the drop height. Small drops and large drops show the same maximum g (between 28 and 32 g). What changes is the duration of this plateau and the total buckled thickness.
• Final compression area: When the full height of the Koroyd is buckled, the dummy is smashing a already buckled pile of plastic, which shows a completely different response.

Illustrations of the 3 Possible Stage Types

• 1/ Intact
• 2/ Partially used: half of the top is buckled, the bottom part is intact. This configuration offers a plateau response.
• 3/ Completely used: the full height has buckled. This configuration offers a final compression response before bottoming out.

Figure 5 - Three stages of Koroyd compression.

Notes

• The orange curve shows a different behavior. The drop was from 275 mm above the ground, which means 200 mm from first contact. We don't explain well why the Jerk is significantly smaller on this drop.
• Keep in mind that the drops at 275 mm (orange), 575 mm (green), and 878 mm (red) were performed on a single pair of blocks so the intact parts of Koroyd were getting smaller and smaller.

Numerical output

Interpretation

The buckling theory behind this type of protector can be observed in practice:

• The bulking force remains the same, directly linked acceleration response; the quantity of buckled material changes with momentum, so mass and speed variations.
• From these observations, it is irrational to think that, for a given protector, a drop from a lower height would lead to more injury. The idea of a minimal threshold that must be exceeded to provide protection is not observed.
• Both SIC and DRI reflect the order of curves being ranked by drop height.
• Assuming a first drop that consumes 25% of the intact height, this will be the situation after each drop on the same protector. For these 4 drops, the acceleration curve should be the same, each drop consuming similar intact amount of material. The 5th drop will show a totally different answer, with higher g.

Figure 6 - Progressive compression over multiple drops.

Honeycomb Thickness Comparison

Protocol

• The dummy is dropped naked on 2 blocks of Koroyd: length: 275 mm, width: 75mm. The initial protector thickness varies for different measurements.
• The pair of blocks are held in place with a thin piece of cardboard, untouched by the dummy

Drop Details for Thickness

Acceleration Curves by Thickness

Figure 7 - EN (1.65m) drop test on Koroyd blocks of different thickness (click for interactive chart).

Analysis

• If the protector is not thick enough, the dummy touches the 'final compression' area and produces a higher max g.

Numerical results

Interpretation

• If used in big enough thickness, free of final plastic compression, the koroyd protector can maintain very low_g (<30g) during high energy impacts.
• The 60mm and 75mm protector shows poorer results in a 165 cm fall, not because of the technology itself but because thiers thickness is insufficient to not enter in the final compression stage.
• The 115 mm protector shows a maximum acceleration below 30 g. Comparable performance at this thickness was not observed with any other non-destructive technologies.

Comparison to Bare-ass 5cm Drop

The idea here is to compare the initial jerk with the bare drop done in this analysis: Jerk Analysis Report

Figure 9 - Comparison between bare-ass 5cm drop and protected drops (click for interactive chart).

Interpretation

• The bare-ass drop first drop peak duration is similar to the ramp up duration of the Koroyd protectors, while having max_g and max jerk much higher.
• It is difficult to consider the initial jerk of the 11.5 cm blue curve as dangerous, given that the 5 cm bare drop (green curve) does not injure a human.

Inflatable Drop Height Comparison

Protocol

• The dummy is dropped naked on an inflatable protector (340mm X 250mm) with a thickness of 130mm. It is was impossible to test thinner protector without bottoming out.
• Multiple drops were performed from different drop heights to characterize the inflatable protector response.
• On some drops (see table below), the air valve opened during the impact, which reduced the acceleration peak. Another tests shows a decrease of about 10-15% in max g when the valve opens during impact.

Drop Details for Inflatable

Acceleration Curves by Height

Figure 10 - Drop test on inflatable protection (13cm) from different heights (click for interactive chart).

Numerical Results

Interpretation

• This inflatable protection shows an almost proportional response to the drop height.
• This particular protector does not pass the current EN certification requirements due to its limited volume and small thickness but this is the best inflatable protector we could make for this given inflated thickness.
• During this experiment valve shown to be an interesting features, by increasing the valve diameter on this same protector the final result can be optimized for the EN drop height and pass the EN test, at the expense of a reduced margin before bottoming out.

Honeycomb vs Inflatable Comparison

This comparison shows the maximum acceleration recorded versus drop height for both particular honeycomb and inflatable protectors, labeled by protection thickness.

Figure 11 - Maximum g vs drop height comparison between Koroyd and inflatable protectors, labeled by thickness (click for interactive chart).

Analysis

• Koroyd behavior: Shows a plateau response across different drop heights with consistent maximum acceleration around 30g. The acceleration is relatively independent of drop height, demonstrating the honeycomb buckling behavior.
• Inflatable behavior: Shows a proportional increase in maximum acceleration with drop height, from ~10g at 330mm to ~68.5g at 1650mm.

Interpretation

• For drop heights below approximately 1 m, this particular honeycomb protector is less comfortable, and the max g remains nearly constant regardless of height.
• This particular inflatable protectors, although more comfortable for drops below 1 m, seem to present higher risk at greater drop heights.
• Comfort from low height drop is not neccessarly representative to safety for higher drops.

Eiband Space Representation

These drops plotted in the Eiband space.

Figure 8 - Eiband space representation of Koroyd drops (click for interactive chart).

Analyse peer review and replication is encourgaed, here the original data:

• Koroyd__167_T-75mm_H-1650mm.csv
• Koroyd__168_T-115mm_H-1650mm.csv
• Koroyd__169_T-75mm_H-275mm.csv
• Koroyd__170_T-75mm_H-575mm.csv
• Koroyd__171_T-75mm_H-878mm.csv
• Koroyd__194_T-60mm_H-1650mm.csv

• Inflatable_182_T-130_H-330mm.csv
• Inflatable_183_T-130_H-638mm.csv
• Inflatable_184_T-130_H-936mm.csv
• Inflatable_186_T-130_H-1363mm.csv
• Inflatable_301_T-130_H-1650mm.csv