IMPACT PROPERTIES OF MATERIALS DURING DROP TESTING

T. Blaine Hoshizaki, PhD

D. Gordon E. Robertson, PhD, FCSB

Andrew Post, MSc

School of Human Kinetics

University of Ottawa, Ottawa, Canada

Introduction

This study investigated the impact characteristics of materials typically used in impact helmets and protective sport equipment to a series of impacts. The materials included several foams (vinylnitriles) and one each of a Expanded Polypropolene (EPP) and a Expanded Polystyrene (EPS). The purpose was to determine whether the attenuating properties of the materials remained consistent after a series of impacts. Certain helmets are designed to withstand single impacts (e.g., bicycle and motorcycle) these typically use EPS liners, the latter are assumed to be more durable; others helmets are intended for repeated impacts (e.g., football) these use vinylnitrile, EPP and EPE.

Methods

• 5-kg load projectile

• dropped down a tube from 10, 20 and 30 cm

• onto an anvil of 5.04 cm diameter

• imbedded with a uniaxial piezoelectric force transducer (ICP force ring, model 203B, PCB Piezotronics)

• each material had the same impact area (????) and had the same thickness of 2.54 cm

Methods

• photo-optical timer measured the velocity of the load prior to impact

• force data were collected at 20 kHz to determine the peak impact force and time to peak force

• each material was tested with a maximum of five repeated trials to observe the changes in the peak impact force and time

• I we need to provide a very good description of the material samples or the audience will not have a very good idea of what the results mean.

Video of Drop Testing

Results

• Figure 1 shows the results for a stiff vinylnitrile (R3953V) for a low drop height of 10 cm. In this case, the material performed poorly on the initial drop but improved its performance for the second and third drops. The peak force for the first drop was over 2500 N but reduced to 1700 N and 1300 N for drops 2 and 3.

• Where is drops 4 and 5????

Results

• Figure 1. Three trials of vinylnitrile for 10 cm drop

0

500

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1500

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2500

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0 0.002 0.004 0.006 0.008 0.01 0.012

Time (s)

Fo

rce (

N)

Trial 1

Trial 2

Trial 3

Results

• Figure 2 shows a relatively compliant material after only four drops from a height of 20 cm. Notice that trial 1 had a different force profile from trials 3 and 4. The material after the first drop yielded a peak force of only 900 N but two trials later the peak had risen to almost 1400 N. Where is trial 5??? and what does this result mean?

Results

• Figure 2

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1400

0 0.005 0.01 0.015 0.02 0.025 0.03

Time (s)

Fo

rce

(N

)

Trial 1

Trial 3

Trial 4

Where is the description?Where is the description?

Results

• Figure 3 shows the average force histories (four trials each) for a 10 cm drop height. Notice that the foam materials (R313V, R326V, VN600) had lower peak forces due to rapid early rises and by spreading the impact forces out over time while the EPS and EPP materials had slower rise times and consequently delivered higher peak forces.

• This is isn’t a direct relationship, I would word it differently and probably expand the explaination to discuss the possible mechanism for managing energy that seems to be reflected in the data.

Results

• Figure 3. Ensemble averages ( # of trials) of five materials to 10 cm drop

-200

0

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1200

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Time (s)

Fo

rce (

N)

R313V

R326V

VN600

EPS 1.5pcf

EPP 2.6pcf P1822B

Discussion

• Unexpectedly materials that were assumed to be durable did not maintain their force attenuation properties even after one impact. Stiffer materials appear to perform better after an initial impact but further tests have shown that their performance also degrades after repeated impacts. One must therefore expect to change helmet liners frequently after severe impacts.

• I think we need to discuss the material characteristics of each sample in order to frame the findings. We also have to discuss why we chose the 5 lb mass and inbound velocity as this has a significant effect on the findings. Finally I think the real signinficant finding of these data is a message to product designers and engineers. That protective characteristics of these materials is not well understood outside the a single impact and peak force paradigm.