20-04-2012, 02:52 PM
Crash Helmet Testing and Design Specifications
Crash Helmet Testing and Design Specifications.docx (Size: 139.08 KB / Downloads: 53)
ACKNOWLEDGEMENT
The satisfaction that accompanies the successful completion of any task would be incomplete without the mention of people whose ceaseless cooperation made it possible, whose constant guidance and encouragement crown all efforts with success. We are grateful to our project guide Prof. Bibhuti Bhusan Mishra for his guidance, inspiration and constructive suggestions that was helpful in the preparation of this project. We also thank the staff of ITER and our friends and classmates who have helped in successful completion of the project.
Abstract
In motorcycle tra_c accidents, the human head is exposed to loads exceeding several times the loading
capacities of its natural protection. Annually, approximately _ve thousand motorcyclists get killed in
Europe as a result of tra_c accidents. They account for 9% of all road fatalities. Wearing a helmet
reduces the risk of fatality with about 50%. Over the years, helmet standards have evolved to be an
e_ective means to assure helmet quality in terms of minimal performance. In general, helmet testing
standards are the result of often rather pragmatic compromises in meetings of technical experts, more
than scienti_c research. As helmet quality improves, the criteria of shock absorption tests are raised
and impact severities are increased.
The shock absorption test of the ECE Regulation 22 (ECE-R.22) is the focal point of this thesis. In this
test, the helmet quality is assessed by measuring the acceleration time history of a headform during a
helmeted headform impact. Helmet quality is expressed in terms of the injury parameters: maximum
resultant translational headform acceleration (amax) of the headform's centre of gravity and Head Injury
Criterion (HIC). The HIC is currently the leading head injury criterion in automotive research, however
the ECE standard is still the only helmet standard that has implemented it in its shock absorption test.
The head model used in this test is an aluminium alloy, "rigid" headform.
The shock absorption test of the ECE-R.22 has two main limitations with respect to injury assessment.
Firstly, the currently used headform may not properly mimic the response of the human head correctly
in helmeted head impact. The rigid headform does not model the _exibility of the human brain and
skull, which may be inadequate to model the helmet-head interaction correctly. Secondly, advances in
injury biomechanics have resulted in other, possibly better, injury criteria that are not or can not be
measured in the current shock absorption test.
To date, little has been done to explore how helmet designs may change as a result of applying these
di_erent criteria. This thesis deals speci_cally with the question:
How could motorcycle helmet designs change as a result of applying new and di_erent head injury criteria in the shock absorption test of ECE-R.22? To implement these criteria in these tests, more quantitative data must be available from the drop test,like rotational accelerations and strains inside the brain. This requires an anatomically more detailed headform or at least a numerical model of such a headform. This holds especially for the strain-based injury criteria, since strains cannot be assessed using a conventional, rigid headform. Furthermore, the interaction between headform and helmet is likely to in_uence the outcome of the test. To better model the head-helmet interaction, the headform must at least have a deformable skull and brain. In this study, an anatomically more detailed, deformable headform is developed that allows more realistic helmet-head interaction. Furthermore, it allows for the assessment of helmet performance using di_erent types of head injury criteria, compared to the standard ECE-R.22 rigid magnesium alloy headform. With this headform, the e_ects of the anatomically more detailed headform on helmetix head interactions are studied as well as the assessment of head injury risk in the shock absorption test of ECE-R.22. Besides the currently applied translational acceleration based head injury criteria, the injury risk is also assessed in terms of rotational velocity/acceleration and strain based head injury criteria. Furthermore, it is studied whether and how the use of the latter injury criteria in the shock absorption test has additional e_ects on helmet design compared to the use of the former. From the results, guidelines are developed for improved helmet quality and the assessment thereof.
Introduction: Crash Helmet Design as Driven by Standards
1.1 Crash Helmet Design Then and Now
c injury mechanisms were studied.
The way helmets were designed was, and still is, heavily dependent on the application of the helmet.
Already in the 15th century B.C., helmets were e_ective means of protecting the head. Figure 1.1 shows
a Corinthian helmet that dates back to the 4th century B.C., weighing about 1.5 kg [Gurdjian, 1975] and
obviously designed to protect against penetration in combat.
Not until the end of the 19th century, it was discovered that serious head injuries could occur without
penetration. It took another 50 years to understand that non-penetrating head injuries are caused by
short-duration accelerations acting on the head and its contents [Holbourn, 1945]. These acceleration
injuries are the most common and dangerous form of injuries for motorcyclists and are often caused
by blunt impact rather than penetration. The early motorcycle helmets were designed accordingly: a
Corinthian helmet that dates back to the 4th century B.C. [
1.1.1 E_ectiveness of Current Day Motorcycle Helmets
Motorcyclists are among the most vulnerable road users. Together with moped riders, they have the
highest risk of getting killed in a tra_c accident [EEVC, 1993]. In car crashes, car occupants are
protected by safety belt systems, airbags, retracting steering systems, the padding of the car interior
and the car body itself. Motorcyclists involved in tra_c accidents are much more vulnerable than car
occupants. Almost the only protection o_ered to a motorcyclist is the crash helmet.
Otte et al. [1982] provide a detailed categorisation by body part of injuries sustained by motorcyclists
involved in tra_c accidents. The injury severity is indicated using the Abbreviated Injury Scale (AIS)
4 CHAPTER 1. INTRODUCTION: CRASH HELMET DESIGN AS DRIVEN BY STANDARDS
1980 there was no helmet law at all. Still, in Europe, where helmet laws have become e_ective since
1975, the number of fatalities is higher than in the USA. Furthermore, the number of fatalities as a
percentage of all road fatalities has increased over the period 1990-1997. This is probably because
the di_erent tra_c situation in Europe compared to the US tra_c situation. Especially in southern
European countries traffic is more chaotic.
1.2 Origin and Outline of the Crash Helmet Standards
To make sure that all motorcycle helmets reach a certain level of e_ectiveness, current helmets are
required to pass a certain standard test before they are allowed to be put on the market. Therefore,
helmets are designed to pass these tests, even though some helmet manufacturers do more than that,
and thus the quality of the helmet depends largely on the quality of these tests. The _rst standard tests
for motorcycle helmets was the British Standard 1869:1952 [BSI, 1952], issued by the British Standard
Institution (BSI). This test applied shock loadings to a helmeted headform. In the test, technicians
would drop a hardwood block weighing ten lbs. (4.5 kg) from a height of nine feet (2.7 m) onto
a helmeted headform.
6 CHAPTER 1. INTRODUCTION: CRASH HELMET DESIGN AS DRIVEN BY STANDARDS
of injury occurs is called a tolerance level (or injury criterion level). Tolerance levels for speci_c types
of injury are obtained from injury risk functions which predict the chance of sustaining an injury at a
speci_c load. The way in which the biomechanical response leads to injury is called injury mechanism.
Injury mechanisms are not the same for the di_erent parts of the head and are not always easy to
understand [Wismans et al., 1994]. For example skull fracture is often caused by direct contact to the
head, whereas Traumatic Brain Injury (TBI) does not necessarily involve direct contact.
So three main factors are of importance in shock absorption tests:
The load: In real accidents, a wide variety of accident con_gurations occurs, resulting in many di_erent
loads onto the helmeted head. The shock absorption test should simulate the most common
accident con_gurations correctly.
The assessment: Criteria must be set to assess the risk of injury. What parameters should be measured,
depends on the choice of the injury criteria.