The Head Injury Criterion (HIC) functional

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Pedestrian Protection System

In the event of a car-to-pedestrian impact the pedestrian can suffer from impacts with the bumper, hood and windscreen. The design of the front-end of the car and the structural stiffness have a significant influence on the kinematics and injury risks of the pedestrian body (Venkatason et.al. 2014). Vehicle crash data in the US, Germany and Japan has shown that the head is the most frequently injured part of the human body (Fredriksson et.al. 2011).
Depending on the length of the front-end of the car, the head impact location will differ. For a standard family car, the head impact for adults will be on the hood whereas in smaller city cars, the head impact will be on the windscreen and/or the hard area between the hood and the windscreen. For small city cars impacts on the A-pillars are also common. (Autoliv, 2014)
In the car industry, a special Wrap Around Distance (WAD) which measures the distance from the ground to a point on the hood along the front structure of the vehicle as shown in figure 1 is widely used when designing a car. (Euro NCAP, 2015) The points on the hood are where the head impact is estimated to occur. For a child or a small adult the head will most likely hit the area from WAD 1000mm to WAD 1500mm and for an adult the head will likely hit the area from WAD 1500mm to WAD 2100mm.
Today the hoods used in cars are commonly made from aluminium. Aluminium is a low density metal which is easily processed and is suitable for cold forming (Ullman, 2003). The soft hood structure have the ability to absorb energy during the impact and give a reduction of the HIC, Head Injury Criterion. (Masoumi et.al 2010). To let the hood deform it needs to be lifted to give a clearance above the rigid engine structure beneath. Studies made by Fredriksson have shown that in a car-to-pedestrian impact in 40km/h the hood must be lifted 100mm to create minimum injuries. This is done by hood lifters with stops that limits the lift height. (Autoliv, 2014)
A headform to windscreen study have been made at Autoliv where the tested windscreens shows HIC values below 1000. HIC values and their meaning will later be described. The windscreen is considered as a soft structure with low HIC and is feasible for a head impact. However the HIC values increase further to the A-pillar area and the cowl. (Wingren, 2009)
An impact on the cowl and also on the hard A-pillars require a pedestrian protection airbag to enable a soft impact. (Autoliv, 2014)

Problem formulation

In smaller cars, a hood lift gives required clearance between the rigid engine structures. Due to the fact that the front end of the car is small, the WAD impact points will be on the windscreen, the cowl or on the A-pillars. The hood lifters do not provide any protection in this area however an airbag, which is a rapidly inflated cushion placed over the rigid structure, will give a soft landing spot upon impact.
Autoliv Sverige AB is today developing a Pedestrian Protection Airbag, (PPA) which lifts the hood itself, replacing the hood lifters.

Purpose

The purpose of this study is to examine how fast the hood can be lifted 100 mm using the pedestrian protection airbag. The definition of a movable hood is: (Euro NCAP, 2015) “All structures connected to the hood that move with the whole assembly when activated in a pedestrian impact. The rear end of the hood is the most rearward point of the movable hood when it is closed.”
A concept generation session will be performed where modifications of the current pedestrian protection airbag, for a faster hood lift, are to be proposed. The generated concepts will proceed to a concepts screening session where positioning and repetitive behavior of the PPA is highly prioritized.
Winning concepts proceed to testing where the performances such as lifting time of 100 mm, positioning and pressure distribution time are tested.

Delimitations

This is an initial study of a hood lift which will lead the way to further development to cover a whole functional system. Exact calculations of the functions in the concepts will not be made. The concepts will not have an optimal performance. Performances in low temperature, -35 ºC, and high temperature, +90 ºC, are not going to be examined. The verification of the concepts are limited to only two tests per concept with no fullscale testing which would include a wiper system and shear pins on the hinges. Also no Free Moving Head, (FMH) tests are made.

Literature studies

Priorities of pedestrian protection – a real life study of severe injuries and car sources

The German In-Depth Accident Study, (GIDAS) is an organization specialized in accident research. GIDAS provides information on vehicle safety to automotive and supplier industries. In the database of GIDAS, information and documentation of real life accidents are stored and available for researches and developers. (GIDAS, 2015)
In GIDAS, information of car-to-pedestrian accidents since 1999 are stored where the injuries of the pedestrians spread from moderate injuries to severe injuries. To classify the injuries, GIDAS uses the Abbreviated Injury Scale, AIS which denote the injuries with a number from one to six depending on the severity of the injury. AIS1 denote a minor injury and AIS6 denote maximal injury, AIS1+ denote at least a minor injury. (Fredriksson, 2010)
In the event of a car-to-pedestrian collision, the collision starts by the pedestrians legs getting impacted by the car bumper. The body of the pedestrian wraps itself around the car body followed by a chest impact. Lastly the head impacts on the hood and/or the windscreen area. According to GIDAS, for severe AIS3+ injuries on adult pedestrians, the windscreen was the major source followed by the hood. The major source of the child pedestrian injuries was caused by the hood followed by the windscreen. (Fredriksson, 2010)
In Fredriksson’s study of severe injuries and car sources, the injuries of the pedestrians were divided into five body regions: Head, neck, chest, arms and legs.
The study is based on data from GIDAS database ranging from 1999 to 2008 where only AIS3+ injuries, which denote a serious injury of the pedestrian, were examined. In 1030 cases of injured pedestrians studied, 161 were AIS3+ classified. The study showed that
• 43% of the pedestrians sustained head injuries
• 37% of the pedestrians sustained chest injuries
• 58% of the pedestrians sustained leg injuries
The injuries in the rest of the body regions were neglected.

The Head Injury Criterion (HIC) functional

The HIC is a tool used by Euro NCAP to measure the head injuries upon an impact with, for pedestrian protection, a vehicle structure such as the hood and the windscreen area. The HIC tool is defined by the acceleration of the head impact of the pedestrian: = { [ ∫ (√ ( )2 + ( )2 + ( )2) ] ( 2 − 1)}
Euro NCAP uses a headform with three perpendicular accelerometers attached to the center of mass of the head to measure the accelerations ax, ay and az to obtain data for the HIC computation. The acceleration measurement data is plotted as a function of time where t1 is the time of an initial head contact and t2 is the time when the head leaves the contact area. (Euro NCAP, 2015; Hutchinson et. al. 1998)
In an impact with a rigid vehicle structure the retardation of the head results in high HIC values. The HIC functional is used by automobile manufacturers to assess the quality of the design of the vehicles to meet the Euro NCAP requirements. The vehicle structures are optimized to absorb the energy of a head impact to achieve minimum HIC values. (Hutchinson et. al. 1998)
The Federal Motor Vehicle Safety Standard, (FMVSS) are standards written in terms of minimum performance requirements for vehicles. According to FMVSS 201, a HIC value over 1000 is not approved. The performance is given under an impact condition featuring a head form which freely moves at a velocity of 40 km/h and impacts on the structure of a vehicle. (FMVSS 201, 1995)

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HIC measurement limitation

In the event of a head impact on the vehicle structure, the impact results in both translational and rotational acceleration of the head. The accelerometers in the head forms used in testing, are limited to linear accelerations where the rotational acceleration is ignored. (Hutchinson et. al. 1998) The rotation of the head leads to high neck injury risks. Even with a HIC value under 1000 the neck may break. (Hutchinson et. al. 1998)
A HIC value measure is not always a reliable method due to its limitations. A lot of engineering judgments need to be implemented for the best results.

Influence of impact speed on head and brain injury outcome in vulnerable road user impacts to the car hood

Improvements of the pedestrian protection and new requirements have changed the way to design and develop pedestrian-friendly vehicles. The Euro NCAP requirement for a HIC value on a head-to-car impact is HIC≤1000 at a velocity of 40 km/h. A HIC value over 1000 is not approved. (FMVSS 201, 1995)
Automotive manufacturers aim to minimize the HIC values. By developing a deformable and soft vehicle front end, the vehicle structure absorbs the energy of the head impact to achieve minimum HIC values.
The result of the testing that is to obtain HIC values below 1000 in a velocity of 40 km/h, the under-hood distance must be at least 100 mm.

Theory

Gas and pressure

For a substance there are usually three different phases – solid, liquid and gas phase, as shown in figure 3. These phases are due to pressure and temperature. In low temperature and high pressure the substances are usually liquid and in high temperature and low pressure the substances are in a gas phase. (Cengel & Boles, 2011).
In a solid phase the intermolecular bonds between the molecules are strong which makes them fixed in a position. In a liquid phase the intermolecular bonding are weaker due to high temperature together with low pressure. The applied energy from the increasing heat makes the atoms in the molecules vibrate in the liquid phase. In a gas phase the intermolecular bonding is the weakest which results in a random movement of the molecules. Due to the random movement, the molecules collide with each other and on obstacles, like for example the walls of a container. (Cengel & Boles, 2011)
To inflate an airbag Autoliv uses two different types of inflators; pyrotechnic and Hybrid inflators. In both types, solid propellants are combusted using voltage applied to a squib. This creates heat which changes the solid phase of the propellants to a gas phase. The heat of the gas created is too high to inflate the cushion and has to be cooled down. The cooling procedure of the inflators differ. In a pyrotechnic inflator the gas is cooled down by a metal filter while passing through. For hybrids, the inflators have, besides the propellants, stored gas. The gas generated by the propellants is mixed with the stored gas and results in a gas mixture with reduced heat. (Odwong, 2003)
When the gas enters the cushion it starts hitting on obstacles, wherever it’s the cushion wall or the air inside the cushion. The wall can be seen as a solid which have a higher density, matter per unit volume, than gas. When the gas collides with the wall the molecules of the gas can’t pass through. Due to the elastic characteristics of the molecules, a bouncing effect forces the gas molecule movement to change direction. The density of the air inside the cushion is lower than the density of the gas. The gas molecules pass through the air molecules and inflates the cushion. This scientific principle is applied on hot air balloons. (Cengel & Boles, 2011)
When the gas is inflating the cushion, a pressure is building up. In the cushion the pressure is due to gas molecules colliding with the cushion wall and exerting a force on it. The pressure generated is a strong function of the density and the temperature. Too much gas in the cushion give high density and high pressure of the gas inside the cushion which lead to a solid characteristic of the cushion and results in high HIC values upon a head impact. If the amount of gas in the cushion is low, both the density and the pressure is low. This lead to the head form traveling far into the cushion and is likely to hit the rigid structure underneath. (Cengel & Boles, 2011)

Material science

When the cushion is inflated, a pressure is building up and the hood is lifted rapidly. If the initial contact area between the hood and the inflated cushion is small, high forces created by the gas pressure will act on the hood. This assumption is based on the formula:
High forces on a small area will bend the hood, creating a plastic deformation in the material upon a hood lift. The plastic deformation is a hardened area of the material which results in high HIC values upon a head impact to the hardened area. In order to reduce risk of a plastic deformation of the hood material, a deeper scientific understanding of the strength of materials and material science is needed.
When shaping the hood in a manufacturing process, the hood is exposed to plastic deformation. The plastic deformation is 2-4 % and give the hood a hardened surface. The hood meets the Euro NCAP requirements but further deformation might lead to not high HIC values.
The general material of the hoods used in today’s vehicles is as described earlier Aluminium. In Aluminium the atoms cooperate through bonds and arrange themselves in crystal lattices. Figure 4 shows an ideal crystal structure. In the atom arrangement however, defects occur partly by natural causes. The defects can be: (Ullman, 2003)
• Vacancies – a missing atom creates an unoccupied place in the lattice
• Substitution atoms – foreign atoms located on a regular atom position in the lattice
• Interstitial – foreign atoms located in the lattice cavity
• Dislocation – a defect in the lattice construction

Table of contents :

Symbols and glossary
1 Introduction
1.1 Autoliv Sverige AB
1.2 Pedestrian Protection System
2 Problem formulation
2.1 Purpose
2.2 Delimitations
3 Literature studies
3.1 Priorities of pedestrian protection – a real life study of severe injuries and car sources
3.2 The Head Injury Criterion (HIC) functional
3.3 HIC measurement limitation
3.3.1 Influence of impact speed on head and brain injury outcome in vulnerable road user impacts to the car hood
4 Theory
4.1 Gas and pressure
4.2 Material science
4.3 Strength of material
4.4 Pedestrian Protection Airbag
4.5 Concept Development
4.5.1 Clarification of the problem
4.5.2 Consultation of experts
4.5.3 Brainstorming
4.6 Concept selection
4.6.1 Autoliv evaluation matrix
5 Concept development
5.1 Parameters
5.2 Concept generation
6 Patent infringements
6.1 Found patents
7 Concept selection
7.1 Equipment
7.2 Body in white, BiW
7.3 Hood
7.4 Hinges
8 Measuring methods
8.1 Accelerometers
8.1.1 Diadem
8.2 Patrick marking
8.2.1 TEMA
8.3 Camera
8.3.1 Falcon Extra
8.4 Pressure measurement
9 Testing and results
10 Analysis and discussion
10.1 Time of hood lift of 100 mm
10.1.1 Repetitive behaviour within the concepts
10.1.2 Comparison of lift times for all concepts
10.2 Pressure
10.3 Positioning
11 Conclusions
12 Future work
References

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