Test components The two components in the tests of this study are a free moving headform and windscreens. These two components are described below.

Full text

(1)

In order to reduce impact severity, an airbag system combined with a pop-up hood function to increase the deformation space could be used. (Bovenkerk et al., 2009) Such a system has the advantage of protecting various body parts due to both the additional deformation space in the hood area and coverage of the hard structure, especially the A-pillars (vertical supports for a car's windshield). A collaboration between Autoliv and VCC (Volvo Car Corporation) in the IVSS (Intelligent Vehicle Safety Systems) program resulted in a Pedestrian Protection Airbag (PPA), like the one Bovenkerk et al. (2009) describes, which when deployed lifts the hood. (Erlingfors & Östling, 2009).

Test components

The two components in the tests of this study are a free moving headform and windscreens. These two components are described below.

Pedestrian headforms

A typical headform impactor has three main parts: a steel base mounted with an accelerometer, a spherical aluminium core, and a PVC skin which shall cover at least half of the sphere. The skin is 12 mm thick for the child headform and 14 mm thick for the adult headform. The adult headform has a weight of 4.5 kg simulating a 50th percentile male and the child headform, simulating a 6 year old child, weighs 3.5 kg. The diameter is 165 mm for both headform impactors. The adult

headform had earlier, according to regulations, a weight of 4.8 kg and this headform is still used sometimes. The impactors are equipped with a damped triaxial accelerometer, with seismic masses within the maximum tolerated distance from their centre of gravity. The x, y, and z component accelerations acquired by this accelerometer are used to calculate a resultant acceleration vs. time trace, which is used to calculate head injury criterion (HIC) from the impact. Figure 4 shows an adult headform prepared for testing. (Stammen & Mallory, 2006; Teng & Nguyen, 2008)

WINDSCREEN STUDY USING

A FREE MOVING HEADFORM

An investigation of windcreen behaviour

when subjected to headform impact

Master Degree Project in Applied Mechanics One year Level 30 ECTS

Spring term 2011 Magdalena Wingren

Supervisor: Thomas Carlberger, HIS

(2)

Preface

This project was carried out at Autoliv Sverige AB, in cooperation with Pilkington, as a Master thesis in Applied Mechanics of 30 ECTS. Autoliv Inc. is the world’s largest automotive safety supplier with sales to all the leading car manufacturers in the world. Autoliv was founded 1953 in Vårgårda, Sweden, and has since the start expanded. Today they have 80 facilities in 29 countries including eleven technical centers with 20 crash test tracks, more than any other automotive safety supplier. Autoliv’s mission is to substantially reduce traffic accidents, injuries and fatalities.

Pilkington was founded in 1826 and is now a leader in the global flat glass industry. They have manufacturing operations in 29 countries on four continents. In 1952, Sir Alastair Pilkington invented the float process which is now the standard for high quality glass manufacture. As a windscreen manufacturer, Pilkington plays an important role in the vehicle development process. I would like to thank all at department 450 and especially my supervisor Håkan Sundmark for making me feel like a part of the group and giving me the opportunity to perform this thesis at Autoliv Sverige AB. I would also like to thank my supervisor Thomas Carlberger from the University of Skövde, for support during this project.

Thanks also to Pilkington, Shirley Sergeant and Matthias Kriegel-Gemmecke, for being a part of this project and providing windscreen samples for testing.

Special thanks go to Hans Andersson, lab technician in the horizontal impactor at Autoliv whom without this thesis could not be completed. Also thanks to all employees at Autoliv who have in some way helped during this process.

Vårgårda, 2 November, 2011

(3)

Abstract

Pedestrian protection performance becomes more and more in focus for the car manufactures and systems to reduce injury risk are under development. A wider understanding of both the present and the future windscreen performance in free moving headform testing is needed to optimize these systems. The purpose of this thesis was therefore to learn and understand windscreen

behaviour when subjected to head impact and to gain knowledge of CAE status for head impact in windscreens from a pedestrian point of view.

A literature review concluded that there are different ways to model a windscreen. It was found that the computer material models for laminated windscreen glass were not capable of fully representing the behaviour of this material under all impact conditions, particularly the non-linear behaviour after fracture or failure.

Experimental testing on three different windscreen models, with a free moving headform in a horizontal impactor, has been performed. Test set up was according to Euro NCAP pedestrian testing protocol and three different windscreen angles were tested. The parameter investigated was curvature and HIC and deformation depth on the windscreen were used as evaluation tools. Deformation was measured with a laser positioned behind the windscreen at impact. Film analysis and integration of headform accelerations were used as comparison. The testing concludes that different curvature alone will not have a big influence on HIC and deformation.

Keywords: PVB laminated windscreen, pedestrian, impact, free moving headform

(4)

Table of content

Introduction ... 1

Problem formulation ... 2

Theoretical frame of reference ... 3

Legislation ... 3

Euro NCAP ... 3

Pedestrian Protection ... 3

Euro NCAP and real world accidents ... 4

Free moving headform (FMH) testing ... 5

Head injury criterion (HIC) ... 5

Pedestrian Protection Airbag (PPA) ... 7

Test components ... 7 Pedestrian headforms ... 7 Windscreen ... 8

Literature review ... 11

FE-models ... 12 Pedestrian headform ... 12 Windscreen ... 12

Parameter study using FE-model ... 14

Method ... 15

Literature review ... 15 Testing of windscreen ... 15

Result ... 19

Literature review ... 19 Experimental testing ... 19

HIC and deceleration ... 20

(5)

Table of figures

Figure 1 Injury distribution in pedestrian accidents per body region (van Rooji, 2001) ... 1

Figure 2 EuroNCAP pedestrian testing (EuroNCAP, 2011) ... 4

Figure 3 Probability of AIS 4+ head injury as a function of HIC (Gabler et al., 2000) ... 6

Figure 4 Pedestrian headform ... 7

Figure 5 Headform certification test (Teng & Nguyen, 2008) ... 8

Figure 6 Finite element model of headform (Teng & Nguyen, 2008) ...12

Figure 7 PVB shear relaxation modulus G(t) as a function of time (Zhao et al., 2006) ...13

Figure 8 Picture of fixture and test setup ...17

Figure 9 Schematic images of horizontal impactor and relative angles ...17

Figure 10 Marking of the head and the windscreen prior to testing...18

Figure 11 Tape on windscreen in test T-86 on the left and test T-89 on the right ...18

Figure 12 Holes in windscreens above impact point. From left to right, T-91, T-92 and T-93 ...19

Figure 13 Distribution of HIC and impact velocity (m/s) of headform ...20

Figure 14 Distribution of HIC at different impact angles and HIC change as a function of impact angle ...21

Figure 15 Average deceleration (g) per impact angle ...21

Figure 16 Average deformation (mm) at impact angle 10°, 20° and 0°, excluding T-88 and T-01 ...22

Figure 17 Average integrated distance (mm) of the headform after contact with windscreen. ..23

Figure 18 HIC as a function of deformation (mm) ...24

Figure 19 Kinematics of head in different impact angles, 20° on upper row, 10° on middle row and 0° on lower row ...26

Figure 20 Crack propagation in fist 14 ms of test T-11043920 ...27

Figure 21 Fracture modes in loop 2. To the left, gradual fracture and to the right, sudden fracture ...28

Figure 22 Crack pattern in T-98 to the left and T-97 to the right. The picture in the middle is a close up on crack pattern in T-98. ...28

Figure 23 Difference in light between loop 1 and loop 2 ...30

Figure 24 Crack in windscreen prior to test ...31

List of tables

Table 1 Velocity and acceleration for each headform (Lawrence, 2005) ... 8

Table 2 Test matrix loop 1 ...16

Table 3 Test matrix loop 2 ...16

Table 4 Average HIC and standard deviation ...20

Table 5 Peak data summary for all tests in test loop 1 ...25

(6)

List of abbreviations used in report

AIS Abbreviated Injury Scale

APROSYS Advanced Protection Systems (Integrated project with initiative of TNO) CAE Computer Aided Engineering

EEVC European Enhanced Vehicle-safety Committee Euro NCAP European New Car Assessment Program

FEM Finite Element Method

FMH Free Moving Headform

GIDAS German In-Depth Accident Study GTR Global Technical Regulation HIC Head Injury Criterion

IVSS Intelligent Vehicle Safety Systems

KTH Kungliga Tekniska Högskolan (Royal Institute of Technology) OICA Organisation Internationale des Constructeurs d´Automobiles PPA Pedestrian Protection Airbag

SHPB Split Hopkinson Pressure Bar

TNO Netherlands Organisation for Applied Scientific Research ULP Louis Pasteur University Strasbourg

VCC Volvo Car Corporation

WAD Wrap Around Distance

WSU Wayne State University

(7)

Introduction

Introduction

Today, there are over 240 million vehicles on the roads in Europe1. Mobility is increasing and safety becomes more and more important. Road crashes kill at least 1.3 million people worldwide each year and injure 50 million. In Europe, the number of fatalities in 2005 were 29 317 where 15% were pedestrians and 6% were cyclists. Pedestrians and cyclists are very vulnerable in traffic environment and most accidents occur in urban surroundings. (Trafikverket, 2011; APROSYS, 2011)

Figure 1 shows the injury distribution per body region in pedestrian accidents. It clearly shows that most severe and life threatening injuries (AIS 5-6) are sustained to the head, followed by thorax, abdomen and spine. Less severe injuries (AIS 2-4) are in 37% of the cases sustained to the lower extremities and pelvis, while the head accounts for 35% and the torso and upper extremities for the remaining 28%. This illustrates the importance of focusing on injuries to lower extremities and head, and to a lesser degree to injuries to the thorax, abdomen, spine and upper extremities. (van Rooji, 2001)

Figure 1 Injury distribution in pedestrian accidents per body region (van Rooji, 2001)

As a result of this the pedestrian protection performance becomes more and more in focus for the car manufactures and systems to reduce injury risk are under development. A wider understanding of both the present and the future windscreen performance in free moving headform testing is needed to optimize these systems, in order to reduce packaging volume and weight. The Pedestrian Protection Airbag (PPA), an airbag mounted on the outside covering the lower part of the

windscreen, protection area required by the car manufacturer is heavily dependent on the vehicle properties i.e. design and styling/geometry as well as the car manufacturer strategy related to pedestrian protection.

(8)

Introduction

Problem formulation

Purpose

Learn and understand windscreen behaviour when subjected to head impact and gain knowledge of CAE status for head impact in windscreens from a pedestrian point of view.

Aim

• Basic inventory of present windscreen design principles (including geometry, shape, attachment techniques etc.) If possible, define future design trends.

• Evaluate present windscreen CAE status. If needed, propose activities (scope and content) to obtain a “good enough” status for CAE driven development of pedestrian protection primarily for windscreen.

• Find2 and evaluate some major windscreen parameters influencing the deformation depth and HIC values.

• Develop test method for testing windscreen behaviour during head impact.

• Generate simple design guidelines (based on the aims above) for pedestrian protection.

Delimitation

• CAE status will be investigated through literature review; no simulations will be performed within the scope of this project.

• In the first experimental test phase, only investigate windscreen currently in production. • Investigate one windscreen specific parameter at a time, in the first phase curvature. • Evaluation parameters are HIC and deformation depth.

• Boundary conditions such as cowl and A-pillar will not be considered in this project.

(9)

Theoretical frame of reference

Theoretical frame of reference

Legislation

In 1998, an agreement was met between 31 countries all over the world. The contracting parties decided to adopt an agreement to establish a process for promoting the development of global technical regulations ensuring high levels of safety, environmental protection, energy efficiency and anti-theft performance of wheeled vehicles, equipment and parts which can be fitted and/or be used on wheeled vehicles. (UNECE, 2011)

Each contracting party to the 1998 agreement has regulations with regards to road vehicle

construction and safety. The purpose of the vast majority of these regulations is to ensure that the construction of the vehicles will provide the occupants with the required security and safety and to reduce injury levels and fatalities. Road accident statistics, however, indicate that a significant proportion of road casualties are pedestrians and cyclists who are injured as a result of contact with a moving vehicle. (Economic Commission for Europe, 2004)

In the late 1980s, the European Enhanced Vehicle-safety Committee (EEVC) began to develop a set of standards that would minimize serious injury to pedestrians in impacts up to 40 km/h. In 1991 EEVC proposed a set of tests representing the three most important locations of injury: head, upper leg and lower leg. Early in 1999, the European Commission announced plans to introduce regulations in order to make the EEVC requirements mandatory for all vehicles weighing less than 2.5 ton within a few years, which was realised 2005 in EU and Japan. EU adopted the headform and legform tests whereas Japan only adopted the headform test. EU directive however requires upper legform tests for mandatory purposes. (EEVC Working group 17, 1998; Directive 2003/102/EC, 2003)

Euro NCAP

Euro NCAP (New Car Assessment Program) provides motoring consumers – both drivers and the automotive industry – with a realistic and independent assessment of the safety performance of some of the most popular cars sold in Europe. Established in 1997, Euro NCAP is composed of seven European Governments as well as motoring and consumer organizations in every European country. All new car models must, by law, pass certain safety tests before they are sold. Euro NCAP encourages car manufactures to exceed these minimum statutory standards. All tested models are evaluated on: Frontal impact, Car to car side impact, Pole side impact, Child protection, Pedestrian protection, Whiplash, Electronic Stability Control ESC, Seat belt reminder and Speed limitations devices. (EuroNCAP, 2011)

Pedestrian Protection

(10)

Theoretical frame of reference

Figure 2 EuroNCAP pedestrian testing (EuroNCAP, 2011)

It is very difficult to assess pedestrian protection using a full dummy. Although it is possible to control the point of impact of the bumper against the pedestrian’s leg, it is virtually impossible to control where the dummy’s head subsequently will strike. To overcome this problem, individual component tests are used. A legform test assesses the protection afforded to the lower leg by the bumper, an upper legform assesses the leading edge of the bonnet and child and adult headforms are used to assess the bonnet top and windscreen area. To protect the head, the bonnet top area needs to be able to deflect. It is important that sufficient clearance is provided to the stiff structures beneath the bonnet. Euro NCAP released a separate pedestrian star rating valid from 1997 to 2009. The pedestrian protection rating was based on the adult and child headform tests and the two legform tests. As of 2009, the pedestrian score has become an integral part of the overall rating scheme; however the technical assessment has remained the same. Different injury criterions have been developed in order to assess cars rating. The legform and upper legform are assessed with five different criterions and for the headform, the Head Injury Criterion HIC is used. (EuroNCAP, 2011)

Euro NCAP and real world accidents

Liers (2009) performed a study, estimating how well Euro NCAP pedestrian rating matches the expected real world benefit. The study was based on the German In-Depth Accident Study (GIDAS) dataset (effective July 2008) and 1821 reconstructed accidents involving a vehicle and a pedestrian could be found. Using a couple of sample criterions a careful selection was performed in order to find the cases that match Euro NCAP test procedure. This resulted in a final master dataset of 667 frontal pedestrian accidents, with the right vehicle type and collision speeds up to 40 km/h. This means that 37% of all pedestrian accidents are addressed by legislation and Euro NCAP tests.

(11)

Theoretical frame of reference

Free moving headform (FMH) testing

Free moving headform testing is the test used by Euro NCAP in their pedestrian rating. Test area is between 1000 WAD (wrap around distance) and 2100 WAD, 1000-1500 WAD with the child headform and 1700-2100 WAD for the adult headform. The wrap around distance is a distance from the ground to a point on the bonnet along the vehicle front structure. The area between 1500 and 1700 WAD is tested with a child headform if the area is down in the bonnet and with an adult headform if the area involves the cowl or the windscreen. When conducting a test, the headform impactor shall be in free flight at the moment of impact, at the required impact velocity and at the required direction of impact. The required velocity is 11.1±0.2m/s and the required direction is 65±2° to the horizontal plane for the adult headform and 50±2° to the horizontal plane for the child headform. The effect of gravity shall be taken into account when the impact angle and velocity is obtained from measurements taken before the first impact. The distance from which the impactor is released shall be enough so that the propulsion system is not struck during rebound of the impactor. (OICA, 2005; EuroNCAP, 2010)

Head injury criterion (HIC)

The head injury criterion has been developed to measure, quantitatively, the head injury risk in crash situations. It has been used as a predictor of head injury risk in frontal impact for over 20 years in the North American motor vehicle safety regulations. It is also the evaluation tool used by EuroNCAP when performing pedestrian headform tests. (Hutchinson, 1998)

In an impact environment, forces and moments are difficult to measure. Load cells are generally large and inconvenient to use, and are frequently interposed between the body and impacting surface, altering the level and characteristics of the contact force. On the other hand, acceleration of body segments is relatively easy to measure and thus form the principle instrument in impact experiments. The normal deceleration of a vehicle imposes forces that are essentially the same as those of actual acceleration, but in reverse. Deceleration in the case of vehicle accidents can be extremely abrupt, however, energy-absorbing objects (such as air bags, collapsible steering wheels, etc.) provide for a more gradual deceleration. The HIC is defined by the following analytic

expression (1) (EuroNCAP, 2010; Hutchinson, 1998)

(

) ( )

                              − ⋅ ⋅ − =

5 , 2 1 2 1 2 2 1 HIC t t dt a t t t t R

where aR is the resultant acceleration:

(12)

Theoretical frame of reference

and ax is the instantaneous acceleration in the Fore/Aft direction, ay is the instantaneous

acceleration in the vertical direction and az is the instantaneous acceleration in the lateral direction. The acceleration is measured in [g]. (EuroNCAP, 2010)

As of 2000, the limits which reduce the maximum time for calculating the HIC are set to 15 milliseconds (HIC15). In EuroNCAP testing HIC below 1000 is regarded as green, although this value will probably be lowered to 650 in the near future. (EuroNCAP, 2010; Hutchinson, 1998) There are some limitations to the HIC:

• No two humans are identical in their physical properties.

• The physical condition and individual reflexes affect what occurs during any potential injury accident. This is not taken into account in HIC.

• The HIC calculation is not an injury predictor. It is a pass/fail baseline measure used for anthropomorphic test dummies. (McHenry, 2004)

HIC is not based on tests where HIC was measured and injuries observed. Head acceleration was measured as a function of time and a value was derived. This means that the HIC value has no specific meaning in terms of injury mechanism. (McHenry, 2004)

Figure 3 illustrates the probability of an injury of AIS 4 or greater (AIS 4+) as a function of HIC. HIC of 1000 means a 20 % risk of sustaining an AIS 4+ injury. The Abbreviated Injury Scale (AIS) is a measure of threat to life which varies from 0 to 6 where AIS 0 corresponds to no injury and AIS 6 corresponds to fatal injury. AIS 4 correspond to severe injury while AIS 4+ includes all severe injuries up to, and including fatal injuries. The graph was developed by Prasad and Mertz in 1985. (Prasad and Mertz, 1985; Gabler et al., 2000)

(13)

Theoretical frame of reference

Pedestrian Protection Airbag (PPA)

In order to reduce impact severity, an airbag system combined with a pop-up hood function to increase the deformation space could be used. (Bovenkerk et al., 2009) Such a system has the advantage of protecting various body parts due to both the additional deformation space in the hood area and coverage of the hard structure, especially the A-pillars (vertical supports for a car's windshield). A collaboration between Autoliv and VCC (Volvo Car Corporation) in the IVSS (Intelligent Vehicle Safety Systems) program resulted in a Pedestrian Protection Airbag (PPA), like the one Bovenkerk et al. (2009) describes, which when deployed lifts the hood. (Erlingfors & Östling, 2009).

Test components

The two components in the tests of this study are a free moving headform and windscreens. These two components are described below.

Pedestrian headforms

A typical headform impactor has three main parts: a steel base mounted with an accelerometer, a spherical aluminium core, and a PVC skin which shall cover at least half of the sphere. The skin is 12 mm thick for the child headform and 14 mm thick for the adult headform. The adult headform has a weight of 4.5 kg simulating a 50th percentile male and the child headform, simulating a 6 year old child, weighs 3.5 kg. The diameter is 165 mm for both headform impactors. The adult

headform had earlier, according to regulations, a weight of 4.8 kg and this headform is still used sometimes. The impactors are equipped with a damped triaxial accelerometer, with seismic masses within the maximum tolerated distance from their centre of gravity. The x, y, and z component accelerations acquired by this accelerometer are used to calculate a resultant acceleration vs. time trace, which is used to calculate head injury criterion (HIC) from the impact. Figure 4 shows an adult headform prepared for testing. (Stammen & Mallory, 2006; Teng & Nguyen, 2008)

Figure 4 Pedestrian headform

(14)

Theoretical frame of reference

be changed from 25-90°. Acceleration is measured at the centre of gravity of the headform. (Teng & Nguyen, 2008)

Figure 5 Headform certification test (Teng & Nguyen, 2008)

Table 1 shows corresponding values for velocity and acceleration for each headform model. Headforms used in EuroNCAP is child/small adult 3.5 kg and adult 4.5 kg. (Lawrence, 2005)

Table 1 Velocity and acceleration for each headform (Lawrence, 2005)

Impactor and mass Certification velocity [m/s] Lower boundary [g] Upper boundary [g] Child/small adult 3,5 kg 7 290 350 Adult 4,5 kg 10 310 410 Adult 4,8 10 337,5 412,4

Windscreen

The windscreen consists of two sheets of glass, sandwiched together by a thin plastic film. The plastic film is usually PVB, short for Polyvinal butyral, due to its optical clarity, excellent performance on adhesion to glass and energy mitigation. (Xu et al., 2010)

Glass is found in a natural state as a by-product of volcanic activity. Today, glass is manufactured from a variety of ceramic materials where the main components are oxides. The main product categories are flat or float glass, container glass, cut glass, fibreglass, optical glass, and specialty glass. Automotive windscreens fall into the flat glass category. (Harper, 2001)

(15)

Theoretical frame of reference

windscreens also usually contains several other oxides: potassium oxide (K2O derived from potash), magnesium oxide (MgO), and aluminium oxide (AI2O3 derived from feldspar). The raw materials are carefully weighed and mixed together and once a batch is made, it is fed to a large tank for melting using the float glass process. The batch is heated to a melted state and fed into a float chamber, which holds a bath of molten tin. The float chamber can be up to approximately 4 meters wide and up to 60 meters long; at its entrance, the temperature of the tin is about 1000 degrees Celsius while at the exit the tin is about 600 degrees Celsius. The glass does not submerge into the tin but floats on top of it, moving through the tank. The melted tin has an almost perfectly flat surface, causing the glass to become flat and the high temperature cleans the glass of

impurities. The decreased temperature at the exit of the chamber allows the glass to harden enough to move into the next chamber, a furnace. This process gives the glass a tin side and a fire side. (Harper, 2001)

After this procedure the glass cools down to room temperature. It is now very hard and strong and ready to be cut into desired dimensions. The cut piece is then bent into shape by placing it into a mold and heating it up to the point where the glass sags to the shape of the mold. After shaping, the glass is hardened. Quenching, were the glass is quickly heated to about 850 degrees Celsius and then blasted with jets of cold air, is a process which toughens the glass by putting the outer surface into compression and the inside into tension. This allows the glass, when damaged, to break into many small pieces of glass without sharp edges. By modifying the tempering procedure, size of the pieces can be altered, making them both bigger and smaller. Bigger pieces allow good vision until the windscreen can be replaced which is better from a safety point of view. (Weissman, 1997) After the glass is tempered and cleaned, it goes through a laminating process. Two sheets of glass are bonded together with a layer of plastic, usually PVB. The lamination takes place in an

autoclave, a special oven that uses both heat and pressure to form a single, strong unit resistant to tearing. When laminated glass is broken, the broken pieces of glass remain bound to the internal tear-resistant plastic layer, and the broken sheet remains transparent. After laminating, the windscreen is ready to be assembled with plastic mouldings so it can be installed on the car.

Known as glass encapsulation, this assembly process is usually done at the glass manufacturer. (van Russelt, 1997)

In order to ensure that the windscreens meet the required standards, a few samples are selected randomly out of the daily production. They undergo visual inspecting looking for optical

distortion, adhesive testing, impact testing and boil and bake test. A more detailed description of the test method can be found in the reference. (van Russelt, 1997)

(16)

Theoretical frame of reference

For that reason, freshly produced float glass has a higher strength compared to grinded and polished plates. (Weissman, 1997)

(17)

Literature review

Literature review

During the literature review, articles concerning both experimental and numerical studies on windscreens have been considered.

An integrated project, called APROSYS (Advanced Protection Systems) was carried out between 2004 and 2009. The project was coordinated by TNO, an independent research organisation. The project included 51 partners and their main objective was to improve passive safety for all

European road users in all relevant accident types and accident severities. The project was divided into 9 sub-projects and as a part of one, SP3, the windscreen was investigated. (APSOSYS, 2011; TNO, 2011)

As a part of the project, double ring bending test on 300mm by 300mm laminated glass samples were conducted. One complication identified was a variation in results between samples depending on the orientation of the glass plates in the laminate and the impacted surface during each test. This arises because each pane of glass has a fire side and a tin side, as a consequence of the manufacturing process - hence one side has a small level of tin impurities. In manufactured windscreens there are no requirements to orientate the glass plates either one way-up or the other. Following the tests on samples of laminated glass, tests on complete windscreens were conducted. Impact locations were selected across the whole surface due to the windscreens doubly curved surface. Scatter in the results between windscreens were also found and one explanation could be the orientation of glass plates. Glass orientation is still not considered in windscreen

manufacturing. (Hardy, 2009)

OICA, Organisation Internationale des Constructeurs d´Automobiles, conducted 5 identical tests at the same impact point on identical windscreens. They observed two different fracture modes:

1. Fracture begins after contact in windscreen, with a short bending phase 2. Sudden fracture occurs after a longer bending phase.

This results in a big scatter of HIC-values. No explanation concerning the difference in fracture modes was found but was thought to be due to internal stresses or micro scratches. (OICA, 2005; OICA, 2011)

Mizuno et al. (2001) performed headform impact tests at various vehicle locations. They investigated, among other things, the deformation necessary to keep the HIC below 1000. Approximation curves were calculated for the windscreen and the car body and based on these curves, a HIC value of 1000 is associated with a dynamic deformation of 76 mm for the car body and around 120 mm for the windscreen.

(18)

Literature review

Pinecki et al. (2011) studied windscreen behaviour when subjected to headform impact. Among other things, they investigated the influence of windscreen thickness. Two thicknesses, 3.96 mm and 4.47 mm, were investigated and the tests show that thickness is not a key parameter for reducing head to windscreen HIC. Also investigated in this study was the same windscreen model from different manufactures.

FE-models

A series of studies has been carried out with the objective of creating a FE-model of headforms and windscreens, in order to investigate head against windscreen impact.

Pedestrian headform

Figure 6 shows a finite element model of the adult headform impactor. The vinyl skin is modelled using solid continuum elements with viscoelastic material properties, and the core is modelled as a steel core with elastic material. All impactor parts use solid elements. Models were developed using LS-DYNA3D and simulation satisfies all WG17 requirements for certification tests. (Teng & Nguyen, 2008)

Figure 6 Finite element model of headform (Teng & Nguyen, 2008)

In order to investigate brain injury mechanisms, a more advanced FE model can be used. Computer technology permits FE models to handle complex geometries and a series of human head models have been developed. A representation of these models includes the Wayne State University (WSU) model, the Strasbourg University (ULP) model and the KTH human head model. (Yao et al., 2008)

Windscreen

Different authors recommend different ways to model laminated glass. Konrad and Gevers (2010), Xu and Li (2009b), Pyttel et al. (2010), Timmel et al. (2006) and Du Bois et al. (2003) all modelled their windscreen with shell elements for the glass plies. All but Konrad and Gevers (2010)

(19)

Literature review

computations. The constitutive relations were embedded in FEA software and compared with classical Hertzian pressure calculations. Xu et al. (2010a) however, found that little difference exists between the models with and without viscoelasticity effects. Thus, PVB can be treated as a material with little or no viscoelasticity. This is consistent with the findings of Wei (2004) and Zhao et al. (2006), which stated that the difference in stresses obtained by treating the PVB as linear

viscoelastic and linear elastic is less than 2%. This is because the impact duration is in the range of milliseconds and the shear relaxation modulus G(t) of PVB changes very little, therefore the PVB behaves like a solid glossy material. This is illustrated in Figure 7. Both hyperelastic and linear elastic models have been investigated, in different studies, and results show good agreement with experimental test with headforms on windscreens.

Figure 7 PVB shear relaxation modulus G(t) as a function of time (Zhao et al., 2006)

In order to get a natural fracture pattern in simulations of windscreens, the mesh needs to be taken into account. Xu and Li (2009b) and Timmel et al. (2006) used hybrid meshes, consisting of

quadrilateral and triangular elements. Konrad and Gevers (2010) used a circular triangular mesh. Pinecki et al. (2011) developed a model with glass plies as shell elements and the PVB as brick elements. This model required an element size of 3.5 × 3.5 mm whereas Timmel et al. (2006) used an element size of 0.1 × 0.1 mm. Both these element sizes are smaller than the usual element size recommendation for an overall car model calculation and this has a strong influence on the time step and therefore the running time of the calculation.

(20)

Literature review

Hardy et al. (2008) reviewed the available material models for the component of passenger cars that are involved in pedestrian-vehicle accidents. This review identified that the computer material models for two materials, laminated windscreen glass and fibre reinforced plastic (as may be used in bonnet structures), were not capable of fully representing the behaviour of these materials under all impact conditions, particularly the non-linear behaviour after fracture or failure. Since

simulations are used by all vehicle manufactures for design and development, it is important to have reliable material models for these components.

Konrad and Gevers (2010) investigation is based on a R&D project using RADIOSS. The model simulates the crack propagation in windscreens when impacted by a headform, side and pole impacts or in roof crush load case. It will be carried over to all mayor explicit FEA solvers, starting with LS-DYNA and continuing with PAM-CRASH and Abaqus. No other study has explicitly stated that the model will be available in any FEA solver.

Parameter study using a FE-model

Not many experimental studies investigating windscreen properties have been done, but Xu et al. (2010b) performed a numerical parameter study on PVB laminated windscreens based on extended finite element method (XFEM). According to their finding, the curvature does play a significant role when it comes to crack propagation. A more curved windscreen reduces the magnitude of stress field upon impact due to reduction of head displacement at the same impact speed. This would favour a shorter crack. A shorter crack implies better PVB energy dissipation capability and fracture resistance at the same head displacement.

In the same study, PVB thickness was investigated. An increase in PVB thickness reduces crack length. Unfortunately the gain becomes less significant as the PVB layer becomes very thick, which implies that enhancing energy absorption and reducing crack length may not be achieved simply by thickening the PVB layer. The possibilities of modifying the PVB material properties were

investigated. The outcome was that in addition to thickening the PVB layer, making it stiffer may also shorten the crack, which implies better pedestrian/occupant protection. (Xu et al. 2010b) Also investigated in this study were aspect ratio of length to width of windscreen, effect of panel size and the effect of boundary conditions. Crack patterns were investigated by keeping the

simulation conditions at regular reference parameters and changing one parameter at a time. (Xu et al, 2010b)

(21)

Method

Method

This thesis is divided into two parts. The first part is a literature review to gain knowledge regarding windscreen properties and the CAE status of windscreens today. The second part is a collaboration with a windscreen manufacturer, Pilkington, were experimental testing is performed on three different windscreen designs with the aim of investigating HIC, deformation depth and crack propagation.

Literature review

A thorough literature review has been carried out. The literature was searched for through the university library databases, external databases and the Internet. Articles from 1995 up until today have been included in the general search. Emphasis was put on a mechanical perspective of the windscreen as opposed to a medical perspective of the pedestrian. Keywords used in the search for literature are found in Appendix.

Testing of windscreen

After the initial literature review a couple of parameters to investigate were suggested. Among these parameters was thickness of windscreen, thickness of PVB layer, curvature, geometric dimensions and tin side versus fire side. After discussions with Pilkington, vertical curvature was chosen for further investigation. The decision to use windscreen currently in production was made because this seemed most efficient in this first phase and this gives a clearer view of windscreen status of today.

Three windscreen designs were chosen by Pilkington, based on curvature and overall geometric dimension. All windscreens has an overall thickness of 4.5 mm with outer glass ply of 2.1 mm, PVB layer of 0.76 mm and inner glass ply of 1.6 mm. One windscreen model, windscreen B, was delivered from Poland and the other two, windscreen A and windscreen C, were delivered from Italy. The different curvatures, flattest to most curved, are as follows:

• Windscreen A, 16 mm XC • Windscreen B, 29 mm XC • Windscreen C, 36 mm XC

The curvature is defined as the cross curve, which is the distance from an imaginary line from top to bottom on the middle of the windscreen.

(22)

Method

Table 2 Test matrix loop 1

# Test No Impact angle Corresponding WS

angle WS 1 T-11043886 10 35 B 2 T-11043887 10 35 B 3 T-11043888 10 35 B 4 T-11043889 10 35 B 5 T-11043890 10 35 B 6 T-11043891 20 45 B 7 T-11043892 20 45 B 8 T-11043893 20 45 B 9 T-11043894 20 45 B 10 T-11043895 20 45 B 11 T-11043896 0 25 B 12 T-11043897 0 25 B 13 T-11043898 0 25 B 14 T-11043899 0 25 B 15 T-11043900 0 25 B

Table 3 Test matrix loop 2

# Test No Impact angle Corresponding WS

(23)

Method

A fixture, consisting of four parts, was built. One part was made to be fixed to the horizontal impactor and three parts were specific frames for each windscreen design. Figure 8 shows the fixed part and the specific Touran part as well as the FMH head used in the test. The pictures shown below were taken before the first test and with the horizontal impactor set on 10° to the horizontal plane.

Figure 8 Picture of fixture and test setup

The test setup was made to simulate EuroNCAP pedestrian headform test. To simplify, a horizontal impactor was used and relative angles were calculated. Instead of tipping the windscreen, the impact angle was altered, but all tests mimicked an impact angle of 65° to the horizontal plane. The different angles of the impactor represented windscreen angles between 25° and 45°, which is a representation of most current car models. In reality, this meant that the windscreen was mounted at 90° to the horizontal plane and the headform was fired from between 0° and 20° to the horizontal plane. Relative angle between impact and windscreen was 70°- 90° in the different cases. A schematic image of the horizontal impactor and the angles can be seen in Figure 9.

Figure 9 Schematic images of horizontal impactor and relative angles

(24)

Method

The deformation depth was measured using a laser which was fixed to the camera support behind the windscreen. The laser has a 200 mm measuring range and uses triangulation. The laser was calibrated according to protocol prior to testing. The laser measuring deformation of the windscreen was positioned perpendicular to the surface. The centre of the windscreens was marked with a cross and in order for the laser to be able to measure the deformation; the

windscreens were also marked with a black circle in the centre as seen in Figure 10. In test loop 2, the marking on the windscreen was done with a grey colour due to its matt quality. Prior to every test, the head was marked with both a crash target sticker and some colour, which made a mark on the windscreen after impact and the exact impact point could be seen.

Figure 10 Marking of the head and the windscreen prior to testing

The windscreens were attached to the fixture by duct tape. In tests T-86 – T-88, the first three tests, the windscreen was taped at three points as shown in Figure 11. Due to an unexpected behaviour of the windscreen in T-88, the windscreens in the following tests had more attachment points around the frame. (Seen in Figure 11)

Figure 11 Tape on windscreen in test T-86 on the left and test T-89 on the right

(25)

Result

Result

Results from both the literature review and the experimental testing are presented below.

Literature review

During the literature review, publications of both experimental testing and simulations were reviewed. Compilation of reviewed literature is found in chapter 2, Theoretical frame of reference, subchapter Literature review.

Experimental testing

A total of 15 tests were performed in the first test loop, 5 at each impact angle, as shown in Table 2. In test T-88 the windscreen was turned more or less inside out due to few points of attachment on the rig. This resulted in a change of attachment in the following tests. In test T-00, the

windscreen cracked in handling before testing. The decision was made to perform the test according to the test matrix, even though cracks were present. No apparent difference could be seen in HIC values between these two tests and all the other, therefore the tests are included in the following analysis. In tests T-91 to T-93, small holes just above the impact point could be seen in the glass. The glass plies had both been crushed and the PVB layer torn by the headform. This is illustrated in Figure 12. This behaviour was not observed in any of the other tests, even though some of the windscreens had crushed outer glass plies. No apparent difference in HIC value or deformation could be seen between these tests and the other tests.

Figure 12 Holes in windscreens above impact point. From left to right, T-91, T-92 and T-93

Test loop 2 consisted of 22 tests, see Table 3. Tests T-14 – T-15, T-17 – T-18 and T-20 were pre-cracked from shipping. Since no apparent difference could be seen on the pre-cracked

windscreen in test loop 1, it was decided to use the pre-cracked windscreens in test loop 2 as well. No complete holes could be seen in test loop 2, even though some tests showed crushed glass on both inner and outer plies. No tear of the PVB layer could be seen.

(26)

Result

Figure 13 Distribution of HIC and impact velocity (m/s) of headform

HIC and deceleration

In Figure 14, average HIC with standard deviation under different impact angles is illustrated for all tested windscreens. Windscreen A show big scatter in HIC values for all tested impact angles. Windscreen B has smallest scatter in all impact angles and also lowest HIC values of the tested windscreens. Windscreen C has the highest HIC values of the tested windscreens. Average HIC and standard deviation is presented in Table 4. As can be seen, all HIC values are well below the Euro NCAP target value of 1000.

Table 4 Average HIC and standard deviation

Windscreen/Angle Average HIC per angle StDev

(27)

Result

Figure 14 Distribution of HIC at different impact angles and HIC change as a function of impact angle

Average deceleration in every impact angle is presented in Figure 15. The first peak of the graph is when the head impactor hits the windscreen and the cracks starts to propagate. A second phase follows, smaller in magnitude and longer in duration which is a result of the windscreen stiffness.

Figure 15 Average deceleration (g) per impact angle

(28)

Result

Deformation analysis

Figure 16 illustrates the average deformation per impact angle in the one point measured by the laser. When calculating the average deformation, results from T-88 and T-01 were excluded due to anomalies in data. The elasticity of the windscreen causes it to rebound and all tested windscreens have a permanent deformation of somewhere between 70 and 90 mm. All tested windscreen models have a similar deformation when tested in 10° to the horizontal. Test angle of 20° to horizontal show largest difference where windscreen B has the largest deformation and windscreen C the smallest. At 0° to the horizontal, windscreen A and C show similar deformation with

windscreen B slightly higher than the other two. There was a lot of noise in the output signal from the laser and it was therefore filtered at channel frequency class (CFC) 180. CFC is a digital filter.

Figure 16 Average deformation (mm) at impact angle 10°, 20° and 0°, excluding T-88 and T-01

A deformation analysis using the integrated value from the accelerometers in the headform has also been performed and the results are presented in Figure 17. The integrated values, especially at impact angle 20°, poorly describe the headforms actual movement and the correlation with the laser measurement is poor. For both windscreen B and C, the integrated values are lower than the measurement from the laser in all impact angles. The integrated values are higher than the

measured laser values in all impact angles for windscreen A. A test by test comparison of all available tests, including graphs from the TEMA analysis, can be found in Appendix E

(29)

Result

Figure 17 Average integrated distance (mm) of the headform after contact with windscreen.

(30)

Result

HIC vs. Deformation

There is no correlation between HIC and deformation when impacting the windscreen at the centre as can be seen for all impact angles in Figure 18. A steeper angle between headform and windscreen at impact will give a smaller deformation. Data from test loop 1 is presented in Table 5 and data from test loop 2 in Table 6.

(31)

Result

Table 5 Peak data summary for all tests in test loop 1

Test No Angle HIC Max deformation

(mm) Time of max def. (ms) Comment T-11043886 10 257 162 32.15 T-11043887 10 296 144 27.05 T-11043888 10 219 175 37.40 Anomalies in data T-11043889 10 168 163 44.35 T-11043890 10 209 153 38.65 T-11043891 20 122 175 41.45 T-11043892 20 152 163 38.10 T-11043893 20 153 163 45.20 T-11043894 20 161 174 24.90 T-11043895 20 189 185 28.30 T-11043896 0 277 178 45.45 T-11043897 0 211 177 45.20 T-11043898 0 240 167 51.00 T-11043899 0 220 182 40.20 T-11043900 0 219 178 47.20 Pre-cracked

Table 6 Peak data summary for all tests in test loop 2

Test No Angle HIC Max deformation

(mm)

Time of max def. (ms)

Comment

T-11043901 10 344 98 - Anomalies in data from

(32)

Result

(33)

Result

(34)

Result

In Figure 19 the kinematics of the head in the different impact angles is illustrated. The sequences are from T-89, T-92 and T-96 respectively. In test angle 0 degree, there is no rotation of the head before impact and it does not start to rotate until the windscreen reaches max deformation and the head has come to a complete stop. The head then rebounds and a small rotation occurs. The kinematics of the head in tests with impact angle 10-20 degree to the horizontal is a bit different. After approximately 5 ms, the head starts to rotate towards the floor. This rotating movement continues throughout the test.

Figure 20 shows the crack propagation of test T-20 during the first 14 ms. Radial cracks appear on direct impact and after approximately 0.67 ms, circumferential cracks start to propagate. Both radial and circumferential cracks continue to propagate during the first 14 ms. This windscreen has a gradual fracture pattern, like all windscreens in loop 1 but two distinctive fracture modes were observed during the second test loop, one with a gradual fracture and one with a more

concentrated and sudden fracture. The sudden fracture gave a more oval fracture pattern as can be seen in Figure 21. This behaviour was found for both windscreen A and C. 6 windscreens showed a gradual fracture pattern, 10 windscreens showed a sudden fracture and 6 windscreen had a fracture pattern of somewhere in between. No obvious trend when it comes to HIC or deformation could be seen.

Figure 21 Fracture modes in loop 2. To the left, gradual fracture and to the right, sudden fracture

All tests in loop1, with the exception of T-98, show similar crack pattern after testing. Figure 22 shows examples of crack patterns in T-97 and T-98. T-98 has a slightly smaller deformation than the other tests in the same angle but no difference in HIC value can be seen.

Figure 22 Crack pattern in T-98 to the left and T-97 to the right. The picture in the middle is a close up on

(35)

Discussion

Discussion

The aim of this project was to learn and understand windscreen behaviour when submitted to head impact and to investigate CAE status for head impact in windscreens from a pedestrian point of view. This meant that the project was divided into two parts, a literature review and experimental testing on windscreens. Both parts will be discussed in this chapter.

Not surprisingly, car buyers are willing to spend money on their own safety but not as willing to spend money on safety systems that protect other road users. This means that the driving forces towards safer cars for pedestrians are legislations or consumer tests where pedestrian protection is a part of the overall rating and therefore, the development of pedestrian safety systems has been slow up until today. Since Euro NCAP’s decision to incorporate the pedestrian rating in the overall rating and since regulations have been approved, much research has been done in the pedestrian area. This research shows that the decision to include the lower leg, the upper leg and the head in EEVC’s standard for pedestrian testing seems to be the right one since the legs and head are the body part most frequently injured. In Euro NCAP, the child and adult headform test area includes the area of the vehicle front structure that falls within the geometric traces of 1000-1500 mm and 1500-2100 mm wrap around distance, WAD, respectively, whereas the current phase of the legislative directives limits the test zone to the hood. Since research shows that many head injuries are a result of head impact on the windscreen, it is important that this area is not forgotten.

Hardy et al. (2008) reviewed available material models for laminated glass and found that they were not capable of fully representing the behaviour of the material under all impact conditions. This still seems to be the case even though some work has been done in the area. Konrad and Gevers are improving their model and it is to be implemented in all major FEA solvers. No other study has an explicit plan to implement the model to any commercial FEA solver and therefore it is not clear if any of them will be available to the public.

Experimental testing

(36)

Discussion

Figure 23 Difference in light between loop 1 and loop 2

The test set up was made to simulate the test method used by EuroNCAP. Even though not exactly the same, the result from this study is comparable with results from a traditional FMH testing. Bovenkerk et al. (2009) argues that the vehicle design will affect the angle of which the head will impact the windscreen. Impact angle when testing sedans and one box in their study was therefore set to 50° to the horizontal plane. This study shows that the relative angle between the windscreen and the impacting headform affect both HIC and deformation. In order to get a comparable result, each car should be investigated individually and a specific impact angle should be obtained. To test all vehicles with 65˚ angle to the horizontal is not satisfactory according to Bovenkerk et al. (2009).

The test set up used in this study made it possible to test a lot of samples in a short period of time. FMH tests on a vehicle would mean the need to glue the windscreen in place and therefore useful time would get lost when waiting on the glue to harden. In discussions with Pilkington the

conclusion was that the glue itself would not affect HIC or deformation and therefore the present test set up with duct tape instead of glue was proposed. Other simplifications to the test set up are the lack of a vehicle mass behind the windscreen, although the fixture was fixed to the horizontal impactor, and that the frame does not follow the exact shape of the windscreen. An illustration of the frames and their placement on the windscreen can be found in Appendix A. Even though the rig does not have the exact shape of the windscreen, there is still contact between the glass and the rig all around. The rig shape was therefore considered irrelevant when it comes to HIC and

deformation of the windscreen.

All tests in this study were performed using the adult headform. As described earlier in this report there is also a child headform with a lower weigh. As of today, there are not many car models where WAD 1000-1500 mm, (the child headform test area) reaches up on the windscreen;

therefore the adult headform was used in all tests. New car models however have a tendency to get shorter hoods and it is not impossible in the future that the child headform test area will be up on the windscreen. To see how the windscreen behaves and the effect on HIC and deformation when using a child headform would therefore be interesting.

(37)

Discussion

estimated to approximately 50%. If a possible safety system is functional even at higher velocities than 40 km/h, the highest velocity used in consumer tests today, more lives could be saved. The first test loop during this study contained 15 windscreens and the tested model was model B. The second test loop contained 22 windscreens, 13 of windscreen A and 9 of windscreen C. The different amount of tested windscreens of each model was due to broken samples from the shipping. One tested windscreen in the first test loop was cracked prior to testing, but no obvious differences in the result could be seen between this windscreen and the others, therefore cracked windscreens were considered useable even in the second test loop. Especially windscreen C had quite a lot of pre-cracked samples but they could not be distinguished from the other tested windscreen in either HIC or deformation. An example of crack in windscreen can be found in Figure 24. This behaviour has also been noticed in other studies where FMH tests have been performed on vehicles. The broken samples disregarded in the study had so many cracks that the windscreens had lost all stability.

Figure 24 Crack in windscreen prior to test

A lot of different parameters were initially suggested to be investigated in this study. It was not possible to test all at once, therefore curvature was chosen for this first phase. Curvature in this case is defined as the depth of curve from the windscreen edge at centre point. This decision was made in collaboration with Pilkington, and available windscreen designs were a contributory factor to the decision. Windscreens currently in production were to be used and therefore it was not possible to investigate parameters that would require altering windscreen properties. There are other parameters that would be interesting to investigate, such as tin side versus fire side and PVB thickness. In previous studies, it is indicated that these could affect the scatter in HIC that can be seen when submitting identical windscreens to the same head impact. (Hardy et al. 2009, Xu et al. 2010b) Investigating the curvature will not give the answer to what properties is affecting the scatter of HIC. Parameters such as adhesion of PVB to glass and moisture content in PVB could be the reason for scatter but is harder to investigate. Excessive moisture in PVB would cause the windscreen to delaminate and that could affect the mechanical properties.

(38)

Discussion

kinematic of the headform also differs from a full dummy and from a real person getting hit. As mentioned above, all vehicles are tested at the same velocity and with the same impact angle and this is not consistent with reality.

The kinematics of the headform differs some between the angles used in this study. A steeper angle will give the headform some rotation. The rotation during the impact is considered too small to affect the result in any big way but some of the kinetic energy of the headform will be

transformed into rotational kinetic energy. Film analysis shows no difference in crack propagation between the tested angles. The evaluation tool used calculates an injury criterion based on linear acceleration which means that it only calculates the risk for injuries caused by linear acceleration. In reality, angular acceleration is a known factor in brain damage. Since HIC is not based on tests where HIC was measured and injuries observed, there is no real correlation between HIC and injury as there is for other injury criterions. Simulated heads give a better understanding for brain injury and a couple of models have been developed. These are a big step closer to reality. It is not possible to only test by simulation, some sort of physical test will be required in order to get a fair and comparable result. Modifications to the headform, like attaching a neck, could be the solution. When attaching a neck, the heads mechanical properties will change and a more natural rotation will occur during FMH.

All tested windscreens show HIC values well below the EuroNCAP limit of 1000. Even if the limit should be lowered to 650, all windscreens in this study would be regarded as green which indicates that these models are soft models and that it is feasible for windscreens to meet pedestrian

regulations. Although regarded as soft, there are differences in stiffness between the models, with model C being the windscreen with the highest stiffness. The fact that model A has a higher stiffness than model B indicates that there are other parameters than curvature influencing the stiffness and therefore HIC value.

The deformation analysis using laser measurements show that a steeper angle between headform and windscreen at impact will give a smaller deformation and the curvature has a more obvious significance at a steeper angle. If the windscreen is mounted at 25º-35º, the curvature of the windscreen will not matter in regards of deformation. The HIC value will be a bit higher for a more curved windscreen, as illustrated in Figure 14. This will also mean that there is no correlation between HIC and deformation. Results show a larger difference in HIC between windscreens B and C even though the difference in curvature is smaller between these windscreens than between windscreens A and B. This is a clear indicator that curvature does not play a crucial role for HIC. This study is performed in the centre of the windscreen, with no hard structures behind for the headform to strike. The integrated deformation curve, using data from the headform

(39)

Discussion

Two different fracture modes have been reported in previous studies, one where fracture begins after contact with a short bending phase and one with a sudden fracture after a longer bending phase. The two different fracture modes resulted in a big difference in HIC (OICA 2005). Two fracture modes were observed in this study too, but unlike the OICA study, HIC did not vary between the different fracture modes. One explanation to the difference in result could be that the windscreens are from different manufactures. A stiffer windscreen than the ones tested in this study is perhaps more likely to show scatter in the results. The different fracture modes occurs independent of impact angle in both studies.

All windscreens tested in this study were windscreens delivered directly from a manufacturer. In a real life accident, the windscreens have most likely been subjected to some wear from both

windscreen wipers and from ice scrapers. It has also been subjected to the sun and the weather and how this affects the windscreen is not known. Micro scratches on the surface should make the windscreen easier to brake in a crash and this could affect HIC.

The literature study showed the lack of a good and robust simulation model available in FEA solvers today. The data collected in this study could be of help if and when a model will be developed.

Conclusions

• Curvature does not seem to play a crucial role when it comes to HIC and deformation. This study however, shows indications that a more curve windscreen together with a steep angle between headform and windscreen will decrease deformation depth.

• All windscreens tested in this study shows HIC well below 1000 and even under 650. • There is no correlation between HIC and deformation in the centre of the windscreen. • As of today, there is no good CAE model for windscreens available in any commercial

software.

Recommendations

To further enhance the knowledge of windscreen behaviour when subjected to headform impact, more tests need to be carried out where new parameters are investigated. One parameter,

(40)

References

References

Abrate S. (1998) Impact on composite structures Cambridge University Press, Cambridge, UK Bovenkerk J., Zander O., Kalliske I. (2009) New modular assessment methods for pedestrian protection in the event of head impacts in the windscreen area. Paper number 09-0159 Directive 2003/102/EC of the European parliament and of the council (2003) relating to the protecting of pedestrian and other vulnerable road users before and in the event of a collision with a motor vehicle and amending Council Directive 70/156/EEC

Du Bois P.A., Timmel M. & Fassnacht W. (2003) Modelling of safety glass for crash simulation.

Computational Materials Science 28 (2003) 675-683

Economic Commission for Europe (2004) Proposal to develop a global technical regulation concerning the protection of pedestrians and other vulnerable road users in collision with vehicles TRANS/WP.29AC.3/7

EEVC Working group 17 (1998, 2002) Improved test methods to evaluate pedestrian protection afforded by passenger cars [Online] [Accessed 2011-03-01] Available:

http://www.eevc.org/publicdocs/publicdocs.htm

European new car assessment programme (November 2010) Pedestrian Testing Protocol Version 5.2

Gabler H.C., Hollowell W. T., Summers S. (2000) Systems modeling of frontal crash compatibility 2000-01-0878 [Online] [Accessed 2011-03-30] Available:

http://www.me.vt.edu/gabler/publications/sae2000-01-0878.pdf

Hardy R. (2009) Final report for the work on “Pedestrian and pedal cyclist accidents” (SP3) AP-90-0003 [Online] [Accessed 2011-03-29] Available:

http://www.aprosys.com/Documents/deliverables/FinalDeliverables/Final%20SP3%20report%2 0AP-90-0003.pdf

Hardy R.N., Watson J.W., Carter E., Neal-Sturgess C., Joonekindt S., Yang J., Herman K., Baumgartner D., Guerra L.J. & Martinez L. (2008) Impact conditions for pedestrians and cyclist AP-SP32-009R [Online] [Accessed 2011-03-28] Available:

http://www.aprosys.com/Documents/deliverables/FinalDeliverables/AP%20SP32%20009R%20 D323%20-%20Jan%202008.pdf

Harper C. (2001) Handbook of ceramics, glasses and diamonds McGraw-Hill Professional Publishing, New York, ISBN: 0-07-026712-X

Hutchinson J., Kaiser M.J. & Lankarani H.M. (1998) The head injury criterion (HIC) functional.

(41)

References

Konrad M. & Gevers M. (2010) Advanced laminated glass modelling for safety FEA LS-DYNA

Forum, Bamberg, Germany, 2010

Lawrence G.J.L (2005) Assessment of the FTSS 4.5 kg aluminium headform as a possible alternative for EEVC WG17 INF GR/PS/148 [Online] [Accessed: 2011-05-05] Available: http://live.unece.org/fileadmin/DAM/trans/doc/2005/wp29grsp/ps-148e.pdf

Liers H. (2009) Benefit estimation of the Euro NCAP pedestrian rating concerning real world pedestrian safety [Online] [Accessed 2011-05-10] Available:

http://www-nrd.nhtsa.dot.gov/pdf/esv/esv21/09-0387.pdf

Liu B., Zhu M., Sun Y., Xu J., Ge D., Li Y. (2010) Investigation on energy absorption

characteristics of PVB laminated windshield subjected to human head impact. Applied Mechanics and

materials 34-35 (2010) pp956-960 DOI: 10.4028/www.scientific.net/AAM.34-35.956

Lozach G. (2003) The use of glass in the automotive industry. Glass Processing Days, Tampere, Finland June 15-18, 2003

McHenry B.G. (2004) Head injury criterion and the ATB. ATB Users´ Group

Mizuno K., Yonezawa H. & Kajser J. (2001) Pedestrian headform impact tests for various vehicle locations. SAE paper 2001-06-0185

OICA (2005) Observation of different windscreen glass fracture modes during headform impactor tests INF GR/PS/164 [Online] [Accessed 2011-03-21] Available:

http://live.unece.org/fileadmin/DAM/trans/doc/2005/wp29grsp/ps-164e.pdf

Pinecki C., Fontaine L., Adalian C., Jeanneau C., Zeitouni R. (2011) Pedestrian protection – Physical and numerical analysis of the protection offered by the windscreen. ESV paper number 11-0432

Prasad P. and Mertz H.J.(1985) The position of the United States delegation to the ISO working group 6 on the use of HIC in the automotive environment. SAE paper 861246

Pyttel T., Liebertz H. & Cai J. (2010) Failure criterion for laminated glass under impact loading and its application in finite element simulation. International Journal of Impact Engineering DOI:

10.1016/j.ijimpeng.2010.10.035

van Rooij L., Bhalla K., Meissner M., Ivarsson J., Crandall J., Longhitano D., Takahashi Y., Dokko Y. & Kikushi Y. (2001) Pedestrian crash reconstruction using multi-body modeling with

geometrically detailed, validated vehicles models and advanced pedestrian injury criteria. [Online] [Accessed 2011-02-21]Available:

http://www-nrd.nhtsa.dot.gov/pdf/nrd-01/esv/esv18/cd/files/18ESV-000468.pdf

Rosén E., Sander U. (2009) Pedestrian fatality risk as a function of car impact speed. Accident

(42)

References

van Russelt M. (2007) Recent trends in automotive laminated glazing. Glass Performance Days Tampere, Finland June 15-18, 2007

van Russelt M. (1997) How to make a good laminated safety glass for windscreens. Glass Processing

Days, Tampere, Finland September 13-15, 1997 ISBN: 952-908959-7

Savineau G. F. (1997) Fundamentals of laminating process and quality requirements. Glass Processing

Days, Tampere, Finland September 13-15, 1997 ISBN: 952-90-8959-7

Stammen J. & Mallory A. (2006) Pedestrian head safety survey of U.S Vehicles: In support of the proposed global technical regulation (GTR). VRTC Technical Report Informal document WP29-144-03

Sun D.-Z., Andrieux F., Ockewitz A., Klamser H. & Hogenmüller J. (2005) Modelling of the failure behaviour of windscreen and component tests. LS-DYNA Anwenderforum, Bamberg, Germany, 2005

Teng T.-L., Nguyen T.-H. (2008) Development and validation of FE models of impactor for pedestrian testing. Journal of Mechanical Science and Technology 22 (2008) 1660~1667

Timmel M., Kolling S., Osterreider P. & Du Bois P.A. (July 13, 2006) A finite element model for impact simulation with laminated glass. International Journal of Impact Engineering 34 (2007) 1465-1478 Weissmann R. (1997) Fundamental properties of float glass surfaces. Glass Proceeding Days,

Tampere, Finland September 13-15, 1997 ISBN: 952-90-8959-7

Xu J. & Li Y-B. (2009a) Study of damage in windshield glazing subject to impact by a pedestrian’s head. J. Automobile Engineering DOI: 10.1243/09544070JAUTO974

Xu J. & Li Y. (2009b) Crack analysis in PVB laminated windshield impacted by pedestrian head in traffic accident. International Journal of Crashworthiness vol. 14, No. 1, January 2009, 63-71

Xu J., Li Y., Ge D., Liu B. & Zhu M. (2010a) Experimental investigation on constitutive behavior of PVB under impact loading. International Journal of Impact Engineering 38 (2011) 106-114

Xu J., Li Y., Chen X., Yan Y., Ge D., Zhu M. & Liu B. (2010b) Numerical study of PVB laminated windshield cracking upon human head impact. Computers, Materials & Continua vol. 18, no. 2, pp. 183-212

Yao J., Yang J., Otte D. (2008) Investigation of head injuries by reconstructions of real-world vehicle-versus-adult-pedestrian. Safety Science 46 (2008) 1103-1114

Figur

Updating...

Referenser

Updating...

Relaterade ämnen :