EFFECT OF OBESITY ON MOTOR VEHICLE CRASH INJURIES THROUGH COMPUTATIONAL MODELING AND SIMULATION IL HWAN KIM

Transcription

1 EFFECT OF OBESITY ON MOTOR VEHICLE CRASH INJURIES THROUGH COMPUTATIONAL MODELING AND SIMULATION by IL HWAN KIM JONG-EUN KIM, COMMITTEE CHAIR DAVID L. LITTLEFIELD ALAN M. SHIH A THESIS Submitted to the graduate faculty of The University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of Master of Science BIRMINGHAM, ALABAMA 2009

2 Copyright by IL HWAN KIM 2009

3 EFFECT OF OBESITY ON MOTOR VEHICLE CRASH INJURIES THROUGH COMPUTATIONAL MODELING AND SIMULATION IL HWAN KIM MECHANICAL ENGINEERING ABSTRACT The growing number of obese individuals in most industrialized countries has become an important social issue. Motor vehicle crashes (MVCs) are the leading cause of injury, yet the role of obesity (either cushion or momentum effect) on MVC injuries is still unknown. The objective of this study is to elucidate the effect of obesity on body injuries in MVCs through computational modeling and simulation. MADYMO, a mathematical dynamic simulation code, is used for model simulation of a vehicle frontal impact with airbag systems, seatbelts and simplified vehicle interior components. Male and female obese dummy models (body mass index greater than 30) are developed based on the MADYMO Hybrid III 50 th percentile male and 5 th percentile female model, respectively. To represent subcutaneous fat geometry and properties, finite element models were created based on the geometry data reconstructed from MRI datasets of obese subjects. The fat model with Mooney-Rivlin hyperelastic properties is integrated into the standard dummy models. Four injury criteria on head, neck, thorax and lower extremity are assessed against various settings of deceleration pulses and occupant restraint systems. From the simulation results, obese males have a much higher risk of injury (especially head and thorax) than standard males, while obese females have slightly increased risk of head and thorax injury. The results are consistent with the findings from real world crash data in literature. Keywords : obesity, motor vehicle crashes, injuries, computer modeling, simulation iii

4 ACKNOWLEDGEMENT This project was supported by a grant from the National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health (grant R01EB A1, Obesity-Related Variables and Motor Vehicle Injury). I appreciate my research committee chairman, Dr. Jong-Eun Kim, for his support, inspiration, and encouragement throughout my studies and with assistance with writing and presenting results. I also extend my gratitude to committee members Dr. David L. Littlefield, for his continual support and guidance, and Dr. Alan M. Shih, for invaluable information, help, and guidance. iv

5 TABLE OF CONTENTS Page ABSTRACT... iii ACKNOWLEDGEMENTS... iv LIST OF FIGURES... vii LIST OF TABLES... ix LIST OF ABBREVIATIONS... x CHAPTER 1 INTRODUCTION... 1 Obesity and Motor Vehicle Crash Injuries... 1 Vehicle Design and Crash Analysis... 4 Objective SIMULATION TOOL AND INJURY CRITERIA... 6 Simulation Tool... 6 MADYMO... 6 System of Units... 9 Injury Criteria Head Injury Criterion Biomechanical Neck Injury Predictor Combined Thoracic Index The Femur Force Criterion MATERIALS AND METHODS Development of Obese Models MRI Data and Imaging process Obese Models Height Models Vehicle Design Simplified Vehicle Model v

6 Seatbelts Airbag system Deceleration Pulses Simulation Studies Case study 1: Pulse Variables Case study 2: Mass flow rates of Airbag Variables Case study 3: Restraint systems Case study 4: Other cases RESULTS MADYMO Simulation Baseline Cases Case Study Case Study 1-1. The baseline pulse times 1.2 cases Case Study 1-2. The baseline pulse times 1.4 cases Case Study Case Study % Depowered Airbag Cases Case Study % Depowered Airbag Cases Case Study Case Study 3-1. Without airbag system Case Study 3-2. Acceleration pulse 1.2 times and; without the airbag system Case Study 3-3. Without seatbelts Case Study 3-4. Without pretensioner and load limiter Case Study 3-5. The maximum force of load limiter set to 2kN Case Study Case Study 4-1. Increased height cases Case Study 4-2. Steering wheel of 10 degree tilted downward Case Study 4-3. Friction coefficient between seat and; dummy set to DISCUSSION CONCLUSIONS AND RECOMMENDATIONS LIST OF REFERENCES APPENDIX A SERIES OF CAPTURED IMAGE IN MVCs B LOAD LIMITER TESTS C IRB APPROVAL FORM vi

7 LIST OF FIGURES Figure Page 1 Joints of MADYMO Model of Male BMI 30 in Prone position Male BMI 30 with image processing data Sample ellipsoid data The process of surface extraction The process of a fat-vest modeling Mesh models of 2D skin and 3D subcutaneous Hybrid model of obese dummy Fat- vests Comparison Dummy model comparison by BMI th percentile male 178 cm and 184 cm Simplified Vehicle Model Point Belts with retractor systems, buckle, lap anchor, and D ring Function of Pretensioner Function of Load limiter Mass flow function of baseline airbag The baseline deceleration pulse Images of crash simulation by time step 30 milliseconds (ms) Spool out by pulse variables at 92 milliseconds vii

8 20 HIC of male BMI Injury criteria of normalized mean values with BMI viii

9 LIST OF TABLES Table.. Page 1 Units in MADYMO Proposed Critical Intercept Values for Nij Material Properties of a Fat-vest Mass distribution of male dummy models Mass distribution of female dummy models Joint Re-Distribution, 50th Male Case (6 cm up) Variables for MADYMO simulation and case studies Male injuries result table of pulse cases Female injuries result table of pulse cases Male injuries result table of airbag cases Female injuries result table of airbag cases Male injuries result table of restraint cases Female injuries result table of restraint cases Male injuries result table of other cases Female injuries result table of other cases Head injury summary data Neck injury summary data Thorax injury summary data Femur injury summary data ix

10 LIST OF ABBREVIATIONS BMI body mass index CTI Combined thoracic index DICOM Digital Imaging and Communications in Medicine FE Finite Element FEM Finite Element Method FFC The Femur Force Criterion FMVSS HIC Federal Motor Vehicle Safety Standards Head injury criterion MADYMO MAthematical DYnamic MOdels mais maximal Abbreviated Injury Scale MB multi body MVCs motor vehicle crashes NASS CDS National Automotive Sampling System's Crashworthiness Data System NHTSA National Highway Traffic Safety Administration VSM Visual Safe MAD x

11 CHAPTER 1 INTRODUCTION Obesity and Motor Vehicle Crash Injuries In 2007, according to the National Highway Traffic Safety Administration (NHTSA), 41,059 people died and 2,491,000 of people were injured in motor vehicle crashes (MVCs) which resulted in an economic cost of $230.6 billion (NHTSA, 2008). Automotive technology has notably developed in the last few decades. Vehicles have been upgraded by strengthening the chassis and installing anti-lock brake systems, electronic brake force distribution systems, tire pressure monitoring systems, traction control systems, limited slip differential systems, and electronic stability programs. Despites these improvements of vehicle technology, more than 40,000 people still died from MVCs and incurring huge medical costs. Meanwhile, the rate of growth for the overweight and obese population, with a body mass index (BMI, kg/m 2 ) calculated by body weight divided by the square of height, of greater than 30, had gradually increased until 1980; however, from 1980 the obese rates increased rapidly. For example, thirty-two states have an obesity rate equal to or greater than 25%. In six of these states (Alabama, Mississippi, Oklahoma, South Carolina, Tennessee, and West Virginia), 30% or more of the residents are considered obese (Center for Disease Control and Prevention, 2008). 1

12 Due to this rise in the levels of obesity in the population, a debate over the role of obesity on MVC injuries has surfaced. Some researchers have examined the association between obesity and body injuries in MVCs. Arbabi et al. (2003) reviewed the database of the University of Michigan Program for Injury Research and Education. Three statistical groups were analyzed: lean, overweight, and obese. They found that the overweight cohort showed a decreased abdominal maximal Abbreviated Injury Scale (mais) score compared with the lean cohort, and the obese cohort was almost identical with the lean cohort. Furthermore, the lower extremity mais score of overweight and obese cohorts increased compared with the lean cohort. For abdominal injury cases, the overweight cohort was safer than the lean cohort, so they concluded that increasing tissue, subcutaneous fat, plays a protective role from MVCs. Wang et al. (2003) investigated the data set of the Crash Injury Research Engineering Network (CIREN). They noticed great individual variability during medical care of traumatically injured patients. For their study, Wang et al. focused on the differences of a human body, for example, bones, fat, muscle and other hard and soft tissues. Among these, the depths of subcutaneous fat were measured by a computed tomography. Women were also found to have significantly greater subcutaneous fat depth than men and also have notably decreased injury severity to the abdominal region. Similar trends were found from male cases, but with no statistical meaning. It was concluded that increased subcutaneous fat may be protective for the abdominal region against MVCs through cushioning. 2

13 Mock et al. (2002) examined the National Automotive Sampling System's Crashworthiness Data System (NASS CDS) from 1993 to1996. Two outcome variables, death within 30 days of MVCs and injury severity score of 9 or higher, were analyzed. This study found an association between increased body weight and increased mortality. From a data set with 27,263 cases, the mean mortality was 0.67%. From the weight category, mortality for those less weighing than 60 kg was 0.37, but those weighing greater than 120 kg was From the BMI category, mortality for a person with a normal BMI (20~24) was 0.60, but for those who were severely obese (40 +) was They found that the odds ratio for death was augmented for each kilogram increase in body weight. The study concluded that increased body weight of occupants was related with increased mortality in MVCs. Neville et al. (2004) examined 242 patients at an academic level I trauma center. They divided the patients to two groups by BMI 30. Two groups, obese and non-obese, were analogous in age, sex, injury severity score, mechanisms of injury, and injury patterns. They found that the obese group had a higher incidence of multiple organ failures (13% vs. 3%) and mortality (32% vs. 16%). They concluded that obesity was an independent predictor of mortality following severe blunt trauma. Zhu et al. (2006) investigated National Automotive Sampling System's Crashworthiness Data System (NASS CDS) with the question of how obesity affected MVCs. They used a data set with 30,667 cases and classified them by a series of criteria: BMI, gender, seatbelt use, velocities, type of collisions, drug or alcohol use, and airbag deployment. They found that the fatality rate for MVCs was 0.87 % and 0.43 % among male and female drivers, respectively. Obese males showed the greatest increase in risk for 3

14 death due to frontal MVCs. Interestingly, they found that male drivers whose BMI is higher than 35 or less than 22 had a significant increase of fatalities, but female drivers did not shows significant association between BMI and fatalities. Cormier (2008) examined the NASS CDS database for the years The variable of age was also added. This study focused on chest injuries and analyzed the association of BMI, gender, age, and delta V from the database. Obese males showed a higher risk ratio than lean males, but obese females did not show prominent risk. Obese females also had a slightly less risk of chest injury. Vehicle Design and Crash Analysis Over the past a few decades, several studies have been conducted regarding MVCs, especially the safety of the occupants. As a result, many safety equipments such as airbag system and seatbelts systems were invented. Today, researchers in many automotive companies have designed new vehicles by using computational aided engineering software such as LS-Dyna, PAM-CRASH, and MADYMO. LS-Dyna and PAM-CRASH simulate the safety of vehicle body in frontal and side collisions and rollover tests. MA- DYMO, a specialized software for injury analysis of occupants, simulates the safety of occupants in vehicle accidents. After designing a desired vehicle model, execute real crash tests are executed using prototype cars. During real crash tests, standard dummy models, the 50 th percentile Hybrid III Male dummy and the 5 th percentile Hybrid III Female dummy, are used. Injuries and contact area of airbag system, knee bolster, and windshield are analyzed afterwards. Problems are found through these crash tests and computer simulations before the manufacturing processing begins. During a new automo- 4

15 tive project, many factors are considered, but added consideration of having an obese or overweight person was not addressed, even though almost 60% of adults in the United States are categorized as overweight or obese. Objective Although many epidemiologic studies have been conducted examining the role of obesity and fat distribution of motor vehicle drivers and its effect on injuries during MVC, (Mock et al., 2002; Arbabi et al., 2003; Wang et al., 2003; Neville et al., 2004; Zhu et al., 2006; Cormier, 2008), this area is still not well understood. The purpose of this study is to examine the association between obesity and body injuries in frontal MVCs through computational modeling and crash simulations. Among MVCs, frontal crashes are one of the most frequent types of accidents and often result in severe injuries (Conroy et al., 2008). In this study, only frontal crashes are considered. MADYMO (TNO, The Netherlands), one of the multi body dynamics codes and a specialized software for injury analysis of occupants, is used in this study. The multi body dynamics method has been an attractive technique because of its capability of analyzing complex kinematics of the human body and vehicle structure with easy modeling and rapid analysis. This thesis will address the entire process of creating obese dummy models, developing a vehicle model, conducting crash simulations with a variety of restraints systems, and analyzing any body regional injury. 5

16 CHAPTER 2 Simulation Tool and Injury Criteria Simulation Tool Crash simulation is used to examine the safety of occupants during the moment of collision. The benefits of this method are that it performs simulations fast and economically when compared with real car crash tests and also optimizes vehicle designs without prototype models. MADYMO (TNO, 2008) MADYMO (MAthematical DYnamic MOdelling) software package was used in this study as the simulation tool. MADYMO provides solutions to injury data of dummy models used in crash simulations. The package includes numerical solvers, dummy, human, and example mathematical models. This package is applicable to both the automotive and aerospace industries. MADYMO provides simulations of multi body (MB), finite element (FE), or both. MADYMO is composed of several systems of bodies. A system of bodies is defined by the kinematic joints, the initial conditions, and the bodies. Among bodies, a MB is a rigid body and it is defined by mass, location of center of gravity, moment of inertia, and products of inertia. A system of bodies includes rectangular plane, ellipsoids, elliptical cylinder, and the FE model. A rectangular plane is defined by the coordinates of 6

17 three points. Fleming s right hand rule with 1 st, 2 nd, and 3 rd points decide the outside of a normal of plane. An ellipsoid is given by following equation: Where a, b and c are the semi-axes of the ellipsoid X, Y, Z, respectively and n is the degree. Default degree is 2 and describes an ellipsoid. If the degree n increases, the ellipsoids will be more rectangular shape. For example, if values of all axes are all 0.1 and the degree is 2 then the ellipsoid will be sphere, but if the degree is 90 then the ellipsoid will be cubic. The relative motion of connected two bodies is restricted by a kinematic joint. Fig. 1 shows most common of joints, such as revolute joints, translation joints, spherical joints, universal joints, cylindrical joints, and planar joints. MADYMO considers every ellipsoid is rigid MB. MADYMO models are composed several ellipsoids. Each ellipsoid is connected by a joint. Some simple models are composed by a single joint and ellipsoid, but most models are composed by several joints. Each joint is connected each other by parent body and child body. For example, parent body of Hybrid III dummy models is a pelvis body and joint. As a result dummy models were easily relocated by parent and child properties. For example, when a pelvis body moves, all the bodies are also moved. While when an upper arm moves, lower arm and hand are moved, but chest, abdomen, and pelvis do not move. 7

18 Fig. 1. Joints of MADYMO Contact is also an important function of MADYMO. MADYMO provides three types of contact such as Contact.MB_MB, Contact.MB_FE, and Contact.FE_FE. Those are divided by types of contact surfaces. MADYMO also provides two contact models the elastic and the kinematic contact models. The Elastic model is available for all contacts and it allows penetration of contacting surfaces with a penetrate function. The Kinematic model is usually used for contact of MB and FE and it does not allow penetration of each contacting surface. It is used for airbag and dummy contact or seatbelts and dummy contacts. 8

19 Contact is defined by master surface and slave surface. Master surfaces are usually MB surfaces except contact of FE_FE. In elastic model, deformable surface also define by MASTER, USER_MASTER, SLAVE, USER_SLAVE, USER_MID_POINT, AND COMBINED. System of Units The system of units used in MADYMO is the International System (SI). The four basic units in MADYMO are kilogram (kg) for mass, meter (m) for length, second (s) for time, and Kelvin (K) for temperature. Table 1 summarizes these units as well as some derived units. Angles are expressed in radians. Table 1. Units in MADYMO Parameter Unit Time s Length m Mass kg Temperature K Velocity m/s Acceleration m/s 2 Force N Torque Nm Moment of inertia kgm 2 9

20 Injury Criteria Head Injury Criterion (Eppinger, 1999) Head injury is a leading cause of fatalities in MVCs. As a result, Federal Motor Vehicle Safety Standards (FMVSS) regulated the value of head injury criteria (HIC) to 700. HIC is calculated by the following equation: The equation was defined by U.S government. Time t 1 and t 2 are arbitrary, but those time interval should be less than equal 15 milliseconds. Acceleration function, a(t), is the resultant head acceleration in g s which is measured at the gravity center of head. Biomechanical Neck Injury Predictor (Eppinger, 1999) The biomechanical neck injury predictor, N ij, is a measure of the injury due to the load transferred through the occipital condyles. This injury parameter combines the neck axial force F z and the flexion/extension moment about the occipital condyles M z. N ij is calculated by following equation Where, the following Table 2 shows values of F int and M int. 10

21 Table 2. Proposed Critical Intercept Values for N ij Dummy Tension (N) Compression (N) Flexion (Nm) Extension (Nm) 50 th Percentile Male th Percentile Female N ij is the collective name of four injury predictors corresponding to different combinations of axial force and bending moment, NTE: tension-extension, NTF: tensionflexion, NCE: compression-extension, and NCF: compression-flexion. Maximum value of those four is N ij and it is limited 1.0 by FMVSS. Combined Thoracic Index (Eppinger, 1999) The Combined Thoracic Index (CTI) is a measure of the injuries of the thorax. It is a combination of the maximum chest deflection D max and the 3ms clip maximum value of the resultant upper spine acceleration Amax. The equation for the calculation of the CTI is given by where A int and D int are constants that A int of both 50th percentile male and 5th percentile female are 90 g, and Dint of 50th percentile male is 103 mm and 5th percentile female is 84 mm. FMVSS restricts that Amax is less than 60g and D max is less than 63 mm and Amax is less than 60g and D max is less than 52 mm for male and female respectively. 11

22 The Femur Force Criterion (Eppinger, 1999) The Femur Force Criterion (FFC) is a measure of injury to the femur. It is the compression force transmitted axially on each femur of the dummy as it is measured by the femur load cell. The FFC injury calculation is applied to the joint constraint force in the bracket joint located at a femur load cell. It is assumed that the coordinate systems of this joint are oriented in agreement with SAE J221/1 because as axial force, the component of the constraint force in the joint ζ-direction is used. A duration curve of this time history signal is made. The resulting femur axial force duration curve must not exceed 10 kn in 50th percentile male and 6.8 kn in 5th percentile female. 12

23 CHAPTER 3 MATERIALS AND MEHODS Development of Obese Models This research aimed at finding association between obese dummy models and body injuries in MVCs. A method, attaching a subcutaneous fat layer to a standard dummy for representing an obese dummy model, was used. The subcutaneous fat layer was modeled by finite element (FE) to assign biomechanical properties to the fat layer. The FE analysis is invented for solving complex elastic problems and it is applied to variety of fields such as bio mechanics, structure analysis, sheet metal forming, and crash analysis. MRI Data and Imaging process To represent a subcutaneous fat layer of obese dummy models in to computational models, segmented tag files acquired from grey-scale MRI datasets were utilized in this study (Shen et al., 2004; Shen and Chen, 2008). Each dataset was contained less than 43 segmented DICOM (Digital Imaging and Communications in Medicine) images, and their resolution is a 256 by 256 pixel. DICOM was produced to support the distribution and viewing of medical images, such as CT and MRI. The 3-dimensional volume rendering technique was applied to connect those segments and converted subcutaneous fat into watertight, no holes or gaps, geometry by triangulated surface in stereo lithography (STL) 13

24 file format. Other organs, muscles, and bones were neglected in this study. Four data sets of male BMI 30 and 35, and female BMI 30 and 35 were obtained and those data were selected by almost identical heights of standard MADYMO dummy models. Fig. 2. Model of Male BMI 30 in Prone position As shown in Fig. 2, however, the image processing data in model were supine or prone position. As a result, the data was not suitable for sitting position dummy models. Fig. 3 shows a dummy model with a subcutaneous fat layer extracted from MRI images. The yellow ellipses illustrate overlapped region of dummy model with the subcutaneous fat regions. Those overlapped or detached regions yielded numerical errors due to contact interface problems. An alternate approach was used in this study. An artificial subcutaneous fat layer based on those image processing data was generated. The details will be explained the following sections. 14

25 Fig. 3.Male BMI 30 with image processing data Obese Models Obese dummy models were based on MADYMO standard hybrid III male (BMI 25, height 1.78 m) and female (BMI 22, height 1.52 m) dummy models. Subcutaneous fat models were generated, named fat-vests. The first step was to extract the surface data from standard dummy models, because visual surfaces displayed on Visual Safe MAD (VSM) were not geometric models. They just owned the information of ellipsoids, as shown in Fig.4. Upper body components, for example the chests, shoulders, abdomen, pelvis, hips, and the lumbar spine, were selected and converted to a FE model by VSM. The converted FE data were changed to geometric data by HyperMesh 9.0, and exported as an IGES (Initial Graphics Exchange Specification) file format. Fig. 5 shows the sample images of the surface converting process for the Hybrid III 50 th percentile male dummy model. The modified surface data, called base surface in this study, were offset by 15

26 1mm for maintaining the tolerance from the dummy model to the fat-vest to avoid initial penetration. Fig. 4. Sample ellipsoid data Fig. 5. The process of surface extraction Each image processing the data for a male with a BMI of 30 and 35, and a female with a BMI of 30 and 35 was divided by 5 layers from pelvis to chest and the thickness of subcutaneous fat were measured. The base surface was also divided into 5 layers at the same location with the image processing data. Based on the thickness data, an outer spline curve was sketched with marked points on each layer and those curves were connected by interpolation method to generate outer surface data. The surfaces of the shoulders and chests were offset by 6 to 10 mm and connected to the outer surface data. The neck and bottom surfaces were also generated and connected to the base surface, so that 16

27 the outer surface with shoulders and chests could enclose all surfaces of the fat-vest. Fig. 6 shows the modeling process for the fat-vest. Fig. 6. The process of a fat-vest modeling 17

28 HyperMesh 9.0 (Altair Engineering) was used to generate the FE mesh data. The surface data were imported to HyperMesh 9.0 and modified by connecting and dividing surfaces to enhance mesh quality during mesh generation. Two-dimensional surface mesh was generated, and then three-dimensional volume mesh was generated by tetrahedra. Fig. 7 shows 2D and 3D mesh models and cross-section views, respectively. Fig. 7. Mesh models of 2D skin and 3D subcutaneous 18

29 Material properties for subcutaneous fat and skin were applied in 3D volume mesh and in 2D surface mesh, respectively. Mooney-Rivlin hyperelastic material model was used for the 3D volume mesh of fat-vests. Vannah and Childress (1996) found the ratio of Mooney-Rivlin coefficients A and B as A = 4B. Todd and Thacker (1994) reported Young s modulus by subcutaneous fat as 64.8 kpa for male and 47.5 kpa for female. The Poisson s ratio was set to Gent (2001) found the relation between Young s modulus and Mooney-Rivlin coefficients to be E=6(C 1 +C 2 ). The density of human fat is measured as 900 kg/m 3 (Fidanza et al., 1953). For the skin, the material properties of linear elastic of Young s modulus of MPa (Agache et al., 1980), a density of 1000 kg/m3 (Alekseev et al., 2008), and a Poisson s ratio of 0.45 (Zheng et al., 1999), were used. The thickness of the skin was set to 2 mm (Seidenari et al., 2000). Table 3 shows the summary of material properties used in this study. Table 3 Material Properties of a Fat-vest 3D Subcutaneous Fat 2D Skin Attribute Unit Male Female Density kg/m Poisson's ratio A N/m B N/m Young s Modulus MPa Density kg/m Poisson's ratio The fat-vest was integrated into the standard dummy as show in Fig. 8. A contact interface between the fat-vest the standard dummy model was assigned by Contact.MB_FE in MADYMO. The mass and sizes of limbs were increased proportionally as 19

30 BMI increased by referring to a report for anthropometry and mass distribution of different-sized males (Armstrong, 1998). Tables 4 and 5 list the mass distribution of body components for males and females, respectively Fig. 8. Hybrid model of obese dummy Fig. 9 shows a comparison of the fat-vests using image processing data. However, the fat distributions could not be matched because the image processing data were either in supine or prone position. 20

31 Fig. 9. Fat- vests Comparison 21

32 Table 4. Mass distribution of male dummy models BMI25 BMI30 BMI35 NAME Mass % Mass % Mass % Head & Neck % % % Thorax & Spine % % % Pelvis & Abdomen % % % Lower Extremity % % % Upper Extremity % % % Fat-Layer % % Total % % % Table 5. Mass distribution of female dummy models BMI22 BMI30 BMI35 NAME Mass % Mass % Mass % Head & Neck % % % Thorax & Spine % % % Pelvis & Abdomen % % % Lower Extremity % % % Upper Extremity % % % Fat-Layer % % Total % % % 22

33 Fig. 10. Dummy model comparison by BMI Height Models The heights of standard dummy models could not represent all drivers. Therefore, two height variations were considered: 184 cm for males and 164 cm for females. The standard dummy models were modified by increasing the distances of joints. A Hybrid III dummy has 26 joints. Each distance of consecutive joints was measured and the ratio of each joint from the shoe joints was calculated; Table 6 shows the initial distance and ratio. To increase height, a ratio of 0.06 from the shoe joint was used for each joint of the 23

34 male cases and a ratio of 0.1 from shoe joint was used for each joint of the female cases. Fig. 11 shows the standard male dummy model and height-modified model. Table 6. Joint Re-Distribution, 50 th Male Case (6 cm up) Original Ratio 6 cm up Neck % Clavicle % Shoulder % RibsJ % LumbarspineU % Elbow % LumbarspineJ % 1.08 Abdomen % LumbarspineL % Hip, pelvis % Wrist % Femur % Knee % TibiaUp % TibiaLow % Ankle % Shoe % 0 Fig th percentile male 178 cm and 184 cm 24

35 Mass information was updated in the same method for the obese dummy model, which maintained a BMI of 30 and of 35. Vehicle Design Simplified Vehicle Model A simplified vehicle model was generated for crash analysis. It has essential parts such as a seat, a steering wheel, an instrument panel, a knee bolster, the floor, seatbelts, and airbag system. Except for the airbag system and the seatbelts, all of the components were modeled by MB (ellipsoid and plane). The seat model was made by 4 ellipsoids for the bottom, lower back, upper back, and head-rest. The knee bolster was designed for knee and tibia contact; it also was defined through contact information with the dummy models. Windshield, roof, and floor were modeled on a plane, and all components were also given contact with the dummy models. The normal direction of a plane was set to the inside of the vehicle for correct contact with dummy models. If the normal direction was outward, then MADYMO recognized that the dummy model and planes were already contacted and penetrated; as a result, the dummy model rebound or moved to outer direction of the plane. The steering wheel was designed with the following information: diameter, rim diameter, hub diameter, number of ellipsoids, number of spokes, and spoke angles. The contact information was given to the steering wheel with a dummy model and airbag system. The instrument panel was modeled by a simple ellipsoid, and the contact information was provided with the hands of the dummy models. Each component was fixed by a lock of an initial joint, because the dummy model on the vehicle was moved only by a deceleration pulse. 25

36 Fig. 12. Simplified Vehicle Model Seatbelts A seatbelt is the most important component among the safety equipments for all motor vehicles. Seatbelts have saved more lives since 1960 than the total of any other crashworthiness design feature (NHTSA, 2005). Seatbelts were composited with 3-point belts, a D-ring, a buckle, a lap anchor, and a retractor system. The D-ring and the lap anchor were modeled by a single ellipsoid and a fixed joint. These play a role for fixing seatbelts during crash simulations; the D-ring is attached on a B-Pillar so it is possible to modify the Z-position, but not the X and the Y. The lap anchor was attached to the seat model, as a result it could be moved by the change of seat position. 26

37 Fig Point Belts with retractor systems, buckle, lap anchor, and D-ring Retractor systems include the pretensioner and load limiter, both of which are an advanced technology of seatbelts. If the crash sensor detected a collision or a strong acceleration, the pretensioner retracts the seatbelts and fastens it to the dummy model. This prevents slack, since sometimes the driver does not tightly tie up the seatbelt or wear a thick outer jacket, which can also make the seatbelt loose. Fig. 14 shows the function of a pretensioner. One of advantages of seatbelts is that it can restrain the dummy models to prevent them from colliding against the steering wheel or windshield. Seatbelts, however, also contributed to thoracic injuries due to its high magnitude holding force. The load limiter was intended to reduce belt related injuries such as rib fractures by allowing forward excursion of occupants torsos when loads on the belt exceed some threshold (Brumbelow et al., 2007). It plays a role in releasing the belt webbing when a great force is applied to the belts, thereby preventing injuries from seatbelts. 27

38 Fig. 14. Function of Pretensioner Fig. 15. Function of Load limiter Airbag system The airbag system was invented as a substitute for seatbelts, and became mandatory since When a crash occurs at 12 mph or more, an airbag sensor triggers the airbag by the combustion of sodium azide, and the 50 liter volume of airbag is filled with nitrogen gases within 0.05 seconds (Anderson et al., 2002). NHTSA estimates that more than 6,018 people are saved by airbags, but also 169 people were killed in MVCs by air- 28

39 Mass Flow bags in 2000 (NHTSA, 2001). As a result, advanced airbag system was introduced for children and light females. To find advantages of the de-powered mass flow rate of airbag system, case studies with variables of mass flow rates were simulated. Three types of mass flow rates of airbags were used for this simulation. One was a baseline airbag (Kiuchi, 1998), another was a 10% de-powered airbag, and the last was a 20% depowered airbag. Kiuchi (1998) simulated the 20% de-powered mass flow rate and the original mass flow rate and found that advanced airbags (20% de-powered) with pretensioner and load limiter showed lower injury criteria on HIC, chest acceleration, and chest deflection. Fig. 16 illustrates the mass flow rate of the baseline airbag. 1.0 Mass Flow Time, s Fig. 16. Mass flow function of baseline airbag Deceleration Pulses Two types of methods were used for the crash simulations. One was a direct impact against a barrier simulation and the other was a deceleration pulse method. A deceleration pulse was acquired from an accelerometer of crash simulation. Three kinds of de- 29

40 Acceleration, g celeration pulses, baseline (delta 56km/h) (Rouhana, 2003), 1.2 times the baseline pulse, and 1.4 times the baseline pulse were used for the MVC simulations. The baseline deceleration pulse was measured by an analog to digital converter. Gravity acceleration (Zdirection) and those pulses (X-direction) were applied to the dummy model, as a result the dummy model moved to X and Z directions. From frontal collisions, acceleration of the Y-direction was also measured, but because it was smaller than the others, it was ignored Time, millisecond Fig. 17. The baseline deceleration pulse Simulation Studies A total of 78 MADYMO simulations were created to examine the association between obesity and body injuries in MVCs. Baseline cases were defined by airbag system, seatbelts with retractor systems, and applied baseline pulse. As shown in Table 7, four categories of simulations for 12 case studies were generated. For example, pulse cases, 30

41 mass flow rates of airbag system, the combination of restraint systems, tilted steering wheel, height variables, and friction of seat and femurs were considered. Table 7. Variables for MADYMO simulation and case studies Gender BMI Pulse Airbag Restraint Extra Male Female BMI 25 BMI 30 BMI 35 Case 1-1 Case 1-2 Case 2-1 Case 2-2 Case 3-1 Case 3-2 Case 3-3 Case 3-4 Case 3-5 Case 4-1 Case 4-2 Case 4-3 Baseline Cases. Baseline cases were composed with the baseline pulse, the baseline airbag and seatbelts with pretensioner and load limiter. Case study 1: Pulse Variables Case study 1 1. The baseline pulse times 1.2 cases Case study 1 2. The baseline pulse times 1.4 cases Case study 2: Mass flow rates of Airbag Variables Case study % Depowered Airbag Cases Case study % Depowered Airbag Cases Case study 3: Restraint systems Case study 3 1. Without airbag system Case study 3 2. Acceleration pulse 1.2 times and without the airbag system Case study 3 3. Without seatbelts 31

42 Case study 3 4. Without pretensioner and load limiter Case study 3 5. The maximum force of load limiter set to 2kN Case study 4: Other cases Case study 4 1. Increased height cases Case study 4 2. Steering wheel of 10 degree tilted downward Case study 4 3. Friction coefficient between seat and dummy set to

43 CHAPTER 4 RESULTS MADYMO Simulation A series of images captured from the baseline simulation, for the standard Hybrid III 50 th percentile male dummy model with the baseline pulse, is shown below. Images were captured every 30 milliseconds. The first image was ready to crash simulation at 0 milliseconds. The dummy was located on a driver seat by regulations of FMVSS 208. In the 2 nd image captured at 30 milliseconds, the airbag system began to deploy, the pretensioner retracted seatbelts began to tighten across the dummy model to prevent slippage, and the dummy model moved forward due to the application of the baseline pulse. In the 3 rd image captured at 60 milliseconds, the dummy is shown right before contact with the airbag system, and the load limiter has released the seatbelts. In the 4 th image captured at 90 milliseconds, the dummy is shown at full contact with the airbag system, with the load limiter having released the seatbelt at its maximum. The final image captured at 120 milliseconds showed that the dummy model has rebounded from airbag system. In almost all of the simulated cases, the same events occurred, but the time at which they occurred varied according to mass, pulses, and restraint conditions. 33

44 Fig. 18. Images of crash simulation by time step 30 milliseconds (ms) For convenient comparison of each injury criteria, normalized values were used. The HIC value was divided by 700, and FFC was divided by 10,000 for males and 6,800 for females. Nij and CTI were already normalized values. If the normalized value is near 0, then it indicates a low risk for injuries, but if it is near to 1 or exceeds it, then it shows a high risk for injuries. Baseline Cases Baseline cases include the airbag system, retractor system, and the baseline pulse. This case study was simulated with six dummy models, male and female, with a BMI of 25(22), 30, and 35, with the above conditions. For cases involving male dummy models, the HIC value for a BMI of 25 was 0.469, for a BMI of 30 was 0.522, for a BMI of 35 was These were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.268, for a BMI of 30 was 0.309, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.636, for a BMI of 30 was 0.706, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the in- 34

45 jury criteria for the femur, for a BMI of 25 was 0.139, for a BMI of 30 was 0.154, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.615, for a BMI of 30 was 0.626, and for a BMI of 35 was 0.583, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.452, for a BMI of 30 was 0.348, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI 22 was 0.739, for a BMI of 30 was 0.777, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.219, for a BMI of 30 was 0.241, and for a BMI of 35 was These were lower than recommended normalized value. Based on the calculated injury criteria for head, neck, and thorax, the rate of increase with BMI were higher for male cases than female cases. For the cases regarding the femur, however, it was different. Case Study 1 Case Study 1-1. The baseline pulse times 1.2 cases Case Study 1-1 included the airbag system, retractor system, and 1.2 times the baseline pulse. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. 35

46 For cases involving male dummy models, the HIC value for a BMI of 25 was 0.964, for a BMI of 30 was 0.983, and for a BMI of 35 was HIC for a BMI of 35 was higher than the recommended value, and others were near the limit value. Nij, the neck injury criteria, for a BMI of 25 was 0.339, for a BMI of 30 was 0.365, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.763, for a BMI of 30 was 0.822, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.173, for a BMI of 30 was 0.187, and for a BMI of 35 was These were lower than recommended normalized value. For cases involving female dummy models, the HIC value for a BMI of 22 was 0.924, for a BMI of 30 was 0.988, and for a BMI of 35 was HIC for a BMI of 35 was higher than recommended value, and others were near the limit value. Nij, the neck injury criteria, for a BMI of 22 was 0.462, for a BMI of 30 was 0.332, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.823, for a BMI of 30 was 0.802, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.245, for a BMI of 30 was 0.295, and for a BMI of 35 was These were lower than recommended normalized value. Compared with the baseline cases, all of injury criteria of Case Study 1-1 showed higher values except for the neck injury of females with a BMI of 30. Among the criteria, an increase of HIC values for the male was more dominant than the other cases. 36

47 Case Study 1-2. The baseline pulse times 1.4 cases Case Study 1-2 included the airbag system, retractor system, and 1.4 times the baseline pulse. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving the male dummy models, the HIC value of a BMI of 25 was 1.536, for a BMI of 30 was 1.551, and for a BMI of 35 was These were higher than the value of 1 recommended by FMVSS. Nij, the neck injury criteria, for a BMI 25 was 0.445, for a BMI of 30 was 0.47, and for a BMI of 35 was These were lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.965, for a BMI of 30 was 0.995, and for a BMI of 35 was The value for the BMI of 35 was higher than the recommended value, and others were near the limit value. FFC, the injury criteria for the femur, for a BMI of 25 was 0.197, for a BMI of 30 was 0.217, and for a BMI of 35 was These were lower than recommended normalized value. For cases involving female dummy models, the HIC value for a BMI 22 was 1.37, for a BMI of 30 was 1.544, and for a BMI of 35 was These were higher than the value of 1 recommended by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.521, for a BMI of 30 was 0.358, and for a BMI of 35 was These were lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.920, for a BMI of 30 was 0.945, and for a BMI of 35 was The value for the BMI of 35 was higher than the recommended value, and others were near the limit value. FFC, the injury criteria for the femur, for a BMI of 22 was 0.278, for a BMI 30 of 37

48 was 0.35, and for a BMI of 35 was These were lower than recommended normalized value. Compared with baseline cases, all of injury criteria for Case Study 1-2 showed higher values. Among them, the increase of HIC values for the male was also more dominant than the other cases, similar to Case Study 1-1. HIC for both male and female cases showed higher increase rates than Case Study 1-1. Fig, 19 shows the magnitude of the spool out of the load limiter, comparing the baseline, the 1.2 times pulse, and the 1.4 times pulse. Fig. 19. Spool out by pulse variables at 92 milliseconds 38

49 Table 8. Male injuries result table of pulse cases Baseline Case 1-1 Case 1-2 Male (1.78 m) BMI 25 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % Table 9. Female injuries result table of pulse cases Baseline Case 1-1 Case 1-2 Female (1.54 m) BMI 22 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 39

50 Case Study 2 Case Study % Depowered Airbag Cases Case Study 2-1 included the baseline pulse, retractor system, and 10% depowered airbag system. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving the male dummy models, the HIC value for a BMI of 25 was 0.439, for a BMI of 30 was 0.486, and for a BMI of 35 was 0.663, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.283, for a BMI of 30 was 0.294, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.646, for a BMI of 30 was 0.632, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.139, for a BMI of 30 was 0.151, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.476, for a BMI of 30 was 0.532, and for a BMI of 35 was 0.504, and these were lower than recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI a 22 was 0.411, for a BMI of 30 was 0.316, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.727, for a BMI of 30 was 0.692, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. 40

51 FFC, the injury criteria for the femur, for a BMI of 22 was 0.217, for a BMI of 30 was 0.235, and for a BMI of 35 was These were lower than the recommended normalized value. Compared with the baseline cases, the overall injury criteria for Case Study 2-1 showed a decrease in values. All of the injury criteria for female dummies show lower decrease rates than male cases. The advanced airbag, or depowered airbag, was invented for a small dummy representing the 5 th percentile female and children, to reduce fatalities related to airbag deployment for this group. This case study demonstrated that a depowered airbag is beneficial for female dummies. Case Study % Depowered Airbag Cases Case Study 2-2 included the baseline pulse, retractor system, and 20% depowered airbag system. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving the male dummy models, the HIC value for a BMI of 25 was 0.418, for a BMI of 30 was 0.431, and for a BMI of 35 was 0.604, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.268, for a BMI of 30 was 0.292, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.629, for a BMI of 30 was 0.634, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.139, for a BMI of 30 was 41

52 0.154, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.375, for a BMI of 30 was 0.474, and for a BMI of 35 was 0.411, and these were lower than recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.411, for a BMI of 30 was 0.316, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.720, for a BMI of 30 was 0.701, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.217, for a BMI of 30 was 0.236, and for a BMI of 35 was These were lower than the recommended normalized value. Compared with baseline cases, overall injury criteria of Case Study 2-2 showed decreased values. All of the injury criteria for the female dummies showed lower decrease rates than male cases. Among them, HIC shows higher decrease rates than Case Study 2-1. It shows that a 20% depowered airbag is better than a 10% depowered airbag when considering head injury for the female dummy models. 42

53 Table 10. Male injuries result table of airbag cases Baseline Case 2-1 Case 2-2 Male (1.78 m) BMI 25 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % Table 11. Female injuries result table of airbag cases Baseline Case 2-1 Case 2-2 Female (1.54 m) BMI 22 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 43

54 Case Study 3 Case Study 3 is a set of simulations related to the restraint variables, for example no airbag cases, 1.2 times pulse with no airbag cases, no seatbelt cases, no retractor cases, and 2 kn load limiter cases. Case Study 3-1. Without airbag system Case Study 3-1 included the baseline pulse and retractor system, but no airbag system. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving the male dummy models, the HIC value for a BMI of 25 was 0.221, for a BMI of 30 was 0.226, and for a BMI of 35 was 0.537, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.328, for a BMI of 30 was 0.339, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.606, for a BMI of 30 was 0.604, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.139, for a BMI of 30 was 0.149, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.332, for a BMI of 30 was 0.241, and for a BMI of 35 was 0.287, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.454, for a BMI of 30 was 0.521, and for BMI of 35 44

55 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.694, for a BMI of 30 was 0.698, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.215, for a BMI of 30 was 0.236, and for a BMI of 35 was These were lower than the recommended normalized value. Compared with the baseline cases, overall injury criteria for Case Study 3-1 showed decreased values except for neck injury. Head and thorax did not come in contact with other equipment; as a result, those injury criteria were lower than the baseline. Neck injury, however, was highly increased in female dummies with a BMI of 30 and 35 because of its severe bend. Case Study 3-2. Acceleration pulse 1.2 times and without the airbag system For cases involving male dummy models, the HIC value for a BMI of 25 was 0.851, for a BMI of 30 was 0.768, and for a BMI of 35 was HIC for a BMI of 35 was higher than the recommended normalized value. Nij, the neck injury criteria, for a BMI of 25 was 0.327, for a BMI of 30 was 0.428, and for a BMI of 35 was Nij for the BMI of 35 was higher than the recommended normalized value. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.767, for a BMI of 30 was 0.876, and for a BMI of 35 was The value for the BMI of 35 was higher than the recommended normalized value by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.172, for a BMI of 30 was 0.187, and for a BMI of 35 was These were lower than the recommended normalized value. 45

56 For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.515, for a BMI of 30 was 0.382, for a BMI of 35 was 0.491, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.469, for a BMI of 30 was 0.562, and for a BMI of 35 was CTI, the injury criteria for the thorax, for a BMI of 22 was 0.760, for a BMI of 30 was 0.787, and for a BMI of 35 was Both Nij and CTI values were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI 22 of was 0.237, for a BMI of 30 was 0.294, and for a BMI of 35 was These were lower than the recommended normalized value. Compared with the baseline cases, overall injury criteria for Case Study 3-2 showed higher values except for the values for the head of the female cases. The male cases showed higher increase rates for the injury criteria than the female cases. Especially, male BMI of 35 showed extremely high injury values for HIC and Nij because of the contact with the steering wheel. Fig. 20 shows the head acceleration data for the male dummy with a BMI of 35. Contact time with the steering wheel shows a drastic increase in acceleration. Even though the HIC value for females was decreased, Nij for females with a BMI of 35 was increased highly due to a severe bend. This case shows the importance of combining the airbag system, the load limiter, and the pretensioner. 46

57 Fig. 20. HIC of male BMI 35 Case Study 3-3. Without seatbelts Case Study 3-3 included the airbag system and the baseline pulse, but no seatbelts. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving male dummy models, the HIC value for a BMI of 25 was 3.227, for a BMI of 30 was 3.531, and for a BMI of 35 was These were higher than the value of 1 recommended by FMVSS. Nij, the neck injury criteria, of BMI 25 was 1.515, for a BMI of 30 was 1.728, and for a BMI of 35 was These were also higher than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 1.049, for a BMI of 30 was1.051, and for a BMI of 35 was These were higher than the recommended value. FFC, the injury criteria for the femur, for a BMI of 25 was 0.715, for a BMI of 30 was 0.798, and for a BMI of 35 was 47

58 These were lower than the recommended normalized value, but value for the BMI of 35 was close to the limit value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.782, for a BMI of 30 was 0.900, and for a BMI of 35 was The value for the BMI of 35 was almost the value of 1 as recommended by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.793, for a BMI of 30 was 0.853, and for a BMI of 35 was These were lower than the value of1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI 22 was 1.219, for a BMI of 30 was 1.194, and for a BMI of 35 was The values for the BMI of 25 and 30 were higher than the recommended value, but the value for a BMI of 35 was near the limit value. FFC, the injury criteria for the femur, for a BMI of 22 was 0.553, for a BMI for 30 was 0.741, and for a BMI of 35 was These were lower than the recommended normalized value. Compared with the baseline cases, these cases showed the worst injury criteria and also indicated the importance of seatbelts. Male dummy models came in contact with the windshield and vented neck. As a result, head and neck injuries of the male dummies were extremely higher thant he baseline cases. Femur injury criteria were also very high values due to this direct contact with the knee bolster. Case Study 3-4. Without pretensioner and load limiter Case Study 3-4 included the baseline pulse and airbag system, but no retractor. The function value for the load limiter was given to 30kN, and the pretensioner was set to zero. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. 48

59 For cases involving the male dummy models, the HIC value for a BMI of 25 was 0.553, for a BMI of 30 was 0.741, and for a BMI of 35 was These all were lower than the recommended normalized value. Nij, the neck injury criteria, for a BMI of 25 was 0.525, for a BMI of 30 was 0.491, and for a BMI of 35 was These were also lower than the recommended normalized value. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.912, for a BMI of 30 was 0.837, and for a BMI of 35 was These were within the normalized value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.169, for a BMI of 30 was 0.148, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving female dummy models, the HIC value for a BMI of 22 was 0.792, for a BMI of 30 was 0.993, for a BMI of 35 was 0.932, and these were lower than the recommended normalized value, but the value for the BMI 30 was near the limit value. Nij, the neck injury criteria, for a BMI of 22 was 0.462, for a BMI of 30 was 0.623, and for a BMI of 35 was These were within the recommended value. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.896, for a BMI of 30 was 0.977, and for a BMI of 35 was The value for the BMI of 35 was higher than the recommended normalized value, and the value for the BMI of 30 was near the limit value. FFC, the injury criteria for the femur, for a BMI of 22 was 0.197, for a BMI of 30 was 0.294, and for a BMI of 35 was These were lower than the recommended normalized value. Overall injury criteria for Case Study 3-5 showed higher values when compared with baseline cases. The load limiter did not release the seatbelt; as a result, the neck was bent severely, and the chest was more highly compressed by the shoulder belt. The head also could not come in full contact with the airbag system. 49

60 Case Study 3-5. The maximum force of load limiter set to 2kN Case Study 3-5 included the baseline pulse and airbag system, and the retractor systems, with the maximum function value of load limiter set to 2 kn. This case study was simulated with six dummy models, male and female, with a BMI of 25, 30, and 35, with the above conditions. For cases involving male dummy models, the HIC value for a BMI of 25 was 0.774, for a BMI of 30 was 0.758, and for a BMI of 35 was These all were lower than the recommended normalized value. Nij, the neck injury criteria, for a BMI of 25 was 0.392, for a BMI of 30 was 0.439, and for a BMI of 35 was These were also lower than the recommended normalized value. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.718, for a BMI of 30 was 0.765, and for a BMI of 35 was These were within the normalized value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.131, for a BMI of 30 was 0.153, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.780, for a BMI of 30 was 0.807, for a BMI of 35 was 0.862, and these were lower than the recommended normalized value, but the value for the BMI of 30 was near the limit value. Nij, the neck injury criteria, for a BMI of 22 was 0.476, for a BMI of 30 was 0.316, and for a BMI of 35 was These were also lower than the recommended normalized value. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.701, for a BMI of 30 was 0.752, and for a BMI of 35 was These were all within the recommended normalized value. FFC, the injury criteria for the femur, for a BMI of 22 was 50

61 0.221, for a BMI of 30 was 0.263, and for a BMI of 35 was These were lower than the recommended normalized value. Overall injury criteria for Case Study 3-5 showed higher values when compared with the baseline cases. A problem with Case Study 3-4 was the limited spool out, but in this case the spool out was not limited. The head came into contact with the airbag strongly, and as a result injury to the head, neck, and thorax of the male dummies increased greatly. Head injury for female cases also increased, but neck and thorax were decreased or slightly increased. The results of Case Study 3-4 and 3-5 show the importance of properly setting the values of the load limiter. Currently, the optimal setting value for the 50 th percentile male dummy model is 4 kn, but that may not be applied for all of the dummy models. 51

62 Table 12. Male injuries result table of restraint cases Baseline Case 3-1 Case 3-2 Case 3-3 Case 3-4 Case 3-5 Male (1.78 m) BMI 25 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 52

63 Table 13. Female injuries result table of restraint cases Baseline Case 3-1 Case 3-2 Case 3-3 Case 3-4 Case 3-5 Female (1.54 m) BMI 22 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 53

64 Case Study 4 Case Study 4 is a set of simulations related to extra variables such as height, angle of the steering wheel, and the contact friction coefficient of the seat and the femur. Case Study 4-1. Increased height cases Height models were utilized in a process identical with the baseline cases. The baseline pulse, airbag system, and seatbelts with retractor systems were used for height cases. The vehicle model was also modified to fit the increased height of the dummy models. For cases involving male dummy models, the HIC value for a BMI of 25 was 0.494, for a BMI of 30 was 0.579, and for a BMI of 35 was 0.650, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.317, for a BMI of 30 was 0.349, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.664, for a BMI of 30 was 0.746, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.153, for a BMI of 30 was 0.162, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving female dummy models, the HIC value for a BMI of 22 was 0.630, for a BMI of 30 was 0.749, for a BMI of 35 was 0.796, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.601, for a BMI of 30 was 0.535, and for a BMI of 35 was 54

65 These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.739, for a BMI of 30 was 0.736, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.200, for a BMI of 30 was 0.299, and for a BMI of 35 was These were lower than the recommended normalized value. Overall injury criteria of Case Study 4-1 showed increased values when compared with the baseline cases. Female cases showed higher neck injury and HIC value than male cases for the BMIs of 30 and 35. Case Study 4-2. Steering wheel of 10 degree tilted downward Tilted steering wheel models were simulated in an identical manner as the baseline cases. The baseline pulse, airbag system, and seatbelts with retractor system were used for height cases. The vehicle model was also modified to fit the increased height of the dummy models. For cases involving the male dummy models, the HIC value for a BMI of 25 was 0.437, for a BMI of 30 was 0.487, for a BMI of 35 was 0.596, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.275, for a BMI of 30 was 0.306, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria of the thorax, for a BMI of 25 was 0.641, for a BMI of 30 was 0.653, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.139, for a BMI of 30 was 55

66 0.155, and for a BMI of 35 was These were lower than the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.599, for a BMI of 30 was 0.583, for a BMI of 35 was 0.600, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.411, for a BMI of 30 was 0.319, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.745, for a BMI of 30 was 0.714, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.218, for a BMI of 30 was 0.240, and for a BMI of 35 was These were lower than the recommended normalized value. Overall injury criteria for Case Study 4-2 showed slightly decreased values when compared with baseline cases. HIC of male cases showed better results than other injury criteria. The angle of the steering wheel affected the injury criteria. Case Study 4-3. Friction coefficient between seat and dummy set to 0.5 Contact friction coefficient modified models were simulated identically as the baseline cases. The baseline pulse, airbag system, and seatbelts with retractor system were the same, but the contact fiction coefficient of for the seat and the femur was set to 0.3 to 0.5. The vehicle model was also modified to fit the increased height of the dummy models. 56

67 For cases involving male dummy models, the HIC value for a BMI of 25 was 0.430, for a BMI of 30 was 0.493, for a BMI of 35 was 0.678, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 25 was 0.271, for a BMI of 30 was 0.32, and for a BMI of 35 was These were also lower than the value of 1 recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 25 was 0.590, for a BMI for 30 was 0.654, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 25 was 0.148, for a BMI of 30 was 0.149, and for a BMI of 35 was These were within the recommended normalized value. For cases involving the female dummy models, the HIC value for a BMI of 22 was 0.618, for a BMI of 30 was 0.585, for a BMI of 35 was 0.579, and these were lower than the recommended normalized value of 1 provided by FMVSS. Nij, the neck injury criteria, for a BMI of 22 was 0.416, for a BMI of 30 was 0.341, and for a BMI of 35 was These were also lower than the value of 1recommended by FMVSS. CTI, the injury criteria for the thorax, for a BMI of 22 was 0.747, for a BMI of 30 was 0.677, and for a BMI of 35 was These were within the value of 1 recommended by FMVSS. FFC, the injury criteria for the femur, for a BMI of 22 was 0.235, for a BMI of 30 was 0.232, and for a BMI of 35 was These were lower than the recommended normalized value. Overall injury criteria for Case Study 4-3 showed slightly decreased values when compared with the baseline. Friction of the seat and the dummy model also affected injury criteria. 57

68 The results of Case Studies 4-2 and 4-3 notified us that interior parts of the cabin affect the injury criteria. Materials and location of interior parts should also be considered to ensure the safety of occupants. Table 14. Male injuries result table of other cases Baseline Case 4-1 Case 4-2 Case 4-3 Male (1.78 m) BMI 25 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 58

69 Table 15. Female injuries result table of other cases Baseline Case 4-1 Case 4-2 Case 4-3 Female (1.54 m) BMI 22 BMI 30 BMI 35 HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % HIC % % Neck % % CTI % % LE % % 59

70 CHAPTER 5 DISCUSSION Currently, some studies insist that subcutaneous fat tissue plays the role of cushioning the abdomen region during MVCs to help prevent injury. Meanwhile, other studies show that the increased mass due to this subcutaneous fat creates momentum effects for these obese passengers. The purpose of this research is to demonstrate those different assertions through computational modeling and simulation technique. Geometric models of fat-vests were generated based on image processing data, and material properties were granted. The hybrid method combines standard dummy models and fat-vest models, and mass information on obese dummy models was updated. Generated obese dummy and the simplified vehicle models were simulated by a variation of pulses, mass flow rates of the airbag system, restraint conditions, and extra cases. The association of obese occupants with each injury criteria that can result from MVCs were found through the baseline and 12 case studies. Table 16, 17, 18, and 19 summarizes the injury data of head, neck, thorax, and femur, respectively, as well as the calculated mean values for a convenient comparison of each injury criteria. All injury data were normalized and Case Study 3-3 was excluded due to its higher injury criteria compared with other cases. 60

71 Table 16. Head injury summary data Table 17. Neck injury summary data Table 18. Thorax injury summary data Table 19. Femur injury summary data 61