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N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics


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Technical paper addressing impact and collision forces incorporated into masters degree program dealing with biomechanical trauma for physicians conducted by Lynn University and the University of Miami Medical Center.

Published in: Automotive, Technology, Education

N - Motor Vehicle Accident Reconstruciton and Biomechanical Physics

  1. 1. Motor Vehicle Accident Reconstruction & Biomechanical Physics Robert C. McElroy, Ph.D. www . ForensicAccident . Com ABSTRACT Accident reconstructionists rely on a wide range of methods to record and analyze motor vehicle accident information. This paper addresses con- temporary methods of obtaining and analyzing collisions with emphasis on G force explanation for biomechanical analysis. INTRODUCTION Traffic accident reconstruction is the effort to determine, from whatever resources are available, how an accident happened. A traffic accident reconstructionist must be familiar with the application of a wide range of mathematics and specialized aspects of vehicle technology. Because of the wide range of knowledge required by the accident reconstructionist, voluntary certification is available through the Accreditation Commission for Traffic Ac- cident Reconstruction. ACTAR certification includes education, work experi- ence, and successful completion of a comprehensive examination. MATHEMATICS FOUNDATION Mathematics are at the core of traffic accident reconstruction. Many different equations are used to determine different aspects of an accident. Sir Isaac Newton developed three mathematical laws of motion which provide the foundation for traffic accident reconstruction. In Newton’first law of motion an important property of matter ap- s pears. It is known as inertia, that property of matter by which an object main- tains a constant velocity in the absence of an unbalanced external force. When an automobile is suddenly stopped, the passengers obey Newton’first law s and continue in their motion with constant velocity until some external Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 1
  2. 2. force changes their state of motion. Seat belts in a automobile can provide such an external force which is much preferred to that exerted by the wind- shield or dashboard. Another statement is the following: A body at rest remains at rest, and a body in motion remains in motion with constant velocity along the same straight line unless acted upon by some outside force. Newton’First Law of Motion: Inertia of Rest & Motion s Newton’ first law states that a body at rest s Newton’ first law also states that a body in s remains at rest unless acted upon by some motion remains in motion with constant velocity external unbalanced force. unless acted upon by some resultant force. Newton’second law states that if a body is acted upon by an unbal- s anced force F, its center of mass will accelerate in the direction of the force. The acceleration, a, is proportional to the force, F, and the constant of propor- tionality, m, is called the mass of the body. Another statement is the following: The acceleration of a body is directly proportional to the resultant force action upon the body and acceleration is inversely proportional to the mass of the body. Newton's second law provides the key relationship between force and acceleration since force and acceleration are vectors and vectors have both magnitude and direction. Mass is only a magnitude, so it follows that the magnitude of force equals the magnitude of acceleration times the mass. Unification of these concepts reveals that force direction must be the same as the direction of the acceleration because mass does not have a directional property. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 2
  3. 3. The second law is written F = ma where the unit of force is the new- ton. One newton produces an acceleration of one meter per second, per second, in a mass of one kilogram. One newton has a value of .2248 lb. Newton’Second Law of Motion: F = m a s Acceleration of a body is directly Acceleration due to a given result- proportional to the resultant force ant force is inversely proportional to acting on the body. the mass accelerated. Newton's third law is equally valid in dealing with bodies at rest or in motion, either uniform or accelerated. The wheels of an automobile in motion push backward on the road, but the road pushes forward on the wheels with an equal force during acceleration. Another statement is the following: Whenever one body exerts a force upon a second body, the second body exerts an equal and opposite force on the first. Newton’Third Law of Motion: Action = Reaction s Action (force exerted on the trailer) is equal to the reaction (force exerted on the car by the trailer). Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 3
  4. 4. SPEED CHANGE Prediction of what happened during a collision by examination of what remains in the form of residual damage can be used to calculate speed change or Delta Velocity (DV) experienced by the vehicles in the collision. DV is one of the best available measures of accident severity. DV assumes that collision stopping force on a vehicle is a linear function of residual crush depth. Up to a certain force level, there is no permanent damage and beyond that point, permanent damage increases with increased force. Two stiffness coefficients, A and B, define the force-damage curve. Fundamental to a solution for speed change are appropriate A and B values for a specific vehicle. A and B values are derived from the collision damage sustained from known velocity changes of a vehicle into a barrier. Therefore, A and B values can be used to calculate speed change based on permanent vehicle crush (see chart). Vehicle tests sponsored by the National Highway Traffic Safety Administration resulted in a series of computer programs released to the public called CRASH. The last public version CRASH3 was revised and released in 1982. Several computer based accident analysis programs are available for the accident reconstructionist, each ultimately stems from this background. A collision analysis project is defined as a series of step by step calculations. The investigator will organize the project into separate events which require solutions for specific unknowns. Normally, each event or step will consist of an equation with only one unknown. The art of collision analysis is to determine which event to solve first and then how to proceed with the next calculation to develop a unified collision sequence analysis. Calculations from a popular AI Tools computer program help to address specific information needed in order to piece together the accident reconstruction. When properly used each calculation will help to fill in a missing piece of information. Here is a list of AI Tools calculations: Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 4
  5. 5. Eq Group Eq # Problem Description Special 45 Tangent Offset Speed 1 Speed from Distance & Drag 46 Radius from Chord and Mid-Ordinate 2 Constant Speed from Distance & Time 47 Critical Curve Speed 3 Speed from Drag & Time 48 Combined Speed from Drag Surfaces 4 Final Speed from Start Speed, Drag & Time 49 Combined Speeds 5 Final Speed from Start Speed, Drag & Distance 6 Start Speed from Final Speed, Drag & Time Airborne 50 Horizontal Launch and Fall Speed 7 Start Speed from Final Speed, Drag & Distance 51 Small Angle Speed at Launch Equation 52 General Projectile Speed Equation Time 10 Time from Constant Speed & Distance 11 Time from Speed & Drag Energy 60 Speed from Linear Kinetic Energy 12 Time from both Speeds & Drag 61 Kinetic Energy from Speed and Weight 13 Time from Distance & Drag 62 Force from Kinetic Energy and Distance 63 Distance from Kinetic Energy and Force Distance 20 Distance from Speed & Drag 21 Distance from Constant Speed & Time Motorcycle 70 Lateral Acceleration Factor from Speed and Radius 22 Distance from Two Speeds & Drag 71 M/C Lean angle from Speed and Radius 23 Distance from Initial Speed, Drag & Time 72 Turning Radius From Speed and lateral acc Acceleration 30 Drag from Speed & Distance Animation 80 Final Speed from Start Speed, Drag and Time Factor 31 Drag from Speed & Time Assist 81 Distance from Start Distance, Speed, Drag and Time 32 Drag from both Speeds & Time 33 Drag from both Speeds & Distance Reaction 90 Total D from P/R time, Speeds & Drag f 34 Drag from Distance & Time Times 91 Total D from Start Speed, Times & f 35 Drag from Initial Speed, Distance & Time 92 P/R Time from Speeds, Total D & f 36 Drag from Road Friction, Brake Efficiency & Grade 93 P/R Time from Start Speed, Total D, f & Time 37 Drag from Horizontal Force & Weight 94 Start Speed from Total D, Times & Drag 95 Start Speed from Total D, Final Speed, P/R Time & f Linear 40 General Two-Dimensional Momentum 96 Drag from Speeds, P/R Time & Total D Momentum 41 Inline - V1 from V2, V3, and V4 97 Drag from Start Speed, Total D & Times 42 Inline - Coefficient of Restitution 98 Total Time from P/R Time, Speeds & f 43 Inline - Plastic, V3 = V4 99 Total Time from P/R Time, Brake D & Drag 44 Inline -Elastic Equation #1 Speed from Distance & Drag. Calculation of vehicle speed made from skid distance and drag factor. Measured skid distance is 120 feet and deceleration factor for the vehicle & road surface combination is .7 G. The vehicle is calculated to have been going 50 mph when the brakes were applied. MOMENTUM Momentum is a restatement of Newton's laws in a form that is useful for the collision analysis or any event which involves very short periods of time. The second law for a single object would be rewritten as F Dt = m DV. The left side of the equation is the Impulse and the right side is the Change in Momentum. This relationship is still a vector relationship. Force has the same direction as the change in momentum which has the same direction as the change in velocity. In summary: Impulse = Change in Momentum If two vehicles collide, the force or impulse on one of the vehicles is equal and opposite to the force on the other. This is a consequence of Newton's third law. Changes in momentum for both collision vehicles cancel. There is no gain or no loss of momentum during a collision. Momentum before the collision Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 5
  6. 6. equals the momentum after the collision and because weight is propor- tional to mass the final equation can be rewritten as: W1V1 + W2V2 = W1V3 + W2V4. On the left is a three car collision. AI Tools Linear Momentum module below, calculates that the Chevy Lumina came into the collision at 27 mph and that the Dodge had an entry speed of 15 mph. Equation #1 was initially used to deter- mine slide to stop, or departure, speeds for each vehicle. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 6
  7. 7. HUMAN DATA Medical personnel involved in an accident investigation can provide valuable injury information that assists in cause analysis. Where possible in fatality accidents, autopsies should be performed to determine the cause of death and record information about the injuries. Nonfatal injury information is also useful to the investigator. Location of broken bones is especially useful when graphically represented in a skeleton diagram in a seated position, on left. Injury illustration could be done for each occupant to produce a composite occupant diagram for the vehicle. Injuries will indicate the direction of crash loading for the vehicle. Location of bruises and contusions can also be addressed, since Injury location identifica- tion from bone fractures these injuries can sometimes indicate use in illustration. or nonuse of a seat belt or shoulder harness. Head injuries are important clues. If an instrument panel, roof pillar, steering wheel or glass has evidence of a head strike (i.e., blood, skin, hair, dent, teeth), that spot should be documented. With a specific location for the strike and the relationship to the occupant’body, the investigator can evalu- s ate the angle of head impact, body position, and restraint system function, see below. It is important to remember that an occupants head motion is exactly opposite to the crash loading direction. Correlation of head strike information between passengers and vehicle’interior. s Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 7
  8. 8. DECELERATION LOADS In a biomechanical investigation approach, the most important task is to determine occupant crash loads and probability of serious injury. Other investigative tasks help the biomechanical investigator to understand physi- cal relationships in the accident which lead up to either receiving a serious injury or not. To calculate an average G force crash pulse, the following equa- tion is used for each axis of occupant travel. V2 Gavg = 2gS In this equation G = the average force on the specific occupant and is expressed as a multiple of occupant weight. Because of crash dynamics, peak G figures will typically be twice the average G. V = velocity change at the major impact, expressed in ft/s g = acceleration of gravity, 32.2 ft/s2 S = deceleration distance, expressed in ft The biomechanical investigator should look for clues in each axis (i.e., roll, pitch, yaw) for velocity change and stopping or deceleration distance to be able to determine a unique crash loading for each occupant. An extreme example of how the different G loading is experienced by different people decelerating over different distances is found in this illustra- tion. To understand the concept, consider a long uniform airplane fuselage that crashes head-on into a cliff at 200 mph with Persons A, B, and C who decel- erate in the crash distances of 5 ft, 20 ft, and 45 ft, respectively. A calculation of the average G-loading experience by Per- sons A, B, and C would be 266G, 66G, and 29G, respectively. The average G calcu- lation for passenger C is: The average forward load on Persons A, B, and C can be calculated by multi- Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 8
  9. 9. plying each persons weight by each persons average G load. If the weights of Persons A, B, and C were 200 lb, 170 lb, and 100 lb, respectively, the average loading experienced would be 53,395 lbf, 11,346 lbf, and 2,966 lbf respectively. The weight load calculation for passenger C would be: This airplane example assumed seats and restraints that held through- out the crash sequence. For such a severe crash, it is not likely for all of the seats and restraints to remain attached. A seat will fail when its maximum load capability is exceeded. Assume seats with integral shoulder harness and lap belts were designed for 25G static loads in the forward direction. Person C of our airplane example would have a seat where the minimum force before seat separation can be expected of 25 times their weight of 170 lb x 25G or 4,250 lbf. The load experienced by Person C was 2,966 lbf average or 5,932 lbf peak. Thus, the seat for Person C would separate when its peak load exceeded the design load capability of the seat. The more damaging loads for Person C will occur later, when he or she undergoes major deceleration. AUTOMOTIVE LOADS A vehicle traveling 35 mph sustained two feet of uniform crush. Substi- tution into the formula reveals the G forces for this accident. V2 352 Gavg = = = 9.5 Gavg x 2 = 19 Gpeak 2gS 2 x 32.2 x 2 Federal Motor Vehicle Safety Standard, FMVSS 207 Seating Systems. Under this standard the seat must be capable of withstanding a force “ times the 20 weight of the seat applied in a forward (S4.2.a) or rear- ward (S4.2.b) longitudinal direction.” Thus if the seat weighs 30 lb then it must not fail at a level below 600 lb when calculated as 20 x 30 lb = 600 lb. It is important to note that occupant weight is not considered. S4.3.2.1 Static force specifies 20 times the weight of the seat back. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 9
  10. 10. VERTICAL CRASH ANALYSIS A more detailed analysis will reveal that the G loading will change throughout the crash sequence. For example, crash loads experienced by an occupant of an extremely high vertical velocity impact are shown on page 10. From point A to B, the crash loads are low (typically 2 to 3 G) as the landing gear deforms. Point B is where the fuselage lower skin contacts the terrain. Loading from point B to C to D to E is extremely high as the aircraft floor comes to rest at point F. If an occupant is sitting on the floor, the loading experienced would have been points B to C to D to E to F. Note the horizontal line, which is an injury load threshold (within time duration limits) above which severe injury is ex- pected. If the occupant is sitting on a seat, the vertical crash loading experi- enced will rise from point B to C and then drop to point G. Loads do not exceed point C, as this is the maximum strength of the seat which fails at Point C. The occupant’load is about zero from point G to H because the occupant is s basically free-falling from a seated position until contacting the floor at point H. Vertical G loading during a vertical crash. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 10
  11. 11. However, the aircraft floor is just about to come to rest at point F. Thus, the occupant impacts a nearly stationary floor and the crash loads experi- enced by the occupant will go from point H to I to J. Unfortunately, occupant load penetration above the injury zone threshold line would indicate that a severe injury would be expected for the example occupant. It is obvious that an average G loading for an entire aircraft is inaccurate and misleading. Understanding the crash loads on an occupant is not possible without good information from the crash survival investigation on injuries, restraints, seat damage, and fuselage damage. AERIAL CRUSH PHOTOGRAPHY On-site aerial photographs taken with the aid of a boom and perimeter grid capture collision data. This system has proven to be extremely useful in addressing collision dynamics. A brief synopsis of this method reveals its use- fulness to the accident reconstructionist and biomechanical expert. Securely place the base against a tire. Elevate & lower the boom gently. Front View of Camera & Grid Top View of Camera & Grid Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 11
  12. 12. COMPOSITE PHOTOGRAMMETRY At night a westbound pickup truck was towing a 2700 pound air compressor. The air compressor dis- connected from the pickup, crossed the centerline of a two lane roadway and struck an eastbound 3,100 pound automobile. The speedometer of the car was “crushed” and read 55 mph. Delta V calculated at 108 mph. Below is a composite photogrammet- ric assembly for this collision. Aerial photographs of the car and air compressor were placed together in order to illustrate maximum engagement at collision. A perimeter grid has been placed around the car. Black markings one foot apart are used for post collision analysis by permitting lines to be drawn across the photograph for documen- tation of crush sustained by the vehicle. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 12
  13. 13. VEHICLE CRUSH DAMAGE Analysis of photographs reveals that the subject vehicle sustained 6.3 feet of crush as a result of the collision. Exemplar Vehicle With Pre Crash and Maximum Engagement Driver to Bumper & Air Compressor Illustration Subject Vehicle With Roof Folded Back Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 13
  14. 14. VEHICLE CRUSH DAMAGE - SIDE VIEW These CAD illustrations show the vehicle side view before and after impact and the relative position of the driver inside the automobile. Post-collision vehicle defor- mation at right illustrates the vehicle contact with the driver. It can be clearly seen that this collision was not survivable. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 14
  15. 15. COLLISION SURVIVABILITY Linear momentum calculations show a closing speed for this head-on collision of approximately 108 mph and Equivalent Barrier Speed (or EBS) of 59 mph. EBS is when a vehicle impacts a massive barrier which absorbs no energy of collision. It is a convenient concept to compare the energy absorbed in crushing vehicles. The National Highway Traffic Safety Administration annually releases its New Car Assessment Program (NCAP) crash test results for current model year vehicles. These tests give occupant injury criterion values for collisions at the 35 mph EBS. In the EBS crash of an exemplar Buick Century, approximately 2' of uniform crush was sustained. As previously addressed, an increase in velocity at impact results in an increase in the energy of collision. The subject vehicle had an EBS of 59 mph which results in an increase of the energy involved in this collision of 184%, or almost three times as much energy being pro- duced, as in the 35 mph crash tests. This massive increase in energy is translated di- rectly into an increase in the forces making this collision unsurvivable. The air compressor has an overall width of 56 inches, 13" less than the Buick, and weighed almost as much. It approached from the driver's side at a slight angle of about 8°. The net result of these factors was primary damage and collision force concentration in an area directly in front of the driver. Force concentration is shown in illustrations included in this report. The previous page however, best illustrates the total amount of intrusion into the driver's side area. Measurements are shown of the distance from the driver to the left front of the vehicle both before and after collision. These measurements reveal that the initial 9.2 ft. of vehicle in front of the driver had been crushed to 2.9 feet. Thus, the area immediately in front of the driver sustained a total crush of approximately 6.3 feet. Extremely high EBS loads coupled with massive collision damage con- centrated directly in front of the driver, made this collision unsurvivable under any circumstances by the driver of the Buick. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 15
  16. 16. LOW SPEED COLLISIONS According to General Motors, in 1986, more than 33% of all automobile injuries occurred in low-speed collisions where the speed difference between the vehicles was less than 20 mph. It has also been estimated that more than 75% of low-speed collisions resulting in injury are rear-end collisions. Over the past forty years, there has been a significant amount of research into the effects of vehicle collisions and resultant occupant movement. However, the majority of this research concentrates on the effects of high collision speeds, typically 30 mph or above. Most staged collisions performed by car manufacturers, insurance institutions (Insurance Institute for Highway Safety), and the United States Government (National Highway Traffic Safety Administration) are frontal collisions at speeds of 30 to 35 mph into a rigid barrier. The four main types of low-speed collisions are 1) rear-end, 2) front-end, 3) lateral, and 4) side-swipe. Rear End Side Front End Lateral Swipe Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 16
  17. 17. In many low-speed collisions, occupants claim common “ whiplash” symptoms such as pain in the neck, shoulders, arms, and low back, despite the absence of vehicle damage. Investigators are frequently asked whether the claimed injuries could have resulted from what appears to be a trivial event. To address this question, investigators must first determine the severity of the collision and then compare that value to human injury tolerance levels. IMPACT SEVERITY Before looking at the ways to determine the severity of a low-speed impact, it is first important to understand the difference between high-speed and low- speed impacts. When two vehicles collide at a high speed, they essentially act like two balls of clay. They deform on impact and remain deformed with little or no crush energy being released after impact. There is little or no “ bounce” or elastic behavior during the impact. The engineering term for this “ bounce” or elastic behavior is restitution. In high-speed collisions, the restitution approaches a minimum value of zero and, therefore, when reconstructing high speed collisions, restitution is usually ignored. When two vehicles collide at a very low speed, they act more like two tennis balls. A large percentage of the deformation of the vehicles is elastic in nature and is released after impact. Only a small percentage remains as permanent crush damage. In low-speed collisions, the restitution approaches a maximum value of one. Therefore, when reconstructing low-speed collisions, the effects of restitution cannot be ignored. The severity of a collision is quantified by the acceleration (or deceleration) experienced by a vehicle during impact. Acceleration is speed change divided by time. If collision duration is assumed essentially constant, Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 17
  18. 18. then velocity change can be used to quantify collision severity. This is a valid assumption for most collisions where the time duration is approximately 1/ 10th of a second. However, in collisions such as underride impacts, the In general terms, severity of an impact is related to vehicle damage. A vehicle that has sustained several inches of rear-end crush has experienced a more severe impact than the same vehicle that has less or even no permanent rear-end damage. However, there are significant differences between the relative strengths of different surfaces of the same car, and between the same surfaces of different cars. For example, the rear ends of two different cars will not be equally strong, so that two different cars with similar damage may not have experienced the same impact severity. BUMPER TECHNOLOGY Where there is bumper engagement with no damage in a rear or front impact, it is often possible to determine impact severity from an inspection of the vehicle bumpers. In many cases, the amount of compression of bumper isolators can be correlated to the vehicle’ V (velocity change) in a minor s front or rear impact. In non-isolator equipped cars, which are increasingly more common, the task of determining severity is more difficult. For many passenger cars in North America, there is often little or no damage after a minor impact. Some Asian cars in particular have quite robust foam-core bumpers that are more damage resistant. Attached is a list of some vehicles equipped with foam core bumpers. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 18
  19. 19. In lateral and side swipe impacts, damage is more noticeable, since body panels, which are much weaker than bumpers, are involved. Body panels dent and horizontal scuffs and creases are easily produced with very minor impacts. Currently in the United States, a Federal safety standard requires that automobile bumpers keep damage away from car bodies in 2½ mph front and rear into flat barrier impacts. Damage is allowed to the bumper itself. These requirements are much weaker than the stronger 5 mph no-damage bumper rule that was in effect during the 1980 to 1982 model years. Neither strong nor weak bumper requirements have ever applied to trucks or vans. Low speed crash tests indicate that many bumpers are built to exceed the standard and in some cases are undamaged at speeds well in excess of those set out in the standards. DAMAGE & SEVERITY ASSESSMENT Damage sustained by vehicles during impacts varies greatly among models and manufacturers. Certain vehicles show no evidence that an impact occurred, even after impacts with severities as high as 10 mph V while others have incurred structural damage during very low severity impacts. These differences in vehicle behavior help explain instances where one vehicle will show large amounts of deformation while the other vehicle will appear undamaged. Often, investigators will underestimate the impact severity because no damage was observed during the vehicle examinations. Similarly, overestimates have also been made when the vehicles show obvious damage. Closer investigation into vehicles’ properties will provide insight into relationships between vehicle damage and the corresponding impact severity. Results from staged human volunteer collisions is significant. From this data, it can be seen that the type of vehicle engagement during collision and type of impact both need to be considered when determining collision severity. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 19
  20. 20. It should be noted that the severity given in the table represents the probable maximum severity. This maximum value is only used if there exists no test data for the same or similar model vehicle, or any other evidence, to indicate a lower damage threshold. Collision severity can also be determined from physical evidence at the scene of an accident, such as vehicle final rest positions and pre-impact acceleration distances, skid marks, etc. Unfortunately, this physical evidence will typically not be recorded by police because the accident is viewed as a “minor collision.” COMPUTER MODELING Most computer programs used to determine speed from crush damage (eg. SLAM, EDCRASH) are written for high-speed collisions and assume a linear (straight line) relationship between crush and speed. They use vehicle crush characteristics, or stiffness coefficients, obtained from crash tests typically performed in the 30 to 40 mph range. In this speed range, for most vehicles, there is essentially a linear (straight line) relationship between crush and speed. Consequently, these computer programs give good results for barrier equivalent speeds of 20 to 50 mph. When these computer programs are used to analyze low speed collisions the same linear relationship is assumed to exist at low speeds. The results obtained from 30 to 40 mph staged collisions are extrapolated backwards. Results from staged low speed collisions indicate that this assumption is incorrect. For most vehicles there is not a linear relationship between speed and crush at low speeds, note the dashed lines above. Therefore, these computer Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 20
  21. 21. programs should not be used to analyze low speed collisions unless great care is used to modify the vehicle crush coefficients. HUMAN INJURY TOLERANCE LEVELS When trying to understand the motion of an occupant subjected to a low speed front- or rear-end collision it is useful to visualize the occupant as a simple head-on-a-stick. Depicted below is a rear-end collision. R o t a t io n Tor que Ar m A c c e l e r a t io n Whenever a vehicle is rear-ended, everything moves forward. This in- cludes the vehicle, the seat, the occupant’torso and the occupant’ head. s s However, there is differential motion. The car and seat move fastest, the torso initially moves more slowly, and the head moves slowest of all. Consequently, the occupant suffers the sensation of “sinking back” into the seat, while their head suffers a rearward rotation (this is the origin of the so-called “whiplash” mechanism). As the torso sinks back into the seat (but actually moving for- ward), the seat compresses like a spring. Ultimately, this compression stops and the torso reaches the same speed as the car. Thereafter, the seat “ unloads” and acts as a damped spring, resulting in the torso moving forward faster than the car. As a result of this “ bounce,” seat the torso can move forward up to about 1.3 times the speed change that the car experiences due to impact. Thus, if the car experiences a velocity change of 5 mph due to a rear impact, the torso can end up moving at up to 6.5 mph. Whether the occupant actually experiences this increase in speed depends on the lockup behavior of the seat belt, especially the shoulder belt. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 21
  22. 22. This composite illustrates the motion of an occupant during a rear-end impact. A potential for injury occurs when the head is fully rotated backwards [6], known as hyperextension, or fully rotated forward [8], known as hyperflexion. A properly positioned head rest can reduce the amount of backward head rotation and hence reduce the potential for head injury. Once the severity of the collision has been determined, then this value can be taken and compared to human injury tolerance levels. Results from daily activities or volunteer exposure to staged low-speed collisions can be used. 1. Daily Activities: The loads that an occupant was subjected to during a collision can only be compared to daily activities if the direction, duration, and magnitude of the loads are the same. For short time durations, less than a second, human injury tolerance is sensitive to the time duration that the load is applied. A higher load can be applied without injury if the time duration is shortened. If an individual jumps off of a table on to a solid floor, then he or she will be subjected to a fairly high deceleration on impact. But injuries typically do not occur because time duration of the impact is of the order of a 1000th of a second. Tests carried out with amusement park bumper cars reveal that, during collisions, vehicle velocity changes as high as 5 m.p.h. can occur. The time interval of these impacts is comparable to the typical duration of a low speed automobile impacts. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 22
  23. 23. 2. Volunteer Exposure: Low-speed collisions with human volunteers have been quantified in the chart below. It should be noted that in the staged rear-end and lateral collisions, none of the reported symptoms lasted longer than three days. Several interesting trends can be observed from their results. In the staged rear end collisions, symptoms started to be reported at collision severities in the 4 to 5 mph range. In the frontal and lateral staged collisions, symptoms started to be reported at about 10 mph. Results indicate that the injury threshold level for a frontal collision is greater than for a rear end collision. This is consistent with typical real life low speed rear-end collisions where occupants of the struck vehicle report injuries, but the occupants of the striking vehicle do not. When using results from volunteer exposure tests, an investigator should be aware of the following limitations: 1. Most of the staged collisions are bumper to bumper impact and not override/underride impacts. 2. Most of the volunteers have been male. 3. Most of the volunteers face forward at the time of impact. 4. Most of the volunteers are under 50 years of age. 5. Most of the volunteers are seat belted. 6. Most of the volunteers do not have preexisting conditions. 7. Most of the volunteers are mentally prepared for the impact. 8. It is difficult but not impossible to insure that a volunteer is subjected to an unexpected impact. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 23
  24. 24. The table below combines MacInnis Engineering's vehicle damage threshold and volunteer exposure results. From the table it can be seen that in a rear-end impact it is possible for occupants to sustain symptoms when the vehicle has no visible damage. SUMMARY AND CONCLUSION Use of results from staged low speed collisions, conducted at known speeds, it is possible to determine collision severity of a real life low speed collision. The determined impact severity can then be compared to results from staged low speed collisions to determine if an injury threshold has been reached. Volunteer exposure tests give a good indication of the collision severity at which symptoms, typically lasting less than 2 to 3 days, start to appear (i.e. the injury threshold). Because human volunteers are used, the severity of the staged collisions is typically not increased beyond the injury threshold severity. Therefore, there is currently little or no data to indicate the relationship between injury severity and collision severity above the injury threshold. Accident reconstruction and biomechanical analysis with their respective mathematical interpretations provide the basis to assess vehicle damage and calculate injury loads sustained by the occupants. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 24
  25. 25. Vehicle damage photographs taken at ground level and with the aid of an aerial boom are important in biomechanical analysis and accident reconstruction. These photographs permit detailed analysis of crush and principle direction of force. Post production photographic techniques can be used to compare the damaged subject vehicle with an undamaged exemplar vehicle. An accident reconstructionist and biomechanical investigator should keep in mind their job is to record facts necessary to mathematically analyze an accident. Some data or clues are perishable and must be obtained or photographed early. Each occupant has a unique level of injury and experiences unique crash loads depending on their location and other accident factors. The purpose of the biomechanical analysis is to determine means and rationale for injuries and improve the chances of survival in future collisions. REFERENCES 1. Daily, John, “Fundamentals of Traffic Accident Reconstruction”Insti- , tute of Police Technology and Management, Jacksonville, FL, 1988. 2. Boddorff, Thomas C. and Ian S. Jones, “Simple Overhead Photography Techniques for Vehicle Accident Reconstruction”S.A.E. Paper No. 900370, , Society of Automotive Engineers, Warrendale, PA, 1991. 3. Baker, J.S., Traffic Accident Investigation Manual, Northwestern University, Evanston, IL, 1975. 4. AI Tools, AR Software. Trantech Corporation, Redmond Washington. 5. McElroy, Robert C., “ Aerial Crush Photography & Analysis For Acci- dent Reconstruction”Special Problems in Traffic Accident Reconstruction, IPTM, University of North Florida, 1994. 6. Fox, Roy G., “Helicopter Crash Survival Investigation”Proceedings of , 23rd International Seminar of the International Society of Air Safety Inves- tigators, Dallas, TX, 1992. Biomechanical Trauma Course on Biomechanics of Motor Vehicle Accidents -- Univ. of Miami & Lynn Univ. -- 2/2000 25