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Low Speed Accidents - Correlating Vehicle
Impact Speeds Using Collision Damage Analysis and Expected Injuries - A Case
Study Introduction Using vehicle crush damage to determine the speed of a vehicle in a collision can be done within certain limitations. Crush damage analysis is dependent upon having reliable crash test data for a specific vehicle. Crush damage analysis can be very accurate if the accident vehicle impacts with a fixed object; such as a concrete bridge abutment, pole or tree. The analysis is accurate as the majority of the impact energy is conserved in deforming the vehicle. There is energy in the rebounding of the vehicle from the fixed object, restitution of the deformed areas, vibration and the generation of heat through the collision. Conversion of energy to heat and lost in vibration is considered minor. In a two vehicle accident, considerable energy is conserved not only in the deformation of the two vehicle�s structures, but in the post impact movement of the vehicles to rest. In this case, the energy expended beyond that found in the deformation of the vehicles is not minor. This energy value can greatly exceed the energy conserved in the crush. It is on this premise that any attempt to determine collision speeds between two vehicles, as in a rear-end type collision, using only crush profiles as the known values, will produce a speed that is lower than is actual and may be significantly lower. The Accident Scenario The case at hand is a two vehicle, in-line accident. The lead vehicle was a 1994 Toyota Corolla. The second vehicle is a 1994 Chevrolet. The Corolla stopped to allow pedestrians to cross at a marked crosswalk when it was struck in the rear by the Chevrolet. Both vehicle�s weigh approximately the same. Both drivers state they were using seatbelts and shoulder harnesses. The police report diagram shows the two vehicles in contact with each other at the crosswalk. The investigating officer did not take any measurements or photographs of the scene. No pre- or post-collision tire marks are reported. A precise point of impact, and the points of rest for each vehicle is not indicated. Various estimates are provided by each driver but cannot be independently confirmed. Both vehicles were moved from the traffic lanes prior to the arrival of the police. Damage estimates of less than $500 for each vehicle are provided in the police report. Photographs of the Corolla have been provided. Two photographs were taken of the rear of the vehicle. These photos depict a limited amount of damage. No photos were taken of the Chevrolet before it was repaired. The driver of the Corolla has claimed injuries. Injuries in the police report indicate "possible injuries" for the driver of the Corolla and "no injuries" for the driver of the Chevrolet.. The driver of the Corolla was transported from the scene by ambulance, treated and released the same day from a local hospital. Defense Expert A report was prepared by an expert for the defense. The opening paragraph of the report identifies the vehicles, drivers and the accident scenario contained in the police report. The expert also reviewed the two photographs of the Chevrolet and medical records of the driver of the Corolla. The defense expert�s report states: "I inspected the vehicle photographs in order to assess the crush damage to the Corolla. Using this information, the data on the physical characteristics of the vehicles and the laws of physics, I calculated that the Chevrolet was traveling forward at a relative, or closing, velocity less than or on the order of 10.5 miles per hour when it made contact with the rear of the Corolla." The remainder of the report uses these speeds to calculate change of velocity (delta V), acceleration and forces applied to the vehicle occupants as a result of the collision. It goes on to detail the forces applied to various parts of the occupant's bodies and opines that injuries could not be sustained at the calculated force levels. Analysis of the Defense Expert Report The defense expert has very limited information to work with. This is stated in the list of items reviewed. The only evidence as to impact speed are the photographs of the damage to the one vehicle. Crush damage analysis to determine impact speed is a viable method under certain conditions. It is not viable under the conditions of an in-line collision between two vehicles. Crush damage analysis is based on the "Conservation of Energy" formula. This principle of physics states that all energy in a collision must be conserved, that is, it cannot be created or destroyed. All the energy in the collision must be conserved. The basic formula is: Total Energy at Impact = Total Energy After Impact + Energy to Crush Vehicles It does not state that all energy in the collision will go to damage. Energy that is not conserved in the damage will be transferred to the post-collision movement of the two vehicles, or be dissipated as heat or vibration. Energy converted to heat and vibration in the collision cannot be accurately quantified. A further breakdown of the formula is: Energy of Vehicle 1 at impact + Energy of Vehicle 2 at Impact = Energy of Vehicle 1 after Impact + Energy of Vehicle 2 after Impact + Energy to Crush Vehicle 1 + Energy to Crush Vehicle 2 The energy conserved through the post-collision movement of the vehicles can be substantial. Any movement of the vehicle requires energy. The most basic of formulas used in the investigation of traffic accidents, the skid to stop formula, is an energy formula. It relies on the principle that the faster is a vehicle is traveling when the brakes are applied, the greater the distance the vehicle will skid before coming to a stop. The energy of the vehicle is conserved by the friction between the road and the tires. The skidding tires generate heat dissipating the vehicle's energy from the forward motion. Determining the energy required to crush the vehicle any amount is based on controlled tests conducted with exemplar vehicles. Crash tests are routinely conducted at speeds in excess of 30 mph and the crush values for lower speeds are extrapolated from these values. But at lower speeds there is a the factor of "restitution" that must be considered. At higher speeds, with the bending of body parts that do not return to their original configurations, restitution can be largely ignored as the value is very small. In lower speed impact, where major body panels are not significantly deformed, this value cannot be ignored. The number of vehicle to vehicle crash tests at moderate impact speeds is limited. There are a large number of variables that are not completely understood. Controlled crash tests, conducted by the U.S. Government, utilize fixed barriers that do not deform. Some of these barriers have a large flat surface and other tests are conducted into angled barriers. Some of the data recorded from these tests is questionable as limited data was recorded. Some barriers were outfitted with 3/4" plywood sheets on the front to protect the barriers. In these tests, the plywood acts to absorb some of the impact energy. Using this in low and moderate speed tests could severely affect the test results. Extrapolating standard barrier crash test data to limited damage speed analysis has been shown to have large error ratios. Manufacturers of computer programs that use the conservation of energy formulas, and rely on crash test profile data, warn the users that the results of the programs may not be reliable under low impact speed conditions. The various auto manufacturers, insurance groups (I.I.H.S, Insurance Institute for Highway Safety), and government highway safety organizations (N.H.T.S.A., National Highway Traffic Safety Administration and Transport Canada) perform controlled car crashes to find the crashworthiness for vehicles and the effects on the occupants. These tests are normally at 30 or 35 m.p.h. into immovable concrete barriers, also called a rigid barrier. The primary purpose of these tests is to determine the effectiveness of the occupant protection systems. Crash tests conducted by NHTSA comply with Federal Motor Vehicle Safety Standard (F.M.V.S.S.) 208. This standard provides the maximum measured loads on test dummies, such as the Hybrid III. In 1995, NHTSA began a series of tests under the New Car Assessment Program (N.C.A.P). This is not a pass/fail test and the results are not obligatory to the auto manufacturers. The tests are conducted at higher speeds than the F.M.V.S.S. tests, involving an energy increase of approximately 36%. The N.C.A.P. results are published under a star rating system with five stars indicating a very good response, less than 10% probability of serious injury as a result of the collision profile, to a one star, indicating a greater than 45% probability of injury. Although the N.C.A.P. tests do not require a passing grade, the adverse publicity of a poor rating could damage sales and this has effectively forced manufacturers to self-guarantee a good crash response. The N.C.A.P test is centered on the provisions of providing protection to the occupants. Since they are performed at speeds of 30 to 35 m.p.h., with limited directions of impact, they do not yield sufficient data to characterize the overall behavior of the vehicle in real world crash conditions. There is no direct correlation between crash performance at high speeds, 30 m.p.h. and higher, and crash performance at lower speeds. There are cases where improvements in high speed crashworthiness, such as stiffer vehicle structures, increased the risk of occupant injuries in lower speed collisions. Some vehicles that perform well in full frontal crashes, as in the F.M.V.S.S. tests, do poorly in off center impact tests as have been conducted by I.I.H.S. Traditional traffic accident reconstruction methodologies concentrate on high speed collisions, those with serious injuries and significant damage profiles. Accident reconstruction programs, such as the CRASH3 program developed by N.H.T.S.A. for the Fatality Accident Reporting System, are incapable of analyzing collision forces at low speeds. Although low speed collisions rare involve immediately visible serious injuries, the number of such accidents have a significant socioeconomic impact. It has been reported that about 20% of all accidents handled by the police involve rear-end collisions, many at low speeds. While this work deals with "low speed" accidents, it must be remembered that this term means any impact that results are minimal vehicle damage. The term "low speed impact" is specific to the relative speeds between two objects at the moment of first contact. Mass, Energy, Momentum and Acceleration Sir Isaac Newton is generally regarded as the father of physics. He formulated three basic laws of motion; as follows: 1. Every body at rest tends to remain at rest while every body in motion tends to remain in motion unless it is acted upon by an unbalanced outside force. This law describes inertia. If you are riding in a car that stops suddenly, the occupant will feel the inertia that tends to resist the stopping. 2. The acceleration of any body is directly proportional to the force acting on the body, while is it inversely proportional to the mass of the body. There are a number of terms used that must be defined; inversely proportional, directly proportional and acceleration. Acceleration is the rate of change over time. Acceleration is expressed as positive, for an increase in velocity, or negative, for a decrease in velocity. Now assume you have an object, A, that is directly proportional to another object, B. Any increase in B results in an equal increase in A. The first part of Newton�s second law states that the "acceleration of any body is directly proportional to the force acting on the body." If a force acting on the body is increased then the acceleration of the body must increase and if the acceleration is increased then the force must increase. For an inversely proportional situation, any increase in A will result in a equal decrease in B. 3. For every force exerted on a mass by another mass, there is an equal but opposite force reacting on the first mass by the second mass. Mass is the amount of matter an object contains. It is different from weight. Weight is how strongly the earth attracts an object of a given mass. If you were to take an stone, weighing 20 pounds on Earth, to another planet with a gravity one-half that of Earth, the stone would weigh 10 pounds on that planet. But the mass of the stone, that is how much stone is present, would be the same. According to Newton�s third law, if the 20 pound stone is placed on a table, then the table is responding with an upward force of 20 pounds; equal and opposite to the stone�s weight. Energy and momentum are both quantities of motion. Kinetic energy is the ability to do work and is possessed by any body in motion. Mechanical work is a force acting through a distance. If an object has a certain amount of kinetic energy, work had to be done to give it that energy. Put another way, to get a stationary vehicle to move requires energy and to get a moving vehicle to stop requires energy. Momentum is a concept of physics that can be confused with energy since both energy and momentum are defined as quantities of motion. Given a certain force, and that force acting through a distance, you have energy. But, given the same force, acting through a period of time, you have a quantity of momentum. Conservation of Linear Momentum When a nail is struck by a hammer, it is the effect of the changing momentum of the hammer that imparts the force required to drive the nail. The energy of the hammer deforms the head of the nail. The energy that does not result in deformation is converted to heat. When two objects collide, the momentum gained by one equals the momentum lost by the other. The impulse of the force is calculated by multiplying that force by the length of time it acts. The impulse, or time rate of change of momentum, is dependent not only on the magnitude of the force, but on the length of time the force acts. In a two vehicle collision, it is not the energy of the collision that acts upon the occupants, but the impulse. The energy of the collision is conserved primarily by physical damage; the momentum is conserved by the movement of the vehicle through the impulse. Theoretically, it is possible to measure the exact movement of all the component structures of a vehicle during a collision and then determine the total collision energy. In reality, the determinations of energy in a two vehicle collision is so complex as to be impossible. Even with thousands of controlled crash tests, the variables within vehicle structures makes it impossible to determine the collision energy within a useful range. The best efforts have an uncertainty of 20 to 30%. Bumpers were first introduced on railroad cars. The primary purpose was the protection of the cargo by reducing the accelerations commonly incurred during coupling and uncoupling and not protection of the railroad car itself. Automotive bumpers are designed to protect the vehicle itself, without regard to the accelerations on the cargo, e.g., passengers. A bumper works by virtue of energy consideration rather than consideration of the momentum. To avoid structural damage, a bumper must be capable of containing the energy of an impacting vehicle below a certain value. This is accomplished primarily with bumper impact shock absorbers. These units store the initial impact forces by means of a spring, gas or hydraulic system. As the resisting force of the impact vehicle is relieved, following the initial compression, there is a significant bounce. This bounce, or rebound, is clearly visible in any controlled crash test as the vehicle comes to rest some distance back from the barrier. In the early 1970�s, there was a standard for bumpers to be able to withstand an impact into a barrier at 5 mph without sustaining damage. This requirement was dropped in 1975 and replaced with the current standard, FMVSS 581. � 581.5 Requirements. a) Each vehicle shall meet the damage criteria of �� 581.5(c)(1) through 581.5(c)(9) when impacted by a pendulum-type test device in accordance with the procedures of � 581.7(b), under the conditions of � 581.6, at an impact speed of 1.5 m.p.h., and when impacted by a pendulum-type test device in accordance with the procedures of � 581.7(a) at 2.5 m.p.h., followed by an impact into a fixed collision barrier that is perpendicular to the line of travel of the vehicle, while traveling longitudinally forward, then longitudinally rearward, under the conditions of � 581.6, at 2.5 m.p.h. The specific performance requirements are contained in FMVSS 581.5 (c)(8). The exterior surfaces shall have no separations of surface materials, paint, polymeric coatings, or other covering materials from the surface to which they are bonded, and no permanent deviations from their original contours 30 minutes after completion of each pendulum and barrier impact, except where such damage occurs to the bumper face bar and the components and associated fasteners that directly attach the bumper face bar to the chassis frame. The definition of "bumper face bar" is in section 581.4. All terms defined in the Motor Vehicle Information and Cost Savings Act, Pub. L. 92�513, 15 U.S.C. 1901�1991, are used as defined therein. Bumper face bar means any component of the bumper system that contacts the impact ridge of the pendulum test device. The concept of no vehicle damage from the test impacts is not true. Damage is allowed and the bumper is still acceptable. Even without damage, a vehicle significant momentum can be transferred. It is the momentum of the collision, not the energy, that causes injuries. If the impacted vehicle does not move, the occupants do not move and injuries are not possible even though there can be significant vehicle damage. The opposite is also true, there can be movement of the vehicle, and consequently movement of the occupants resulting in injury with no damage to the vehicle. The barrier test does not replicate conditions of real impacts between two vehicles. The barrier is flat and the surface is not steel. The face of the barrier is covered by a sheet of plywood to prevent damage to the barrier. This plywood sheet is damaged in the impact and absorbs some of the impact damage. A vehicle to vehicle collision is seldom in a perfectly straight line, as in the barrier tests, and the impacting vehicle does not have a large, flat surface. Real-world impacts between vehicles includes variables such as eccentric collision forces, one vehicle off line of the other to some degree. Vehicle occupants are not always sitting in the best position, facing straight ahead, as are the test dummies. Comparing real-world accidents to barrier test impacts with test dummies is like comparing apples and oranges. There are many variables that cannot be resolved between the collisions. Biomechanics The 1953 Mercedes-Benz model 180 is considered the first vehicle to incorporate crashworthiness measures to protect the occupants. The car was constructed to absorb energy by means of a deformable front structure and the interior was padded with energy absorbing materials. The concept was to reduce the acceleration forces on the occupants and thereby reducing the injury levels. There is limited knowledge about injury mechanisms. Impact biomechanics differs from "classical biomechanics" by dealing with transient events of very short duration; less than 100 milliseconds. At such loadings, human tissues have a different set of properties. There is a substantial knowledge base on structural injuries to bone tissue but there is a lack of data on injuries to the joints and functional injuries in general. Viano4 listed the state of knowledge on various regions of the human body. The skull, face and thoracic regions propensity to injury are fairly well understood; understanding of injuries to the spine and brain are hypothetical or largely not understood. Injuries which have been generated in a car collision are due to the deformation beyond the failure limits of tissues. Such deformations either result in damage to the anatomic structures or alteration in normal functions. Structural injuries are likely to heal without long term effects. Functional injuries to the nervous system are of concern as there is a marked risk the function will never return. There are three ways to cause injuries: 1. Body compression when the deformation exceeds elastic tolerances. 2. Body acceleration when the motion of the internal organs lags behind the motion of the skeleton resulting in injuries due to bursts of the relative organ. 3. Exceeding viscous tolerance levels. Biological tissues are viscouelastic, their response and tolerance to forces is time sensitive. For example, if a fluid filled organ is compressed at a slow rate, the applied energy is absorbed without damage. If the organ is compressed rapidly, a rupture may occur before a sufficient change in shape takes place. Research has shown that injury levels, for certain regions of the body, can be predicted using viscous criterion but the knowledge base is extremely limited. Correlating Damage and Predicted Injuries It is not possible to match damages to a car to the forces imparted on an occupant. Crash test data is limited to the test protocols. Comparing a crash test, as with a vehicle into a rigid barrier, to a collision between two vehicles, neither one fixed in place, results in the conservation of the energy by the movement of the impacted vehicle. Any energy conserved through the movement of the vehicle is not available to create a damage profile. Examination of a vehicle that has been impacted in line with another vehicle will not reveal the impact forces. Without specific information on the forces imparted to the occupant, and their seated position, it is not possible to state what injuries they should or should not have received in the collision. Reasonably accurate information regarding the pre-collision trajectories, collision positions, post-collision trajectories and positions of rest for each vehicle is normally not available except in fatality accidents. A Volvo Safety Report, "An in-depth study of neck injuries in rear-end collisions," presented at the 1990 IRCOBI Conference, states: "The results of this study also indicate that the shape of the impact pulse has a greater influence on the severity of the neck injury than the amount of transferred energy. It is reasonable to believe that during the first part of the acceleration, a 'stiff' impact pulse will cause greater movement of the cervical spine before the head hits the restraint than a 'soft' impact pulse."
Figure 1 shows the relationship of a stiff, or time short, impact pulse to a softer, or time long, impact pulse. Both impacts are of the same magnitude but the stiff pulse results in injuries and little, or no, damage. Recent Developments In late 1998, the Volvo Car Corporation introduced the WHIPS seat in their Model S80 sedan. This seat is designed to rotate backwards slightly during a rear-end impact. The rotation of the seat during the collision reduces the forces applied to the occupant. The seat is designed specifically to reduce neck injuries. The Volvo Whiplash Protection Study, which is the basis for the design, was conducted over a ten year period. Volvo researchers followed the occupants of their vehicles involved in rear-end collisions for extensive periods. The report states, "...injury risk is shown to be almost constant irrespective of the degree of vehicle deformation. Severity measures based on deformation depth are not good predictors of neck injury risks."
Figure 2 - Neck injury risk compared to collision damage deformation depth. Volvo Car Corporation Safety Report, 16th ESV Conference, Paper No. 98-S7-O-08. This research is consistent with evidence found by other researchers that indicated people suffered neck injuries even in impacts with very low severity. Continuing studies on the WHIPS seat system, as the seat is introduced into the general vehicle population, will reveal the ability of the seat system to reduce some of the more common low speed impact injuries. |
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