FORENSIC ENGINEERING ANALYSIS of MULTI-VEHICLE IMPACT SEQUENCE (original) (raw)
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Methods of reconstructing motorcycle collisions have traditionally been limited to speed from skid marks, speeds from scrapes or gouges, speed from rider ejection, speed from linear momentum, or sometimes speed from witness observations. Oftentimes, the data necessary for analysis is either misunderstood or misinterpreted. This paper tests the applicability of using rotational mechanics and specific models for motorcycle front fork deformation and vehicle deformation when determining motorcycle impact velocity. Additionally, the results of these methods are statistically tested for significance and reliability against independent motorcycle impact test data. NAFE Journal June 2012
MASS PROPERTIES and AUTOMOTIVE CRASH SURVIVAL
74th SAWE International Conference, Alexandria VA, 18 May 2015
Problems in dynamics may be solved by any one or more of three basic methods: force and acceleration (F=m a), work and kinetic energy (Fd=½ mV^2), impulse and momentum (〖Imp〗_(1→2)=mV_2-mV_1); these are just different ways of looking at a common underlying reality . The method(s) used to investigate a particular dynamics problem depends upon the specific nature of the problem. Problems involving that most severe form of automotive longitudinal deceleration, crashing, are no exception. Even at the most elementary level, as represented by the previous equations, the unifying role of mass properties is evident. Notable in the basic formulae of all three methods for the solution of problems in dynamics is the common parameter “m” (mass). However, this represents just the “tip of the iceberg”; at the detailed level representative of actual engineering problems the full role played by mass properties is often revealed to be far more complicated than that indicated by such simple basic equations. For instance, an automobile traveling at a particular velocity will possess a certain amount of kinetic energy which must be dissipated for the vehicle to come to a stop. The dissipation can be controlled and orderly as in the case of braking a car to a stop at an intersection, or it can be somewhat more violent as in the case of a collision with a concrete abutment. In both cases the outcome is directly dependent upon the magnitude of the kinetic energy involved. Initially the mass properties involvement seems to be very simple: the kinetic energy of any body of mass “m” moving at a velocity “V” is expressible as “½ mV^2”; to come to a stop that energy must be dissipated through the work done by a deceleration force “F” times the distance “d” traveled during the deceleration. However, the kinetic energy possessed by an automobile is much more than would be indicated by a simple determination of its mass “m” from its weight (“m= W/g”). Many components of an automobile possess not only translational kinetic energy, but rotational as well. Thus the simple mass “m” is not the appropriate value needed for kinetic energy determination; there is a greater value “me”, termed the “effective mass”. The calculation of “me” involves the rotational inertia of such components as the wheels, tires, brakes, shafts, bearings, etc. Thus not only the mass of the automobile as a whole, but that of various components, come into play when calculating the amount of kinetic energy which, in turn, determines the magnitude of the deceleration forces required to affect a complete stop in a certain distance. When the deceleration is a matter of braking, certain other vehicle mass properties come into play: the vehicle longitudinal, lateral, and vertical CG. When the deceleration is a matter of crashing, then the vehicle mass density and mass density distribution also have significance. The purpose of this paper is to make explicit the exact role that all the mass properties play in determining the automotive deceleration performance during a crash. This has a direct bearing on the survivability of a crash, which can be enhanced through thoughtful mass properties engineering.
Modern methods for analyzing motor vehicle deformation rely upon a force-deflection analysis to determine deformation work energy. Current methods provide acceptable accuracy when calculating the velocity change of vehicles involved in a collision but require significant modification to accommodate oblique and low-velocity collision events. The existing algorythms require vehicle-specific structural stiffness coefficients for each colliding vehicle, determined from full-scale impact testing. The current database of vehicle structural stiffness values is generated mainly through government safety standard compliance testing and is quite extensive for frontal impacts involving passenger cars and many light trucks and SUVs. However, the database is devoid of specific crash testing necessary for deformation analysis of rear and side structures of many vehicles. Additionally, there remains a dearth of structural stiffness coefficients for heavy commercial vehicles, buses, recreational vehicles, heavy equipment and motorcycles, rendering the application of the current force-deflection analysis approach useless for many impacts involving such vehicles. The research presented, known as the Generalized Deformation and Total Velocity Change System of Equations, or G-DaTA∆V™, develops an accurate, reliable and broadly-applicable system of equations requiring knowledge of the structural stiffness coefficients for only one vehicle, rather than both vehicles involved in a collision event, regardless of the impacted surfaces of the vehicle. The developed methodology is inclusive of non-passenger vehicles such as commercial vehicles and even motorcycles, and it also accommodates impacts with objects and surfaces not supported by the current structural stiffness coefficient database. The G-DaTA∆V™ system of equations incorporates the linear and rotational collision contributions resulting from conservative forces acting during the impact event. The contributions of the G-DaTA∆V™ system of equations are as follows: 1. Consideration of non-conservative contributions from tire-ground forces and inter-vehicular frictional energy dissipation commonly present during non-central collision configurations. 2. Ability to solve for collision energy of a two-vehicle system using a single structural stiffness for only one of the colliding vehicles using work/energy principles. 3. Determination of the total velocity change for a vehicle resulting from a given impact event, which results from conservative and non-conservative force contributions. 4. The ability to predict the time period to reach maximum force application during an impact event, allowing for the determination of the peak acceleration levels acting on each vehicle during an impact. The results of applying the G-DaTA∆V™ to full-scale impact tests conducted as part of the RICSAC collision research will be presented. Additionally, analysis of real-world collision data obtained through the National Automotive Sampling System demonstrates a close correlation with the collision values recorded by the vehicle event data recorders (EDRs) as part of the supplemental restraint system airbag control moducles