Validation of Damage Analysis in Low Speed Impacts

Questions/Topics related to Damage and Crush Analysis
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MSI
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Validation of Damage Analysis in Low Speed Impacts

Post by MSI »

Q: I’m involved in a low speed case and the opposing expert has said in so many words that using crush (based on NHTSA test data) isn’t real reliable and shouldn’t be used. I’m looking for papers that validate crush (i.e.-looked at crash tests with known speeds, did crush analysis, then compared them to each other and have listed the percentages of accuracy). Are you aware of any studies like that?

A: Probably a good place to start since it is a low speed impact is our restitution paper:
  • Abstract:Effects of restitution on damage interpretations are compounded by the fact that restitution reduces the residual deformation while increasing the total impact speed change. This paper presents a revised analytical procedure to include restitution effects for the CRASH program and refinements to the restitution modeling within the SMAC program. The conversion of vehicle impact test results into inputs for the two revised programs is also included. The effects of the refinements to the damage analysis procedures on reconstruction results are illustrated by direct comparisons with corresponding results produced by the original SMAC and CRASH programs and with measured data from full scale vehicle impact tests.
In the paper we demonstrate that due to lack of consideration of restitution in damage analysis, you will tend to underestimate the speed change in a damage analysis particularly in low speed collisions due to restitution. The reason I point that out is that it may be the ‘worst case’ scenario for damage analysis.
Since you have a low speed impact you are in the restitution phase for damage analysis and so need to consider it.
Therefore be sure to consider a “range of speeds” in lower speed impacts due to restitution.

In addition, there have been a lot of papers on low speed impacts, restitution, damage analysis, for example: Getting back to basic damage analysis, some validation papers on damage analysis:
  • Further Validation of EDCRASH Using the RICSAC Staged Collisions, SAE paper 89-0740
    • I caution you that the creative 'comparison' techniques for the EDCRASH trajectory analysis validation by EDC is somewhat sketchy, see Validation for vehicle to vehicle collision models? for a discussion of problems with their use of ‘combined impact speeds’ and ‘95% confidence’ to perhaps hide 40% error problems with the trajectory solution of EDCRASH. However i think the damage analysis validation in the paper isn't subjected to that same treatment.
    Neptune has made a cottage industry out of crush coefficients, vehicle specs and damage analysis techniques, some of which may be useful and/or what you are looking for in your situation.
It's been a several years since we've done research in the specific area of damage analysis (we are working on some other things) so i haven't recently reviewed all these references so be sure to read each to form your own opinion as to the veracity and usefulness of the information contained in the papers.
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MSI
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Re: Validation of Damage Analysis in Low Speed Impacts

Post by MSI »

March 2015: In another forum a thesis and a paper from the 90's were brought to our attention which may be of interest for low speed impact analysis.
The paper:
  • Low Speed Rear Impacts and the Elastic Properties of Automobiles
    D.P. Romilly, R,W Thomson,Departmenot of Mechanical Engineering, F.P.D. Navin, M.J. Macnahb
    • Abstract
      • Many low speed, rear impact accidents produce occupant neck injuries which have become a concern to the insurance industry and the medical profession. The authors,with the cooperation of the Insurance Corporation of British Columbia (ICBC), conducted approximately 30 low speed pendulum tests to measure the elastic and plastic properties of selected vehicles in rear end collisions. Displacement/time traces were generated from high speed video recordings of elected points on the vehicle and an anthropometric dummy occupant.The authors noted that a high impact speed
        was required to produce perceptible damage to the test vehicle. This speed caused violent movement to the test dummy neck. Discussioon of these results with insurers indicates there is a conflict between bumper stiffness (required for the Canadian 8 km/h bumper standard) desired by the material damage section of the insurance industry, and the need for vehicle compliance for occupant protection.Investigation into the vehicle's elastic and plastic properties will also aid in the design of Civil Engineering roadside structures and provide a better understanding of injury causation at low impact speeds
and The thesis
  • An investigation into low speed rear impacts of automobiles
    Author: Thomson, Robert William, Degree Master of Applied Science - MASc, Program Mechanical Engineering, Date: 1990
    Abstract:
    • A substantial number of whiplash injuries are reported for motor vehicle accidents which produce little or no structural damage to the automobile. These injuries are predominantly associated with rear-end type accidents affecting passengers of the struck vehicle. Since passengers of the striking vehicles are not reporting as many injuries for the same accidents, occupant and vehicle dynamics experienced during low speed-rear impacts were proposed to be a major source of the whiplash claims. A review of previous research revealed that little information exists for this type of accident. In general, vehicle safety research and government regulations have been directed towards occupant mortality - not injury - in frontal collisions. Occupant dynamics research has been limited to sled testing, using modified seat structures, or out-of-date vehicle models. Full scale, rear impact, crash testing has concentrated on high impact speeds (above 30 km/h) where significant structural deformation occurs. A research program was designed to investigate the occupant and vehicle dynamics during low speed - rear impacts. Experimental research was undertaken to document the structural performance of vehicles, noting the impact speeds necessary to initiate the crush mechanisms in the rear portion of the vehicle. To facilitate this testing, a pendulum impactor, based on the government test procedures, was designed and built to consistently reproduce impact speeds below 20 km/h. A total of 56 rear impact tests were conducted with 1977-1982 Volkswagen Rabbits. The vehicle wheels were locked to represent a vehicle stopped in traffic - the most commonly reported whiplash producing accident. An anthropometric test dummy was used to represent a front seat passenger during the tests. High speed video recordings of the tests were digitized to provide kinematic information on the occupant and vehicle response. Accelerometers were incorporated into the last 24 tests to monitor the acceleration levels at the bumper mount, seat mount and within the dummy. Information obtained from this testing suggested that permanent structural damage was only visible when an impact speed between 14 and 15 km/h was experienced by the vehicle. Very little frame deformation occurs for impact speeds below this value. Below this threshold, the vehicle frame can be considered rigid; vehicle response being dominated by the compliance of the bumper and suspension systems as well as sliding of the locked wheels. The accompanying occupant response was a differential rebound of the head and shoulders off the seatback and head restraint. This relative motion between the head and torso was evident in each test and increases the potential for injury. Typical occupant response observed consisted of an initial loading and deflection of the seatback due to the occupant's inertia followed by the release of this stored spring energy as the occupant was catapulted forward. It is this elastic behaviour of the seatback which is the likely cause of whiplash injury. Resulting head velocities were found to be in the order of 1.5 - 2 times the resulting vehicle speed. Initial occupant postures which increased the distance between the torso and seatback tended to increase the dynamic loading experienced by the passenger. Analytical modelling of the vehicle was initiated as the groundwork for full occupant-vehicle simulation. A finite element model of the vehicle frame, bumper, and suspension was created. Previously obtained empirical information suggested that a non-linear bumper and suspension system connected to a rigid frame would be an acceptable approximation. A parametric analysis of bumper stiffness and braking conditions was conducted in a 30 simulation matrix. General kinematic trends of the tests were observed in the simulations, however, limitations in the material properties introduced a much stiffer response than that experimentally observed. Results from this study show that little protection is offered to an occupant during a rear end collision. Impact energy management within the vehicle may not be adequate to prevent injury. Improved occupant protection requires the highly elastic behaviour of the vehicle frame and seatback to be attenuated. This will eliminate the amplification of vehicle motion through the seatback to the occupant.
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