1996-02-01

Energy and Major Diversion in Accident Reconstruction 960888

Special methods of momentum-based accident reconstruction often are required when path diversions are major, that is, in excess of about 45°.
An energy-based component is well known to be required for the maximal path diversion of 180″; with parallel unknowns the two momentum equations become identical, so that an independent equation based on the observed damage is required. Also-as we demonstrated in a previous paper-in intersection impacts, even with modest path diversions, momentum loss to earth during impact, while classically neglected, appreciably affects the inferred momentum of approach. The present study shows further that, with major diversion and brief travel to rest, conventional neglect of travel during impact may greatly distort the assigned post-impact travel direction and/or distance and, in consequence, the momentum solution.
This paper first checks the validity of “observed” EDSMAC damage, prior to its use as a reference in accident reconstruction, by the reconstruction of speeds and times using elementary physics, for both normal and oblique deformation. Functional peculiarities are discovered in simulations of severe fully symmetric frontal impact.
The paper then checks, against known experimental impacts, the validity of the reference EDSMAC simulations established by Woolley in 1994. Functional peculiarities are discovered in those reference simulations of offset impact.
Checking against the remainder of this reference family, it is shown that for frontal, comer, and side impact the methods of CRASH usually give substantial and sometimes gross errors, mainly due to classical neglect of travel and rebound during' impact, while its extension, CRASHEX, gives results usually quite close to those of EDSMAC.
Comparison of various approaches with the reference cases thus indicates that a fully developed, properly calibrated method of reconstruction can substantially match the reference method of simulation at its best, while sometimes in relevance and always in speed of solution it clearly outperforms the reference.

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