What is the "Effective Mass" in the CRASH equations?

[MSI]

I have posted pages from CRASH3 manual for those interested in what is the 'effective mass'.

• Please see EFFECTIVE MASS in Momentum

Also see pg 54-57 of 1976 report User's Manual for the CRASH program

• Here are the pages of interest:
  • Effective Mass Concept Related page from 1976 Crash Manual

May 2024: yikes realized that the 2nd post not available to unregistered guests, REGISTER...but put it here for now
For a recent paper which re-examines the "Effective Mass' concept

• "Effective Mass" first presented by Ray McHenry as part of his CRASH program development, see the following:
  • 1976 report pg 54-57 User's Manual for the CRASH program
  • 1975 SAE Paper 750893, pg 10-11 A Comparison of Results Obtained with Different Analytical techniques for Reconstruction of Highway Accidents”

A recent SAE paper which re-examines the "Effective Mass' concept is as follows:

• SAE 2004-01-1181 An examination of the CRASH 3 effective mass concept
  • SAE paper 2004-01-1181, Rose, Fenton & Ziernicki
    The paper is essentially a review and presentation of the concepts originally created and presented by McHenry, see NHTSA CRASH3 Technical Manual
    Abstract:
    • This paper examines the validity of the effective mass concept used in the CRASH3 damage analysis equations. In this study, the effective mass concept is described, the simplifying assumptions that it entails are detailed, and the accuracy of the concept is tested by comparing DVs calculated from the CRASH3 equations to results of numerical simulations with a non-central impact model. This non-central impact model allowed the effective mass concept to be tested in isolation from other assumptions of the CRASH3 program. The results of this research have shown that the effective mass concept accurately models the effects of collision force offset when certain conditions are met. These conditions are discussed, along with their implications for damage interpretation. This paper also presents an analytic expression that relates damage energy to closing speed (initial relative velocity) for the general case of non-central collisions. Equations relating damage energy to closing speed for the case of central collisions have been discussed extensively in the literature. However, a comparable equation for the general case of vehicle-to-vehicle noncentral impacts has not been reported. The effective mass concept is used to generalize the relationship between closing speed and damage energy.
    CONCLUSIONS:
    • This study has confirmed that, as long as a reasonable representative collision force moment arm can be obtained, the effective mass concept accurately captures the effects of collision force offset. The greatest accuracy is achieved by selecting the representative moment arm of the collision force as close to maximum deformation as possible. A damage energy/closing speed relationship was derived for the general case of non-central collisions, where the line of action of the collision forces does not pass through the vehicle centers of gravity. Second, this generalized damage energy/closing speed relationship was integrated with an energy balance equation, via a derived relationship between damage energy and post-impact rotational kinetic energy, to provide a complete set of equations for vehicle speed analysis.

June 2021: Addendum....The SAE paper cited above is based on Nathan's Master Thesis:

• Rose, Nathan A., “An Examination of the Effective Mass Concept for Eccentric Motor Vehicle Impacts in the CRASH3 Algorithm,” Master’s Thesis, University of Colorado – Denver, Spring, 2003.