Introduction: Experiments are conducted in which something representing a human strikes (or is struck by) a blunt object Measurements are taken that reflect the likely severity of injury: acceleration, force and deformation over the milliseconds of impact, and summaries of these, such as maximum acceleration The experiment may be focused on the effectiveness of protective equipment An instrumented headform might strike a vehicle exterior or interior, or a helmeted headform might strike a floor, for example
Conditions of an impact in the real world may differ from those in the experiment in respect of speed (which is probably the variable of most interest), mass, stiffness and so on Yet, it will be desired that the results of the experiment are relevant to the real world The Head Injury Criterion (HIC) is a well-known method of summarising the acceleration of a headform; how much will HIC increase by if impact speed is doubled, for example
Theory is fairly primitive, surprisingly The cost of experimental work is appreciable, so data is sparse Maximum use needs to be made of what there is
Theory and implications for data analysis: Suppose we assume an equation for the forces acting between the human and the object (A simple example would be that force is proportional to distance of deformation ) Is it possible to work out the consequences, in the sense of equations for outputs (such as maximum acceleration, HIC, and maximum deformation) in terms of inputs (such as impact speed, mass and stiffness)? For some assumed equations, the answer is yes
The Hunt and Crossley equation is an important example describing human acceleration when striking something deformable such as sheet metal or foam polymer This includes the linear spring as a special case The general form has a type of nonlinearity (a power function with exponent n) and damping If the Hunt and Crossley equation describes the force, then various outputs are power functions of various inputs, with exponents that are determined by n
An important limitation on the applicability of such theory is bottoming out, which refers to great increase of force consequent on (for example) cushioning becoming completely crushed Consequently, as well as danger from high stiffness, there is danger in a cushion (of given thickness) being too soft
Experiments are also conducted in which something representing a human is penetrated by a projectile Measurements might include the length and volume of the cavity representing a wound Theory about the forces acting is different from the blunt injury case, but issues concerning data are very similar Force may be a power function (with exponent q) of the speed with which a projectile is moving through a medium Then distance of penetration is a power function of impact speed (and the exponent is 2 - q)
Examples: Power functions are convenient for data description and analysis: take the logarithm of the input and output variables and a straight line is predicted; estimate the slope; from the slope, n or q can be calculated
Examples of data description and analysis will be given for three types of data
• An instrumented headform (a proxy for a human head) hits something deformable, such as a helmet lining
• A low-mass object hits an instrumented proxy for a human chest, which deforms
• A projectile penetrates into a simulant of human tissue
The usefulness of experiments on injury is increased by data analysis that is based on credible theory Prediction can be made of results in conditions – for example, at impact speeds – that are not directly tested in experiments
Biography
Jeffrey Dutschke has been a Research Fellow at the Centre for Automotive Safety Research (CASR), University of Adelaide, since 2015. He completed his PhD there in 2012, his thesis being on understanding the criteria used to assess head injury in impact testing. On returning to the CASR after experience in a start-up software company in Canada, Jeffrey’s work has included analysis of mass accident data, at-scene crash investigation, mathematical modelling and biomechanics.
Corresponding Author:
J K Dutschke
Corresponding Author’s email:
jeff@casr.adelaide.edu.au |