Automotive Accident Reconstruction

Practices and Principles, Second Edition

Donald E. Struble, John D. Struble

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000764741
• Edition: 2
• Topic: Technik & Maschinenbau
• Subtopic: Bauingenieurwesen

This fully updated edition presents practices and principles applicable for the reconstruction of automobile and commercial truck crashes. Like the First Edition, it starts at the very beginning with fundamental principles, information sources, and data gathering and inspection techniques for accident scenes and vehicles. It goes on to show how to analyze photographs and crash test data. The book presents tire fundamentals and shows how to use them in spreadsheet-based reverse trajectory analysis. Such methods are also applied to reconstructing rollover crashes. Impacts with narrow fixed objects are discussed. Impact mechanics, structural dynamics, and conservation-based reconstruction methods are presented. The book contains a comprehensive treatment of crush energy and how to develop structural stiffness properties from crash test data. Computer simulations are reviewed and discussed.

Extensively revised, this edition contains new material on side pole impacts. It has entirely new chapters devoted to low-speed impacts, downloading electronic data from vehicles, deriving structural stiffness in side impacts, and incorporating electronic data into accident reconstructions

1 General Principles

An Exact Science?

Science is the endeavor of examining the world around us, developing hypotheses that may explain its behavior, testing those hypotheses, and thereby obtaining a deeper understanding of how that world works. Notice that the word “exact” was not used in this description of science. Science seeks exactness, but there are always limits. In fact, Werner Heisenberg pointed out that in the limit, the very act of observing one property degrades our knowledge of another.¹ The best science can do is approach exactness as closely as possible, within the limits of time, money, and practicality.

Engineering is different from science in that it seeks to apply the knowledge of science in the design, development, testing, and manufacture of new things. Certainly, motor vehicles, roads, and roadside appurtenances are engineered things that must be understood by the reconstructionist. Motor vehicle crashes are events out of the ordinary that occur outside of the laboratory (and outside the presence of the reconstructionist), without many (or even all) of the measurement and observation tools available to the scientist. Very often important information is entirely missing.

So, reconstruction is neither exact; nor is it a science. It is partly engineering in that it deals with engineered things. It is also an art, significantly shaped by experience and intuition. It is not the purpose of this book to emphasize this latter aspect, since that is covered more thoroughly elsewhere, although certain practices and observations from the authors’ experience will be introduced where they may be helpful. Rather, it is hoped that fundamentals essential to reconstruction will be set forth, and illustrative examples included, so that the reconstructionist can put numbers on things and ensure that his opinions are consistent with the physical evidence and the laws of physics and are therefore as close to the truth as he or she can make them. After all, it was Sir William Thompson, Lord Kelvin, who said, “When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced it to the state of science.”² So in this book, we will be concerned more with the quantitative than the qualitative.

Units, Dimensions, Accuracy, Precision, and Significant Figures

To put numbers on things, we must speak a common language. It is the case that the Système International d-Unitès (International System of Units), or metric system, abbreviated SI, has been adopted and used throughout the world. However, reconstructionists must speak the language understood by nontechnical persons such as judges, juries, and attorneys. This requires compromises. For example, the metric unit of force is the Newton (not the kilogram), but these authors have yet to encounter a common force-measuring device, the bathroom scale, that reads in Newtons. It is also the case that the English speakers populate the majority of court rooms around the world. Indeed, the Technical English System of Units is the language spoken by most reconstructionists, which will be used in this book.

The base units used in this book are force (pounds, abbreviated lb), length (feet, abbreviated ft), and time (seconds, abbreviated sec). Metric equivalents will be provided on occasion. In vehicle crashes, times are often discussed in milliseconds (thousandths of a second, abbreviated msec). Derived units are obtained from the base units. For example, area is a measurement derived from length and is reported in square feet (abbreviated ft²). Velocity is derived from length and time, and is measured in feet per second (abbreviated ft/sec), as is acceleration, measured in feet per second squared (abbreviated ft/sec²).

A Word about Mass

Mass is a measure of the amount of substance—that which resists acceleration. It is a derived unit; namely the amount of mass which would require the application of 1 lb of force to achieve an acceleration of 1 ft/sec². This amount of mass, called a slug, would weigh about 32.2 lb on the surface of the earth. (But on the moon, one slug would weigh about one-sixth as much, because the moon’s gravity is about one-sixth the Earth’s.) By Newton’s Second Law, we see that m = F/a, and so 1 slug equals 1 lb·sec²/ft. Since lay persons usually have no concept of a slug mass, it has been these authors’ practice to speak only of weight (units of force) and reserve the slug for applying Newton’s Laws. The concept of pound mass does not relate to base units, is easily confused with pound force, and is not used herein.

Length

Generally, length quantities for vehicles are reported in inches (abbreviated in.). This includes the all-important (to reconstructionists) measurement of crush and stiffness. This practice is retained herein, but calculations regarding the laws of physics are applied in length units of ft. For example, energy is expressed in foot-pounds (abbreviated ft-lb), and moment of inertia is calculated in slug-in.² (because vehicles are measured in in.) but converted to slug-ft² when used in physics computations. For consistency, physics laws are applied in ft, even though inputs and outputs relating to vehicle dimensions are expressed in in. to maintain familiarity for the user and the consumer of the results. For example, vehicle crush is expressed in in. and crush stiffness in lb/in. or lb/in.². In the technical literature, metric stiffness values have been seen in kilopascals, but most lay persons would be baffled by them.

Velocity

Finally, lay persons generally understand feet per second when applied to velocity. However, the speedometers in their vehicles read in miles per hour, so it has been this author’s practice to use miles per hour (abbreviated mph) when communicating about speed. Of course, conversions to ft/sec are used for computational purposes. Similarly, lay persons have some understanding of angle measurements in degrees (abbreviated deg), but radians (abbreviated rad) used in calculations are mostly unknown to them. Therefore, angles are communicated in degrees, and angle rates (such as roll rate and yaw rate) are communicated in degrees per second (abbreviated deg/sec).

Precision and Accuracy

The ability to detect small changes of a property is known as precision and is often related to the resolution (degree of fineness) with which an instrument can measure. A set of scales may report a weight of 165.76 lb, but if those scales cannot detect a difference between 165 and 166 lb, it is misleading to report weights to 0.01 lb when the precision of the instrument is only 1 lb. The use of two decimal places would imply more knowledge than is actually present. This effect is seen when examining computer files for crash barrier load cells, which may show multiple readings that are identical to six decimal places! Close examination of the data may reveal an actual precision of about 1.5 lb, which is understandable for a device intended for measurements up to 100,000 lb.

Precision is not to be confused with accuracy, which reflects the degree of certainty inherent in any measurement. Uncertainty means that the true value is never known. The best we can do is make an estimate of the true value, using an instrument that has been calibrated against a standard (whose value is known with some published precision).

Significant Figures

The precision of calculations in the computer is a function of the hardware and software in the computer. Excel 2016, running under Windows 10, claims a number precision of 15 digits, for example. This is far more than needed to avoid round-off error during calculations, and it is hardly representative of the pre...