Licensed Professional Engineers

FORENSIC CLUES # 30 - "Accident Reconstruction " by John L. Ryan

FORENSIC CLUES -A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.

Volume 30 February/March 2009

“Accident Reconstruction”

By John L. Ryan

Accident Reconstruction

Accident reconstruction involves attempting to determine the sequence of events of an accident and is a crucial part of product liability cases. Understanding exactly what occurred in an accident gives an engineer the best chance of preventing the accident from occurring again. Often there is limited information to base conclusions on what really happened in an accident. Understanding what goes into accident reconstruction will help attorneys understand what information is crucial to this process. Witness testimony is unreliable at best but must be analyzed and cross referenced with the other available information.

Some types of accidents are easier to reconstruct than others. Machine guarding accidents where a person is exposed to power transmission hazards are normally simple to reconstruct. Guarding accidents are often very traumatic, and evidence of the point of contact frequently exists in the form of blood or clothing. Other accidents may involve a broken component of a product or machine. The end result of the failure is usually evident by injuries to the victim, but the events leading up to the failure can be very difficult to determine.

Pieces of the Puzzle

Information used to reconstruct accidents is varied. The different pieces of evidence must be gathered to be able to produce an accurate accident reconstruction. Witness testimony is usually the best place to start an accident reconstruction. If the victim of an accident is the only witness, his or her statements may contain some clues to what was happening right before the accident. Many people involved in accidents have difficulty remembering details of what occurred before, during, and after an accident. Victim’s statements must be studied closely for any clues to what was occurring prior to the accident and what could have initiated the accident. If there were third-party witnesses, there will be a much better picture of what was happening before the accident and how the accident occurred. All testimonies must be cross referenced with each other as well as with the other information available in the case for the best accuracy.

Witness marks are any noticeable marks, gouges, or other physical abnormalities that are evident after an accident. Marks that may have been on the product prior to the accident should be identified, although this can be a difficult process. Witness marks offer more information when trying to piece together the cause of an accident. Witness marks can give information like point of impact of product in a fall situation, direction and magnitude of forces in an accident, and other information. Accident site characteristics can also provide clues, however usually someone would have had to take photographs of the accident site immediately after or there will likely be little useful evidence. Photographs of the site can offer information like resting position of the victim or exact point of contact with hazardous machinery. Blood stains are very useful, and tests for traces of blood can be performed long after the accident to provide additional information.

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Damage to product helps reconstruction

Tire skid marks are important data for recreating automobile accidents and obtaining information on the speed and direction of travel of the vehicles.

With catastrophic failure of materials, the cause of the material failure must be determined. The product may not have been designed to withstand the forces that can be encountered, a material with inadequate material properties could have been used, or there may be material defects of unknown origin. Determining if a material was overloaded involves finding out what the potential forces were that would be involved. Hand calculations or finite element analysis (FEA) software can be used to determine if a certain design is adequate or if the design would be overstressed under certain circumstances. If a design is adequate, the next possibility is to determine the properties of the material used. Manufacturer records will reveal the material specified for a product. Testing the material itself will reveal if the material is different from what the manufacturer specified. Hardness tests can provide the approximate yield strength of a material to identify material defects or different material than was specified for production. Spectroscopy can reveal specific elemental components of different materials. Many aluminum alloys are composed of a number of different elements. Identifying the presence or absence of these elements can help determine if a material meets manufacturer specifications.

Microscopic investigation of evidence can help reveal information on material failure as well as offer additional clues to the accident scenario. Microscopic images can show the absence or presence of clothing fibers to help establish point of impact. Microscopic images can help determine whether a metal failure was caused by overloading or by fatigue.

Wear and dirt patterns can help establish the composition of a machine or product. Determining if a guard was being used on a piece of machinery at the time of the accident can be as simple as inspecting the connections for the guard to determine recent use, looking for paint fading patterns or dirt/grease patterns, or even by testing for blood traces on the guard if there was bleeding involved in the accident.

Injuries to victims provide a lot of information as well. In fatal accidents, these injuries provide critical information in lieu of personal testimony by the victim. Location of the body can help reveal mechanism of injury, initiating event, and other crucial information. Injury location and severity provide important information on the point of contact and what kind of forces were involved, as well as what secondary movements and injuries occurred after the initial trauma.


Models of the accident scene and of the products and people involved in the accident can help explain the sequence of events that occur in an accident. Models allow us to process the accident information visually. Making a model and experimenting with possible different scenarios can lead to the right scenario that accounts for all available data. If a jury member can actually see how an accident occurred with the use of a model

Models are also extremely useful to demonstrate an accident for the courtroom. that person will have a more accurate representation of the elements of an accident.

Models can also be used to demonstrate alternate design technology that could have prevented the accident. Safety Engineering Resources specializes in producing accurate, scale models of products involved in accidents.

These models are equipped with the original configuration, as well as alternate designs that safeguard the hazardous elements.

These models are invaluable to demonstrate how the accident occurred, to give people a visual concept of the accident. The effectiveness of alternate designs are also demonstrated, as well as the simplicity of implementation. The technical descriptions of how an alternate design works make much more sense when combined with a working visual aid.

Safety Engineering Resources has an in-house model building specialist. Safety Engineering Resources develops models at the request of our clients. We can also develop models for attorneys who already have designated their experts.


Explaining safety devices with models


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