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Knowing when a personal injury case is due to the action or inaction of another party, or when responsibility falls fully on the injured party, can be a difficult distinction to make. At Mechanical and Safety Engineering, we have been investigating personal injury cases for decades. Along the way we have learned to make this distinction. This issue of Forensic Clues will examine the principles behind this litmus test.

As professional engineers, our highest ethical responsibility is to protect the health, welfare, and safety of the public. We take this seriously. When someone is injured, as an engineer identifying the reason for the injury is paramount. Identifying the mechanism of injury can help identify hazards, design defects, manufacturing defects, material defects, or whether the injury was due to a failure to maintain some product or property. Identifying these hazards and providing alternate solutions helps to protect other people from suffering the same fate. Haphazardly assigning blame however is not helping to protect the health, welfare, and safety of the public. There are many personal injury cases that have no true validity. These cases undermine valid cases involving real problems. Some cases may be valid, but there is inadequate evidence or data to make such a conclusion.

Data That We Review

The more information that is available concerning an accident, the more thorough an accident investigation and reconstruction will be. If the accident scene can be examined or photographed immediately after an accident, important details can be identified that later will be lost. Often as expert witnesses, we are called in long after an accident occurs. We must base our investigation on the available data, which includes examining the accident scene for anomalies, and examining the product, structure, or place that the accident has been attributed to. Extensive photographs, measurements, hardness, and other property characteristics are taken to be analyzed at a later date. Witness statements are reviewed for additional information.

Is There Evidence of Premature Failure?

In accident cases involving products, one of the first benchmarks is determining whether there was premature failure of a product component that led to the accident. These can be obvious, such as the fracture of a control arm in an automobile, or they can be subtle, such as when the failure of an overhead door reversing edge eventually leads to free fall of the door due to cable spool-out that occurs when an object is impacted with a broken or defective reversing edge. Identifying structural components that have failed and identifying control or sensor failure can quickly establish responsibility for a product injury. Once the specific failures are identified, the cause of failure must be examined.

What Causes Premature Failure?

The cause of catastrophic product failures must be determined. Considering only the mechanical failure of a product, structure, or component, the failure may be due to inadequate component sizing, inappropriate material selection, fatigue failure, design defects, the failure of a different component, overload, or other reason. Engineering calculations and design software can identify design weaknesses. Details such as dimensions, materials, and connections can be determined from exemplar products, if they are still in the stream of commerce. Inspection of the subject product can determine if the material met design specifications. Specific failure characteristics can show if the failure was due to fatigue, and what type of forces were acting on the product to cause failure. Direction of loading can often be determined, which can help establish a specific defect, as well as aid in accident reconstruction.

What About Misuse?

Misuse and overuse are often blamed for product failures. This is an attempt to put responsibility back on the product user. There are times when accidents are simply due to obvious misuse or overuse. While it can be easy to put all blame on the product user in these situations, often the misuse of a product is foreseeable. Foreseeable misuse requires prevention or safeguarding by the manufacturer to prevent injury to the user. An example of product misuse involves automobiles that were manufactured in the 1990's era that had automatic seatbelts which involved an automatic torso seatbelt, and usually a completely separate lap belt. Drivers predictably would not fasten the lap belt, instead relying on the shoulder belt. Severe frontal collisions with only the shoulder belt resulted in severe injuries and death, since the system was designed to be used with a lap belt. It was foreseeable that people would not use the additional lap belt. Designers failed to see this, and the end result was numerous needless injuries and deaths. While the cause can be attributed to misuse, the failure of the manufacturer to safeguard a foreseeable use makes the manufacturer responsible from an engineering perspective.

Factor of Safety

Some manufacturers claim overloading their products or structures beyond a specific weight rating absolves them of responsibility. These claims may or may not have validity. A toddler's chair that fails when a 250 pound man sits in it would probably not be considered to be defective by most engineers. A toddler's chair that fails when a 10 year old sits in it may be a different story. Engineering design involves a factor of safety. Simply put, a design must be able to withstand forces in excess of what it is ever expected to experience. Factors of safety are necessary for many reasons, including variability of materials, design inconsistencies, range of use, uncertainty in service conditions, impact loading, corrosion, temperature change, lack of adequate analysis, and difficulty in analysis due to complexity of shape. The most important aspect of factor of safety involves consequences of a failure. A failure that results in an inconvenience can be tolerated where a failure that results in large financial loss or serious injury/death must be prevented from ever occurring.

Is There a Standard Violation?

Industrial standards vary from worthless documents that define terms to actual performance standards that have extensive requirements. Some industry standards establish a certain minimum standard for specific products or structures. These requirements can involve strength requirements for load supporting products, stability requirements for vehicles and structures, safeguarding of product hazards, product hazard warning requirements, and more. Testing of a specific product involved in an accident is often not possible due to evidentiary concerns. Testing an exemplar version of a product involved in an accident is often possible and establishes whether the product adheres to industry standards. Failure to meet standard criteria shows that the manufacturer did not make the product as safe as possible and identifies product defects.

The Concept of Failsafe

Another important concept in this discussion of accident case validity is the concept of failsafe. A design is failsafe if the failure of a component or structure does not result in a hazard to the product user. For many machines the failure of a safety switch or component will result in the entire machine becoming inoperable until the safety feature is restored to specifications. An example of a failsafe design occurs in overhead doors.

Some doors have failsafe reversing edges that cause the door to be inoperable if the reversing edge shorts out or otherwise fails. This prevents a sequence of events that leads to cable failure and/or door freefall. Machines that continue to operate when a safety feature malfunctions are not failsafe. A hydraulic press that continues to operate when its light curtain malfunctions or loses power is not failsafe. Manufacturers try to balance safety with convenience. If a product is not failsafe that could easily be made failsafe, that product will likely be judged to be unsafe by engineers.

Witness Statements

Witness statements help identify potentially problematic cases. Discrepancies between observed facts and presented information may be simple inaccurate recall or may indicate attempts to hide elements of the case.


Attorneys are ethically required to advocate for their clients. As engineers, we are required to be impartial, basing any opinions and conclusions on facts, and inferences from these facts as well as our experience. This makes it critical to identify early in a case the cause of the accident and whether the manufacturer or other party may have caused or contributed to the accident. Let us help.

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John L. Ryan, BSME, P.E. is a Mechanical Engineer who provides general Mechanical and Structural Engineering expertise. Mechanical and Safety Engineering (MASE) provides full service analysis and accident reconstruction of products involved in accidents. Mr. Ryan's services have been requested for attorneys and insurance companies needing forensic engineering expert witness testimony to determine whether machinery and products involved in injury cases were adequately designed or whether they have a Design, Manufacturing, or Material Defect. All products are lab-tested on site to determine adherence to industry standards and engineering design protocol. Alternate preventative designs are developed when none exist commercially.

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