Forensic engineering has been defined as “the investigation of failures – ranging from serviceability to catastrophic – which may lead to legal activity, including both civil and criminal”.It therefore includes the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury, damage to property or economic loss. The consequences of failure may give rise to action under either criminal or civil law including but not limited to health and safety legislation, the laws of contract and/or product liability and the laws of tort.
The field also deals with retracing processes and procedures leading to accidents in operation of vehicles or machinery. Generally, the purpose of a forensic engineering investigation is to locate cause or causes of failure with a view to improve performance or life of a component, or to assist a court in determining the facts of an accident. It can also involve investigation of intellectual property claims, especially patents.
As the field of engineering has evolved over time, so has the field of forensic engineering. Early examples include investigation of bridge failures such as the Tay rail bridge disaster of 1879 and the Dee bridge disaster of 1847. Many early rail accidents prompted the invention of tensile testing of samples and fractography of failed components.
Vital to the field of forensic engineering is the process of investigating and collecting data related to the materials, products, structures or components that failed. This involves inspections, collecting evidence, measurements, developing models, obtaining exemplar products, and performing experiments. Often testing and measurements are conducted in an Independent testing laboratory or other reputable unbiased laboratory.
Failure mode and effects analysis (FMEA) and fault tree analysis methods also examine product or process failure in a structured and systematic way, in the general context of safety engineering. However, all such techniques rely on accurate reporting of failure rates, and precise identification, of the failure modes involved.
There is some common ground between forensic science and forensic engineering, such as scene of crime and scene of accident analysis, integrity of the evidence and court appearances. Both disciplines make extensive use of optical and scanning electron microscopes, for example. They also share common use of spectroscopy (infrared, ultraviolet, and nuclear magnetic resonance) to examine critical evidence. Radiography using X-rays (such as X-ray computed tomography), or neutrons is also very useful in examining thick products for their internal defects before destructive examination is attempted. Often, however, a simple hand lens may reveal the cause of a particular problem.
Trace evidence is sometimes an important factor in reconstructing the sequence of events in an accident. For example, tire burn marks on a road surface can enable vehicle speeds to be estimated, when the brakes were applied and so on. Ladder feet often leave a trace of movement of the ladder during a slipaway, and may show how the accident occurred. When a product fails for no obvious reason, SEM and Energy-dispersive X‑ray spectroscopy (EDX) performed in the microscope can reveal the presence of aggressive chemicals that have left traces on the fracture or adjacent surfaces. Thus an acetal resin water pipe joint suddenly failed and caused substantial damages to a building in which it was situated. Analysis of the joint showed traces of chlorine, indicating a stress corrosion cracking failure mode. The failed fuel pipe junction mentioned above showed traces of sulfur on the fracture surface from the sulfuric acid, which had initiated the crack.
Extracting physical evidence from digital photography is a major technique used in forensic accident reconstruction. Camera matching, photogrammetry, and photo rectification techniques are used to create three-dimensional and top-down views from the two-dimensional photos typically taken at an accident scene. Overlooked or undocumented evidence for accident reconstruction can be retrieved and quantified as long as photographs of such evidence are available. By using photographs of the accident scene including the vehicle, “lost” evidence can be recovered and accurately determined.
Forensic materials engineering involves methods applied to specific materials, such as metals, glasses, ceramics, composites and polymers.
Historic examples :
There are many examples of forensic methods used to investigate accidents and disasters, one of the earliest in the modern period being the fall of the Dee bridge at Chester, England. It was built using cast iron girders, each of which was made of three very large castings dovetailed together. Each girder was strengthened by wrought iron bars along the length. It was finished in September 1846, and opened for local traffic after approval by the first Railway Inspector, General Charles Pasley. However, on 24 May 1847, a local train to Ruabon fell through the bridge. The accident resulted in five deaths (three passengers, the train guard, and the locomotive fireman) and nine serious injuries. The bridge had been designed by Robert Stephenson, and he was accused of negligence by a local inquest.
Although strong in compression, cast iron was known to be brittle in tension or bending. On the day of the accident, the bridge deck was covered with track ballast to prevent the oak beams supporting the track from catching fire, imposing a heavy extra load on the girders supporting the bridge and probably exacerbating the accident. Stephenson took this precaution because of a recent fire on the Great Western Railway at Uxbridge, London, where Isambard Kingdom Brunel’s bridge caught fire and collapsed.
One of the first major inquiries conducted by the newly formed Railway Inspectorate was conducted by Captain Simmons of the Royal Engineers, and his report suggested that repeated flexing of the girder weakened it substantially. He examined the broken parts of the main girder, and confirmed that the girder had broken in two places, the first break occurring at the center. He tested the remaining girders by driving a locomotive across them, and found that they deflected by several inches under the moving load. He concluded that the design was flawed, and that the wrought iron trusses fixed to the girders did not reinforce the girders at all, which was a conclusion also reached by the jury at the inquest. Stephenson’s design had depended on the wrought iron trusses to strengthen the final structures, but they were anchored on the cast iron girders themselves, and so deformed with any load on the bridge. Others (especially Stephenson) argued that the train had derailed and hit the girder, the impact force causing it to fracture. However, eyewitnesses maintained that the girder broke first and the fact that the locomotive remained on the track showed otherwise.
Product failures are not widely published in the academic literature or trade literature, partly because companies do not want to advertise their problems. However, it then denies others the opportunity to improve product design so as to prevent further accidents. However, a notable exception to the reluctance to publish is the journal Engineering Failure Analysis, which publishes case studies of a wide range of different products, failing under different circumstances. There are also an increasing number of textbooks becoming available.
Another notable publication, dealing with failures of buildings, bridges, and other structures, is the Journal of Performance of Constructed Facilities, which is published by the American Society of Civil Engineers, under the umbrella of its Technical Council on Forensic Engineering.