Forensic engineering

thumb|Broken gear teeth on a piece of machinery

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". The forensic engineering field is very broad in terms of the many disciplines that it covers, investigations that use forensic engineering are case of environmental damages to structures, system failures of machines, explosions, electrical, fire point of origin, vehicle failures and many more.

Investigation

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. The next step would be to do an initial incident appraisal, this is done before any analysis is done and they take a quick observation of what the solution is at hand.

Forensic materials engineering involves methods applied to specific materials, such as metals, glasses, ceramics, composites and polymers.

Organizations

The National Academy of Forensic Engineers (NAFE) was founded in 1982 by Marvin M. Specter, P.E., L.S.; Paul E. Pritzker, P.E., and William A. Cox Jr., P.E. to identify and bring together professional engineers having qualifications and expertise as practicing forensic engineers to further their continuing education and promote high standards of professional ethics and excellence of practice. It seeks to improve the practice, elevate the standards, and advance the cause of forensic engineering. Full membership in the academy is limited to Registered Professional Engineers who are also members of the National Society of Professional Engineers (NSPE). They must also be members in an acceptable grade of a recognized major technical engineering society. NAFE also offers Affiliate grades of membership to those who do not yet qualify for Member grade. Full members are board-certified through the Council of Engineering and Scientific Specialty Boards and earn the title "Diplomate of Forensic Engineering", or "DFE". This is typically used after their designation as Profesional Engineer.

Examples

thumb|left|200px|Failed fuel pipe at right from a road traffic accident.

thumb|left|200px|Close-up of the broken fuel pipe from a road traffic accident.

thumb|left|200px|Close-up of the broken fuel pipe.

The broken fuel pipe shown at left caused a serious accident when diesel fuel poured out from a van onto the road. A following car skidded and the driver was seriously injured when she collided with an oncoming lorry. Scanning electron microscopy (SEM) showed that the nylon connector had fractured by stress corrosion cracking (SCC) due to a small leak of battery acid. Nylon is susceptible to hydrolysis when in contact with sulfuric acid, and only a small leak of acid would have sufficed to start a brittle crack in the injection moulded nylon 6,6 connector by SCC. The crack took about 7 days to grow across the diameter of the tube. The fracture surface showed a mainly brittle surface with striations indicating progressive growth of the crack across the diameter of the pipe. Once the crack had penetrated the inner bore, fuel started leaking onto the road.

The nylon 6,6 had been attacked by the following reaction, which was catalyzed by the acid:

:image:amide hydrolysis.svg

Diesel fuel is especially hazardous on road surfaces because it forms a thin, oily film that cannot be easily seen by drivers. It is much like black ice in its slipperiness, so skids are common when diesel leaks occur. The insurers of the van driver admitted liability and the injured driver was compensated.

Applications

Most manufacturing models will have a forensic component that monitors early failures to improve quality or efficiencies. Insurance companies use forensic engineers to prove liability or nonliability. Most engineering disasters (structural failures such as bridge and building collapses) are subject to forensic investigation by engineers experienced in forensic methods of investigation. Rail crashes, aviation accidents, and some automobile accidents are investigated by forensic engineers in particular where component failure is suspected. Furthermore, appliances, consumer products, medical devices, structures, industrial machinery, and even simple hand tools such as hammers or chisels can warrant investigations upon incidents causing injury or property damages. The failure of medical devices is often safety-critical to the user, so reporting failures and analysing them is particularly important. The environment of the body is complex, and implants must both survive this environment, and not leach potentially toxic impurities. Problems have been reported with breast implants, heart valves, and catheters, for example.

Failures that occur early in the life of a new product are vital information for the manufacturer to improve the product. New product development aims to eliminate defects by testing in the factory before launch, but some may occur during its early life. Testing products to simulate their behavior in the external environment is a difficult skill, and may involve accelerated life testing for example. The worst kind of defect to occur after launch is a safety-critical defect, a defect that can endanger life or limb. Their discovery usually leads to a product recall or even complete withdrawal of the product from the market. Product defects often follow the bathtub curve, with high initial failures, a lower rate during regular life, followed by another rise due to wear-out. National standards, such as those of ASTM and the British Standards Institute, and International Standards can help the designer in increasing product integrity.

Historic examples

thumb|right|350px|[[Dee bridge disaster.]]

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 said that they saw the girder break first, and that the locomotive and tender were still on the track at the far side of the bridge.

Publications

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.

The journal Engineering Failure Analysis (), published in affiliation with the European Structural Integrity Society, publishes case studies of a wide range of different products, failing under different circumstances.

A 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.

The Journal of the National Academy of Forensic Engineers is a peer-reviewed open access journal that provides a multi-disciplinary examination of the forensic engineering field. Submission is open to NAFE members and the journal's peer review process includes in-person presentation for live feedback prior to a single-blind technical peer review.