Forensic engineering is the investigation of materials,
products, structures or
components that fail or do not operate or function as intended, causing personal
injury or damage to property. The consequences of failure are dealt with by
the law of product liability. The field also deals with
retracing processes and procedures leading to accidents in operation of
vehicles or machinery. The subject is applied most commonly in civil law cases, although it may be of use
in criminal
law cases. 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.
History
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.
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. 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.
Analysis
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.
Examples
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 or 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, hence the van driver should have seen the leak well before the crack
grew to a critical size. He did not, thereby resulting in the accident. 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:
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
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, eye
witnesses maintained that the girder broke first and the fact that the locomotive
remained on the track showed otherwise.
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. 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.
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