Irrespective of the failure, the pattern of a forensic analysis
is well established. While the tests and analyses selected for
individual cases can vary widely based upon materials of
construction, the overall method of proceeding from naked eye to
detailed microscopic examinations remains the same.
Guidelines of general forensic procedures
The primary role of a failure analyst in determining the root
causes of a failure is to identify the sequence of events involved.
At the beginning of an investigation the failure analyst should
start with an open mind without any bias, and let the data and
evidence collected paint a picture of the incidents which caused or
led to the failure. The generally accepted basic steps in an
investigation are:
- Identifying the different damage patterns/modes present
- Planning and performing different tests on the components to
find the actual mechanisms which contributed to the failure
- Differentiating between primary and secondary mechanisms
involved in the subject failure
- Identifying the possible root causes after analysis of the data
collected from the different experimental procedures
- Identifying the primary root cause leading to the failure by
refining the possibilities
Many times during an investigation the data may suggest a
different root cause for the failure than the one initially
hypothesized. All the physical evidence, background information and
experimental data has to lead to the actual failure mechanism.
Often we call the failure investigation a "puzzle," and after the
analysis all the puzzle pieces have to fall into place.
The failure analyst/forensic engineer needs to understand the
physical behavior of the failed component and what different
scenarios could cause that particular type of failure. All the
physical evidence present in the failed component, in the mating
component(s) and the adjacent areas requires careful documentation
to identify the failure mechanism. Sometimes the expert witnesses
or failure analysts are contacted by lawyers in the beginning of an
investigation. This is the best case scenario. However, many times,
the experts are brought into a legal case after the initial phase
of the investigation. As long as the photographs and all the
evidence can be reviewed, the experts may get a good idea of the
initial scenario.
Generally the lawyers, investigators and insurance companies
involved in the initial phase of examination keep the evidence for
later examination. As long as the evidence is kept in proper
condition, most or all of the required testing can be performed. It
is of utmost importance to keep the components involved in an
investigation in a protected environment so that the failure
features of the components are not altered during the storage. In
most of the cases, the pieces are not examined right away and it
may take a few years before the actual examination can happen. For
example, a failed iron pipe, if kept in an outside environment will
rust, destroying valuable fracture information. However, if the
failed pipe pieces are kept in a controlled environment, most of
the features associated with the failure can usually be analyzed
and identified.
When a laboratory receives components for a legal investigation,
it is the lab's responsibility to inform the lawyer or the client
if destructive tests are going to be performed on the components.
All interested parties should be invited to participate in an
investigation that includes destructive testing. A specification
provided by the American Society of Testing Materials (ASTM E 860)
clearly identifies this requirement. If the lawyer involved is not
aware of this, it is the investigator's or lab's responsibility to
educate him/her. The lawyer is then expected to notify all
interested parties involved in the particular case and agree upon
on a date on which all the parties can meet at the particular lab
for the analysis. The lab performing the analysis provides a
protocol outlining the different steps necessary to determine the
root cause of the problem. This protocol is then submitted to the
lawyer who circulates it to the other interested parties who may or
may not have any feedback or comments. These comments can be
addressed before or during the investigation. Many times after
doing one or a few steps of the investigation, the proposed
protocol may have to be changed due to some evidence present on the
components which was not expected before the examination.
Generally the investigation starts with photographic
documentation, dimensional measurements and identification of the
overall macro-features associated with the failure. Then the
different pieces of evidence are examined carefully and primary
areas of interest are identified. In-detail examinations of the
identified areas are performed by sectioning the pieces as needed
and examining them closely.
It should be mentioned here that a failure analyst or forensic
engineer should not be a "hired gun" of the lawyer. The role and
opinion of the failure analyst should be independent of any bias
and should be strictly based on the facts and data gathered from
the investigation. It is a general practice for the expert to
discuss the findings with the lawyer before writing a report. We
have had many cases where the lawyer did not want a complete report
with all the data and findings together with a conclusion as the
findings were not in favor of the side that lawyer was
representing.
After the investigation and reporting, the next phase might be
to provide a deposition or appear as an expert witness in court. At
this time the expert witness is expected to present the findings in
a simple enough manner so that the lawyers, the juries and the
judges can understand the report and the findings. This is very
important for an expert to do correctly as the outcome of the case
may be altered based on these presented facts.
Case History Example
A few years ago, Massachusetts Materials Research, Inc. (MMR)
performed a litigation investigation on the failed back of a
wheelchair that was designed to provide support to the user. The
seat back was attached to the upper canes of the wheelchair via
four cane clips. The upper two clips contained a latching mechanism
to hold metal pins that protruded from the sides of the seat back.
The two bottom clips were notched to hold the metal seat back pins.
The clips were attached to the canes with a hex bolt which, when
tightened, pulled the two sides of the clip toward each other and
maintained a tight fit against the cane. The failure of one of the
cane clips caused the subject seat back to fall out while the
wheelchair was occupied. Head trauma and subsequent hospitalization
led to the death of the 40 year old owner.
Material analysis identified the clips to be a blend of
polycarbonate/acrylonitrile butadiene styrene (PC/ABS). This was
the intended material of the clips per the manufacturer. Additional
extraction type analysis performed on a fractured clip piece
yielded an aliphatic liquid. This compound indicated some kind of
lubricant, e.g., mineral oil, etc. Note that this was not detected
on pieces analyzed away from the fracture. The composition of the
clip material was the same at and away from fracture, which was
confirmed with different analysis techniques.
The owner bought the wheelchair in February and the accident
happened during Thanksgiving weekend in November. There was a
presence of spider-web cracks (known as craze patterned cracks) at
the failed locations. The presence of mineral oil at the primary
failure location would indicate an environmental stress cracking
(ESC) situation. This type of oil can be found in sunscreen,
moistening lotion, etc. Our analysis indicated that exposure to an
ESC agent caused numerous surface cracks from which cracks
progressed with application of normal service loads. Note that
these surface cracks would act as stress raisers. Polycarbonate
plus ABS type material is known to show ESC when exposed to
sunlight, temperature variations and mineral oil type compounds. As
personal care products can reasonably be expected to come in
contact with these clips, the ESC failure here indicates a poor
design vis-a-vis material selection.
The owner used his chair very regularly both indoors and
outdoors including transportation. He weighed about 180 pounds and
was very active. The seat back was also designed to recline.
Combination of an inadequate design and improper material selection
caused ESC failure of the seat back and death of the owner. It
should be noted here that a few years later the manufacturer
completely changed the design of the clips and used aluminum metal
instead of the polymer material.
The investigation performed at MMR helped the family of the
deceased person to win the case.