FAILURE ANALYSIS OF A PLASTIC TOY HELICOPTER
Dale B. Edwards, P.E., Engineering Systems Inc., Aurora, IL
Dennis B. Brickman, P.E. Engineering Systems Inc., Aurora, IL
Charles A. Fox, Ph.D., ESI Demonstratives, Ames, IA
Rodney A. Brewster, ESI Demonstratives, Ames, IA
Abstract
A failure analysis was performed on a plastic toy
helicopter that exhibited several fractures of both the
rotor blades and the rotor hub sections. Fractography and
flight testing were utilized to determine whether or not
the toy helicopter fractures were due to operation and
flight of the helicopter or due to other causes. The
multiple fractures were found to be inconsistent with
normal operation and flight of the helicopter and the
analysis showed that the fractures were likely due to
manual manipulation of the rotor blades and hub sections
prior to use of the helicopter.
Introduction
In 2012, an adult male allegedly sustained an eye
injury while attempting to launch a manual pull cord
consumer plastic toy helicopter for the first time after
assembly. Two additional incident descriptions, not
consistent with the allegations, were contained in the
medical records including that the patient sustained his
eye injury while fixing the toy helicopter and another
stating that the patient sustained his eye injury when a
screw came flying from a door and hit the left eye. The
toy helicopter was packaged as separate components and
required assembly by the user in accordance with the
step by step instructions on the back of the package.
Figure 1 depicts a properly assembled exemplar toy
helicopter with the rotor blade insignia facing up. An
independent third party testing lab determined that the
toy helicopter model passed the requirements of the
ASTM Standard Consumer Safety Specification for Toy
Safety F963-08. [1] The U.S. Consumer Product Safety
Commission conducted an investigation regarding this
reported toy helicopter incident and did not believe a
recall of the product was necessary. [2]
According to the injured party, he (an adult male)
assembled the toy helicopter involved in the incident. He
reportedly installed the rotor blades with the insignia
facing down, which is not the correct placement of the
blades. It is important to note that the blades will remain
in the hub without the hub cover when installed correctly
(with insignia facing upward), but the blades will not
remain in the hub without the hub cover when installed
upside-down. When the blades are installed upside-
down, the rotor blades contact the tail section of the toy
helicopter, as shown in Figure 2. Inspection of the
subject helicopter tail section revealed no damage from
the blades striking it. Testing was conducted on an
exemplar helicopter with the blades inserted upside down
and there was no blade breakage and no tail section
damage, even though the blades initially contacted the
tail section prior to launch when improperly installed in
this way.
At the time of the incident, the user stated that he
held the starter above his head with his vertical left hand
and pulled the starter cord horizontally with his right
hand. Some portion of the toy helicopter allegedly
struck the user in the eye immediately after the initial
launch, but the injured user did not actually see what
portion of the helicopter contacted his eye. After the
event, the fractured hub sections and the three rotor
blades were found in the condition shown in Figures 3
and 4. Two of the three rotor blades exhibited damage
and all three hub sections revealed fractures. The
fractured pieces of the two rotor blade tab sections and
the separated fractured elements of the hub were not
recovered after the incident. The remainder of the toy
helicopter, including the hub cover, was found to be
intact without failure.
In this investigation, the authors have considered all
of the available evidence when conducting the failure
analysis. Review of background information was an
important part of the process. This information at times
can be misleading, however, and the fracture evidence
present in the part must take precedence. All of the
available information should be weighed and harmonized
in order to arrive at the correct explanation for the
failure. [3] An accurate and complete accident
reconstruction using the available data must be consistent
with the laws of physics, and the physics of interaction
between the man, product/machine, and the environment.
[4] In this case, the numerous fractures in the hub and
blade sections of the toy helicopter could not be
explained by a single flight of the helicopter.
Testing of Exemplar Helicopters
An exemplar toy helicopter was flight tested using
the same launch technique reportedly utilized by the
injured user. The injured party demonstrated on video
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with an exemplar helicopter his body position and the
manner in which he pulled the cord. This technique was
replicated during our exemplar helicopter testing. The
exemplar toy helicopter was tested three times in the
properly assembled condition with the rotor blade
insignia facing up as shown in Figure 1. Then the
exemplar toy helicopter was tested three times with the
rotor blade insignia facing down and the blade tab fully
inserted into the hub, as illustrated in Figure 2. Finally,
the exemplar toy helicopter was tested three times with
the rotor blade insignia facing down and the blade tab
placed in an incorrect position, towards the outside of the
hub. After the starter cord was pulled, the toy helicopter
flew away from the user. The toy helicopter did not
contact the user during all nine launch sequences. The
nine flight tests resulted in no failures of the rotor blades
or of the hub. All nine flight tests were documented by
video. Figure 5 demonstrates a sample flight test
sequence with the rotor blade insignia facing down and
the blade tab fully inserted into the hub.
Material
The hub and rotor sections of the toy helicopter
were molded from polypropylene resin that incorporated
calcium carbonate filler and this was confirmed by
Fourier Transform Infrared (FTIR) analysis. This
material is used in a large variety of consumer products
ranging from toys to lawn chairs. Polypropylene is
commonly used for toys due to its relatively high
stiffness, compared to polyethylene, and its low cost.
The material has good strength and stiffness properties,
and elongation at break values ranging from 10 to 80
percent, depending on the filler level.
Parts for these different products can be easily
molded in large quantities by injection molding.
Potential molding defects, such as voids and porosity
due to poor venting of the mold, can lead to premature
failure of the parts if these features are large, numerous,
or present in a highly-stressed area of the part. As will be
seen in the following analysis of the failed toy helicopter
parts, there was no evidence that any molding defects
caused the failures.
Fractography
The fracture surfaces of the plastic helicopter were
examined using a optical stereomicroscope in order to
determine the origins of the fractures in the various failed
parts and the fracture surface morphology. This was done
as part of the failure analysis in order to determine
whether the fractures were due to defect(s) in the molded
parts, fractures due to assembly, fractures due to flight of
the helicopter, or fractures due to other causes. There
were several different fractures of the hub and rotor
sections of the helicopter, including fractures of two of
the three rotor blades and at all three of the hub
locations. It was apparent from that the various fractures
that were present in the helicopter parts that they were
not a result of a single incident or a crash and that the
fractures did not result from normal operation and flight
of the helicopter, even if the blades were installed upside
down (as shown in the testing discussed above). The
multiple fractures were not consistent with a crash of the
helicopter. The forces generated in a crash would not
have created the fractures of different portions of the hub
and no failures occurred during flight testing of an
exemplar helicopter when it crashed to the ground. The
fractures of the hub sections, in particular, were all very
different and resulted from high, localized stresses that
involved bending in different directions that would not
be consistent with normal operation of the helicopter.
The bending failures were ductile in nature and did not
appear to be the result of impact loading. Several of the
hub fractures (Hub B and Hub C) were replicated by
manual bending the blade/hub areas of an exemplar
helicopter.
Blades 1, 2 and 3
The three rotor blades of the failed helicopter are
shown in Figure 3. Two of the blades had fractures, one
fractured at the tab section junction with the blade and
one with an angular fracture in the tab section. The third
blade did not exhibit a fracture. The blades were
arbitrarily assigned numbers 1, 2 and 3: Blade 1 did not
exhibit a failure, Blade 2 had the tab section fractured
completely off and Blade 3 exhibited an angular fracture
in the tab section of the blade.
Hub Section Fractures
The hub sections all exhibited different fractures.
The hub sections were arbitrarily designated as Hub A, B
and C, as shown in Figure 4.
Hub A
Hub A exhibited a fracture of one of the ribs at the
outer part of the hub and a partial fracture of the side and
underside of the hub, as shown in Figures 6 and 7,
respectively. The rib and hub body partial fractures both
exhibit stress whitening and micro-ductility on the
fracture surfaces that indicates that the material was
stressed beyond its yield strength. The fracture of the rib
initiated on the inner portion of the rib (at arrows in
Figure 6) due to an outward force applied to the rib. The
partial fracture on the hub initiated on the under side of
the hub body, tearing from the outer edge, due to a very
localized downward force. The likely loading scenario
was forcing of the tab portion of the blade into the hub
with upward bending of the rest of the blade.
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It was surmised that the tab portion of the rotor
blade likely had not been fully inserted in the hub, but
was placed towards the outside of the hub as shown in
Figure 8. The reason for this positioning of the end of the
tab is that it could explain loading of the outermost rib by
the tab end, due to interference between the tab end and
the rib section.
Hub B
Hub B exhibits a fracture of one of the innermost
ribs and a complete fracture of the end of the hub body,
as shown in Figure 9. These fracture surfaces also
exhibited micro-ductility at the fracture origins,
indicating that the material had been stressed beyond its
yield strength. The end fracture exhibited ductile fracture
and stress whitening where the end of the hub was torn
off. There are areas outside of the ductile initiation points
that exhibit more brittle fracture characteristics.
It was surmised that the tab portion of the rotor
blade likely had been fully inserted into Hub B, as the
innermost tab was broken (see Figure 10). The end of the
tab portion of the rotor blade could load this rib during
assembly as well, as shown for an exemplar helicopter in
Figure 11. The blade fractured at the transition between
the tab section and wider section of the rotor blade. This
area was likely damaged due to bending it downward
while trying to force the upside-down blade into the hub.
It is unknown whether the blade was intact at the time of
the supposed flight.
Hub C
Hub C exhibits an angular fracture in the hub body
and a fracture of one of the innermost ribs, as shown in
Figure 12. The fracture surfaces exhibit stress whitening
and micro-ductility again indicating that the material had
been stressed above its yield strength. The tab of the
rotor blade was likely fully inserted into the hub since
the innermost rib had been fractured.
Figures 13 and 14 depict the fracture surface of the
rib and hub body, showing stress whitening and microductility at the fracture origins. The fracture in the Hub C
body initiated on the top surface due to a downward
bending force with a twist, causing the fracture. The tab
end loaded the rib, causing it to fracture as well.
The hub cover that holds the rotor blades in place
exhibited no damage whatsoever, as shown in Figure 15.
If the fractures of the hub body had occurred with the
hub cover in place, some indication and failure of the
cover would also have been present, likely in the form of
a fracture of the hooks that snap onto the hub at all three
hub locations. Failure of one of these hooks would not
cause the hub cover to pop off since two others would
remain. The rotor cover also did not detach during the
testing of the exemplar helicopters.
Animation/Simulation
In an effort to explain how the fractures occurred,
based on the fracture surfaces, an animation of each hub
fracture was developed. The construction and geometry
of the toy helicopter hub and rotor blades utilized in the
animation sequences were captured using FaroArm
portable measuring machine technology. [5] Figures 16
through 18 are sequences from each animation that
demonstrate the failure of the hub sections. These
sequences clearly show different fracture scenarios for
each hub and blade fracture. The green and yellow
arrows in the animation sequences represent the applied
forces during the failure sequences. The elements
highlighted in red in the animation sequences show the
fractures in the respective rotor blade and hub sections.
The fourth frame in each animation sequence includes
photographs of the actual fractured rotor blade and hub
sections of the toy helicopter involved in the incident.
The loading needed to cause these overload failures did
not occur as a result of flight or crashing of the helicopter
to the ground, as was verified during exemplar flights of
the helicopter where it crashed to the ground and no
failures of the rotor blades or hub occurred.
Discussion
The helicopter reportedly had been assembled and
flown only once, causing the multiple fractures in the
hub and blade sections as a result of this flight. The
fracture surfaces present in the hub and blade sections
are not compatible with this scenario. The flight testing
of an exemplar helicopter, described above, also
indicates that this scenario is not correct, as no fractures
occurred to the hub or blades due to flight of the
exemplar helicopter, even when the blades were installed
upside down or when the blades were inserted into the
hub short of the rib.
The localized bending that caused the fractures in
the hub and blades was different for the three hub
locations. The fractures in Hub A likely occurred by
pushing the tab portion into the end of the hub while
bending the blade upward. The fractures in Hub B likely
occurred due to downward bending of the tab/hub at the
end of the hub and the fractures in Hub C were due to a
combination of downward bending and twisting. This is
shown on the fracture surfaces of the hub sections and in
the animation sequences. Again, these fractures all could
not have occurred as a result of a single flight of the
helicopter. It appears more likely that they occurred
during an attempted assembly of the helicopter, with
manual manipulation of both the blades and hub areas,
potentially using a tool. It is likely that the helicopter
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would not have been able to fly if all of these fractures
had been present prior to the reported launch of the
helicopter.
Conclusions
The failure analysis that was conducted on the toy
helicopter indicated that the failures of the rotor hub and
blades were due to overload and were not consistent with
the flight of the helicopter or impacts from crashing,
even if the helicopter was assembled incorrectly. Flight
testing conducted on an exemplar toy helicopter in
multiple assembly states (blades installed properly,
blades installed upside down and blades installed upside
down and partially inserted into the hub) indicated that
the helicopter did not contact the user and there were no
failures of the rotor blades and the hub after all test trials.
Fractography clearly showed that the fractures involved
intense localized bending that caused yielding of the
material. The different fractures resulted from bending in
different directions and could not have occurred as a
result of a one-time flight of the toy helicopter. The lack
of damage present on the rotor cover indicates that it
likely was not in place when many of the fractures took
place. Animation of the fracture process, including actual
photos of the failed sections clearly demonstrated the
failure mode and is an excellent tool for failure analysis.
References
1.
2.
3.
4.
5.
ASTM F963-08, Standard Consumer Safety
Specification for Toy Safety, West Conshohocken,
PA, 2008.
U.S.
CPSC,
Report
#20121031-FDE8B2147462356, www.saferproducts.gov, 2012.
M.D. Hayes, D.B. Edwards, A.R. Shah,
Fractography in Failure Analysis of Polymers,
Elsevier, Oxford, United Kingdom, 2015.
E.H. Knox, et al, Methods of Accident
Reconstruction: Biomechanical and Human Factors
Considerations, Proceedings of the ASME 2015
International Mechanical Engineering Conference
and Exposition, Houston, TX, 2015, pp. 1-11.
FaroArm, www.faro.com, 2015.
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Figures
Figure 1. Overall view of properly assembled exemplar
toy helicopter.
Figure 3. Designation of blades from failed helicopter.
Figure 2. Overall view of improperly assembled
exemplar helicopter with rotor blades installed upside
down (note blade contacts tail section).
Figure 4. Designation of Hub locations and the likely
position of the blades from the failed helicopter.
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Figure 5. Exemplar helicopter flight test sequence with rotor blade insignia facing down and rotor blades installed upside
down and blade tab fully inserted into hub.
Figure 6. Close-up of fractured rib on the outer portion of
Hub A (initiation at arrows).
Figure 7. Partial fracture of the body of Hub A due to
downward force applied to end of hub.
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Figure 8. Hypothesized position of blade tab in Hub A
where end of tab loaded broken rib (at arrow).
Figure 11. Example of tab inserted fully into hub, as was
likely the case for the subject Hub B (note that hub cover
covers this area and keeps the blades in place).
Figure 9. Fractures of innermost rib and end of Hub B.
Figure 12. Portion of Hub C fracture showing angular
fracture of hub and fracture of innermost rib.
Figure 10. Fracture of innermost rib on Hub B.
Figure 13. Close-up of fracture origin area of innermost
rib showing micro-ductility.
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Figure 15. Subject hub cover exhibiting no damage.
Figure 14. Portion of Hub C fracture surface, initiating at
top surface of hub from downward bending.
Figure 16. Animation sequence for Hub A/Blade 1 fracture scenario.
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Figure 17. Animation sequence for Hub B/Blade 2 fracture scenario.
Figure 18. Animation sequence for Hub C/Blade 3 fracture scenario.
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