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Collapsing Restaurant Chair Dumps Lady
A lady fell to the floor as a
restaurant chair collapsed. The chair, when viewed
following the accident, looked remarkably usable. Was this an accident looking for a place to
happen?. The subject chair is shown in Photograph 1.
Photograph 1 - Overall view of
subject chair AFTER
collapsing accident.
Close inspection revealed four
areas of the chair which were
cracked, fractured and/or were areas of
interest. Those areas are described below and are
denoted in Photograph 2.
Photograph 2 - Left front quarter view of the
subject chair with cracks/fractures and areas
of interest noted.
The cracks, fractures and areas
of interest are delineated below:
1. The far right lumbar spoke was
missing (see #1 in Photograph 2)
2. A three centimeter (3 cm) long
multifaceted abrasion/scar was noted on the left
side of the leg reinforcement bar (LRB) near the
left front leg
(see #2 in Photograph 2).
3. The fillet weld, attaching the
chair leg to the leg reinforcement bar (LRB) had
completely failed (see #3 in Photograph 2).
4. The upper end of the leg tube
to front seat cross brace weld was fractured. The
other left front leg to seat cross brace weld was
cracked. Additionally, the seat cross brace, a piece
of wrought steel strap, was also fractured (see #4 in
Photograph 2).
Additional visual indications of
possible cracking were observed in several other
welds on the subject chair.
A close-up view of the welds on
the seat ring that had provided the lower right
lumbar spoke weld attachment point is shown in
Photograph 3. White arrows denote area on the back
of the seat ring where the far right lower lumbar
spoke welds failed
Photograph 3 - Close-up view
of lower failed lumbar spoke welds. Arrows denote
failed weld beads and dirt and debris packed around
the failed welds.
It is clearly evident from the
dirt and debris packed around the failed lumbar spoke
weld
shown in Photograph 3 that the far right lumbar
spoke has been missing for a significant time
period.
A close-up view of the three
centimeter (3 cm) long, multifaceted abrasion noted
on the left side of the leg reinforcement bar (LRB),
just behind the left front leg, is shown in Photograph 4.
Photograph 4 - Close-up view
of a 3 centimeter long abrasion/scar noted on the
leg reinforcement bar (LRB).
A close-up view of the fillet
weld attaching the chair leg to the leg
reinforcement bar (LRB) which has completely failed
is shown in Photograph 5. The view in Photograph 5
is looking up the inside of the left front leg from
the floor. It is clear from Photograph 5 and
photographs to follow that the weld failed in the
heat affected zone
and not in or through the weld
bead.
Photograph 5 - A close-up
view, looking up the
chair leg from the floor, of a
failed weld
connecting the leg to the leg
reinforcement bar.
The upper end of the leg tube to
front seat cross brace weld was also found to be
fractured. Additionally, as a result of the leg tube
to seat cross brace weld failure (see upper white
arrow denoting weld fracture in Photograph 6), the
seat-cross brace, a piece of wrought steel strap,
also fractured and appears torn. See lower white
arrow in Photograph 6.
Photograph 6 - Weld failure in
the tube end of the chair leg to cross brace strap
fillet weld (see upper white arrow). Also, note tear
in the cross brace strap (see lower white arrow).
A close-up view of the inside
fillet weld joining the leg to the front seat strap
is shown in Photograph 7.
Photograph 7 - Close-up view
of the cracked fillet weld joining the leg to the
front seat strap. Crack is noted with white arrow.
The white arrow in Photograph 7
denotes cracking at the weld toe in the leg to seat
cross brace fillet weld.
Close visual examination of the
failed leg to leg reinforcement bar weld indicates
that the weld failure is probably the result of
periodic and/or cyclic overstress. Thus, the subject
leg weld was cracked and had been in the process of
fracturing for an extended time period.
The end of the failed leg to LRB
weld exhibits a very dark, discolored, fracture
surface as shown in Photograph 8.
Photograph 8 - Overall view of
the end of the failed leg to LRB weld exhibits a
very dark, discolored, fracture surface.
This very dark, discolored,
fracture surface is proof that the leg to LRB weld
has been fractured for a long period of time and did
not fail on the day of the chair collapse accident. A
close-up view of the weld nugget cavity in the leg
clearly shows the darkened fracture surface.
The darkened fracture is denoted
with white arrows in Photograph 9
Photograph 9 - Close-up view
of the weld nugget cavity (in the leg tube) showing
a
darkened discolored fracture surface indicative of
older (as opposed to fresh) fracture. White arrows
denote darkened, discolored, long standing fracture.
A close-up view of the failed
weld in Photograph 10 reveals that the edge of the
weld nugget cavity area of the leg tube, from which
the weld separated, exhibits a marked amount of "lipping"
(deformation around the lip or edge of the weld
nugget cavity).
Photograph 10 - Close-up view
of the weld nugget cavity (in the leg tube) showing
the "lipping" (deformed edge of the nugget cavity)
on the edge of the nugget cavity.
It can be seen in Photograph 10,
that the edge of the nugget cavity is lipped and
corroded and that the cavity surface is dark and
discolored from long standing oxidation and/or
fracture surface pounding (the two fracture surfaces
are pounded/mashed against each other as the chair
is used). Additionally, a profile view of the weld
nugget fracture reveals embedded metal debris which
has been forced into the weld nugget fracture by
opening and closing of the fracture
(pounding/mashing) during use of the chair. These
areas of embedded metal debris are denoted with
arrows in Photograph 11.
Photograph 11 - Close-up view
of leg to LRB fillet weld showing embedded particles
on the weld nugget fracture surface. Arrows denote
embedded/impacted particles of debris.
The fact that there are two
failed welds associated with the left front leg
allows the sequence of weld failures to be
determined.
- In order for the left front leg
to cross seat brace weld to experience significant
stress, especially bending stress, the leg to LRB
weld would have to fail first.
- The fact that the (outside) leg
to cross seat brace weld has failed indicates that
that it had to have a source of bending stress.
- The fact that the cross seat
brace strap has also failed indicates that it also
had to experience stresses sufficient to fracture
the brace strap. Visually, the cross brace strap
crack fracture surface is polished. This indicates
that the crack has opened and closed numerous times,
as the chair was used, thereby polishing the crack
fracture surface as a result of relative motion
between the two crack fracture surfaces.
Stresses
of a level sufficient to fail the leg to seat cross
brace weld and the cross brace strap itself are not
present when the leg to LRB weld is in place and
functional. Thus, the leg to LRB weld
(#3 in Photograph 2) had to fail first.
After this weld failed completely, then and only
then could:
- Sufficient bending stress be applied to the leg to
seat cross brace weld (#4 in Photograph 2) to cause
it to fail.
- As the leg to seat cross brace weld is failing, then
and only then can the front left leg move rearward
along the surface of the LRB creating the
abrasion/scaring noted as #2 in Photograph 2.
Thus, the failure sequence that is
revealed by the subject chair is:
- The leg to LRB fillet weld fails over a long period of time, probably as
a result of progressive cyclic overload or
metal fatigue.
- This failure and separation of the leg to LRB fillet weld takes a
long time and finally the leg to LRB weld nugget separates from the leg.
- Continued use of the chair results in lipping of the edge of the weld nugget
cavity (see Photograph 10).
- Continued use of the chair also resulted in metal debris being embedded
in the leg to LRB weld nugget fracture surface (see Photograph 11).
- At some point in time, the left front leg moves rearward creating the first
LRB scar.
- These facts attest to the long standing nature
of the leg to LRB weld failure.
- Thereafter, sufficient stress, resulting from
use of the chair with a failed left front leg to LRB
weld, is transferred to the leg to seat cross brace
welds to cause time dependant failure, of the leg to
one of the seat cross brace welds, again probably by
metal fatigue or cyclic overstress.
- The failed leg to seat cross brace weld fracture surface also is
observed to contain rust and/or grease buildup
attesting to a long term
progressive failure of the weld, i.e., not a failure on the day of the subject
accident.
As a
result of the visual examination of the subject
failed chair involved in the accident certain
opinions can be stated. These opinions are based on
visual examination only. The opinions are:
1.
The welding of the subject chair, which
failed, was not defective and the welding design was
not defective. Therefore, the failure of the
subject chair was not the result of a manufacturing
defect.
2.
The failure and absence of the far right
lumbar spoke on the back of the subject chair
confirms that at least one lumbar spoke weld
probably had to have been physically broken by the
owner, a user and/or employees of the owner after it
left control of the manufacturer.
3.
The magnitude of dirt and debris packed in
and around the lower lumbar spoke rod weld bead
remnants attest to the fact that the lumbar spoke
weld failure is not recent and certainly pre-existed
the subject accident.
4.
The three centimeter (3 cm) long
abrasion/scarring on the left front side of the leg
reinforcement bar (LRB) consists of abrasion/scaring
from more than one event.
5.
The existence of a multifaceted scar on the
LRB confirms that the left front leg to leg
reinforcement bar fillet weld had entirely and
completely fractured prior to the subject accident.
6.
The fact that the left front leg tube to seat
cross brace weld failure fracture surface contained
rust and/or grease attests to the fact that this
fracture was long standing.
7.
Cursory inspection by the owner would have
revealed that the leg to LRB weld had failed.
Cursory inspection would have confirmed that the
chair should have been repaired and/or taken out of
service.
8.
The leg to seat cross brace weld would not
experience significant bending stress until, unless,
and after the leg to LRB weld failed.
9.
Failure of the subject chair leg to LRB weld
transferred periodic bending stress to the leg to
seat cross brace weld as the chair was used. These
periodic or cyclic bending stresses caused a metal
fatigue and/or cyclic overload stress failure of the
leg to seat cross brace weld to occur.
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