The collapse of a chair spurred an investigation and a root cause analysis. The chair, when viewed following the accident, looked remarkably usable, but further inspection showed that the chair was far from capable of being able to bare repeated loads of substantial weight. The subject chair is shown in Photograph 1.
Photograph 1 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 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 Lower failed lumbar spoke 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 3 centimeter long abrasion noted on the leg reinforcement bar.
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 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.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 Cracked fillet weld joining the leg to the front seat strap.
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 End of the failed leg to LRB weld exhibits a dark, discolored, fracture.
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.
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 Weld nugget cavity showing lipping (deformed 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 Weld showing embedded particles on the weld nugget fracture.
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 dependent 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.