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Load
Binder Chain Failure
Metallurgical failure analysis of a load binder
chain which failed during usage was requested. The
load binder is shown in Photograph A
Photograph A: Load binder chain which failed during use on an automobile transporter.
A closeup view of the failed chain link is shown in
Photograph B.
Photograph B: Overall photograph of failed chain link.
It is clearly evident that the chain link failed in
the "closure" weld. Chain links are manufactured
from straight rod, which is bent around an oval dye to
form the individual chain link. The link is fully
formed or closed by welding the ends of the rod
together with a "closure" weld. The closure weld is
usually formed by resistance welding. The next chain
link in the chain is made by passing the straight
rod through the chain link just manufactured and the
process is repeated. It is well
established that the weld in a chain link should be
the strongest part of a chain link. Thus, when a
chain link fails in the link weld, further
investigation is fully warranted.
In the present instance, the legal process had
reached the stage where no cleaning of the fracture
nor destructive testing was permitted. A closeup
view of the failed chain link weld fracture surface
revealed some very interesting and obvious facts.
The failed fracture surface is shown in Photograph
C.
Photograph C: Overall view of failed chain link weld. Dark
left half of fracture is clearly old, pre-rusty
fracture surface.
As can be seen in Photograph C, the left half of the
fracture surface is discolored (dark) and presents a
featureless fracture. The lower right also presents
a gray, featureless surface.
Stereomicroscopic
examinations of the failed chain link fracture
surface confirmed that these were "Pasty" welds,
i.e., welds made with very little or no adhesion
across the weld line. "Pasty" welds are a defect, well
known in the chain industry. However, testing by the
chain manufacturer at the time the chain is produced may or may not reveal these
"Pasty" chain
welding defects.
The failed chain link was examined with the scanning
electron microscope (SEM) without cleaning the
fracture surface (per court order). The SEM
examination did reveal evidence of dendritic
solidification on the chain link weld fracture.
Dendritic solidification (where the dendrites are
visible) is evidence that melting occurred, but that
insufficient molten chain link weld metal was present to create a
fully sound, solid weld when the molten metal
solidified. The Pasty weld defect is the
result of insufficient melting and/or insufficient
"squeeze" pressure when the chain link weld is
closed. The
scanning electron micrograph showing dendritic
solidification (lack of sufficient molten metal in
the weld) is shown in Photograph D.
Photograph D:
Scanning Electron Micrograph showing dendrite
solidification (lack of sufficient molten metal in
weld).
A finite element analysis (FEA) of the failed link
"pasty weld" zone was performed. The FEA net is
shown in Photograph E.
Photograph E: FEA net of failed "pasty weld" in chain link.
The visual, stereomicroscopic, and SEM examination
proved that the "pasty weld" region covered over 50%
of the chain link fracture and thus the chain link
weld lacked the strength to perform properly and
thus was defective. The FEA analysis proved that
with a chain link weld defect of this size, the
welding defect would, under the influence of normal
operational stress, grow by metal fatigue until
insufficient weld remained and final instantaneous
and catastrophic fracture would occur.
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Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- 