Metallurgical failure analysis of a load binder chain which failed during usage was requested. The failed load binder is shown in Photograph A.
Photograph A Load binder chain which failed during use on an automobile transporter.
A close-up view of the failed chain link from the binder chain 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. 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 close-up 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 Failed chain link weld. Dark left half of fracture is clearly old fracture surface which predates the final chain link separation.
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 the closure weld was a pasty weld, i.e., a weld 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.
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, thus the chain link weld lacked the strength to perform properly, and therefore 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.