A metallurgical failure analysis was requested after an accident occurred during the construction of a water tower. A mobile crane, located outside the water tower, was used to remove a water tank crane boom, which had been used inside the water tower tank during construction. As removal of the crane was proceeding, the internal crane boom bolts used to attach the three crane boom sections failed resulting in the separation of one of the crane boom sections from the main crane boom.
J.E.I. Metallurgical, Inc. was retained to conduct visual and non-destructive examination on the failed crane's bolts, nuts, bolts fragments, fractures and metallographic mounts previously prepared by other investigators. Visual examination of a come-along and broken come-along chain was also requested. The come-along had been used to stabilize the internal crane boom during the removal process. The question of which one happened first had to be addressed; did the bolts fail first, allowing the come-along to break thus causing the accident, or did the come-along fail first allowing the internal crane boom to slide and fall vertically, causing the crane boom bolts to fail?
Visual, microscopic, stereomicroscopic, metallographic and scanning electron microscopic (SEM) examinations were all utilized in the investigation. Finite element analysis (FEA) was also used to evaluate the bolt stress resulting during the various failure sequences. These investigative methods indicated that the subject crane bolts had failed in a variety of different fracture modes. Several bolts had failed in a pure tensile ductile dimple rupture fracture mode with very limited shear lip (see Photograph A below) resulting from excessive joint constraint.
Photograph A Bolt that failed in a pure tensile ductile dimple rupture mode.
Threads on one bolt/nut combination were actually stripped, illustrating improper assembly of the derrick boom. A close-up view of the nut, with stripped threads, is shown in Photograph B below.
Photograph B nut with stripped threads.
The remainder of the bolts failed in a combination tensile overload / tensile bending overload. An example of the tensile overload / tensile bending fracture topography is shown in Photograph C below.
Photograph C Bolt that failed by tensile overload/tensile bending fracture.
During the accident reconstruction, it was determined that the come-along chain failed first. As a result of the come-along chain failure, the bolts holding each crane section failed in a variety of fracture modes. Thus, the final separation of the derrick boom sections occurred as a result of the failure of the section attachment bolts from stresses applied as the subject crane boom dropped and struck the water tank wall.