Chain Link Failure Analysis Example
A welded chain link failed while securing heavy equipment. The failed link had been placed over the edge of the equipment causing a bending moment in the link.
Manufacturer's instructions, provided with the equipment the link was attached, cautioned against placing the link on the edge of the secured equipment.
The link was examined in the laboratory. A weld defect open to the surface was present in the link. This was identified by optical fractography.
The link material was determined to be carbon steel with an ultimate strength of 125,000 psi.
A finite element analysis was performed to determine the stress levels in the link. Three scenarios were examined:
- Link with weld defect
- Link in contact with corner of secured equipment
- Link with weld defect in contact with corner of secured equipment
The analysis showed that the link would not have failed if either there was a manufacturing defect present in the weld and no bending or the link loaded in bending on the corner of the secured equipment and there was no defect in the weld.
However, the link would fail if both a manufacturing defect were present in the weld
and the link was loaded in bending over a corner of the secured equipment.
The conclusion of the failure analysis was that the link failed as a result of the combination of a manufacturing defect in the weld and improper use. The images below were used to support this conclusion.
Stereomicroscope view of the fracture surface of a chain link.
Enlargement showing the intitiation point of the fracture at weld defect.
Metallographic cross section of chain link showing weld area.
Finite element analysis with a simulated weld defect present in the chain link. Stresses are below ultimate strength of the material at maximum allowed load.
Cross section at weld area with simulated defect.
Maximum stress is approximately 70000 psi.
Analysis with link bent over bar. Stresses are below ultimate strength of the material at maximum allowed load.
Cross section at weld area with bending load.
Maximum stress is approximately 60000 psi.
Analysis with link bent over bar with simulated weld defect present. Stresses are above ultimate strength of the material at maximum allowed load indicating failure of the link would occur.
Cross section at weld area with simulated defect and bending load.
Maximum stress is approximately 135000 psi.