Examination of a fractured D shaped shackle
At ESR Technology we carry out a considerable number of Root Cause Failure Analysis (RCFA) examinations each year. Always the aim is to understand why a component failures so that a change can be made so that hopefully it won’t happen again or we will know when it will happen so it can be replaced before the failure occurs (planned replacement).
Components that have been examined can be from a wide range of sources, oil and gas, transport, energy generation, bridges, manufacturing plant to name a few and include just about every component you can think off including machinery ie shafts, bearings, gears, pumps, valves pistons and static structures, pipework, vessels, frames, fasteners, castings.
Characterisation of the damage to determine the failure mechanism is the first stage in the understanding. There are many failure mechanisms and some are complex and very difficult to pin down, but usually the more difficult the failure the more interesting it is or sometimes when the failure could have had a serious impact on an individual.
In this case study the failure although to a small component could have had serious consequences. The failure was to a D shackle bolt that was on one of the rigging lines supporting a yacht’s main mast. The yacht was navigating around Anglesey in poor weather when the shackle broke. Failure of the rigging could have meant losing the mast and a possible capsize.
Figure 1: Yacht at North Wales ready to sail.
Figure 2: Location of D shackle part of the rigging for the main mast.
The shackle had fractured into three sections with several partway through wall fractures present on the inside of the bend, Figure 3. The surface of the shackle was covered with patches of brown stain.
Figure 4 shows the fractures in more detail; they appeared to be dark in colour, rough and granular. The clearest through wall fracture was selected to examine in more detail.
Figure 5: Fracture surface with areas of corrosion product and grain boundaries visible.
Figure 6: Detail of the intergranular fracture.
Figure 7: EDX analysis showing the chemical elements present, Cr, Fe, Ni typical of a stainless steel. Note no Mo detected.
Optical Microscopy
Sections were cut from the shackle to include a through wall crack, mounted and metallurgically prepared and examined in the as polished and etched condition using an optical microscope. Figure 8 to Figure 10 show images taken of the cross sections. The image show that the cracking was primarily intergranular and branched typical of preferential corrosion ie stress corrosion cracking.
Figure 9: Detail of the fracture face with branched cracks preferentially attacking the grain boundaries.
Figure 10: Etched micros structure showing the grain boundaries and associated cracking.
Vickers Hardness Measurements
Vickers hardness measurements were performed on polished cross-sections using a Vickers indenter and a load of 20 kg (HV20). The hardness was variable as expected for a deformed drawn material. The average hardness was 215Hv20.
Table 1: Vickers hardness results.
- The fractured shackle was from an ocean going yacht and therefore subject to salt water. Salt water contains chloride ions that are particularly corrosive to stainless steels.
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The shackle was manufactured from stainless steel 304 stainless steel that has a lower corrosion and pitting resistance especially in sea water compared to 316 stainless steel due to the absence of the alloying element molybdenum.
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The microstructure was equiaxed with strain lines typical of a worked / drawn material. A drawn material although stronger will have a lower corrosion resistance due to stored energy into the microstructure
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The shackle was fractured into three sections in the most heavily deformed area ie the U bend.
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The surface of the shackle was discoloured (brown) an indication of corrosion.
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The SEM showed that the fracture face was primarily rough and inter-granular.
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Optical microscopy showed the cracks had propagated through the material in an intergranular and branched manner. This typical of cracking is typical of stress corrosion cracking in stainless steel material.
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In conclusion the D bolt shackle failed by stress corrosion cracking. The mechanism occurred due to the presence of an applied (in service) or residual stress (bend section), with a corrosive environment (chloride ions from sea water).
For more information about failure investigation or corrosion issues please contact Eirwyn Davies, Principal Metallurgist, on 01925 843490 or This email address is being protected from spambots. You need JavaScript enabled to view it..