Investigation Of Failures
Authors: Ross Robert B. 
Published: March 2016

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Components fail because they have been subjected to adverse conditions or they have some defect of design, material or construction.

The reasons for carrying out a failure investigation should be carefully examined before embarking on what could possibly be a very expensive operation. possibly be a very expensive operation. The most important reason for requesting a failure investigation, in my opinion, is that those involved have a genuine desire to identify the reason in order that future failures can be eliminated.

It is also possible that those involved are desirous of recovering any financial loss caused by the failure. Technical curiosity to satisfy oneself regarding the reason can be quite legitimate and quite often can supply information which is useful.

It is generally possible to give some indication of the complexity and therefore the probable cost of an investigation immediately following the visual examination, and certainly this should be possible with a degree of accuracy after some metallurgical investigation before becoming involved with expensive mechanical tests and chemical analysis, etc.

At this stage it will usually be possible to satisfy the person with the possible to satisfy the person with the technical curiosity, but will very often not supply sufficient information to give conclusive information of the cause.

Once the investigation becomes involved with the legal fraternity then the costs can spiral as very often this will require two independent laboratories or organizations carrying out completely separate investigations and lawyers will require very positive proof. proof. In many instances components operate successfully for lengthy periods with relative massive defects. It is the duty of the failure investigator to firstly identify the mechanism of failure and then to find the reason for the failure.

We have recently examined a pin failure used to retain an expensive sophisticated component. It gave several years successful service prior to failure. Our investigation found:The correct material was used in the wrong condition.There was a poor radius under the head.There was clear evidence of stress corrosion at this poor radius.Fatigue propagation existed from the stress corrosion at the radius.The design was incorrect resulting in heavy loading at the radius.

It was reported that with an adequate radius the pin would still be in service, surviving successfully with poor heat treatment, a fluctuating poor heat treatment, a fluctuating tensile load in a corrosive atmosphere. with poor design.

Some time ago an anchor chain link failed and was investigated. This had been in service for over 10 years, had a 10–15mm manufacturing defect, with poor heat treatment resulting in very poor heat treatment resulting in very low impact values. It also had clear evidence of quite severe service damage.

The recommendation was that the readily found manufacturing defect was the prime cause should be identified and prime cause should be identified and removed from any link in service. The brittle chain would then be capable of withstanding the handling damage.