What is a failure analysis? That's a good question. Forty years ago, the minister of my church asked me what I did for a living. I answered, "I am a consulting engineer." With a puzzled face, he asked, "What does a consulting engineer do?" I told him most of my work involved conducting studies that were generally called failure analyses. He looked at me and said, "You know, I do a lot of failure analyses as well. I didn't realize that is the type of work engineers do." At that point, it was clear an engineer's concept of a failure analysis was not a universal concept.
For many, a failure analysis is the determination of why something broke. From an engineering perspective, a failure analysis is an investigation or study that is performed to determine how a component or system failed and the factors that contributed to the failure.
A typical failure analysis consists of four key components: background information, inspection and testing, research and analysis, and conclusions. All of the components of the analysis are interdependent. To illustrate the interdependency of each component of a failure analysis, I will review the analysis of leaking, integrally finned copper chiller tubes.
Background information is a critical component of the analysis. Information the failure analyst may need includes specifications for the component that failed, the sequence of events that led up to the failure, the environment in which it was functioning, the service history and the manufacturing process used to fabricate the component.
For this illustration, background information tells us the tubes were in service for two years and developed pinhole leaks. The chiller was retubed, and the new tubes developed leaks two days after the chiller was placed back into service. Two weeks prior to the original failure, the water supply was temporarily changed to on-site well water. Additional information provided includes inspection and maintenance histories and material specifications.
Inspections and tests performed depend on the type of failure analysis. A failure analysis can include the selection of samples to be analyzed, nondestructive testing and destructive testing. In most cases, the most critical component of inspection and testing is the selection of samples.
Several representative tubes from various failed locations in the chiller were selected to be tested. It was important we selected representative tubes from both the original installation and the new installation. The tests performed included visual and optical examinations, microscopy, corrosion analysis, and examination of the chemical and mechanical properties of the tubes.
The research and analysis phase includes the evaluation of the results of the tests performed, analysis of the physical system that experienced the failure, collecting additional information from literature and service data, and the formation of conclusions. The evaluation of the test results may indicate additional tests need to be performed or additional samples need to be evaluated.
With the failed tubes, inspection and testing were not enough. Additional information on the water supply as well as causes of pitting corrosion were also reviewed. The corrosion products in the pits were redgreen in color and crystalline in appearance with a high sulfur content.
The conclusion phase is the important step that identifies the mode of failure, or why a structure or system broke. Sometimes, the conclusions will include a new method or new material necessary to prevent the failure from happening again.
The results of the analysis of the failed chiller tubes were consistent with microbiological induced corrosion. The water supply was not the source of the bacteria that was causing the corrosion. Concentrations of the bacteria associated with the failure of the tubes were detected in the well water.
Each component of a failure analysis is interdependent and complex. A failure analysis cannot be completed and the "why" question cannot be answered without the consideration of all four components.
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