Going back to our potential fracture analysis, in addition to the crack itself, other locations on the component and mating components of the assembly often contain key features that allow the failure to be understood. Many failure-analysis references provide lists of background questions that should be answered during investigation of the physical failure. A thorough physical investigation supported by a thorough background and documentation investigation is the FOUNDATION of any failure-analysis project. However, the failure analyst that stops with a description ofhowthe component failed has only completed the first step in explaining whythe component failed.
Many components that are subjects of a failure investigation have suffered from multiple categories of damage. Material loss or property degradation may proceed unnoticed until an undesired material shape change calls attention to the preceding situation. By the time the investigator gets to examine the component or assembly, the sequence of damage may be very difficult to determine. That is part of what makes failure analysis work so challenging, and it is part of why doing a simple certification check is not a valid approach to failure analysis. Post-failure damage may cause a component that was “in spec” to go out of spec.
In general, a thorough failure analysis will include characterization of properties at the failure location, analysis of service conditions in as quantitative a manner as possible and an analysis of as much of the design and manufacturing process as seems relevant.
Sometimes it is not possible to analyze the properties at the failure location. This is often the case in wear, erosion or corrosion failures where the material that was originally in contact with the environment is gone. In such cases, the test locations must be selected with care and based on sound, well-documented reasoning so others can understand why certain specimens were selected as opposed to others. Picking a test location just because it is convenient may lead to misleading data.
If you need to know WHY something failed, you have to have a conversation with the people who will be doing the work. There are many levels of “thoroughness,” and it is important for you to communicate what the detailed purpose of the investigation is and who will be reading the report or attending the presentation of data. The potential failure analyst should ask YOU, the client, a lot of questions. Only in this way can you be sure that the investigation has a good chance of providing the desired information. This does not mean you will always get clear, definitive answers. As someone said, sometimes the best conclusion to a failure investigation report is:
“We have not succeeded in answering all of your questions. In some ways we feel as confused as when we started. However, we are now confused at a higher level and about more important things!”