Speaking with greater clarity, why did the subject of the investigation deform or break? Why did some of its material disappear? Why have the strength characteristics become degraded?
We always have a true answer that we can provide to the question of why something broke or was deformed. It is because the stress exceeded the strength. We always have a true answer to the question of why something has lost some portion of its original mass through wear or corrosion. For wear, or mechanically induced material loss, the answer is the same as for fracture and deformation. The now-familiar baseline answer is because wear is essentially a complex process that involves deformation or fracture at the microscopic scale.
For corrosion, the baseline answer is that the environmental conditions were more severe than the component could resist. For the last question, strength characteristics depend on the microstructure, and certain environmental factors can change the microstructure over time. Usually, these changes result in a degradation of useful characteristics when compared to the carefully optimized properties that the manufacturer first produced.
In a sense, failure analysis may be viewed as an attempt to get more specific, and thus useful, answers to the questions of why something is no longer serviceable. In order to answer that question in terms of the basic physical changes that led to the diagnosis of damage or failure, we must determine the specifics of how the damage happened.
The core of that technical process starts with classifying the basic damage category into mechanical, environmental or both. Environmental damage may result from thermal exposure or impingements by other types of physical energy (light, radiation, etc.). The other main category of environmental damage is classical corrosion (i.e., exposure to incompatible chemicals or incompatible concentrations or combinations of chemicals).
All of the interactions of the component with its environment, including experiences resulting from chemical, physical (energy impingement) and mechanical (loads or forces) interactions, are complex. The temperature affects how forces are transformed into stress fields and how aggressively a substance may attack a material. Even if the environment is air, we need to consider that air is a substance that can change the way the component responds to the damaging event. Air is a different environment than a hard vacuum. For many materials, air may be considered a default; for other materials, air may be a hard vacuum. The vacuum lacks any active elements that could potentially attack the material.
The answers that we are able to extract, like a confession from a criminal, will be more useful if we are thoughtful about how we go about getting the information. Just as torture often leads to incorrect information provided by suspected criminals, damaging the evidence from a failure event can also mislead.
In order to do a good job as a failure analyst, we need to cultivate multiple skill sets in addition to knowing how to apply our technical knowledge. This includes learning about the domain of human factors.
Sticking to a process will go a long way toward gaining confidence from others who may have natural tendencies to mistrust. Understanding when someone might be tempted to hide something is a crucial skill in some situations. Knowing how to tell that someone is unaware of the significance of an important factor is also critical. The most powerful and useful failure-analysis processes are flexible enough to modify when a roadblock to the investigation appears.