Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Glossary of Engineering Terms


Beach Mark When studying the surface of a fracture, many times there are a series of concentric half-moon shaped lines; they can also be nearly straight and parallel. These lines indicate where the crack front actually stopped in its progress. The crack does not start again until the load, which initiated the crack in the first place, is applied again. If the load is removed then the crack stops and a new mark is made. The term came from the observation one can see on an ocean beach as the tide goes out and the level of the ocean lowers, any debris is left at the last previous high level. All of this debris being placed at the same ocean level creates a line of debris, or to coin the phrase "beach mark." The beach mark is always oriented perpendicular to the direction of the stress, so a failure analyst can tell exactly how the failed part was loaded and where the crack started. Another way to look at how crack growth works versus the beach marks left, is to imagine the circular ripple formed in a calm bucket of water when a small pebble is dropped into it. The pebble creates a circular wave which emminates outward from the epicenter. All of this information is very useful in failure analysis.
Chevrons Cracks start on a microscopic level as voids. If they are on the same plane they will eventually join to become a tiny crack with a relatively smooth surface. If however these tiny cracks do not all lie on the same plane, then as they get bigger and get closer to each other there will be a point when they will join from a tearing action. Visually consider if you will, two columns of 10 dots each. Now start a crack at the upper left dot and progress downward along the same column connecting the dots; at the same time start a crack at the lower right dot and progress upwards, again, connecting the dots in the same column. You can see that as the cracks approach each other there is less solid area between them. At some point, depending upon the type of material it is, there will not be enough material left between them and the two cracks will join. The location of this change in direction is known as a "chevron mark." On a very large scale these chevron marks join and they clearly show to the naked eye the direction of travel of the crack. This is very important when trying to determine where the crack started in the first place.
Corrosion Fatigue It is a combination of two failure mechanisms working together. The corrosion mechanism reduces the effective cross sectional area of a component until the working stresses exceed the fatigue limit of the metal. A crack develops and then progresses. The rate at which the crack progresses is proportional to the stress. The rate of corrosion is also dependent upon whether the crack and its progression interferes with the ability of the metal to develop and maintain its protective oxide film.
EDX An anacronym for Energy Dispersive X-ray. EDX can determine the chemical composition for regions as small as 0.1 micrometers. The depth of analysis can also be as little at 0.1 micrometers. These two parameters are functions of beam voltage and sample compostion.
Fretting When two normally tight fitting components begin to loosen up they will begin to bang against each other. The constant banging will cause local spalling, or flaking away of the material; this leaves very light craters the size of flakes. Most of the time this local fatigue is accompanied with a red oxide layer. The oxide is from the physical interaction which takes place between the two surfaces.
OEM Accronym for Original Equipment Manufacturer
Sensitization A phenomena whereby a stainless steel in the presense of about 700 to 1100 degrees F. will precipitate chromium carbides at the grain boundaries. The grain boundaries now being depleted of much needed corrosion resisting chromium are prone to chemical attack. Cracks will follow these corrosion prone grain boundaries.
Stabilized One way to minimize chromium carbide precipitaion is to include alloying elements in the metal that preferentially combine with carbon before chromium has a chance. In this way, the chromium is left in the matrix to provide corrosion resistance. Elements that do this are columbium and/or tantalum.