This term raises some vital issues that the engineer should be aware of when using it. First several definitions are available and thus it is perceived differently by different groups and individuals. From a legal standpoint, in many states, using the term to describe a product that was involved in an incident involving an injury means that the designer or manufacturer of that product may be financially liable for the injury. Engineers sometimes use the term simply to indicate an imperfection in a product without intending its frequent legal use. Further the engineer realizes that no material and no finished product is perfect. However, these variations from perfection do not necessarily mean they were cause for failure and thus the injury that may occur in a given incident. Some product liability attorneys may not readily acknowledge or understand this distinction.

Use of the "defect" term is thoroughly discussed in the very useful reference cited at the end of this blog post. Given here is a brief summary of some of the points from that article that I found to be most valuable.

Precise use of certain terms can be difficult. For example - root cause failure analysis - implies there is a single fundamental reason a product failed. Actually engineering failure analysts know that very often there are multiple causes of physical failures of products. Some of these causes may be primary and others are lesser but still contributing factors. Further, often imperfections exist in a material or product that are real but a competent failure analysis will conclusively show that they played no part - either as a primary or as a contributing factor - to the given failure. Thus referring to these imperfections as defects can be dangerous.

The key point is that all defects are imperfections BUT all imperfections are not defects. Examples of material or product imperfections might be small geometric or dimensional variations from a drawing, minor variations in the microstructure of a metal or a variation from the specified surface finish on a product when none of these variations had any role in the type of failure that occurred. However, the surface finish example could be correctly identified as a "true defect" if the product was subjected to cyclic stresses in normal service and it was conclusively shown that it failed by fatigue. Another example would be a small variation in the level of an alloying element specifically added to a metal for corrosion resistance that failed solely by static stress overload while in a corrosive environment. In that case the chemical composition variation of the metal would be an imperfection but not necessarily a defect unless it was shown that the variation in composition diminished the metal's mechanical properties.

An imperfection in a product generally means it deviates from some criterion used to specify it. The imperfection only becomes a true defect if it is a primary or a contributing cause of the failure as proven by a rigorous failure analysis. Therefore, for self- preservation, it is essential that the original designer clearly and explicitly details the required criteria for each feature that must be met for safe and reliable use of the product in its intended service. If this is done and a failure occurs due to one of these critical features then the designer is not responsible. The liability then falls to others, e.g., a material or sub-component supplier. If the design engineer is loose and non-specific with the specifications regarding critical features then he or she is responsible. Of course the liability goes to the user if he abuses the product by applying it for a non-intended use. A classic example of the latter is using a screwdriver as a prying bar. The issue then becomes a "defective user"!

W.T. Becker, R.J. Shipley and D. Aliya, "Commentary: Use of the Term Defect", Journal of Failure Analysis and Prevention, April 2005, pages 16-19, (published by ASM International).