Most often the ultimate failure of a component or device is because of the physical failure of its material. The material (or materials) represents the most fundamental aspect of a product and thus several causative factors may have been involved in the failure. In general, physical failures occur because of four broad categories of reasons: an inadequate design, a defective manufacturing process, a defective material or unintended usage. Because of this range of possibilities, several areas of technical knowledge are often required to effectively identify the root-cause of the failure, the contributing factors and rule out other causes.

One of the first questions the investigating engineer will ask is what materials were involved. Here the possibilities are metals or their alloys, polymers (i.e., plastics or elastomers), ceramics or a composite material that combines two of the previous classes. This discussion will be concerned with metallic materials. In most applications metal alloys are still the most frequently used materials.

The following knowledge areas are used in root-cause failure analysis of metal alloys – in no particular order of importance.

Physical and mechanical metallurgy is useful for several reasons. First using information from these areas and initial facts about the application, the failure analyst will be able to recognize the general characteristics, e.g., strength, fabrication characteristics, resistance to particular service environments, etc., of the material used and form some tentative ideas on causation. These indications will then guide him or her to select and complete particular metallurgically related laboratory evaluations and/or standardized tests to clearly define the material failure mechanism and contributing factors. These techniques often include microscopy, metallography, and a variety of ASTM or other standardized analytical and mechanical property tests. These procedures can also often confirm or refute that the given material or product was defective due to, for example, chemical composition, incorrect heat treatment or poor quality control during fabrication.

Failure by one of the several forms of corrosion is very common and effective analysis depends on using additional knowledge areas. First there must be a clear understanding of the several different mechanisms of attack plus the physical, chemical and electrochemical conditions that promote each and how different metallic alloys react in different applications. From this knowledge the failure analyst can use results from metallurgical tests and standardized laboratory electrochemical tests to confirm or refute specific corrosion mechanisms.

Stress analysis from mechanical engineering design is frequently an important area for the failure analyst to use. Comparing the type and levels of service stresses acting at the time of the failure and the resisting strength of the material at its failure site informs the analyst about multiple factors. These include the suitability of the material selected by the original designer, the actual versus the designed-for stresses, the role of unrelieved residual stresses due to manufacturing error and the role of cyclic stresses and mechanical or corrosion fatigue. Knowledge of  metal fatigue is very important because of the common occurrence of this failure mechanism.

The analyst needs to know – or be able to find out and fully understand – how a great variety of mechanical components and equipment are designed, are intended to function and  their limitations. These include, for example, bearings, bolted fasteners, pumps, valves, pressure vessels, gears, welded joints, brakes, springs – and more. This knowledge coupled with his or her other findings often allows the analyst to conclude whether or not the given component or equipment was being used and operated as designed and intended or whether it had been abused or otherwise incorrectly used.

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