Metallurgical failures can be extremely costly, resulting in replacement costs, downtime and, in the worst-case scenario, injury or loss of life. Thus, preventing as many of these failures as possible is of significant importance to industry. As with any engineering problem, the first step in preventing an occurrence is to understand what causes that occurrence. What follows is a list, in order, of the most prominent metallurgical failure modes, along with a brief description of the failure mode or cause of failure.
Fatigue is the term used to refer to failures that occur under conditions of dynamic loading where the maximum stresses are lower than the tensile strength of the material. Fatigue failures are estimated to account for approximately 90 percent of service failures attributed to mechanical causes. In layman's terms, these failures occur due to repetitive loading, such as vibration, pressure pulsation, rotation, etc. Fatigue failures account for the largest percentage of failures because of the nature of fatigue. It is relatively straightforward to design for yield and tensile strength, but fatigue loading can cause failures at stress levels far below yield, and things like unexpected vibration or minor material defects can result in fatigue cracks and eventual failure.
Corrosion, the breakdown of materials through chemical reactions, is estimated to cost the U.S. economy over $1 trillion annually, of which over $50 billion is estimated to directly impact the oil, gas and petrochemical industries. Unlike environmentally assisted cracking (EAC), the only thing that needs to be present for corrosion to occur is a corrosive environment, which explains the prevalence of corrosion failures on a national scale. The common nature of corrosion, combined with the high occurrence of corrosive environments found in the oil, gas and petrochemical industries, makes it unsurprising that corrosion is the second-most-common failure mode.
EAC is the failure of a material, generally at stresses significantly lower than the tensile or even the yield strength of the material, due to interaction with the environment - generally in the form of corrosive elements, in which case the cracking is referred to as stress corrosion cracking, or hydrogen, in which case the cracking is referred to as hydrogen embrittlement. For EAC to occur, three things have to be present: First, you need an environment conducive to EAC (corrosive or containing hydrogen). Second, you need a material susceptible to the environment. And third, you need a source of stress. In the absence of any of these three components, EAC will not occur. However, due to the prevalence of corrosive and hydrogen- containing environments in the oil, gas and petrochemical industries, EAC remains one of the most common failure modes.
Overload is failure of a part due to a single traumatic event that causes some part of the material to exceed its yield or tensile strength. Overload failures can be some of the most dramatic failures observed, resulting in catastrophic damage. Additionally, many failures that are not directly caused by a single overload event will still end with an overload failure, e.g., the final overload of a fatigued part. Overload generally takes two forms: ductile and brittle. While the majority of overload failures occur on a relatively short time scale, ductile overloads tend to occur more slowly than brittle overloads.
Material defects are interesting because they do not represent a failure mode, but they are a cause of failure. Material defect-related failures are things such as manufacturing defects, off-specification materials, welding defects, use of the wrong material, etc.
Wear, erosion and erosion corrosion: Any time two materials are in contact, there are frictional forces between those materials. When the materials are both solid and the frictional forces cause damage, it is called "wear," but when one of the materials is liquid or gas, it is called "erosion." When erosion occurs in a corrosive environment, there is a possibility for the erosion and corrosion to mutually accelerate each other, leading to a rapid failure mechanism known as "erosion corrosion."
Hopefully, this list provides insight into the types of failures most likely to cause trouble.
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