Fatigue, one of the most common mechanisms leading to component failure, refers to the cracking or deformation that occurs in materials as a result of exposure to stress cycles. This stress comes in many different forms, such as compression, expansion, tension, or torsion. If undergoing significant cyclical stress, even highly ductile materials may be subject to fatigue failure over time.
Component fatigue life can be difficult to predict, as fatigue often results from repeated exposure to stresses that are far below the maximum predicted yield strength of a given material. Put simply, even moderate repetitive stresses can cause a part to wear out and crack; Further stresses will compound this damage until the crack expands and leads to potentially catastrophic part failure.
The harbingers of fatigue mechanics often escape notice, even to highly educated engineering professionals. In this blog, we will discuss the various types of fatigue failure and the warning signs for which engineers and manufacturing professionals should look.
Recognizing Fatigue
While fatigue is typically recognized as the most common form of failure, there are many specific types of fatigue. Understanding the properties of each individual type will assist engineers in determining which fatigue risks may affect their application. The most common types of fatigue failure include:
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Mechanical Fatigue
This failure type is usually caused by a mechanical strength issue and is typically attributed to poor raw material choice, manufacturing defects, weak welds, or design flaws.
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Creep Fatigue
Caused by accumulative cyclical or thermal stresses that deform the material, creep occurs slowly over a long period of time until the deformity becomes so severe that the part cannot function as intended.
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Corrosion Fatigue
Corrosion happens when a reaction occurs between a given material and an external element – usually water or other environmental factors. As oxidation changes the properties of the original material, it becomes brittle and more susceptible to cracking caused by cyclical stress.
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Thermo-mechanical Fatigue
An example of crossover between fatigue types, thermo-mechanical fatigue is the combined result of mechanical, creep, and corrosion fatigues. As temperature extremes cause creep and related environmental factors (i.e. humidity) create corrosion, materials become more prone to damage and cracks from cyclical loading.
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Fretting Fatigue
Unique to applications where contact occurs between separate surfaces, fretting refers to the wear that occurs on one or both surfaces after repeated cyclical contact.
What You Need to Know
No matter which type of fatigue occurs, the process leading up to failure can be broken down into three steps:
1. Initial Cracking
The longer a component is utilized in an application, the less ductile it becomes. As the material’s ductility gets lower, the chance of a stress cycle inducing a crack at the molecular level increases.
2. Expansion
Further cyclical stress will aggravate the existing microscopic crack, causing it to expand. If a crack is visible, the part or component may be at risk of imminent failure.
3. Failure
Ultimately, the part will fail. Based on the type of fatigue responsible for the failure, the part will either break as it becomes too brittle to withstand stress or deform to the point where it cannot function properly.
The risk of fatigue failure is most common in industries that place frequent cyclical stresses on their parts and components. Examples of such industries include:
- Aerospace
Aerospace components typically operate under intense stress, extreme temperature fluctuations, and demanding environmental conditions. This makes material choice and prototyping especially important, as aerospace parts are particularly susceptible to nearly all types of fatigue.
- Medical
Medical tools and devices run a higher risk of corrosion due to frequent contact with liquids, bodily fluids, and cleaning agents, among other risk factors.
- Automotive
Much like aerospace, materials used in the manufacture of automobiles experience a broad range of environmental and operational risk factors and are thus vulnerable to all types of fatigue.
- Energy
Power generation often involves intense heat, leaving parts at increased risk of thermo-mechanical fatigue and creep.
- Oil & Gas
Pipelines used to move gases, oil, and other refined materials are subject to constant environmental exposure and intense pressure, leaving them at risk for corrosion, creep, and thermo-mechanical fatigue.
Fatigue life can be extended dramatically through the careful selection of raw materials, the application of appropriate coatings, and by thoroughly testing designs for flaws and weak points before implementation. Additionally, materials should be run through a series of fatigue tests in advance of production to ensure they will be suitable and cost-effective for a given application.
Partnering with IMR Test Labs
IMR Test Labs offers a full range of testing services to help customers determine the fatigue life of the materials and components they use daily. We offer mechanical fatigue testing, high and low temperature fatigue testing, high and low cycle fatigue testing, metallurgical analyses, and a diverse range of other testing services for both metals and non-metallic materials.
Our global network of state-of-the-art testing facilities enables us to provide testing services at any point in a product’s development cycle. Highly educated and uniquely experienced, our chemists, PhDs, engineers, and support staff help our customers determine the best material for use in any application within their unique operations.
To learn more about failure analysis, download our eBook: Case Study Guide to Failure Analysis. To discover how our testing services can help you avoid catastrophic product or material failures, please contact us or request a quote.
