A Short Guide to Fatigue Failure in Machine Design

At a Glance:

An overview of fatigue failure and how it affects mechanical systems.
How FEA software enables engineers to predict fatigue failure points on structural designs, before the manufacturing process.

From the dawn of the Industrial Revolution, humanity has become increasingly reliant on machines in daily life. This frequent use results in mechanical structures having to compete not only with the erosion of time, but also the fatigue stemming from repetitive loads being placed on them.

Defined as the initiation and propagation of cracks due to cyclic loads, fatigue failure can affect even the most well-built contraptions. If left unnoticed for long enough, these cracks can snowball into much larger structural deformities, leading to severe damage in mechanical components that might otherwise seem to be operating well within their limits.

This article provides an overview of what fatigue failure is, explores notable real-world examples and outlines the critical principles that every engineer should understand to help prevent this pervasive issue. It will also look at the relationship between fatigue failure and FEA software.

A Notable Example of Fatigue Failure in the Aerospace Industry

Before segueing into a discussion of fatigue failure, a good exercise would be to consider the repercussions of mechanical failure. A notable real-world example to aid this is the 1954 crash of the De Havilland Comet. As the world’s first commercial jetliner, the Comet was the crown jewel of Britain’s aerospace industry, representing a symbol of British aviation prowess…until a string of accidents changed its course.

Starting with BOAC Flight 781, two fatal Comet aircraft disasters were the subject of a multi-year investigation, with authorities concluding that metal fatigue due to design defects led to explosive cabin decompression mid-flight. In particular, the square design of forward-facing windows created opportunities for stress accumulation at the corners, the effect of which was exacerbated by riveted window supports (instead of glued).

Together, these design decisions triggered fatigue cracks following cyclic cabin pressurization, eventually leading to propagation of cracks and subsequent violent decompression of the aircraft.

Fundamentals of Fatigue Failure

Building on the example of what happens when structures are not designed with fatigue failure in mind, we can further explore the principles of fatigue failure and its identifying features. In a nutshell, fatigue failure refers to the critical weakening of a material that has been subjected to repeated loads. At a high level, there are several stages that a fatigued structure goes through on its way to complete failure.

As noted above, the first thing a fatigued structure goes through is…well, fatigue. Through exposure to fluctuating loads (e.g., wheels on rails or compression/decompression of an airplane cabin), mechanical systems can start to develop superficial cracks or scratches in areas of high stress (known as initiation).

These areas can include features such as holes, corners or fillets. Over time, these small cracks grow incrementally with every load cycle (propagation), eventually reaching sizes that allow for sudden structural failure.

A diagram of a fracture surface where a crack was triggered by fatigue.

In the image above, different stages of fatigue failure are presented. The “fatigue origin” shows the development of a superficial crack, which then propagates as “beachmarks.” Eventually, the “beachmarks” cause an “overload” of the structure, leading to collapse.

Two important types of fatigue failure are high-cycle and low-cycle fatigue. High-cycle fatigue refers to relatively low stress loads acting on a mechanical component that leads to fatigue failure over millions of load cycles. On the other hand, low-cycle fatigue relates to higher stress loads that lead to failure over a fewer number of cycles (thousands).

Factors to Consider in Reducing the Risk of Fatigue Failure

Now that the concept of fatigue failure has been well defined through fundamentals and an example, a logical question to ask is: “How can engineers and designers address the causes of fatigue failure and prevent them?”

An obvious answer relates to the choice of material used in manufacturing. By thoroughly selecting appropriate materials, engineers can ensure that mechanical components can withstand a wide variety of load cases and environmental conditions. Additionally, manufacturers can account for residual stresses—such as those introduced by welding or machining—in the design process to delay crack initiation.

Furthermore, managing loading conditions is crucial in reducing the risk of fatigue failure. When designing components, engineers must evaluate the scale, direction and frequency of cyclic loads to predict where cracks might initiate. A similar sentiment guides efforts to predict the effects of temperature-related stresses. Extreme and fluctuating temperatures can weaken materials over time, increasing the rate at which cracks can propagate.

A common method engineers employ to aid their design process is to simulate how a product or material will react by employing FEA software.

The Relationship between FEA and Fatigue Analysis

Finite Element Analysis (FEA) is an important part of an engineer’s toolbox, especially when it comes to predicting fatigue failure in mechanical designs. By creating a mesh of smaller elements out of complex geometries, FEA software can calculate detailed stress and strain distributions under realistic load cases. Engineers can then identify areas subject to high stress, which are often the first parts of a design that experience crack initiation.

Advanced FEA software can combine simulation results with fatigue analysis, creating illustrations such as S–N (stress vs. number of cycles) curves. These graphs can paint a picture of a material’s fatigue life, giving designers insights into which materials are best suited for the task at hand.

Through this fatigue analysis, FEA can highlight fatigue-prone regions and enable engineers to make targeted improvements, ultimately leading to more reliable and longer-lasting components.

Optimize Performance from Lessons Learned

The idea of fatigue failure is an important area of study in many engineering disciplines and has been evolving over time. Technological advancements in software today have helped engineers conduct fatigue analysis in more precise ways than ever before, leading to better and safer structures being built.

While disasters such as the De Havilland Comet crashes will always be a residual risk associated with air travel, the combined use of FEA software with advanced CAD programs allows engineers to mitigate this risk as much as possible. Nonetheless, the lessons learned from the Comet’s failures remain relevant today, highlighting the need to address fatigue failure from the earliest stages of machine design.