Fatigue (material)

Fatigue is the process where a material develops cracks due to cyclic loading. Cyclic loading means a force is applied repeatedly over time. This process is important because it causes materials to fail even when the force is much lower than their actual strength. Fatigue is not just a phenomenon in metals. Most materials, including plastics, ceramics, and composites, can experience fatigue-related failure.

Unlike a single heavy impact, fatigue is a cumulative and irreversible process. This means the material cannot recover its original state even after the load is removed.

The mechanism of fatigue follows a specific sequence of stages. It begins with crack initiation, where a tiny crack first forms. In metals, this often occurs at stress concentrations like holes, sharp corners, or grain boundaries.

After initiation, the material enters the crack growth phase. This phase is divided into two parts: Stage I and Stage II. During Stage I, cracks propagate slowly along crystallographic planes where shear stresses are highest. Once the crack reaches a critical size, it enters Stage II. In this stage, the crack grows quickly in a direction perpendicular to the applied force. Finally, the material reaches ultimate failure, often through a sudden and brittle fracture.

In metallic samples, the initiation process involves four discrete steps. First, the material develops cell structures and hardens in response to the applied load. This hardening increases the amplitude of the applied stress because it creates new restraints on strain. Eventually, these structures break down to form persistent slip bands (PSBs). These bands cause localized slip, creating surface features called intrusions and extrusions. These features make the surface look like the uneven edge of a deck of cards. This surface roughness acts as a stress concentrator, which helps the crack nucleate.

Scientists categorize fatigue into two main types based on the number of cycles. High cycle fatigue occurs when a material undergoes more than 10,000 cycles before failing. In this type, the stress levels are relatively low and primarily elastic. Low cycle fatigue involves fewer than 10,000 cycles and occurs under higher stress. This higher stress causes significant plasticity, meaning the material undergoes permanent shape changes.

To predict how long a part will last, engineers perform fatigue tests using small samples called coupons. They apply constant amplitude cyclic loading and measure the crack growth over thousands of cycles.

The history of fatigue research is tied to industrial accidents. In the nineteenth century, many metal railway axles failed suddenly. In 1842, a locomotive axle failure caused the Versailles rail accident.

In 1843, Joseph Glynn investigated a locomotive tender axle and identified a keyway as the crack origin.

By 1870, August Wöhler summarized his work on railroad axles. He concluded that the cyclic stress range was more important than the peak stress. He also introduced the concept of the endurance limit. In 1903, Sir James Alfred Ewing demonstrated that fatigue failures actually begin with microscopic cracks.

Many environmental and mechanical factors can change how fast a crack grows. For example, higher mean stress and increased moisture both increase the rate of crack growth. In aluminum, moisture can cause hydrogen embrittlement at the crack tip.

Interestingly, cracks growing inside a material in a vacuum grow much slower than surface cracks. Other factors include the "short crack effect," where cracks smaller than 1 mm grow faster than expected. Overloads can also change growth rates. An overload greater than 1.5 times the maximum load initially increases growth, but then leads to a long period of reduced growth.

Understanding fatigue is vital for modern engineering and safety. A famous example occurred in 1954 with the de Havilland Comet jetliner. Several planes broke up in mid-air due to fatigue failures.

This disaster led manufacturers to redesign aircraft. They replaced square windows with oval ones to reduce stress concentrations. Today, fatigue is recognized as a stochastic process, meaning it has a degree of randomness. Even identical samples can show different fatigue lives in controlled environments. This makes careful testing and design essential for everything from bicycles to jet engines.