Fatigue (material)

Things can get tired.

Metal can get tired too. This is called fatigue. It starts with tiny cracks. These cracks grow every time we use the object. They grow a little bit with each use.

The cracks grow until they are too big. Then the metal can break suddenly. This can happen to metal, plastic, or even glass. It is hard to see the cracks at first. This makes the break a big surprise.

We can test metal to see how it works.

Things can get tired. This is called fatigue.

Fatigue happens when a material breaks from repeated use. This can happen to metal, plastic, or ceramics.

It starts with tiny cracks. These cracks often form at stress concentrations. A stress concentration is a spot like a hole or a sharp corner. These spots make the force feel stronger in one place.

Once a crack starts, it grows in steps. First, the crack moves slowly. It grows a small amount with each load. This part is called crack growth. You might see tiny lines called striations on the surface. These lines show where the crack was after each use.

Eventually, the crack reaches a critical size. At this point, the crack grows very fast. This leads to a sudden and complete break. This break often looks brittle, which means it snaps suddenly. Fatigue damage is permanent. The material does not get better even when it rests. Scientists use special tests to study how fast cracks grow. This helps them predict how long a part will last.

Fatigue is a way that materials break from repeated use.

It is not a one-time break from a single heavy load. Instead, it happens because of cyclic loading, which means a force is applied over and over. This can happen to metals, plastics, and even ceramics.

Even if the force is much smaller than the material's strength, it can still cause damage. This damage is irreversible, so the material cannot recover even when it rests. This makes fatigue a very important thing to study for safety.

The way it works happens in several clear steps. First, a crack must begin, which is called crack initiation. In metals, this often starts at stress concentrations like holes or sharp corners.

Next comes crack growth, where the crack moves slowly through the material. Each time the load is applied, the crack grows a tiny bit more. You might see tiny lines called striations that mark each step of this growth. Finally, the crack reaches a critical size. Once it is big enough, it grows very fast and the whole object snaps suddenly.

People have been studying this for a long time. In 1837, Wilhelm Albert wrote the first article on fatigue. Later, in 1839, Jean-Victor Poncelet described metals as being "tired" during his lectures.

In the mid-1800s, many railway axles failed, which helped scientists learn more. For example, in 1843, Joseph Glynn reported on a locomotive axle failure. He found that a keyway helped the crack start. By 1870, August Wöhler showed that the range of the force was very important for fatigue.

Scientists use many facts and numbers to predict how long a part will last. They use fatigue tests on small pieces called coupons.

These tests apply a constant load over thousands of cycles to see how fast cracks grow. They also look at different types of fatigue. High cycle fatigue involves more than 10,000 cycles with low stress. Low cycle fatigue happens when the stress is higher and causes the material to change shape. They even study how things like moisture can make cracks grow faster in aluminum.

You can see how fatigue affects the world around you. It is why engineers must be very careful when designing machines.

For instance, after the de Havilland Comet jetliners had accidents in 1954, engineers changed how planes were built. They replaced square windows with oval ones to help prevent cracks. This is similar to how you might avoid putting too much weight on a paperclip by bending it back and forth. If you bend a paperclip many times, it will eventually snap just like a metal part experiencing fatigue.

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.