What is Metal Fatigue? Causes, Forms, and Prevention

Metal fatigue happens when repeated stress causes metal parts to weaken and crack over time. 

Even if everything seems fine after each use, this slow damage can lead to sudden failures in machines, vehicles, or structures, often when you least expect it. 

Understanding how metal fatigue works is crucial for maintaining the safety and longevity of your equipment.

So how do you spot the warning signs before it’s too late? And what can engineers and manufacturers do to prevent it? Let’s break it down.

What Is Metal Fatigue?

Metal fatigue occurs when metals gradually weaken due to repeated stress or loading. Over time, cracks form and grow, and the metal may break even if the force is relatively weak.

Metal fatigue typically occurs in parts that undergo repeated motion or are subjected to repeated use. 

You can see it in things like:

• Airplane wings and engines
• Car parts such as wheels and axles
• Bridges and railway tracks

The process works like this:

• Small cracks start in the metal after enough cycles of stress.
• Each time the part is used, the cracks get bigger.
• Eventually, the metal can break without warning.

Metal fatigue does not mean that metal just gets "tired". Instead, it's about how the structure inside the metal changes incrementally.

Which Metals Are Most Prone to Fatigue?

Certain metals are more susceptible to fatigue due to their properties and common applications. 

Here’s a breakdown:

Metals Commonly Affected by Fatigue

🟢 Steel

• Strong yet heavy, it is widely used in bridges, buildings, and heavy machinery.
• Prone to fatigue over time, especially under repeated stress.

🟢 Aluminum

• Lightweight, so it’s popular in aircraft, cars, and bicycle frames.
• Less durable than steel under cyclic loads, making fatigue a key concern.

Metals with High Fatigue Resistance

🟢 Titanium & Nickel Alloys

• Used in demanding applications like jet engines and turbines.
• More resistant to fatigue, but also more expensive.

📌 Key Takeaway: Selecting the right metal depends on the application; balancing strength, weight, and fatigue resistance is crucial for ensuring safety and longevity.

How Does Metal Fatigue Occur?

When metal is subjected to repeated or fluctuating loads over time, its structure can become weakened. 

This process typically occurs in three main stages: microscopic cracks form, these cracks grow, and ultimately, the metal breaks.

1. Crack Initiation

In the first stage, tiny flaws or defects start to appear inside the metal. These are often so small you cannot see them without special tools.

Crack initiation often begins at points of stress concentration, such as:

• Surface scratches or dents
• Sharp corners
• Notches or holes
• Grain boundaries within the metal

Repeated loading causes these minor imperfections to become weak spots. Each stress cycle increases the size and number of these tiny cracks.

Material defects, such as air bubbles or impurities from the manufacturing process, also play a role. Poor design and harsh environments, such as exposure to corrosion, can accelerate the initiation process.

2. Crack Propagation

After small cracks start, they begin to grow. This occurs when the metal is still in use and is experiencing stress.

Every time you use the part, the crack grows a bit more. Over thousands or millions of cycles, the crack moves deeper into the metal. The crack usually develops in the same direction as the main stress.

Factors that affect crack growth include:

• The size and type of load
• Number of load cycles
• Temperature
• The presence of corrosive agents

As the crack grows larger, the remaining metal must carry a greater load. This causes the metal to weaken over time, thereby increasing the risk of failure.

3. Final Fracture

At this stage, the crack reaches a critical size. The metal can no longer handle its load, even if the stress is not very high.

Failure happens suddenly. You may not notice any outward signs until the part breaks. This stage is fast compared to the other stages.

The key details of final fracture are:

• Break usually occurs at the most significant crack
• The break surface often looks rough or jagged
• The structure loses strength and cannot recover

You can prevent reaching this stage by regular inspections and replacing parts before cracks become dangerous.

📌 Also Read: Different Types of Metal Used in Welding

Common Causes of Metal Fatigue

Metal fatigue occurs when specific conditions are present, increasing the likelihood of cracks and failure. The most significant issues involve repeated stress, harmful environments, and flaws in the manufacturing or design of parts.

1. Cyclic Loading

Cyclic loading is a primary reason metals wear out. This happens when you apply repeated stress, even if each load is not very large.

For example, parts such as airplane wings, car axles, and bridges are always in motion or vibrating. Even if the movement is slight, the metal bends back and forth over thousands or millions of cycles.

These repeated actions make small cracks inside the metal grow larger. Over time, the cracks can get big enough to break the part, sometimes with very little warning.

Key points about cyclic loading:

• Most fatigue cracks start on the surface.
• Loads much lower than the metal’s maximum strength can still cause fatigue.
• High-stress areas and places with lots of vibration are most at risk.

2. Corrosion & Environmental Factors

Metal exposed to moisture, salt, or chemicals can suffer from corrosion. This weakens the surface, allowing cracks to start and grow faster.

For example, if you use steel in a damp place or near the ocean, rust can form quickly. The corrosion eats away at the metal, making it thinner and full of pits or small holes.

These damaged spots act as starting points for fatigue cracks. Even light or regular use can cause cracks to deepen when corrosion is present. Chemical exposure can exacerbate the damage, breaking down the metal’s structure over time.

Main factors:

• Rust, humidity, and chemical spills speed up fatigue.
• Corrosion and fatigue together create “corrosion fatigue,” which is more dangerous than either alone.
• Protective coatings and regular cleaning help slow this process.

3. Poor Design & Material Defects

If a part is designed with sharp corners, thin edges, or holes in the wrong places, it becomes much easier for cracks to start.

Stress is often highest at the edges of holes or notches. These are referred to as “stress concentrators.” 

If the shape of the part is not smooth, or if the material has flaws such as impurities or tiny air pockets, cracks can form more quickly.

Sometimes, problems come from the manufacturing process, such as tiny cracks or inclusions inside the metal. These defects are often invisible but can weaken parts from the inside out.

Key design and material issues:

• Sharp corners and sudden changes in shape should be avoided.
• Defects within the metal, such as slag or bubbles, reduce its fatigue life.
• Careful design and inspection help prevent these problems.

📌 Also Read: What Temperature Does Aluminum Melt: Welding Metal Tips 2024

Signs and Symptoms of Metal Fatigue

Metal fatigue happens when repeated stress weakens metal over time. 

Here’s what to look for:

• Visible cracks – Small cracks often appear near stress points like welds, joints, or bends. These can grow larger if not addressed.
• Surface deformations – Dents, uneven areas, or warping can signal internal weakening.
• Unusual noises – New or worsening sounds (such as clicking, rattling, or ticking) may indicate that parts are wearing down due to stress.
• Increased vibrations – Machines or vehicles shaking more than usual may indicate fatigued components.
• Sudden failure – In some cases, metal fatigue causes parts to break without obvious warning, especially if cracks go unnoticed.

Regular inspections help catch these signs early and prevent unexpected failures.

📌 Also Read: What is Porosity in Welding

How to Prevent Metal Fatigue

Using the right material is one of the best ways to prevent metal fatigue. 

Choose metals or alloys that are resistant to fatigue, such as certain steels, titanium, or aluminum alloys. These materials can handle stress changes better and will last longer under repeated use.

Design is very important. Focus on reducing stress concentration points. Smooth out sharp edges and avoid sudden changes in the metal's shape. 

Rounded corners or gentle curves help distribute the load and reduce the risk of cracks forming.

It helps to use bulleted lists to remember key steps for stress reduction:

• Smooth all sharp edges and corners
• Avoid holes or notches unless necessary
• Transition changes in thickness gradually

Regular inspections are important for catching early signs of fatigue. Non-destructive testing (NDT) methods enable you to check for cracks without damaging the metal. Common NDT methods include:

• Ultrasound testing: sound waves pass through the metal to detect gaps or cracks
• X-ray inspection: shows hidden cracks inside the metal
• Dye penetrant testing: a dye highlights cracks on the surface

Routine maintenance is key. Check parts regularly and address any minor issues promptly. Staying alert to signs of wear, corrosion, or loading beyond safe limits helps you avoid sudden failure.

By combining smart material choices, effective design, and regular inspections, you can lower the risk of metal fatigue in your equipment or structures.

📌 Also Read: 9 Common Weld Defects: Causes, Types, and Prevention

Metal Fatigue Testing & Analysis Methods

When you want to check how a metal part will hold up under repeated stress, there are several main testing and analysis methods you should know.

The S-N curve, also called the stress-life curve, is used to show how many cycles a metal can handle before it breaks. This curve is made by repeatedly applying a specific stress to test samples and recording the number of cycles until they fail. The S-N curve helps you compare different metals and pick the best one for the job.

Fatigue life prediction models let you estimate how many cycles a metal can go through before it cracks. Common models include the Basquin and Coffin-Manson equations. These models use data from real tests to give you practical numbers for safe product design.

Cyclic loading is a test where stress is applied and released over and over again. This shows how a piece of metal will behave in real life, such as in bridges or airplane wings.

Computer simulations like finite element analysis (FEA) allow you to model complex parts under repeated load without having to make expensive prototypes. Using FEA, you can spot weak spots and predict failures before building the actual part.

Some key steps in metal fatigue testing and analysis are:

• Preparing samples of metal or parts.
• Applying repeated or cyclic loads using special machines.
• Measuring how many cycles parts survive at each stress level.
• Comparing results to prediction models or simulation data.

Conclusion

To prevent metal fatigue, choose high-quality materials, avoid sharp stress points, and regularly inspect for cracks. 

Proper welding and fabrication techniques also help identify and eliminate weak spots that can lead to failure.

When working on metal projects, using the right equipment is crucial. ARC Captain’s durable welders ensure strong, fatigue-resistant joints. 

Check out our 110V MIG welders for light-duty work, 220V welders for heavy-duty jobs, or a 110V stick welder for versatile repairs. Invest in reliable tools to build long-lasting metal structures.

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Frequently Asked Questions

Metal fatigue can occur in various metals when they are subjected to prolonged stress. Detecting and managing fatigue early can help prevent failures and maintain safety.

Can Metal Fatigue Be Repaired?

Metal fatigue usually cannot be fully repaired, but the damaged part can sometimes be reinforced or replaced. Welding or patching may work for small cracks, but it doesn’t restore full strength. In most cases, it's safer to replace the fatigued metal. Regular checks help catch fatigue early before it leads to failure.

How Long Does Metal Fatigue Take to Develop?

Metal fatigue develops over time after repeated stress or loading. It can take days, months, or even years depending on how often the metal is used and the amount of stress it experiences. Tiny cracks start to form and grow with each load cycle. Eventually, the cracks lead to failure if not found early.

Does Metal Fatigue Affect All Metals Equally?

No, metal fatigue does not affect all metals equally. Some metals like titanium and certain steel alloys are more resistant to fatigue. Softer metals or those with lower strength can develop fatigue more quickly. The material’s quality, design, and how it’s used all affect its fatigue life.

Can You Detect Metal Fatigue Before Failure?

Yes, metal fatigue can be detected before failure using methods like visual inspection, ultrasonic testing, or X-ray scanning. These tools help find small cracks or weak spots. Regular maintenance and inspection are crucial for identifying fatigue early. Catching it early can prevent accidents and extend the life of the metal part.

What Causes Fatigue in a Material?

Fatigue in a material is caused by repeated loading and unloading, even if the force is below the breaking point. Over time, this repeated stress causes small cracks to form. Factors like vibration, bending, or temperature changes can make fatigue worse. Eventually, the cracks grow and lead to failure.