What is Metal Fatigue?

Metal fatigue can cause catastrophic failures without warning, such as a fan blade separating from a jet engine, causing damage or even death. To understand and prevent metal fatigue, we must first understand something about stresses in metals.

Stress can be applied in different ways. For example, tension stress develops on the outer radius of a bent piece of metal wire. At the same time, compression stress occurs at the inner radius of the bend. Reversing the bend reverses the compression and tension stresses.

When repeated over and over, stress concentrations like these will cause microcracks. If the stresses continue, the cracks will grow. And because the cracks are small, there may be little or no visible warning. The result can be an unpredictable metal fatigue failure.

Stresses that Cause Metal Fatigue

In addition to bending, or radial stresses, other types of stresses can cause metal fatigue. There may be a defect caused by the manufacturing process or within the material itself. Increased metal fatigue also can occur due to corrosion, part rotation, temperature, wear, or structural design. For example, the edges of holes tend to concentrate stress, but the hole could be placed elsewhere making the part less susceptible to fatigue.

The stresses that cause metal fatigue are usually lower than the material’s ultimate tensile strength. Engineers designing a part must understand how much repeated stress the part can handle, and that partly depends on the fatigue strength of the metal.

Forms of Material Fatigue Failure

Metal fatigue failure can be reflected in different forms of fatigue, including:

• Thermal fatigue failure. This type of metal fatigue occurs due to temperature changes. These changes can be caused by environmental factors as well as temperature fluctuations from applications being turned off and on.
• Corrosion fatigue failure. Commonly due to corrosive environments that damage the metal. Corrosion can initially cause cracks which can cause mechanical damage and fatigue.
• Vibration fatigue failure. As the name implies, vibration fatigue is due vibrational damage that leads to cracks and stresses when equipment is functioning at levels that are out of operational standards.
• Mechanical failure. This type of metal fatigue is due to stresses occurring over time and includes corrosion and vibration fatigue failure.

Design Considerations for Determining Metal Fatigue Strength

Different materials have different fatigue strengths. To determine the fatigue strength of a material, engineers will test multiple identical specimens under different cyclic loads until they break. Many such data points can then be plotted on a graph to determine the fatigue limit of the material.

Using this known value, structural engineers can perform a software fatigue analysis of a part design. If needed, they can redesign the part to minimize internal stresses. Or they could specify a different material that would be more resistant to fatigue stress.

Engineering designs where metal fatigue from repeated stresses can cause problems include:

• Jet engine turbofans with rotating propellers
• Airplane body parts
• Off-road bikes
• Bridges with traffic and wind vibration
• Automotive suspensions
• Manufacturing equipment
• Any component under vibrational stress

Metal Material Considerations

Selecting the proper metal material for your application can greatly reduce and prevent metal fatigue. Metals that are corrosion-resistant and allow for heat conductivity are better suited for applications that are subjected to challenging environments. We are able to send material test reports for all Aluminum products, Stainless Steel products and Steel Sheet and Plate as well as long products.

Aviation Metal Fatigue

Metal fatigue in aircraft is a common occurrence due to the cyclical nature of pressure and stress on aircraft parts and components. Over time small cracks can increase in size and scope, where metal fatigue can become a contributing factor to mechanical and structural failure.

Pressure, atmospheric exposure, and general flight conditions can weaken aluminum, carbon steel, and stainless steel aircraft components. Main areas where metal fatigue occurs in aircraft:

• External areas such as skins that function under structural pressure
• Internal areas where load-bearing components are subjected to high stress environments

Routine maintenance and inspection can help offset aviation metal fatigue while polishing the aircraft’s metal surfaces can also help slow the effects of cracks and fatigue.

Prevent Metal Fatigue with High-Performance Metal Materials