Improving Fatigue Resistance

Fatigue involves localized, permanent damage to metals exposed to cyclic stress. In a previous article I discussed the fatigue mechanism. This article covers factors that can be addressed to improve high-cycle fatigue life

Factors that influence fatigue life

Several design, material, and fabrication factors influence component and joint fatigue life, including the following:

• Applied stress
• Metal strength
• Mechanical design features that are stress concentrators
• Non-metallic inclusions
• Fabrication defects
• Surface residual stress
• Surface roughness
• Metal fracture toughness

Applied stress

Fatigue life is inversely proportional to the stress on a component or joint. Sometimes, the easiest way to improve component and joint fatigue life is to reduce the load and/or increase component or joint cross-section.

Metal strength

Increasing an alloy’s strength increases the number of cycles before a crack forms. Strength can be increased by adding alloying elements, cold working, and/or heat treating. Steels can be made so strong that fatigue cracks do not form.

Keep in mind the trade-offs between strength and fracture toughness. For an alloy with a certain microstructure, as its strength increases its fracture toughness decreases and the crack length before final overload fracture decreases. See the discussion in this article on strength and toughness. So, while increasing strength can increase the number of cycles before crack formation, increasing strength too much can lead to fracture after a small crack has formed.

Mechanical design features that are stress concentrators

Notches, holes, changes in x-section, and laser or scribed surface identification marks are examples of component features that are stress concentrators. Eliminating them or designing them to reduce the stress concentrating effect are ways to improve fatigue life.

Inclusions

Inclusions are nonmetallic and sometimes intermetallic particles in a metal that acts as stress concentrators and fatigue crack initiation sites. They are usually simple oxides, sulfides, nitrides, or their complexes in ferrous alloys and can include intermetallic phases in nonferrous alloys. Inclusions are the product of chemical reactions and contamination that occurs during metal melting and pouring.

Some alloys are produced using special processing and control over impurity levels to reduce the number of inclusions. Also, control over supply base is important – make sure metal comes from mills that have good control over their processes.

Fabrication defects

Fabrication defects include voids that form during metal casting and laps and seams that form during hot working processes. These defects are stress concentrators that can become crack initiation sites.

Surface residual stress

Residual stresses are locked-in elastic stresses within a metal, even though it is free of external forces (see this article on residual stress). Residual stresses can be tensile or compressive.  In fact, tensile and compressive residual stresses co-exist within a component. Tensile residual stress at the surface of a component add to the tensile stress being applied, leading to reduced fatigue resistance. Compressive surface residual stress normally increases fatigue resistance because they subtract from the applied stress.

Cold working, steel through hardening (quench and temper), electroplating and other coatings, and welding are examples of processes that can result in tensile residual stresses at a component’s surface. Shot peening and other surface forming processes result in compressive surface residual stress and are used specifically for that purpose. Stress relief heat treating is used to reduce elastic stresses in components and weld joints

Surface roughness

Surface roughness acts as stress concentrators, reducing the number of cycles to initiate a fatigue crack compared to a smooth surface. The rougher the surface, the worse the fatigue resistance is for a metal. Different component fabrication methods result in different levels of surface roughness.

Fracture toughness, K_IC

Fracture toughness is a measure of the ability of a material under load to withstand fracture when a crack is present. For two metal samples with the same applied load, the sample with the higher fracture toughness will be able to tolerate a larger crack before fracturing. Fracture toughness depends on the composition and microstructure of a metal.

Engineering for fatigue

Many approaches are available for designing and fabricating components and joints that have the reliability needed to withstand exposure to fatigue conditions. The trick is to identify the fatigue requirements for a component or joint and use the design and fabrication approaches that are easiest and least costly to implement.

Learn from failures

Finally, if you have components that are failing by fatigue, perform a failure analysis to determine the metallurgical and mechanical factors that are contributing to the failures to give you a better sense of the approaches to use to prevent the failures.

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