Guide to Fatigue Failure

Fatigue failure is a progressive, localised and permanent damage which appears in those parts under fluctuating stresses and strains. Above certain stress levels, fatigue gives rise to cracks or fractures after a sufficient number of cycles have elapsed. It can be considered as a combination of cyclic stress, tensile stress and strains and, if any of those factors is absent, fatigue failure will not initiate or propagate. Many fatigue cracks are initiated and grow from structural defects, so that the theoretical fatigue life is reduced.

This page covers fatigue failure under the following headings:

Fatigue Life

Fatigue life can be defined as the number of stress cycles required to cause failure; being a function of many variables; stress level, cyclic wave form, metallurgical condition of the material, etc. This wide range of variables makes analytical prediction of fatigue failure difficult. Many repeat tests on similar components in service has been shown as the only available procedure. Laboratory tests, however, are essential in understanding fatigue behaviour.

Fatigue Process

Failure problems which result from fatigue generally follow three phases:

• Phase I: Initiation - Fatigue failure leads to crack nucleation and crack propagation. This initial phase never extends over more than five grains around the origin. Sometimes phase I may not be discernible, depending on material, environment, etc.
• Phase II: Propagation - Progressive cyclic growth of a crack until the remaining un-cracked cross section becomes too weak to sustain the loads imposed.
• Phase III: Fracture - Remaining cross section suddenly fractures as a result of the loads imposed.

Fatigue Failure Types

Several different variables are involved in the failure process, resulting in different types of fatigue failure. Three main types of fatigue can be found:

• Fatigue Cracking: Results from cyclic stresses that are below the static yield strength of the material. It is the most important fatigue failure process.
• Thermal Fatigue: Is the result of temperature cycling; thermal expansion and contractions cause thermal stress.
• Contact Fatigue: Parts under pressure contact with each other, usually rolling or sliding, suffer metal fatigue.
• Corrosion Fatigue: in metal parts under cycling stress in a corrosive environment. The corrosive environment accelerates formation and propagation of cracks, even where individual variables can produce stress.

Effects of Material Condition on Fatigue

Localised plastic deformation is responsible for crack propagation, and microstructure of the material can affect crack growth, either inhibiting or modifying it. Some metal conditions which affect fatigue are:

• Grain Size: Fatigue life is independent of grain size under low cycle fatigue conditions for many metals, and is increased when grain size reduces under high cycles.
• Alloying: The influence of chemical composition on fatigue is approximately proportional to its influence on tensile strength.
• Second phases: These affect crack propagation due to the strain caused by the presence of the second phase, the stress concentration of the second phase (shape, distribution) and the nature of the bond.
• Work hardening: Work-hardened alloys show lower crack propagation rates and small deformation increases during fatigue. Fatigue strength can be increased by cold working.
• Heat Treatment: Fatigue strength is generally increased by any heat treatment that increases tensile strength.

Effects of Manufacturing Practices on Fatigue

Manufacturing practices influence fatigue performance by affecting the intrinsic fatigue strength of material near the surface, by introducing or removing residual stress in the surface layers, and by introducing or removing irregularities on the surface that act as stress raisers.

• Machining: Heavy cuts, residual marks, etc can promote fatigue failure.
• Drilling: The fatigue strength of components can be reduced merely by the presence of a drilled hole.
• Grinding: Proper grinding practice produces a smooth surface that is essentially free of induced residual stresses or sites for the nucleation of fatigue cracks. However, abusive grinding is a common cause of reduced fatigue strength
• Surface compression: Compressive residual stress increase fatigue life. This can be obtained by shot peening, producing visible marks on the surface, such as dimples.
• Plating: Electroplating can impair the fatigue strength by virtue of hydrogen embrittlement.
• Cleaning: Some alkaline solutions are not satisfactory because they attack the surface.
• Welding Practices: Can have a effect on the fatigue strength of a metal at and below the surface.
• Identification marks: A high stresses may be introduced into components by identification marks (date, part number, etc).

Reducing Fatigue Failure

Manufacturing processes are known to have significant effects on fatigue properties of parts. The effects are either detrimental to fatigue properties or beneficial, represented by the chart below:

Effect on Fatigue Life of Manufacturing Processes

Shot peening is the most effective in reducing fatigue failures. Tests of shot peened rods versus polished rods show the life cycle of the peened parts to be up to 10 times longer.