Fatigue is damage to materials caused by repeated cyclic stresses which eventually lead to crack formation and eventual failure. This material destruction can occur even when the stress intensity is below the fracture strength of the material.
Fatigue crack growth behavior is dependent on a number of parameters. In region 2 of the Paris equation (Eqn. 2) the mean stress and frequency of cyclic loading are more influential than the microstructure.
Ductility is a material’s ability to bend or deform to a certain degree without rupturing. It is an important factor in the fatigue resistance of metals because it allows them to withstand a significant amount of strain before breaking.
The ductility of a metal is determined by its strength and composition. It is also affected by surface treatment, heat treatments and coatings. The higher the ductility of a metal, the better its fatigue resistance will be.
Several factors can affect the ductility of a steel, including the amount and distribution of iron in the alloy. While iron increases the strength of the alloy, it drastically decreases its ductility. Adding other elements such as copper, nickel, cobalt and molybdenum can help increase the ductility of an alloy. However, these metals tend to reduce the corrosion fatigue strength of magnesium alloys. Using an epoxy coating is an effective way to improve the corrosion fatigue strength of magnesium alloys.
Fatigue involves localized damage to metals under cyclic stress and it is irreversible. This process is stochastic which can lead to large scatter in results from seemingly identical samples under well controlled conditions. This scatter is known as fatigue life scatter.
The size and dispersion of inclusions in steels has a significant influence on fatigue strength. It is believed that the globular, small size and sparse distribution of the calcium-based inclusions in mischmetal treated steels contribute to improved fatigue performance.
It is also believed that the calcium treatment helps reduce the occurrence of striations – marks that indicate crack propagation and are a key factor in fatigue. This is due to the low inclusion-matrix interface strength of classical inclusions and a high stress intensity at the crack tip (Stage I crack growth).
In addition, the elongation of inclusions during the rolling and quenching processes results in higher fatigue toughness. However, striations may still occur and the quality of the notches and holes in components can have a major impact on fatigue behavior.
The expansion of a metal due to a temperature change is known as thermal strain. The expansion of a material is determined by the ratio of its thermal strain to its mechanical strain and is related to its elastic modulus, or Poisson’s ratio.
Oxide inclusions play an important role in very high cycle fatigue crack initiation in many steels. Their effect is controlled by their shape, size, and position in the microstructure and can be significantly reduced by modifying the cooling conditions during melting.
Inclusion engineering and the concept of ‘inclusion relative plasticity’ are important factors to consider when evaluating the fatigue behaviour of steels. Inclusions can form a variety of phases which differ from the matrix, such as oxides, sulphides, oxy-sulphides, phosphates and carbides (including carbo-nitrides). The relative plasticity of these inclusions varies with their composition and temperature. Consequently, their deformation behaviour is complex and difficult to predict. However, data published by Avesta/Outokumpu suggests that as a general rule, austenitic and duplex stainless steels have fatigue limits in air around their tensile 0.2% proof strength levels.
When oxidation is reduced, it improves the corrosion fatigue strength of the steel. This is because the oxidation reduces the interaction between the material and the corrosive environment. This means that there is less cyclic stress and corrosion reaction, and that the material can resist external attacks.
Oxygen is one of the worst impurities that can affect the quality of steel, but it is difficult to remove from the molten metal during the steelmaking process. Oxygen can cause hot brittleness, temper brittleness, crack and fracture, and weaken welding performance.
Adding calcium to steel can help to reduce oxidation and improve its performance. It can also refine the grain, partially desulfurize, and change the composition, quantity, and form of non-metallic inclusions. This can improve the fatigue resistance, impact toughness, high-temperature and low-temperature performance, casting, and machining performance of steel. In addition, adding calcium can increase the surface hardness and wear resistance of the steel. It can also decrease the tendency of the material to oxidize and form brittle chromium and molybdenum nitrides.
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