Fatigue of metals is one of the most important failure mechanisms in engineering. Tensile tests have been used to study the fatigue behavior of metals. The reduction in load-recycling load ratio, or the fractional toughness, can be used as a measure of fatigue strength. The fractional toughness is obtained by comparing the stress at the beginning of a cycle to the stress at the end of the cycle. In general, a lower fractional toughness indicates a material is more susceptible to fatigue failure.
When a metal experiences full saturated stress reversal cycles, the fatigue strength typically decreases with each cycle. This phenomenon is often referred to as fatigue strippage. It is caused by the progressive breaking down of the microstructure of the metal. As the number of cyclic loadings increases, the size of the microstructural defects increases, eventually leading to fatigue failure.
The fatigue strippage mechanism is due to the cyclic loading of the metal which results in the formation of cracks at a microscopic level. This damage is known as crack initiation. As the number of cycles increases, the cracks gradually increase in size and eventually reach a critical size at which the material breaks. As the material deforms, the stress concentration at the crack tip increases and the metal begins to spall, or peel away from the surface. This ultimately creates a fatigue strippage pattern on the surface of the material which is characteristic of a metal that has been subjected to high cycle fatigue loading.
The fatigue strippage mechanism is a complex process. It involves several different components, including microstructure, deformation, loading, and residual stresses. For example, the microstructure of the metal can affect how quickly a crack will grow. Additionally, the size and shape of the crack tip can be affected by the rate of loading, the magnitude of the applied load, the shape of the load curve, and the amount of material surrounding the crack tip.
The fatigue strength of a metal is usually represented by its fatigue strippage limits. This is the maximum level of stress which the material can endure before experiencing strippage failure. As the number of cycles increases, the fatigue strippage limits of a metal tend to decrease. This is due to the continuing microstructural degradation caused by the repeated cyclic loading.
Most materials experience some degree of fatigue strippage when subjected to cyclic loading. To prevent fatigue strippage failure, engineers must consider the fatigue strippage limits of the material when designing structures. Additionally, they must be aware of the effects that the loading conditions and the microstructure of the material can have on the fatigue strippage mechanisms. By understanding the fatigue strippage mechanisms, engineers can design safer structures which are less likely to experience fatigue failure.
