The fatigue strength of seamless steel tube material from Shandong Derunying is extremely sensitive to various external and internal factors, wherein external factors include shape, size, surface smoothness, and service condition or the like of the parts, and internal factors include composition, texture, purity, residual stress and so on of the material itself. Subtle changes of these factors will cause fluctuations or even significant difference in the fatigue performance of the material.

The influence of the factors on fatigue strength is an important aspect of fatigue research. This research will be helpful in the design of appropriate part structures, the selection of correct seamless steel tube materials and the formulation of various rational cold and hot processing techniques, thereby ensuring high fatigue performance of the parts.

1. The influence of stress concentration
Conventionally, fatigue strength is obtained via measurement using elaborate smooth sample. However, different notches, such as steps, keyways, threads, and oil holes etc., inevitably exist in actual mechanical parts. The existence of these notches results in stress concentration, which makes the maximum actual stress at the root of the notch much greater than the nominal stress borne by the part, and often starts the fatigue failure of the part.

Theoretical stress concentration coefficient Kt: a ratio of the maximum actual stress to the nominal stress at the root of the notch obtained according to the elastic theory under ideal elastic conditions.

Effective stress concentration coefficient (or fatigue stress concentration coefficient) Kf: a ratio of the fatigue limit σ-1 of a smooth sample to the fatigue limit σ-1n of a notch sample.
The effective stress concentration coefficient is influenced not only by the size and shape of the component, but also by the physical properties of the material, processing, heat treatment and other factors.

The effective stress concentration coefficient increases with the notch sharpness, but is usually smaller than the theoretical stress concentration coefficient.
Fatigue notch sensitivity coefficient q: the fatigue notch sensitivity coefficient indicates the sensitivity of the material to the fatigue notch and is calculated by the following formula.
The data range of q is 0-1, and the smaller is q, the less sensitive is the seamless steel tube material to the notch. Experiments show that q is not purely a material constant, and is still related to the notch size; q is basically unrelated to the notch only when the notch radius is greater than a certain value, the radius value being different for different materials or processing status.

2. The influence of size
Due to the texture heterogeneity and internal defects of the material, the increase in size will enlarge the probability of material failure, thereby reducing the fatigue limit of the material. The existence of the size effect is an important issue in applying the fatigue data obtained via measurement of the small sample in the laboratory to the part of actual size. It is impossible to completely and similarly represent the stress concentration, the stress gradient or the like on the part of actual size, so the laboratory results and the fatigue failure of some specific parts are disconnected with each other.

3. The influence of surface processing status
Uneven machining marks always exist on the machined surface. These marks are equivalent to tiny notches causing stress concentration on the surface of the material, and will reduce the fatigue strength of the material. Tests show that, for steel and aluminum alloys, the fatigue limit of rough machining (rough turning) is lower than that of longitudinal fine polishing by 10%-20% or more. The higher is the strength of the material, the more sensitive it is to surface smoothness.

Post time: Aug-06-2020