The phenomenon of stress concentration is widespread, which has a great impact on failure.
Technical supervision should be strengthened and strict inspection should be carried out to eliminate some unnecessary stress concentration factors (such as processing defects).
At the same time, certain technical measures should be taken to minimize the degree of stress concentration in the design and processing.
1. Effect of reducing stress concentration from strengthening materials
Local strengthening is adopted to improve the fatigue strength of the material at the stress concentration, so as to reduce the harm of stress concentration.
(1) Surface heat treatment strengthening
Surface heat treatment strengthening includes surface induction hardening, carburizing, nitriding and composite treatment, which can obtain soft (high toughness) center and hard surface, and there is residual compressive stress on the surface, thus reducing the effect of stress concentration.
(2) Shell quenching
Short shaft parts with large diameter and section changes, such as low hardenability steel, can form a thin surface hardened layer after intensive quenching, and there is residual compressive stress in it, which can reduce the effect of stress concentration.
Compared with surface induction quenching, thin shell quenching has a more favorable aspect, that is, for similar parts, induction rate fire tends to make the transition zone (such as shaft shoulder) with changed section unable to be quenched, resulting in residual tensile stress, which increases the harmful effect of stress concentration.
(3) Shot peening strengthening
It can strengthen the metal surface and produce large residual compressive stress, thus reducing the harm of stress concentration.
When the surface roughness of high-strength materials is large or there are defects, shot peening treatment has a more obvious effect on reducing the stress concentration.
The effect of stress shot peening is better than that of general shot peening.
(4) Rolling strengthening
The surface of the part will be deformed and strengthened to produce residual compressive stress, thereby reducing the harmful effect of stress concentration.
The effect is related to the rolling parameters and the microstructure and properties of the material itself.
2. Reduce the stress concentration factor from the design aspect
(1) For the transition of the variable section part, the fillet of the transition part shall be enlarged as much as possible to make the transition area close to the streamline shape, and the processability shall also be considered.
You can change the transition method. Elliptical transition is better than circular transition, or you can use other transition methods.
(2) According to the stress direction and position of the part, select the appropriate opening position.
The hole shall be generally opened in the low stress area. If it must be opened in the high stress area, reinforcement measures shall be taken.
The major axis of the ellipse should be parallel to the direction of the principal stress to reduce the stress concentration factor.
(3) Adding notches and round holes in the low stress area near the stress concentration area can make the stress streamline smooth, thus reducing the maximum stress peak value.
For example, the stress concentration factor shown in Fig. a is 3, while the stress concentration factor shown in Figure b is 2.63.
Similarly, in the low stress area near the stress concentration area, the addition of unloading trough (Fig.) can also improve the stress concentration.
Effect of adding unloading trough on stress concentration
A large number of failure analysis shows that tool marks in processing, defects in welding, non-metallic inclusions in dangerous sections, and unsmooth arc transition are often the direct factors that cause part failure.
Therefore, stress concentration cannot be ignored in failure analysis.