Shot peening is a crucial metal part processing technology that can effectively extend the fatigue life of parts. The basic principles of shot peening are elaborated from six aspects as follows.
- Cyclic Load
A cyclic load refers to a load whose magnitude and direction change periodically or vary randomly over time. Most parts are subjected to cyclic loads, especially those used in the automotive and aerospace industries. The load has four notable characteristics: frequency, variability, load magnitude, and load type. Shot peening is most effective when the cyclic load is mainly a repeated bending force. - Cyclic Stress
The cyclic load of any part can cause corresponding cyclic stress. The stress alternates between tensile stress and compressive stress. It is the action of tensile stress on the part that leads to fatigue failure of the part.
Under the action of cyclic stress, the microstructure in the weak areas of the material gradually changes, resulting in damage accumulation, such as dislocation movement and grain slip, and then cracks are formed. These cracks continue to expand until the part fractures. Even when the cyclic stress is much lower than the static load strength limit of the material, this kind of fatigue failure can still occur. An important element of cyclic stress is "stress propagation", and the function of shot peening is to reduce the stress propagation effect of indentations, which can significantly improve the fatigue resistance of the material. - Fatigue Curve (Without Shot Peening)
Metal materials themselves have the ability to resist cyclic stress. It is usually represented by a fatigue curve, as shown in the figure.
4. Fatigue Curve (With Shot Peening)
Figure 2 Fatigue Curve (With Shot Peening)
At a specific number of cycles, the cyclic stress level of the shot - peened part is increased. Compared with the fatigue curve without shot peening, the stress level after shot peening has increased significantly.
5. Shot Peening Intensity
Shot peening intensity is usually characterized by the arc height value of the Almen strip. The Almen strip is a specially treated thin steel sheet. After shot peening it together with the workpiece, the strip will be bent and deformed by the impact of the shot. By measuring the arc height of the bent strip, the corresponding shot peening intensity value can be obtained.
Shot peening intensity is mainly affected by process parameters such as shot diameter, shot flow velocity, shot flow rate, and shot peening time. The larger the shot diameter and the faster the velocity, the greater the momentum of the shot when it collides with the workpiece, and the higher the shot peening intensity. Shot peening intensity can directly control the thickness of the residual compressive stress hardened layer. The residual compressive stress formed by shot peening can reach about 60% of the tensile strength of the part material, and the depth of the residual compressive stress layer usually reaches 0.25mm, with a maximum limit of about 1mm. A higher shot peening intensity will generate a greater residual compressive stress on the workpiece surface. This residual compressive stress can offset a part of the tensile stress generated by the working load, thereby improving the fatigue resistance of the workpiece. However, if the shot peening intensity is too high, the residual stress may exceed the yield strength of the material, resulting in stress relaxation and instead reducing the effect of shot peening strengthening. In addition, shot peening intensity is also related to factors such as the material, shape, size of the part, and the required strengthening effect. These factors need to be comprehensively considered to determine the appropriate shot peening intensity.
6. Shot Peening Coverage
Shot peening coverage intuitively reflects the degree of the action of the shot on the workpiece surface. A higher coverage rate means that more areas of the workpiece surface are impacted by the shot, thus more fully introducing residual compressive stress, improving surface roughness, and increasing surface hardness, etc. This can effectively improve the fatigue resistance, wear resistance, and corrosion resistance of the workpiece, and extend the service life of the workpiece.
At a specific number of cycles, the cyclic stress level of the shot - peened part is increased. Compared with the fatigue curve without shot peening, the stress level after shot peening has increased significantly.
5. Shot Peening Intensity
Shot peening intensity is usually characterized by the arc height value of the Almen strip. The Almen strip is a specially treated thin steel sheet. After shot peening it together with the workpiece, the strip will be bent and deformed by the impact of the shot. By measuring the arc height of the bent strip, the corresponding shot peening intensity value can be obtained.
Shot peening intensity is mainly affected by process parameters such as shot diameter, shot flow velocity, shot flow rate, and shot peening time. The larger the shot diameter and the faster the velocity, the greater the momentum of the shot when it collides with the workpiece, and the higher the shot peening intensity. Shot peening intensity can directly control the thickness of the residual compressive stress hardened layer. The residual compressive stress formed by shot peening can reach about 60% of the tensile strength of the part material, and the depth of the residual compressive stress layer usually reaches 0.25mm, with a maximum limit of about 1mm. A higher shot peening intensity will generate a greater residual compressive stress on the workpiece surface. This residual compressive stress can offset a part of the tensile stress generated by the working load, thereby improving the fatigue resistance of the workpiece. However, if the shot peening intensity is too high, the residual stress may exceed the yield strength of the material, resulting in stress relaxation and instead reducing the effect of shot peening strengthening. In addition, shot peening intensity is also related to factors such as the material, shape, size of the part, and the required strengthening effect. These factors need to be comprehensively considered to determine the appropriate shot peening intensity.
6. Shot Peening Coverage
Shot peening coverage intuitively reflects the degree of the action of the shot on the workpiece surface. A higher coverage rate means that more areas of the workpiece surface are impacted by the shot, thus more fully introducing residual compressive stress, improving surface roughness, and increasing surface hardness, etc. This can effectively improve the fatigue resistance, wear resistance, and corrosion resistance of the workpiece, and extend the service life of the workpiece.
