Experimental Study on the Effect of Shot Peening and Re-Shot Peening on the Residual Stress Distribution and Fatigue Life of 20CrMnTi

Abstract

As a surface peening technique, shot peening introduces residual compressive stresses to the surface of the part, which effectively increases the fatigue life of the structure and material. However, when structures are subjected to alternating loads, this can lead to stress relaxation on the material surface, weakening the effectiveness of the shot peening process. In addition, reasonable shot peening parameters are essential. In this paper, the effects of shot peening pressure and shot coverage on the fatigue life of materials during shot peening were investigated, followed by fatigue tests on 20CrMnTi specimens using a high-frequency fatigue testing machine to study the effects of shot peening and re-shot peening on the fatigue life of shot-peened materials after different load cycles. The results show that a reasonable shot peening pressure and coverage rate can significantly improve the fatigue life of the material, while a shot peening pressure higher than 0.4 MPa will reduce the fatigue life of the material 20CrMnTi. Coverage rates of 100% and 200% can both improve the fatigue life of the material, while a 200% coverage rate has a better strengthening effect. Re-shot peening removes the residual compressive stress relaxation on the surface of the material caused by cyclic loading and improves the fatigue life of the material. The maximum value of the residual compressive stress on the surface of the test material after shot peening is 443 MPa, and after a certain number of fatigue loads, the residual compressive stress on the surface is reduced to 203 MPa, which is subjected to secondary shot peening, and the residual compressive stress is restored to 415 MPa, and the fatigue life is significantly increased. When the second shot peening time is taken as 25% of the fatigue life of the initial shot blasting of the material, the shot peening effect is better.

1. Introduction

Shot peening is a surface strengthening technique used in practical engineering [1,2], which is mainly reflected in the generation of material strengthening and stress strengthening on the material, which can effectively improve the fatigue resistance of the material and prolong its service life [3,4,5,6]. Material strengthening mainly affects crack generation, and stress strengthening mainly affects crack expansion [7,8,9].

Shot peening can introduce residual compressive stresses on the surface of the part. Since in practical engineering, most fatigue cracks in parts occur on the surface or subsurface, residual compressive stresses are beneficial in inhibiting crack generation and expansion [10,11,12]. Properly set shot peening process parameters can induce a residual compressive stress layer on the surface of the part without producing excessive surface roughness, thus effectively improving the fatigue life of the part [13,14,15,16]. Gundgire et al. [17] subjected additive manufacturing 316 L samples to shot peening and severe shot peening with different parameters and investigated the residual stresses, surface morphology, microhardness, and corresponding microstructure of the shot-peened reference and AM samples. The results showed that severe shot peening induced higher values of compressive residual stresses deeper in the sample. Gao [18] investigated the residual compressive stress field of the material after shot peening for TC18 material and concluded that the fatigue crack source would be generated on the subsurface due to the presence of the compressive stress field, which resulted in the improvement in the fatigue life of the material. Zhang et al. [19] performed shot peening on three different metallic materials and found a linear relationship between the maximum depth of the residual compressive stress field after shot peening and the shot peening strength of the material. Nakagawara et al. [20] investigated the effect of strain-induced cyclic transformation on the surface microstructure of SUS 304 austenitic stainless steel. The strain-induced cyclic transformation was performed by repeated shot peening tests at room temperature and 200 °C. The surface hardness of the specimens treated with strain-induced cyclic transformation using SP (shot peening) was higher compared to the specimens treated with conventional SP, and the SP-influenced Strain-induced cyclic transformation can improve the surface mechanical properties of steel. Huang et al. [21] reviewed the literature on the application of shot peening in corrosion prevention and concluded that the integrity of the shot-peened surface may deteriorate when inappropriate shot peening parameters are used, which further weakens the corrosion performance of the surface. Therefore, it is very important to optimize the shot peening process. Xia et al. [22] investigated the high cycle fatigue (HCF) life of 50CrMnMoVNb spring steel with different SP strengths and conditions, and compared the differences in microstructure and crack initiation points. The results show that SP treatment is beneficial for improving the fatigue life of heat-treated specimens with higher strengths, and the optimal SP strength is 0.15∼0.25 mmA. However, SP deteriorates the HCF life of specimens with lower strengths. Several papers have performed numerical simulations of the shot peening process. Through the simulation, compressive residual stress profiles were obtained, and the effects of speed and shot peening coverage were investigated. The results show that the residual stress distribution is highly dependent on impact velocity and material properties [23,24,25]. Some studies in the literature have used X-ray diffraction analysis and other means to study the effect of shot peening intensity and coverage on the fatigue properties and obtain the stress–strain relationship of the surface layer. The results show that the effect of shot peening on improving fatigue properties does not vary simply monotonically with shot peening intensity and coverage, and that surface integrity and fatigue resistance are relatively high within a certain range of shot peening parameters. When choosing shot peening parameters, it is inappropriate to blindly pursue high strength or shot peening coverage while ignoring the surface integrity of the metal material [26,27,28].

The effect of re-shot peening and multiple shot peening on the surface residual stress and roughness of the material has been studied by several scholars. Chen et al. [27] performed multiple shot peening treatments on SAF 2507 duplex stainless steel. Residual stresses, microstructure, and strain-induced transformation in the shot-peened specimens were evaluated by X-ray diffraction line profiling. The results point out that multiple shot peening contributes to the development of deep, highly compressive residual stresses and moderate work hardening, and that multiple shot peening significantly reduces surface roughness and contributes to surface quality. The effect of re-shot on the fretting fatigue life and residual stress recovery of the initial shot-peened Ti-6Al-4V material was investigated at room temperature and high temperature. The residual stresses relaxed to the range of 20%–50% after enduring about 40% of the expected life, or only at 370 °C under thermal exposure [29]. Unal et al. [30] compared the effects of conventional shot peening, severe shot peening, re-shot peening, and precision grinding on AISI 1050. The depth and magnitude of residual stresses on the surface were quite high for aggressive shot peening and re-shot peening. Wang et al. [31] investigated the changes in the surface integrity of WC-Co carbide after conventional shot peening and double shot peening treatments. The study showed that double shot peening improved the uniformity of residual stresses at macroscopic and microscopic levels; however, the surface roughness increased significantly after conventional shot peening and double shot peening due to the accumulation of elastic–plastic deformation. Wang et al. [32] investigated the effect of the re-shot peening process on the residual stress and room temperature fatigue life of TC18 titanium alloy, and the results showed that a suitable re-shot peening cycle could improve the fatigue life of the material. Zhao et al. [33] found that the fatigue load level has an important effect on the relaxation rate, relaxation degree, and relaxation range of the residual compressive stress field of shot-peened TC4 titanium alloy, and the re-shot peening cycle also has a significant effect on the fatigue life of TC4 titanium alloy. Ongtrakulkij et al. [34] investigated the effects of media type and media size on hardness, roughness, and residual stresses when using a secondary shot for double shot peening. The study found that conditions with 80-μm silica media as the second shot produced the highest hardness and residual compressive stress on the surface, with 14% and 53% increases, respectively, and a 20% decrease in roughness compared to single shot peening. Yang et al. [35] used ceramic beads for wet shot peening (WP) of Ti-6Al-4V (TC4) alloy, compared with dry shot peening (SP). The results showed an increase in roughness values from 0.596 μm (Ra) to 1.594 μm (Ra) induced by the WP and SP treatments. The shot peening treatment induced a micro-hardness gradient and compressive residual stress (CRS) distribution along the depth, with cracks tending to initiate at the subsurface rather than at the treated surface. Qian et al. [36] studied the method of random multiple shot peening simulation analysis and investigated the effect of different parameters on the peening effect of shot peening; the results showed that the random multiple shot peening simulation method is more reasonable and more accurate.

In this paper, the standard 20CrMnTi specimens were first shot peened with different process parameters to study the distribution of surface residual pressure under different shot peening parameters. On this basis, group fatigue tests were carried out on the shot-peened specimens to investigate the effect of shot peening pressure and shot peening coverage on fatigue life. Then, the 20CrMnTi specimens that had undergone different fatigue cycles were shot peened again to study the effect of re-shot peening on the recovery of residual stresses, and then the effect of re-shot peening time on fatigue life was investigated to determine a reasonable re-shot peening time.

2. Experimental Procedure

The experimental material is 20CrMnTi. 20CrMnTi is a low-carbon alloy structural steel with good fatigue resistance and impact resistance, which is usually used to manufacture parts such as automotive transmission gears and shafts. Its chemical composition and mechanical properties are shown in

Table 1. Chemical composition of 20CrMnTi in weight %.

C	Si	Mn	P	S	Cr	Ni	Cu	Ti
0.18	0.24	0.96	0.013	0.005	1.12	0.01	0.04	0.06

and

Table 2. Mechanical properties of 20CrMnTi.

${\mathit{\sigma }}_{\mathit{s}}$/MPa	${\mathit{\sigma }}_{\mathit{b}}$/MPa	$\mathit{\delta }$/%	$\mathit{\psi }$/%	${\mathit{A}}_{\mathit{K}\mathit{U}2}$/J
835	1080	13	60	99

[37]. In Table 2, ${\sigma }_s$ is yield strength, ${\sigma }_s$ is tensile strength, $\delta$ is elongation, $\psi$ is section shrinkage and $A_{KU2}$ is impact absorption energy. The smooth specimens are round bar specimens with the structure size shown in Figure 1. The smooth specimens are machined on a CNC lathe and the machining parameters strictly follow the relevant standards.. The specimen’s surface was carefully polished to avoid the influence of varying surface roughness, and the surface roughness of the gauge section was no more than Ra0.4.

The shot peening equipment used is ROSLER VP100BP pneumatic shot peening machine. The shot peening projectile is 0.6 mm diameter steel wire cut shot, whose hardness is greater than that of 20CrMnTi material. The shot pressure is 0.1 MPa, 0.2 MPa, and 0.3 MPa, respectively, and the shot is spraying perpendicular to the surface of the specimen with a distance of 150 mm from the nozzle to the specimen, and the shot coverage is 100% and 200%, respectively. Figure 2 shows the field situation of the shot peening test.

The QBG-100 high-frequency fatigue tester was used for the fatigue test, and the fatigue test was conducted on the specimens after shot peening and the specimens without shot peening, respectively. The test was axially loaded with a loading frequency of 220 Hz, stress ratio was 0.1, and the upper limit of cycle number was set to 10⁷. The stress ratio is set to R = 0.1 here because the fatigue tester we use is an electromagnetic resonance type high-frequency fatigue tester, and setting it to a positive stress ratio makes it easier for the tester to vibrate. Branco et al. [38] analyzed the effect of different stress ratios on fatigue life in detail, and as this paper focuses on the effect of shot peening and re-shot peening on residual stress distribution or fatigue life, the effect of stress ratio is not investigated. The fatigue tests were conducted at 630 MPa and 700 MPa stress levels to study the effects of shot peening pressure and shot peening coverage on fatigue life. On this basis, fatigue tests were conducted on the re-shot-peened specimens at 630 MPa stress level to investigate the effect of the number of load cycles on the fatigue life before re-shot peening.

The surface residual stresses of specimens treated with 0.1 MPa, 0.2 MPa, and 0.3 MPa shot peening were examined, and the residual stresses on the surface of the specimens were detected by the X-ray diffraction method combined with the chemical corrosion method. For the specimens treated with different shot peening process parameters, the μ-360s model residual stress measuring instrument was applied for the detection of residual stresses, as shown in Figure 3. In this paper, the chemical etching method is used to detect the residual stresses in different depths of the material, and the schematic diagram of the peeling layer is shown in Figure 4, in the figure t indicates the thickness of the peeled layer. The peeling depths are 25 μm, 50 μm, 75 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm, 250 μm, and 300 μm, and the residual stresses on the material surface are measured using μ-360s model residual stress measuring instrument for each layer of material removed to obtain the distribution curve of residual stresses along the depth direction. Since the material removal depth is small compared to the specimen size, the residual stress error due to delamination can be ignored.

3. Results and Discussion

3.1. Residual Compressive Stress Distribution on the Material Surface after Shot Peening

Figure 5 shows the measured curves of the residual stress distribution of the material with depth. The analysis of the curves in Figure 5 shows that with the gradual increase in the shot peening pressure, the value of the residual compressive stress on the surface of the material after shot peening has a tendency to gradually decrease, but the specific value does not differ much. The maximum value of the residual compressive stress is first increased from 612 MPa to 654 MPa, an increase of 42 MPa, and then decreased to 641 MPa, a decrease of 13 MPa; the overall difference is not significant, but the depth location of the maximum residual compressive stress appears at a gradually increasing distance from the surface.

The location of the depth of the maximum residual compressive stress introduced at 0.3 MPa is 75% deeper than the location of the maximum depth of residual compressive stress introduced at 0.1 MPa. The depth of material deformation is mainly determined by the strength of the shot peening and the properties of the material itself.

3.2. The Effect of Shot Peening on Surface Morphology and Organization

The surface of the material will be shot-peened to produce plastic deformation, and different shot peening parameters cause the surface roughness of the material to be different. The size of the surface roughness will affect the fatigue life of the material, and inappropriate shot peening will cause the surface roughness of the material to increase, which in turn will produce stress concentration, and it promotes the initiation of cracks, which in turn will lead to a reduction in the fatigue life of the material. Therefore, in the shot peening process, one should choose reasonable shot peening parameters and control the size of the surface roughness.

Figure 6 shows the surface morphology of the specimen at different shot peening pressures as seen under a 50× microscope. Figure 6a shows the unpeened specimen 1, which can be observed having clear machining marks using an optical microscope; specimen 2, specimen 3, and specimen 4 are the specimens shot peened under the shot peening pressure of 0.1 MPa, 0.2 MPa, and 0.3 MPa, respectively, with a shot peening coverage of 100%, and the surface morphology of each specimen is shown in Figure 6b–d.

The surface morphology of the specimens was observed using a 3D laser profiler, and the results for different parameters are shown in Figure 7. It can be seen from Figure 7 that the original specimen has a smooth surface, and with the increase in the shot peening pressure, the value of the crest and trough of the surface profile of the shot-peened specimen gradually increases and becomes more rough and uneven [39].

Figure 8 shows the surface morphology of the specimens with different coverage rates under the same shot peening pressure. Specimen 1, specimen 3, and specimen 5 were selected to study the effect of different shot peening coverage on the surface roughness of the shot-peened specimen. Specimen 1 is an unpeened specimen, specimen 3 is a shot-peened specimen with a pressure of 0.2 MPa and a coverage of 100%, and specimen 5 is a shot-peened specimen with a pressure of 0.2 MPa and a coverage of 200%; the surface morphology of each specimen observed by optical microscopy is shown in Figure 8a, Figure 8b, and Figure 8c, respectively. The surface profile of the specimen with 100% shot peening coverage is similar to that of the specimen with 200% coverage, but the 200% profile is more uniform and has slightly smaller peaks and valleys than the 100% profile.

Figure 8 shows the surface profile of the specimen measured by the laser profiler. It includes the surface roughness of the original specimen and the surface roughness of the specimens subjected to shot peening at different shot peening coverage rates. The roughness curves at different coverage rates are shown in Figure 9. As can be seen from Figure 9, the surface roughness of the original specimen is Ra = 0.27 μm, the surface roughness of the specimen at 100% coverage is Ra = 2.73 μm, and the surface roughness of the specimen at 200% coverage is Ra = 2.59 μm. A comparative analysis shows that the shot peening treatment causes changes in the surface roughness of the material surface. When shot peening is carried out, the surface roughness of the specimen increases gradually with the gradual increase in the shot peening coverage, and when the coverage reaches 100% and exceeds 100%, the change in the surface roughness of the specimen starts to decrease gradually, and even the surface roughness decreases slightly when the coverage reaches 200%. A continuous increase in shot coverage does not cause significant changes in surface roughness, so in order to reduce the cost of the shot peening process, it is worth considering the selection of a suitable shot peening coverage.

3.3. The Effect of Shot Peening Pressure on Fatigue Life

In order to examine the strengthening effect of different shot peening pressure treatments, the specimens treated with different shot peening pressures were divided into four groups A, B, C, and D; each group contains three specimens, corresponding to no shot peening treatment and 0.1 MPa, 0.2 MPa, and 0.3 MPa shot peening pressures, respectively. The shot coverage of the specimens was 100%, and half of the specimens from each of groups B, C, and E were fatigue tested at 700 MPa and 630 MPa stress levels, respectively.

The four groups of specimens were subjected to fatigue experiments at stress levels of 700 MPa and 630 MPa, respectively, and the measured fatigue lives are shown in

Table 3. Fatigue life under different shot peening pressures.

Group Number	Shot Peening Pressure/MPa	Fatigue Life (Mean) (700 MPa)	Fatigue Life (STD) (700 MPa)	Difference (%)	Fatigue Life (Mean) (630 MPa)	Fatigue Life (STD) (630 MPa)	Difference (%)
A	unpeened	7.78 × 104	8558	-	1.69 × 106	219,700	-
B	0.1	1.02 × 105	12,240	31.1	2.43 × 106	376,650	43.8
C	0.2	1.25 × 105	16,350	60.7	2.81 × 106	519,850	66.3
D	0.3	4.09 × 104	3681	−47.4	6.94 × 105	76,340	−58.9

. Comparisons of the fatigue life of the specimens at different shot peening pressures are shown in Figure 10a,b.

For the fatigue life data at 700 MPa stress level, the fatigue life of specimens in groups B and C increased by 2.42 × 10⁴ and 4.72 × 10⁴, respectively, compared with the fatigue life of the original specimens. For the fatigue life data at 630 MPa stress level, the fatigue lives of both groups B and C specimens were improved by 7.4 × 10⁵ and 1.12 × 10⁶, respectively, compared to the fatigue life of the original specimens. This is due to the lower shot pressure, so the shot velocity is smaller, and the surface roughness produced on the specimen surface is not very high, which brings less negative effect than the positive effect from the residual compressive stress site and improves the fatigue life of the material.

The fatigue lives of the specimens in group D were both reduced compared to those of the original specimens by 3.69 × 10⁴ and 9.6 × 10⁵, respectively. This is due to the higher shot peening pressure, so the velocity of the projectile is higher, resulting in larger surface roughness on the specimen surface as well. Although the higher shot peening pressure will make the material surface produce a deeper residual compressive stress field, which is conducive to improving the fatigue life of the parts, the negative effect brought by the surface roughness at this time is greater than the positive effect brought by the residual stress field, resulting in a reduction in the fatigue life of these two groups of specimens.

3.4. The Effect of Shot Peening Coverage on Fatigue Life

The shot peening effect is also different for different shot peening coverage rates.

The specimens with different shot peening coverage were divided into three groups, A, C, and E, corresponding to unpeened, 100%, and 200% shot peening coverage, respectively, and the three groups of specimens were subjected to fatigue experiments at stress levels of 700 MPa and 630 MPa, respectively, and the measured fatigue lives are shown in

Table 4. Fatigue life under different shot peening coverage rates.

Group Number	Coverage (%)	Fatigue Life (Mean) (700 MPa)	Fatigue Life (STD) (700 MPa)	Difference (%)	Fatigue Life (Mean) (630 MPa)	Fatigue Life (STD) (630 MPa)	Difference (%)
A	unpeened	7.78 × 104	8558	-	1.69 × 106	219,700	-
C	100	1.25 × 105	16,350	60.7	2.81 × 106	519,850	66.3
E	200	1.43 × 105	20,023	83.8	3.63 × 106	907,500	114.8

. The comparison of the fatigue life of the specimens with different shot peening coverage is shown in Figure 11a,b.

The fatigue lives obtained for the specimens without shot peening at 700 MPa stress level and 630 MPa stress level were 7.78 × 10⁴ and 1.69 × 10⁶, respectively. The specimens were shot peened at 0.2 MPa for 100% and 200% coverage, respectively, and half of the specimens from each of groups C and E were fatigue tested at 700 MPa and 630 MPa stress levels, respectively. The fatigue experiments were performed at 700 MPa stress level, and the fatigue lives of the specimens in groups C and E were increased by 4.72 × 10⁴ and 6.52 × 10⁴, respectively, compared with the fatigue lives of the original specimens. The fatigue life of C and E specimens was increased by 1.12 × 10⁶ and 1.94 × 10⁶, respectively, compared with that of the original specimens when fatigue tests were performed at 630 MPa stress level.

At a shot peening pressure of 0.2 MPa, the variation of the specimen surface roughness was within a reasonable range, so the fatigue life of the material was significantly improved at 100% shot peening coverage. When the shot peening coverage reaches 200%, the subsequent shot peening will play a certain role in dressing the craters produced by the previous shot peening impact, making the craters more uniform and slightly reducing the surface roughness, so the fatigue life is a little longer than the coverage of 100%.

3.5. The Effect of Re-Shot Peening on Fatigue Life

After the initial specimens were shot peened, they were divided into four groups to conduct fatigue experiments on the fatigue test machine; the fatigue experiments were stopped when the number of cycles reached 3 × 10⁵ and 1 × 10⁶, respectively, and the surface residual stresses were detected on each shot-peened specimen. Then, a group of shot peening specimens was taken for secondary shot peening, and the residual stresses were detected on the secondary shot peening specimens, and the relevant residual stress detection results are shown in

Table 5. Residual stress.

Pressure/MPa	Coverage/%	Stress Level/MPa	Cycles	Surface Residual Compressive Stress/MPa
0.2	200	630	0	443
0.2	200	630	3 × 105	314
0.2	200	630	1 × 106	203
0.2	200	630	1 × 106	415 (Re-shot peening)

.

Table 5 shows that the surface residual compressive stress is relaxed during the service period of the part, and the relaxation of the residual stress becomes more and more serious with the increasing number of cycles. After the second shot peening of the specimen, the test shows that the surface residual compressive stress of the part is restored, proving that the second shot peening can effectively restore the surface residual compressive stress of the material.

After the initial specimen is shot peened, the fatigue test is performed on the fatigue test machine. The fatigue test is stopped when 25%, 50%, and 75% of the fatigue life of the blasted specimen is reached, respectively. After being subjected to a certain number of cycles of load, the second shot peening is performed, and the coverage rate of secondary shot blasting is at 100%.

Then, the fatigue test is continued on the specimen under the same test conditions to obtain the number of cycles when the specimen breaks, which is the fatigue life of the second shot-peened specimen. The number of cycles before the second shot peening and the number of cycles after the second shot peening before the fracture of the specimen are added together as the total fatigue life of the specimen, as shown in

Table 6. Total fatigue life.

Number	Coverage/%	Stress Level/MPa	Cycles before Re-Shot Peening	Total Cycles
0	200	630	0	3.63 × 106
1	200 + 100	630	0.9 × 106	3.87 × 106
2	200 + 100	630	1.8 × 106	3.71 × 106
3	200 + 100	630	2.7 × 106	3.56 × 106

.

In order to more visually compare the effect of secondary shot peening on the fatigue life of the specimen, the fatigue life obtained from Table 6 was organized in the form of a bar graph, as shown in Figure 12.

The fatigue life obtained after fatigue testing of the specimen after a single shot peening is 3.63 × 10⁶. After fatigue testing a shot-peened specimen for a certain number of cycles, the test is suspended, and the specimen is shot-peened again, and then the fatigue test is continued. When the number of fatigue cycles before re-shot peening again was 25% of the fatigue life of the single shot-peened specimen, the total fatigue life of the specimen reached 3.87 × 10⁶, which is an increase of 2.4 × 10⁵ compared to the fatigue life obtained by shot peening.

When the number of fatigue cycles before re-shot peening is 50% of the fatigue life of the single shot-peened specimen, the total fatigue life of the specimen reaches 3.71 × 10⁶, an improvement of 0.8 × 10⁵, which is a relatively small improvement. When the number of fatigue cycles before re-shot peening is 75% of the fatigue life of the single shot-peened specimen, the total fatigue life of the specimen reaches 3.56 × 10⁶, and compared with the primary shot peening specimen, its total fatigue life not only did not improve but also decreased, down by 0.7 × 10⁵.

After a comparative analysis of the test results, it can be seen that the secondary shot peening, to a certain extent, restored the residual stresses relaxed during the service period of the parts and was thus able to improve the fatigue life of the parts, and that the selection of a reasonable number of fatigue cycles before the secondary shot peening is very important. As the focus of this paper is on the effect of the parameters of the shot peening and the timing of the secondary shot peening on the residual stress distribution and fatigue life of the material, no detailed fracture surface morphology analysis has been carried out. In fact, a fracture surface morphology analysis is also relevant for determining the mechanism under fatigue, and Macek et al. [40] have analyzed in detail the relationship between fracture morphology analysis and the causes of fatigue failure. Subsequent studies will investigate the fracture morphology of different shot-peened specimens in order to clarify the fatigue fracture specificity of shot-peened materials.

4. Conclusions

By testing and analyzing the residual stress of 20CrMnTi specimens under different shot peening parameters, as well as fatigue life analysis tests of specimens under different shot peening parameters and secondary shot peening tests, the following conclusions were obtained:

(1)

The test results show that by shot peening, residual compressive stresses can be introduced on the surface of the part, thus effectively increasing the fatigue life of the material. An increase in shot peening pressure has an effect on causing an increase in the surface roughness of the material, while the effect on the surface residual stress field is not a continuous increase but only an increase in the depth of the residual stress field. Thus, when the shot peening pressure is too large, the negative effect of surface roughness is greater than the positive effect of the residual stress field, but will reduce the effect of shot peening so that the fatigue life of the material is reduced. Therefore, it is necessary to reasonably choose the shot peening pressure;

(2)

In the initial stage of shot peening, the coverage of the projectile gradually increases. At the same time, the surface roughness and residual stress of the material also gradually increase. When the coverage reaches 100%, the fatigue life of the material will be significantly increased. When the coverage rate increases again, the surface roughness of the material starts to become stable or even decreases slightly, and when the coverage rate reaches 200%, increasing the coverage rate again will not significantly improve the fatigue life of the material;

(3)

The shot-peened material will undergo stress relaxation during service due to alternating loads, resulting in the weakening or even disappearance of the residual compressive stress field on the surface of the material. Secondary shot peening can effectively restore the residual compressive stress on the surface of the shot-peened material, eliminating the stress relaxation of the material in the process of service. The research in this paper shows that the maximum value of the residual compressive stress on the surface of the test material after shot peening is 443 MPa, and after 10⁶ cycles of fatigue loads, the residual compressive stress on the surface is reduced to 203 MPa, which is subjected to secondary shot peening, restoring the residual compressive stress to 415 MPa.

However, the timing of the implementation of the second shot peening is very critical; the best results were achieved when the second shot peening was applied at 25% of the fatigue life of the specimen. With an increase in the number of fatigue cycles before the second shot peening, the secondary shot peening effect gradually reduced.