# Comparison between Shot Peening, Cavitation Peening, and Laser Peening by Observation of Crack Initiation and Crack Growth in Stainless Steel

## Abstract

**:**

## 1. Introduction

_{th}and the stress intensity factor range increment ∆(∆K) were improved by cavitation peening [30]. The load controlled plane bending fatigue tester was used in these experiments to evaluate crack initiation and growth in surface layers treated by shot peening, cavitation peening, and laser peening. Please note that K-decreasing test was carried out in order to determine whether crack initiation or crack growth is important, and also the improvement of the fatigue life within short crack by cavitation peening was dominant [31].

## 2. Experimental Apparatus and Procedure

#### 2.1. Crack Growth Test

_{a}was calculated from the bending moment M, the width of the specimen W and the thickness of the specimen t using the following equation:

_{a}is 2 σ

_{a}, and J, ϕ, S and H are shape factors. Although the crack propagates in three dimensions, the depth of the crack b is estimated from the length of the crack 2a.

_{0}and b

_{0}are the length and the depth of the notch, and C

_{1}and C

_{2}are constants obtained from experimental data using the least squares method. For the present experiments, 2a

_{0}, b

_{0}, C

_{1}and C

_{2}were 2, 0.25, 2.51 and −0.51 for the K-decreasing test and 5, 0.25, 2.26 and −0.28 for the crack growth test at constant applied stress, respectively.

_{th}. The crack length was measured after each 10

^{5}cycles using an optical microscope, then da/dn and ∆K were calculated. Please note that the secant method [28] was used for computing the crack growth rate da/dn. A K-decreasing test was carried out, and ∆K

_{th}is given by the value of ∆K at da/dn = 10

^{−10}m/cycle.

^{−9}m/cycle to da/dn = 10

^{−8}m/cycle. da/dn is plotted against ∆K on a log-log scale in this domain in Figure 4 [30]. The intercepts A and B at da/dn = 10

^{−8}m/cycle, and C and D at da/dn = 10

^{−9}m/cycle were obtained for non-peened and peened specimens, respectively. The average value m of the values of AC and BD was determined. ∆(∆K) is defined as 10

^{m}. For the crack growth test at constant applied load, the applied stress was 160 MPa for the non-peened specimen and 200 MPa for the peened specimen.

_{p}= 0.88 s/mm were tested at the K-decreasing test and the constant applied stress test. In Section 3. Result, the averaged value and error bar, which show the maximum value and minimum value, were shown.

#### 2.2. Submerged Water Jet System

_{c}was 3 mm, the injection pressure was 30 MPa and the standoff distance was 222 mm, the same as used in a previous study [17]. Cavitation develops as cloud cavitation, and becomes ring vortex cavitation at the surface, before collapsing. Thus, the specimens were placed in a recess so that the whole surface was flat.

_{p}is defined as follows.

_{p}= 8 s/mm, the same as in the previous study [17].

#### 2.3. Submerged Pulse Laser System

_{a}and partly in water s

_{w}. The maximum energy, the beam diameter, the pulse width, and the repetition frequency of the laser pulses used in the experiments for this study were 0.35 J, 6 mm, 6 ns and 10 Hz, respectively. The focal length of the final convex lens was 100 mm and the spot size of the laser on the target was about 0.8 mm in diameter in these experiments, the laser pulse density was chosen to be 4 pulse/mm

^{2}, the same as in the previous study [17].

#### 2.4. Recirculating Shot Peening Accelerated by Water Jet System

_{p}was chosen to be 0.18, 0.29, 0.58 and 0.88 s/mm considering the results obtained and presented in the previous report [17].

## 3. Results

#### 3.1. Improvement of ∆K_{th} and ∆(∆K) by Peening

_{p}= 0.88 s/mm, the 1st set was shown by closed symbols and the 2nd set was shown by open symbols. In Figure 8, the number after “Shot peening (SP)” shows the processing time per unit length. For example, “SP0.18” means shot peened at t

_{p}= 0.18 s/mm. As shown in Figure 8, the ∆K-da/dn curves of the peened specimens have shifted to the right. ∆K

_{th}was calculated from these curve at da/dn = 10

^{10}m/cycle, and the values are 3.71 ± 0.11 MPa$\sqrt{\mathrm{m}}$ for the non-peened specimen, 6.33 ± 0.20 MPa$\sqrt{\mathrm{m}}$ for the laser peened specimen, 6.46 ± 0.25 MPa$\sqrt{\mathrm{m}}$ for the cavitation peened specimen and 6.52 ± 0.17 MPa$\sqrt{\mathrm{m}}$ for the shot peened specimen at 0.88 s/mm, as shown in Table 3. In the case of shot peening with short processing times per unit length, t

_{p}= 0.18, 0.29 and 0.58 s/mm, ∆K

_{th}is 5.51, 5.74 and 6.49 MPa$\sqrt{\mathrm{m}}$, respectively. As reported in the previous reference [17], the optimum value of t

_{p}is 0.88 s/mm, thus ∆K

_{th}cannot be improved by reducing t

_{p}. Thus, it can be concluded that peening at the optimum value of t

_{p}increases ∆K

_{th}by more than 70%.

_{p}= 0.88 s/mm, the 1st set was shown by closed symbols and the 2nd set was shown by open symbols. As shown in Figure 9, the number of cycles to fracture was extended by peening. The longest number of cycle to fracture was obtained with cavitation peening, followed by shot peening at t

_{p}= 0.88 s/mm.

^{−9}m/cycle to da/dn = 10

^{−8}m/cycle. The constants of Paris’ law and correlation coefficient of the data in Figure 11b,d are shown in Table 4. The number of data points and the probability of non-correlation were also shown in Table 4. The probability of non-correlation was less than 1% except shot peening at t

_{p}= 0.18 s/mm. Namely the relationship in Figure 11 is worth to discussing. The constant C was reduced by shot peening, cavitation peening and laser peening remarkably. In addition, the constant m of cavitation peening and laser peening increased. This result suggested that the crack growth at initial stage was reduced by cavitation peening and laser peening. As shown in Figure 11a–d, the ∆K-da/dn curves shift to the right and down after peening. This means that at equivalent values of ∆K, da/dn decreases after peening. If equivalent values of da/dn are compared, ∆K increases after peening. ∆(∆K) was calculated as described in Section 2.1, and is 2.05 ± 0.23 for laser peening, 2.21 ± 0.05 for shot peening at t

_{p}= 0.88 s/mm, and 2.32 ± 0.05 for cavitation peening. These values are also shown in Table 3. At the present condition base on the longer fatigue test changing with processing time per unit length and pulse density [17], both ∆K

_{th}and ∆(∆K) of laser peening were slightly smaller than those of shot peening at t

_{p}= 0.88 s/mm, as shown in Table 3. If the laser peening condition was changed, the result would be changed. In the case of shot peening at shorter t

_{p}, ∆(∆K) is 1.68 ± 0.32 for t

_{p}= 0.18 s/mm, 1.69 ± 0.10 for t

_{p}= 0.29 s/mm and 1.99 ± 0.11 for t

_{p}= 0.58 s/mm. As with ∆K

_{th}, ∆(∆K) does not improve with shorter t

_{p}. Thus, both ∆K

_{th}and ∆(∆K) increase with t

_{p}under the conditions used here.

_{p}= 0.18 and 0.29 s/mm. Please note that aspects of the peened surface treated by cavitation peening, laser peening and shot peening at t

_{p}= 0.88 s/mm were shown in the reference [17]. As shown in Figure 12, in the case of SP0.18 and SP0.29, plastic deformation pits introduced by the shots did not cover whole area, and not-peened surface was observed. This is one of reasons why data of SP0.18 was more scattered compared to the others.

_{th}and ∆(∆K). As ∆(∆K) is an improvement ratio against non-peened specimen, ∆K

_{th}is revealed by ∆K

_{th}’ which is normalized by non-peened one. As shown in Figure 13, ∆K

_{th}’ and ∆(∆K) are roughly in a linear relationship. Please note that the correlation coefficient for the 7 points is 0.963. This means that the probability of non-correlation is less than 0.05%. Thus, it can be said that the relationship between ∆K

_{th}’ and ∆(∆K) is highly significant. The values of ∆K

_{th}’ for cavitation peening and shot peening are similar. On the other hand, in the case of ∆(∆K), this is slightly larger for cavitation peening than for shot peening as shown in Table 3. This suggests that the effect on reducing crack growth is slightly larger for cavitation peening than for shot peening.

#### 3.2. Effect of the Mechanical Properties on ∆K_{th} and ∆(∆K)

_{th}and ∆(∆K), Figure 14 and Figure 15 show ∆K

_{th}and ∆(∆K) as functions of the mechanical properties, including (a) the Vickers hardness HV, (b) the maximum height of the roughness R

_{z}and (c) the surface residual stress σ

_{R}. The correlation coefficients for each of these curves are 0.855 for Figure 14a, 0.830 for Figure 14b, 0.924 for Figure 14c, 0.912 for Figure 15a, 0.658 for Figure 15b and 0.930 for Figure 15c. These results show that both ∆K

_{th}and ∆(∆K) increase with increasing Vickers hardness and compressive residuals stress. As is well known, peening is a mechanical surface treatment used for work hardening and introducing compressive residual stress by generating local plastic deformation. The surface roughness is increased by shot peening, laser peening and cavitation peening. This is why ∆K

_{th}and ∆(∆K) increase with increasing R

_{z}. However, the increase in roughness also increases crack initiation. Thus, a small increase in surface roughness by peening is best for improving the fatigue properties.

_{th}and ∆(∆K) are similar to each other as shown in Table 3. As shown in Figure 14a and Figure 15a, the Vickers hardness after shot peening, laser peening and cavitation peening are very close to each other. On the other hand, the surface roughness after cavitation peening is smoother than that after shot peening and laser peening, as shown in Figure 14b and Figure 15b. This shows that ∆K

_{th}and ∆(∆K) can be improved by cavitation peening with a smaller increase in surface roughness compared with shot peening and laser peening. When the compressive residual stress at the surface is compared as shown in Figure 14c and Figure 15c, the largest value is for shot peening, with cavitation peening second and laser peening the smallest. It can be concluded that the smallest possible surface roughness is obtained with cavitation peening, and that shot peening introduces the largest compressive residual stress, but with the greatest increase in surface roughness.

## 4. Discussions

_{fs}as a function of ∆K

_{th}and ∆(∆K), respectively. The value of the fatigue strength of austenitic stainless JIS SUS316L was obtained from reference [17], and is shown in Table 3. In Figure 16 and Figure 17, σ

_{fs}is revealed by σ

_{fs}’ which is normalized by non-peened one. As mentioned above, there are no big differences between the values of ∆K

_{th}’. The correlation coefficient between σ

_{fs}’ and ∆K

_{th}’ is 0.806. This means that the probability of non-correlation is larger than 20%. On the other hand, the correlation coefficient between σ

_{fs}’ and ∆(∆K) is 0.884, and the probability of non-correlation is about 12%. Thus, it can be concluded that the decrease in the crack growth rate rather than in crack initiation is the main reason for the improvement in fatigue strength.

## 5. Conclusions

_{th}and the stress intensity factor range increment ∆(∆K) obtained by means of a K-decreasing test and a constant applied stress test, respectively. Please note that in the present experiments, a load controlled plane bending fatigue tester was used. The results obtained for the material under test, JIS SUS316L, which were treated by shot peening, cavitation peening and laser peening, can be summarized as follows.

- (1)
- The values of ∆K
_{th}’ and ∆(∆K) are roughly in a linear relationship, even though the specimens were treated using different peening methods. Please note that ∆K_{th}’ is ∆K_{th}normalized by non-peened one. The reduction in crack growth after cavitation peening is larger than that after the other peening methods at equivalent values of ∆K_{th}’. - (2)
- The correlation between ∆(∆K) and the fatigue strength of the stainless steel specimens treated by the various peening processes is better than that between ∆K
_{th}’ and the fatigue strength. - (3)
- ∆K
_{th}and ∆(∆K) are increased with increasing surface hardness and compressive residual stress. - (4)
- The values of ∆K
_{th}and values of ∆(∆K) of the specimens after treatment by the different peening methods are each roughly the same. When the mechanical properties of the peened specimens were compared, it was found that the cavitation peened specimen was smoother than the others. The compressive residual stress at the surface introduced by shot peening was larger than that introduced by the other peening methods.

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 4.**Schematic diagram illustrating the definition of the stress intensity factor range increment, ∆(∆K) = 10

^{m}.

**Figure 8.**Relationship between the stress intensity factor range ∆K and the crack growth rate da/dn obtained by the K-decreasing test.

**Figure 9.**Crack length as a function of number of cycle obtained by the constant applied stress test.

**Figure 10.**Aspect of fractured surface after the constant applied stress test of specimens of (

**a**) non-peened, (

**b**) shot peening, (

**c**) cavitation peening and (

**d**) laser peening.

**Figure 11.**Relationship between the stress intensity factor range ∆K and the crack growth rate da/dn obtained by the constant applied stress test (continued).

**Figure 13.**Relationship between the stress intensity factor range increment ∆(∆K) and the normalized threshold stress intensity factor range ∆K

_{th}’.

**Figure 14.**Relationship between and the surface mechanical properties and the threshold stress intensity factor range ∆K

_{th}.

**Figure 15.**Relationship between the surface mechanical properties and the stress intensity factor range increment ∆(∆K).

**Figure 16.**Relationship between the normalized threshold stress intensity factor range ∆K

_{th}’ and the normalized fatigue strength σ

_{fs}’.

**Figure 17.**Relationship between the stress intensity factor range increment ∆(∆K) and the normalized fatigue strength σ

_{fs}’.

C | Si | Mn | P | S | Ni | Cr | Mo |
---|---|---|---|---|---|---|---|

0.014 | 0.63 | 0.97 | 0.030 | 0.004 | 12.03 | 17.45 | 2.05 |

Yield Strength (0.2%) | Tensile Strength | Elongation |
---|---|---|

304 MPa | 576 MPa | 52% |

**Table 3.**Threshold stress intensity factor range ∆K

_{th}, stress intensity factor range increment ∆(∆K) and fatigue strength σ

_{fs}.

Process | ∆K_{th}$\mathbf{MPa}\sqrt{\mathbf{m}}$ | ∆K_{th}’ | ∆(∆K) | Fatigue Strength σ_{fs} MPa | σ_{fs}’ |
---|---|---|---|---|---|

Non-peened | 3.71 ± 0.11 | 1.00 ± 0.03 | 1.00 ± 0.05 | 278.9 ± 5.2 | 1.00 ± 0.02 |

Shot peening | 6.52 ± 0.17 | 1.76 ± 0.07 | 2.21 ± 0.05 | 325.0 ± 8.7 | 1.17 ± 0.04 |

Cavitation peening | 6.46 ± 0.25 | 1.74 ± 0.09 | 2.32 ± 0.05 | 348.1 ± 8.4 | 1.25 ± 0.04 |

Laser peening | 6.33 ± 0.20 | 1.71 ± 0.07 | 2.05 ± 0.23 | 303.2 ± 8.4 | 1.09 ± 0.04 |

Heading | Paris’ Law Constants | Correlation Coefficient | Number of Data Points | Probability of Non-Correlation (%) | |
---|---|---|---|---|---|

C m/cycle | m | ||||

Non-peened | 3.8 × 10^{−10} | 1.4 ± 0.4 | 0.67 | 18 | 0.23 |

Shot peening (t _{p} = 0.18 s/mm) | 6.5 × 10^{−10} | 1.1 ± 1.5 | 0.38 | 5 | 53 |

Shot peening (t _{p} = 0.29 s/mm) | 5.5 × 10^{−11} | 2.0 ± 0.4 | 0.80 | 19 | 0.004 |

Shot peening (t _{p} = 0.58 s/mm) | 1.2 × 10^{−10} | 1.4 ± 0.4 | 0.55 | 29 | 0.2 |

Shot peening (t _{p} = 0.88 s/mm) | 7.6 × 10^{−11} | 1.6 ± 0.3 | 0.66 | 52 | 0.001 |

Cavitation peening | 3.7 × 10^{−11} | 1.8 ± 0.4 | 0.66 | 33 | 0.3 |

Laser peening | 5.3 × 10^{−11} | 1.8 ± 0.3 | 0.71 | 36 | 0.013 |

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**MDPI and ACS Style**

Soyama, H. Comparison between Shot Peening, Cavitation Peening, and Laser Peening by Observation of Crack Initiation and Crack Growth in Stainless Steel. *Metals* **2020**, *10*, 63.
https://doi.org/10.3390/met10010063

**AMA Style**

Soyama H. Comparison between Shot Peening, Cavitation Peening, and Laser Peening by Observation of Crack Initiation and Crack Growth in Stainless Steel. *Metals*. 2020; 10(1):63.
https://doi.org/10.3390/met10010063

**Chicago/Turabian Style**

Soyama, Hitoshi. 2020. "Comparison between Shot Peening, Cavitation Peening, and Laser Peening by Observation of Crack Initiation and Crack Growth in Stainless Steel" *Metals* 10, no. 1: 63.
https://doi.org/10.3390/met10010063