# Effect of Severe Shot Peening on the Very-High Cycle Notch Fatigue of an AW 7075 Alloy

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## Abstract

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^{7}–10

^{9}cycles. At higher loading amplitudes, the fatigue properties tended to decrease, most likely due to accelerated fatigue crack initiation on the surface damage features created by the peening process and also by rapid residual stress relaxation.

## 1. Introduction

^{8}of loading cycles. In several works [1,11,12,16,19,20,21], authors have shown that the application of the shot peening increases the fatigue strength and reduces the notch influence. In [22], the effect of the shot peening and severe shot peening treatment on smooth and notched fatigue performance of the X70 microalloyed steel was compared. Rotating bending fatigue test (run-out number at 10

^{7}cycles) results showed that while identical peening parameters were used to treat the smooth and notched specimens, its influence on fatigue performance was very different. In the case of smooth specimens, the conventional shot peening increased fatigue life by 5% and the severe shot peening by 10%. However, in the case of the notched specimen, the improvement was 20% for the conventional shot peening and 50% for the severe shot peening. The significant difference in the peening effect between the smooth and notched specimens was attributed to the fact that the shot peening can redistribute the steep stress gradient in the notch root.

## 2. Experimental Material and Methodology

#### 2.1. Material and Specimens

_{x}CuFe

_{y}, MgZn

_{2}, and Al

_{x}Fe

_{y}Si types [24,25]. The AW 7075 alloy in T6511 state is mostly strengthened by the coherent η’ particles [26,27,28]. As in this study, no additional heat treatments were applied, and no more attention was paid to the study and characterization of the strengthening particles.

#### 2.2. Surface Treatment

_{α}radiation with an irradiated area of 0.5 mm

^{2}. The diffraction signal from {222}

_{α}planes was collected at 2θ = 156.9°. The measurements were carried out using the sin

^{2}ψ method, with nine inclinations between ±39°. The measurements were carried out in axial (ϕ = 0°) and tangential (ϕ = 90°) directions. To obtain the depth profile of the residual stress distribution, the surface was gradually removed by electrolytic polishing.

#### 2.3. Fatigue Life Analysis

^{6}to 10

^{9}cycles. After reaching 10

^{9}cycles without fracture, the tests were terminated. During the tests, the specimens were cooled by the stream of dry air.

_{d}= 71.7 GPa, Poisson’s ratio ν = 0.33, and mass density ρ = 2800 kg/m

^{3}. The 3D model was prepared for modal and harmonic analysis. Modal analysis was used in order to design the specimen so that its intrinsic frequency of longitudinal vibrations is as close as possible to 20 kHz. The 3D model was important, especially in this step (modal analysis), because it allowed revealing other spatial shapes (not only the longitudinal one) and frequencies of intrinsic vibrations. It was necessary to ensure that other intrinsic vibration modes had their frequencies outside the interval 19.5–20.5 kHz. In the harmonic analysis, cyclic displacement with the amplitude of 20 μm and with the intrinsic frequency 19,995 Hz was applied at one of the ends of the specimen. Note that the actual intrinsic frequency was calculated in the modal analysis. The full method of the harmonic analysis was chosen.

## 3. Results

#### 3.1. Stress Analysis in the Notch

_{n}= 19,995 Hz (calculated within the modal analysis) was performed to describe stress concentration in the notch vicinity. Firstly, the basic approximation of the stress concentration at the notch was performed by the equivalent ellipse concept introduced by Murakami [32]. The equivalent ellipse concept estimates the stress concentration factor K

_{t}in the form of the following equation:

_{t}= 3.2.

_{xx}stress component (direction of the specimen’s axis) are shown in Figure 4. In Figure 4a, the stress concentration in the gauge length of the specimen is apparent. Detail of the stress concentration in the notch is shown in the section in Figure 4b—the highest stress is in the notch root, as expected. The stress distribution across the diameter in the middle of the specimen is shown in Figure 4c. The figure shows that the highest σ

_{xx}stress in the notch root is 457.32 MPa (for the displacement amplitude of 20 μm). The stress factor S

_{f}expresses the level of stress σ

_{xx}in the central part of the specimen caused by the displacement amplitude of 1 μm. This means that the maximum stress factor is S

_{f}(max) = 22.866 MPa/μm. The average stress across the diameter is calculated according to equation (2):

_{xx}(avg) = 186.308 MPa (again for the displacement amplitude of 20 μm). Consequently, the average stress factor is S

_{f}(avg) = 9.315 MPa/μm. Finally, the stress concentration factor following from FE analysis is: K

_{t}= σ

_{xx}(max)/σ

_{xx}(avg) = 2.455.

_{t}= 3.2) and from FE analysis (K

_{t}= 2.455), it is obvious that the analytical solution is more conservative and predicts higher stresses in the notch root. For determination of the stress level in the tested specimens, the stress factor S

_{f}(avg) = 9.315 MPa/μm was used. The S

_{f}(avg) multiplied by the amplitude of loading displacement corresponds to the nominal stress in the cross-section, where failure occurs. In this study, the nominal stress was used, so it is comparable with other works where smooth specimens were tested.

#### 3.2. Effect of Shot Peening on the Surface State

#### 3.3. Fatigue Test Results

^{6}and 10

^{9}of loading cycles is less than 40 MPa. This number is even lower when considered severely shot peened specimens (Δσ

_{a}less than 20 MPa). The fatigue limit (for 10

^{9}cycles) for NP specimens was recorded to be 93 MPa. The applied shot peening process is found to be beneficial in terms of the fatigue limit, as, in the case of the SSP specimens, the fatigue limit was recorded to be 107 MPa (an increase of 15%). The regression curves of the NP and SSP specimens are crossing at approximately 10

^{7}cycles, revealing that the positive effect of the shot peening process on the fatigue properties of notched specimens is limited only for the very-high cycle region, while for the lower lifetimes (and higher loading amplitudes), the severe shot peening reduces the fatigue strength.

## 4. Discussion

^{9}cycles is increased from 93 MPa (for NP) to 107 MPa (SSP), which represents an increase of 15%. The analysis of regression curves shows that the beneficial effect of the SSP is recorded for the region beyond 10

^{7}loading cycles. On the other hand, for the shorter lifetimes and higher loading amplitudes, the SSP causes worsening of the fatigue properties. According to the regression curve coefficients, the fatigue strength for the 10

^{6}cycles is, in the case of the SSP specimens, 7% lower than for the NP specimens. Similar behavior is also observed in other works, in which ultrasonic fatigue testing of NP and SP specimens has been performed [3,33]. In these works, the phenomenon is attributed to a higher residual stress relaxation rate due to higher applied loading amplitudes at a lower number of cycles of interest. Since the elastic residual stresses are mostly resulting from increased dislocation density, the application of dynamic loading causes further slip of the bonded dislocations. In this case, the fatigue properties improvement by the residual stress effect is lost in the first few thousand loading cycles, and only the effect of the grain refined layer takes place [34]. However, the possible positive effect of the grain refined layer is strongly opposed by the negative effect of the increased surface roughness, which causes acceleration of the fatigue crack initiation. Under higher loading, also the fatigue crack propagation increments are higher, so the crack relatively quickly grows through the grain refined layer, and its effect becomes negligible. The fatigue curves of both states exhibit low steep character. Benedetti et al. [12] tested fatigue properties of the same alloy after the shot peening process, applied on the specimens with various notches. They reported a similar low steep character of the fatigue curves. On the other hand, they recorded “knee” behavior, with run-out specimens after approximately 10

^{6}cycles for the specimens with a notch with similar and higher stress concentration factor. After the application of the shot peening process, they observed a change in the behavior, and only specimens with the sharpest notch exhibited such behavior. It should be mentioned that completely different specimen shapes and loading modes (four-point bending with R = 0.05) and also different shot peening parameters were used, resulting in the different residual stress profiles and surface texture. This can explain the differences in the observed behavior from the results presented in this study.

^{5}cycles. On the contrary, for the notched specimens, the crossing of the fits of S-N curves is recorded at approximately 10

^{7}cycles. This can be attributed to the strong notch sensitivity of the AW 7075 alloy due to its high strength in the artificially aged condition.

## 5. Conclusions

^{7}–10

^{9}of loading cycles. The severe shot peening process introduced the compressive residual stress field, which, together with the grain refinement, successfully increased the number of cycles necessary for fatigue crack initiation and also slowed down the propagation of the short cracks.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**The microstructure of the experimental material in the as-delivered state, longitudinal section, (

**a**) Electron backscatter diffraction method (EBSD) inverse pole figures (IPF) map with corresponding pole figures; (

**b**,

**c**) Electron channeling contrast imaging (ECCI) images, showing sub-grain structure.

**Figure 3.**(

**a**) The 3D model of the notched ultrasonic fatigue specimen; (

**b**) detail of the element’s distribution in the vicinity of the notch.

**Figure 4.**Analysis of the stress distribution throughout the cross-section under the notch. The amplitude of displacements A = 20 μm. (

**a**) stress concentration in the gauge length of the specimen, (

**b**) detail of the stress concentration in the notch, and (

**c**) stress distribution across the diameter in the middle of the specimen.

**Figure 5.**SEM micrographs, showing the surface of the notch area of the not peened (

**a**,

**b**) and severe shot peened specimens (

**c**,

**d**).

**Figure 6.**ECCI images of the near surface region; (

**a**,

**b**) NP (not peened) specimen; (

**c**,

**d**) SSP (severe shot peening) specimen.

**Figure 7.**The residual stress profile measured in the notch of not peened (NP) and severe shot peened (SSP) specimens.

**Figure 8.**The results of the fatigue tests of the notched specimens; open symbols with arrows represent the run-outs.

**Figure 9.**Fracture surface of the NP specimen fractured at σ

_{a}= 102 MPa, N

_{f}= 1.06 × 10

^{8}. (

**a**) overall view, the dashed red line encloses the area of final fracture; (

**b**) area of the fatigue crack initiation and propagation; (

**c**) detailed view of the area of stable fatigue crack propagation; (

**d**) area of the final fracture.

**Figure 10.**Fracture surface of the NP specimen, fractured at σ

_{a}= 130 MPa, N

_{f}= 2.1 × 10

^{6}. (

**a**,

**b**) overall view, the dashed red line encloses the area of final fracture; (

**c**) area of the fatigue crack initiation and propagation; (

**d**) detailed view of the area of stable fatigue crack propagation.

**Figure 11.**Fracture surface of the severely shot peened specimen fractured at σ

_{a}= 112 MPa, N

_{f}= 6.91 × 10

^{8}. (

**a**) overall view, the dashed red line encloses the area of final fracture; (

**b**) area of the fatigue crack initiation and propagation; (

**c**) detailed view of the initiation site; (

**d**) detailed view of the area of stable fatigue crack propagation.

**Figure 12.**The fracture surface of the SSP specimen, broken at σ

_{a}= 130 MPa, N

_{f}= 2.1 × 10

^{6}. (

**a**) overall view, the dashed red line encloses the area of final fracture; (

**b**) area of the fatigue crack initiation and propagation; (

**c**) the initiation site with clearly visible deformed layer; (

**d**) area of stable fatigue crack propagation.

**Figure 13.**The comparison of the fatigue properties of AW 7075 alloy after severe shot peening—notched and smooth (reproduced from [3], with permission from Springer Nature, 2017) specimens.

**Figure 14.**Transversal section of the run-out SSP specimen in the notch area, showing the presence of the discontinuities in the hardened layer (ECCI). (

**a**) discontinuities in the hardened layer and (

**b**) detail of hardened layer.

Al | Zn | Mg | Cu | Cr | Fe |
---|---|---|---|---|---|

87.1–91.4 | 5.1–6.1 | 2.1–2.9 | 1.2–2 | 0.18–0.28 | max. 0.5 |

Mn | Si | Ti | Ti + Zr | Other-each | Other-total |

max. 0.3 | max. 0.4 | max. 0.2 | max. 0.25 | max. 0.05 | max. 0.15 |

**Table 2.**Basic mechanical properties of the experimental material [7].

UTS (MPa) | Elongation (%) | Reduction of area (%) | Hardness (HV10) |
---|---|---|---|

631 | 4.9 | 15.7 | 175 |

Regression Coefficients | Fatigue Strengths Calculated Based on the Regression Coefficient | |||||
---|---|---|---|---|---|---|

Specimen | Coefficient of Fatigue Toughness σ _{F} (MPa) | Exponent of Fatigue Life Curve b | Number of Cycles | NP | SSP | Difference (SSP vs. NP) (%) |

(MPa) | ||||||

NP | 257.9 | −0.046 | 10^{6} | 136 | 127 | −6.62 |

10^{7} | 123 | 121 | −1.63 | |||

SSP | 170.15 | −0.021 | 10^{8} | 111 | 116 | +4.50 |

10^{9} | 99 | 110 | +11.11 |

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**MDPI and ACS Style**

Jambor, M.; Trško, L.; Klusák, J.; Fintová, S.; Kajánek, D.; Nový, F.; Bokůvka, O.
Effect of Severe Shot Peening on the Very-High Cycle Notch Fatigue of an AW 7075 Alloy. *Metals* **2020**, *10*, 1262.
https://doi.org/10.3390/met10091262

**AMA Style**

Jambor M, Trško L, Klusák J, Fintová S, Kajánek D, Nový F, Bokůvka O.
Effect of Severe Shot Peening on the Very-High Cycle Notch Fatigue of an AW 7075 Alloy. *Metals*. 2020; 10(9):1262.
https://doi.org/10.3390/met10091262

**Chicago/Turabian Style**

Jambor, Michal, Libor Trško, Jan Klusák, Stanislava Fintová, Daniel Kajánek, František Nový, and Otakar Bokůvka.
2020. "Effect of Severe Shot Peening on the Very-High Cycle Notch Fatigue of an AW 7075 Alloy" *Metals* 10, no. 9: 1262.
https://doi.org/10.3390/met10091262