# Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens

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## Abstract

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## Featured Application

**7475-T7351 aluminum alloy are widely used for structural components in aerospace applications.**

## Abstract

## 1. Introduction

^{m}. The rate of crack advance per cycle is related to the stress intensity factor range ΔK. C and m are constants that depend on the material, environment and stress ratio. Crack closure is considered a very good approach to explain the influence of mean stress on the fatigue crack growth rate [6,8]. Bergner and Zouhar [6] showed that crack growth rates of various aluminum alloys varied by a factor of about 20 for some ΔK values, suggesting that the main factor to explain that discrepancies was the crack closure effect and the environment. Fatigue cracks tends to grow into a material region which has experienced large plastic strains due to its location in the crack tip plastic zone. Typically, this material is deformed beyond its elastic domain in the direction normal to the crack flanks. The trace of the plastic deformation produced is left in the crack’s path. It acts in the same way to an additional wedge stick between flanks, thus pre-straining them and partly protecting the crack tip from the action of posterior loads. This phenomenon is called plasticity induced crack closure and tends to decrease the effective stress intensity range thereby resulting in slower crack propagation rates [8].

## 2. Materials and Experimental Procedures

#### 2.1. Materials and Samples

_{UTS}= 490 MPa and σ

_{YS}= 414 MPa, respectively.

_{a}, root mean square (RMS) roughness R

_{q}and mean roughness depth R

_{z}. Table 2 summarizes the roughness parameters showing an increasing of more than 300% in the three roughness parameters for the peened surfaces.

_{3}, 1.5% HCl, 1% HF, and 95% H

_{2}O (volume) (Coventry, UK)) and taken micrographs using an optical microscope Leica DM 4000 M LED (Wetzlar, Alemanha). Figure 2 shows typical micrographs indicating that base material microstructure (Figure 2a) with elongated grains in the rolling direction. The plane selected to take micrograph was normal to the loading direction to demonstrate the shot-peening effect. Around the shot peened surface (Figure 2b), an increasing of grain deformation and roughness was observed.

_{0.05}= 157 for MP surfaces and HV

_{0.05}= 167 for SP surfaces. Therefore, shot peening surface hardness increased is more than 6%.

#### 2.2. Fatigue Tests

_{int}cycles, as shown schematically in Figure 3. The main purpose of these tests is to obtain the a-N and da/dN curves as a function of the stress intensity factor range ΔK to analyze the effects of the shot peening, specimen thickness and stress ratio.

## 3. Results and Discussion

_{1}and a

_{3}are the crack lengths at the specimen’s surfaces, a

_{2}is the current crack length at the center and a

_{0}is the initial crack length. The tunnel effect is a well-studied manifestation in fatigue crack propagation. Specimens stress state affect fatigue crack propagation, thus propagation rate is distinct at the crack flanks front relatively to specimens’ central points. The effect of stress state is usually explained by crack closure mechanisms. Typically, a plane stress state occurs at the surface that promotes crack tip plastic deformation and accordingly plasticity induces crack closure [32]. In turn, inside the specimen, there is a tri-axial stress state which prevents plastic deformation. As fracture surface roughness may be different, promoting roughness induces crack closure, especially for low values of ΔK [33]. This stress state effect on fatigue crack propagation slows down crack growth at the surface and hence promotes the tunnel effect. Several different parameters are used to understand to what extend tunnel affects the specimens’ behavior. The simplest and most common parameter is d/B (Figure 7c). Other used parameters are used and presented in the literature [34,35]. Note that the stable shape of the crack front has a uniform distribution of effective stress intensity factor range.

_{2}–a

_{0}). As expected, shot peening increases the retardation of the surface crack propagation observed by a higher tunnel effect parameter for crack length lesser than 10 mm. As mentioned above, tunnel effect can be caused by residual stresses profiles.

_{int}of 7500 and 15,000 cycles, as shown in Figure 3. The results obtained were compared with the reference constant amplitude loading tests. Figure 10a–d shows the collected results from the tests performed in specimens with 8 mm thick. The typical transient behavior after overloads is not detected in all blocks because of the reduced transient zone and the crack measuring method. The analysis of the figure shows that for MP specimens the fatigue crack growth rate reduction reaches the maximum value for N

_{int}= 7500 cycles, while for the SP specimens the crack growth rate continues to decrease, although slightly, when N

_{int}increases from 7500 to 15,000 cycles. For MP specimens, fatigue crack growth decreases more for 7500 cycles because induced plasticity of crack closure retardation is more critical (Figure 10d). This behavior cannot be confirmed when crack closure is not measured. For the SP specimens with R = 0, the behavior is similar. This effect is more noticeable for N

_{int}= 7500 cycles then for 15,000 cycles.

## 4. Conclusions

- -
- As a result of its small influence depth, the beneficial effect of shot peening on da/dN-ΔK curves is negligible, particularly for R = 0.4. However, this effect seems to increase near the threshold condition.
- -
- For both mechanically polished and shot-peened samples, a specimen’s thickness has only marginal influence on the stable crack propagation regime.
- -
- A significant effect of the mean stress was observed, particularly in near- threshold region.
- -
- Periodic overload blocks promote a reduction of the fatigue crack growth rate. For MP specimens, the reduction reaches the maximum value for the interval between blocks of 7500 cycles, while, for SP specimens, the crack growth rate continues to decrease for intervals of 15,000 cycles.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**Figure 1.**(

**a**) Shot peening machine; (

**b**) shot peened specimen; and (

**c**) dimensions of Compact Tension (CT) specimens in mm.

**Figure 5.**Thickness effect on the da/dN-∆K curves for specimens: (

**a**) MP, R = 0.05; (

**b**) SP, R = 0.05; (

**c**) MP, R = 0.4; and (

**d**) SP, R = 0.4.

**Figure 6.**Shot peening effect on da/dN-∆K curves for specimens: (

**a**) B = 4 mm, R = 0.05; (

**b**) B = 8 mm, R = 0.05; (

**c**) B = 4 mm, R = 0.4; and (

**d**) B = 8 mm, R = 0.4.

**Figure 7.**Impression marks of the crack path and tunnel effect: (

**a**) MP specimens; (

**b**) SP specimens; (

**c**) schematic indication for tunnel effect parameters; and (

**d**) tunnel effect value distribution for MP and SP specimens.

**Figure 8.**Residual stresses profile and X-ray diffraction peak breadth against the depth from surface for SP specimens. “○” corresponds to the residual stress in rolling direction (MPa) and “□” to the diffraction peak breadth (°).

**Figure 9.**R effect on da/dN-∆K curves for specimens: (

**a**) B = 4 mm, MP; (

**b**) B = 8 mm, MP; (

**c**) B = 4 mm, SP; and (

**d**) B = 8 mm, SP.

**Figure 10.**Block overload effect on da/dN-∆K curves for 8 mm thick specimens: (

**a**) R = 0.05, MP; (

**b**) R = 0.05, SP; (

**c**) R = 0.4, MP; and (

**d**) R = 0.4, SP.

Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti | Others | Al |
---|---|---|---|---|---|---|---|---|---|

0.1 | 0.12 | 1.2–1.9 | 0.06 | 1.9–2.6 | 0.18–0.25 | 5.2–6.2 | 0.06 | 0.15 | Remaining |

**Table 2.**Surface roughness parameters for Mechanically Polished (MP) and Shot Penned (SP) specimens.

Specimen | Parameter | Mean Value ± Standard Deviation (μm) |
---|---|---|

MP | Ra | 1.22 ± 0.02 |

Rq | 1.50 ± 0.02 | |

Rz | 7.74 ± 0.13 | |

SP | Ra | 3.70 ± 0.17 |

Rq | 4.60 ± 0.21 | |

Rz | 23.50 ± 2.00 |

B [mm] | Specimen | R | C | m | Validity [MPa m^{1/2}] | Correlation Factor |
---|---|---|---|---|---|---|

4 | MP | 0.05 | 1.41 × 10^{−8} | 3.94 | 7–13 | 0.995 |

4 | MP | 0.4 | 2.42 × 10^{−6} | 2.04 | 12–24 | 0.996 |

4 | SP | 0.05 | 2.95 × 10^{−7} | 2.94 | 8–14 | 0.970 |

4 | SP | 0.4 | 2.70 × 10^{−7} | 3.05 | 5–10 | 0.982 |

8 | MP | 0.05 | 2.72 × 10^{−8} | 3.89 | 7–12 | 0.996 |

8 | MP | 0.4 | 1.96 × 10^{−6} | 2.16 | 13–22 | 0.998 |

8 | SP | 0.05 | 2.53 × 10^{−7} | 2.97 | 9–16 | 0.991 |

8 | SP | 0.4 | 2.63 × 10^{−7} | 3.25 | 5–17 | 0.973 |

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**MDPI and ACS Style**

Ferreira, N.; Antunes, P.V.; Ferreira, J.A.M.; D. M. Costa, J.; Capela, C.
Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens. *Appl. Sci.* **2018**, *8*, 375.
https://doi.org/10.3390/app8030375

**AMA Style**

Ferreira N, Antunes PV, Ferreira JAM, D. M. Costa J, Capela C.
Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens. *Applied Sciences*. 2018; 8(3):375.
https://doi.org/10.3390/app8030375

**Chicago/Turabian Style**

Ferreira, Natália, Pedro V. Antunes, José A. M. Ferreira, José D. M. Costa, and Carlos Capela.
2018. "Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens" *Applied Sciences* 8, no. 3: 375.
https://doi.org/10.3390/app8030375